Bottlehead Forum
General Category => General Discussion => Topic started by: mediumjim on December 30, 2010, 05:42:05 PM
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I had my Parasound HCA-1200II in storage and pulled it out last week with the plan to build a 2nd system. This particular Parasound was a good one. Well I get around to hooking it up and the left channel is distorted and was cutting out. I do a simple DC-Offset test on it and everything was in proper spec. I cleaned the contacts but it still was a no go.
This led me to procuring a clean and recently serviced original Dynaco ST-70. The power supply has been recapped, otherwise is all original. From what I've read, these are great sounding amps. Does anyone here have first hand experience with them? If so, what can I look forward to? It will be to me in about a week or so as it is shipping cross country to me.
It will be mated with a McIntosh MX-110 Preamp/Tuner. The speakers I plan on using are Meadowlark Swifts that are 8 ohm and a true 89db. The other possibility is a restored pair of JBL L100A Century's that are 91db. Any thoughts on what speakers will perform best?
Many thanks and Happy New Year!
Jim
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Well, you should have plenty of power for either of your speaker choices.
Beyond that, almost everyone has their favorite modification or new parts. This was a very popular amp in its day, so there are still plenty around, and they end up in a lot of hands.
Mine is pretty much stock, too, and I love it for what it is: A very good push-pull tube amp.
Not the ultimate this or that, just a very good amp.
This means that the bass will likely not impress like a good Class A Mosfet amp, but it will be very enjoyable to listen to if the load is not too difficult, and you're not asking it to fill a stadium with 32' organ pipes!
Make sure you have nice matched output tubes, because you will be setting bias per pair, not per tube.
And, if you have pets or kids, keep the cage on; it isn't as pretty, but with the cage off, high Voltages are accessible on the top of the driver board.
If you do a search here, there was a discussion of this amp just a few days ago, with lots more information and opinions.
Enjoy!
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jc is correct it is a good amp-you will like it-i had 3 why i left the last one go i don't know--i got all crazy and redid all of them soup to nuts-ahhhh now i remember --HOW I GOT HERE- using the dynaco and their incredible following and documentation as a learning tool and then looking for something else led to bottlehead kits-a bottlehead foreplay ----you may get more bass response by changing the coupling caps to .47 or 1 uf-you can get more info at the dynacodoctor forum or dynacotubeaudio forum --there is a great deal of info on them and a large assortment of mods and tweaks
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Both Paully and I have ST-70s. Mine is the stock circuit. His uses Magnequest output transformer and a different power transformer. I don't remember about his power supply caps. Mine is an Antique Electronic supply drop in replacement. Both have all new resistors. The chassis, iron and circuit board are all that are original in mine but the circuit is only slightly modified.
Maybe Paully will post on his modifications.
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Thanks for all the great input. Mine was done by a guy in New Jersey named Carl and from I can gather he has been restoring them for a long time and has a good loyal group of followers. The one I got is pretty much stock. Many years ago when I was a just a teenager, I friend of a friend had built one and drove a pair of A7 VOTT's to impressive levels.
I do know about tubes and I will be replacing the JJ's that it will be with if they don't do it for me. The funny thing is that replacing the EL34's will cost more than the amp if I go with Mullards or Fat Bottle RCA's. They are supposed to bias a 1.56 and I hear that running them a bit cooler, say 1.45 will increase the tube life and provide a better soundstage...will have to experiment.
Happy New Year!
Jim
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Jim,
Sorry, I just realized that I didn't tell you what to expect. Expect good power. I had speakers that were mid 80s dBW in sensitivity and they rocked with the ST-70. This was in a room of enormous volume. These speakers, Fuselier 3.8Ds, are the other end of the scale from Altec A7s. So power is there.
All the tubed goodness you expect was there. There wasn't the SET purity but soundstage, imaging, detail, smooth high frequencies and pretty good bass was there. The ST-70 and a Krell KSA-250S were swapping in and out of my system. The ST-70 didn't compare under 250 Hz. But the Krell ended up being a 145 pound ST-70 amp stand.
Bias can be where you want it. There are two bias resistors. As long as you measure and hand match a pair you just divide by 10 and you have the magic point. As a matter of fact you should measure the resistors that are in the amp and go with 1/10 of the resistance in volts for an accurate measurement. Many of us who don't need the maximum power have tweaked the bias lower than Dynaco's spec.
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Hi Grainger:
Thanks for the advice on how to properly Bias them. I often run my Marantz 9's Triode and with my KEF 104/2's that even though KEF says 92db, they're closer to 87db...if I really push them I get sibilence and some distortion. The good thing about original Dynaco's is that they did use great iron and as you said, plenty of current.
If the bass is lacking, I may run a pair of powered subs to address that issue. Another choice for speakers is a pair of Energy 22 Reference Connoisseurs that have tight bass down to 40hz and can reach 32 or so if set up correctly and are 8ohm 89db so they will be perfect for the job.
At some point, I will get another ST-70 and set them up as close as possible and run them as Mono Blocks...
Jim
P.S. I was going through my tube stash as came upon a quad of Groove Tubes GT-EL34B's that are power rated 7 (late breakup). These are made in Czechoslovakia at the old Tesla facility and are from Telefuken tooling. Also found was a NOS Tung Sol 5U4GB. I will need to test the EL34's (means I need to dig out my Eico 667) and see how close they match up for Mutual Conductance, Leaks, Shorts. If they pass at this point, I wiill heat them up and check the ma on each.
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I used an ST-70 for years.
Assuming a stock circuit expect it to sound rather romantic, with "lush" rather then deep lows and soft rather then extened highs. It's faults tend toward the pleasant and musical, more so if still running the tube rectifier.
The selenium stack in the bias supply should have been replaced with a modern diode by the fellow doing the rebuild. The selenium diode WILL fail if not replaced.
Do make sure the bias resistors measure at 15.6 ohm if you are setting bias via the original Dynaco instructions. If the rebuilder used a different value make sure he tells you how to set bias.
The 7199 driver tubes in the orginal circuit are getting hard to find. Sovtek did do a run a couple of years back but the orignal Dyna marked USA sourced tubes were better sounding. There are tube sockets available to use 6U8 tubes in place of the 7199. Make sure the modder has not done this already.
http://www.dynakitparts.com/store/product.aspx?id=277
I think a pair of ST-70 will be better used for biamping then as mono-blocks. I don't know how loud you listen but I would guess the Meadow Larks or JBLs will go as loud as you need with one ST-70. If you really need more grunt I would look at getting a pair of Mark IIIs instead of running ST-70s in mono.
With updated driver and power supply boards, which can make the unit less "romantic", a ST-70 is competative in sound with very good modern amps.
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Hi Wanderer:
Here's a couple of pics showing the power supply from both sides.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimagehost.vendio.com%2Fpreview%2Fbi%2Fbigjimmead%2FST-70a.jpg&hash=cc1ad04e14f6c4f8daca89fc931c3309e66a21bb)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimagehost.vendio.com%2Fpreview%2Fbi%2Fbigjimmead%2FST-70.jpg&hash=fbe48e09dc46db1b5c69ba7cffce86b20c7635d5)
Thanks,
Jim
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That 5U4 will most definitely work as a sub, but I think you will enjoy the GZ-34 more.
Dynaco originally used precision 15.6 Ohm resistors to feed the cathodes of each pair of EL-34s to facilitate a quick calibration of the Volt meters of the era before setting the bias. 1.56 Volts was the Voltage of a fresh carbon-zinc flashlight battery, so you set your Volt meter with that, then set the bias on your amp.
This set the bias, as you may have surmised, to 100 mA per pair, or 50 mA per EL-34, if they were well matched.
So, yes, you may indeed find that this setting is a little hot for your tastes.
Then again, maybe not. Marshall guitar amps specified that one used a scope to set bias, and watched the scope until the last vestige of cross-over distortion disappeared on an amp being run just under clipping. This method generally worked out setting the bias remarkably close to that 50 mA point.
You're in for some fun!
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OK, that looks like the original Selenium rectifier for the bias supply. If it is still in use, you will want to replace that with silicon. I am told that you don't want to breath the results of the Selenium going 'poof'.
I just mounted a 3-lug terminal strip to the screw that holds the Selenium stack in place, and mounted a generic 1A silicone diode to that. That way, I just removed the wires from the Selenium and moved them "up" to the terminal strip.
It looks like it has new electrolytics in the bias supply, a good thing.
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I breezed through the posts, not sure if what I would suggest was mentioned. If that is not a new can cap, that should be replaced sometime in the very near future. The old ones go bad. The old tube sockets can get loose and lose contact, so whenever I have rebuilt one I have replaced those. Just something to watch if there is an issue. Do some reading on using a 5u4, is that one that can tax the power transformer? I can't remember to be honest. But the rectifier does matter so use a good one (5AR4 probably being the best and what the amp was designed around) and make sure it isn't rated too high for the old transformer which is only 3A if you use something else. The new ones are usually 5A.
The 7199 are expensive. I didn't want a new circuit for the driver board so I bought a new board using the old circuit for new parts and then also bought adapters from ebay to be able to use 5GH8A and I like those as much as the 7199, as far as I could tell. And thems 5GH8A tubes is cheap!!! So that is something you could try without changing anything other than buying an adapter and a few tubes.
I love my Dynaco. Even though I use Paramounts now, I have no intention of letting it go. Nice well rounded sound. It is a great amplifier. The output transformers are what everyone fights over, so you can mod other things and improve the sound. I would recommend upgrading the power supply via Triode Electronics power supply or some such and then you don't have to worry about the can cap, which is something to worry about otherwise.
Don't forget the Dynaco Doctor forum and the Vintage Tubes asylum. Lots of help and reading there. Curcio, DynakitParts, and the Vintage asylum have all the parts, upgrades, and help you could want and then some.
That's all I can think of at the moment.
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Thanks JC/Paully:
It is being shipped to me with a 5U4GB...I do have some nice Mullard 5AR4's that I can try. The rebuilder is actually from NY, sorry and is well known for his excellent work. With that said, if he felt the Selenium Circuit was risky or bad, it would have been replaced as he typically does. At some point I will update the Selenium to Silicon as I will Speaker Binding Posts. What's interesting, I'm seeing that he typically uses 5U4GB's.
7199's are still out there and I plan on getting a backup pair and keeping it original.
Jim
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Jim,
Your idea of "recently serviced" and mine are different. No component there looks like it has been replaced with the possible exception of the bias caps, not the B+ cap.
The 10 ohm resistors that have caused problems with other ST-70s and Foreplays are on the circuit board dead center up near the input jacks. You can try a jumper around them to see if it decreases hum.
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I think the Dynaco Doctor has replacements for the screw terminal strips for the speaker connections, if you want to go that way. Otherwise, I've seen people mount standard 5-way binding posts onto a small strip of plexiglass, and replaced the terminal strips with those. They should also have a download available of a .pdf of the original owner's/construction manual at Dyanaco Doctor. You'll definitely want to hunt that up.
The screw terminals, of course, allow for having all the different impedances from the output transformer available at the rear of the amp.
The 5AR4/GZ-34 was indeed the original in the amp, though, IRRC, the 5U4 was even in the manual as a usable replacement. I know I used one once, when my last Mullard GZ-34 decided to give up one night.
Of course, that's the neat thing about tubes: If the manufacturer is OK with it, it's an easy matter to find out which you prefer!
OH, BTW, the Voltage to the plates of the output tubes will change, depending on which type of rectifier tube you choose. So, be prepared to reset the bias each time you swap.
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Correct me if I am wrong, but everything I remember reading was that the 5U4GB was in the manual as nothing more than a temporary sub. It pulls 3 amps versus the 5AR4 1.9 and is therefore taxing the old power transformer which is rated for a max of 3. Also it is direct heated and starts conducting very quickly also taxing the power supply. It is not a long term option for a Dynaco without an upgraded power trannie. I have the upgrade so I wouldn't be concerned. When I had the old power transformer I looked into it and decided it wasn't a good idea. It is cheap so people are tempted to use it but you have to do some upgrades to be safe doing so. At least that is what I recall.
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Well, I believe you are correct about the respective heater currents, but I would have to dig up my manual to ascertain what Dynaco said about the matter. Hey, I don't even remember the transformer specs off the top of my head!
Hopefully, I will run across that info before too much time has passed.
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Grainger:
Thanks, I will have to give it a listen to to determine if anything needs to be corrected/replaced. This guy has rebuilt/restored hundreds of Dynaco's and does only what needs to be done to put them back in spec or better than spec.
Jim
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Was going through my tube stash and a found a sleeve of NOS Richardson 5AR4's that are really Red Bank Bendix 6106's. Made in 1987. I also found a Amperex Holland Metal Base GZ34/5AR4. The guide pin is broken, but it is nice and strong.
On the other front, the GT-EL34B's tested strong for mutual conductance from 116 to 138, with 80 being borderline weak. Not a tight match for mutual conductance. Next test will be to get them in the ST-70 and do a Mutual conductance test under real load.
Okay, time for this west coast kid to prepare to celebrate the new year!
See you all next year!
Jim
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I dug up the Stereo 70 manual, and the 5U4 is indeed listed under the "emergency" replacements section. I believe both it and the 5AR4/GZ-34 are directly heated rectifiers, so I don't think that part applies.
I have not yet located my specs on the PT 5V winding. I can only say that I used a 5U4 in mine daily for some time while locating/waiting for a replacement 5AR4/GZ-34 without any evident issues.
Happy New Year, all!
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Happy New Year, survived another one!
The guy who serviced it indicated that he feels that 5U4GB's are more stable that older 5AR4's. I will use a GZ34/5AR4. It should also be noted that when real clean and original units comes his way he will service them from ones he is restoring, e.g., use resistors, caps, etc., that are well within spec from them. This apparently is the case with mine. This is more than fine with me.
It will ship Monday and I should have it by the following Monday.
I might need to sell part of my vintage tubes to afford a quad of Mullard XF2's or similar and a good duet of 7199's...only if the ST-70 has the goods in the tone department.
Jim
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I can't speak from real experience, but man I don't think a matched quad of Mullard xf2 are even remotely worth it! Output tubes today are, by and large, considered every bit as good as most of the historic counterparts. There are so many really good output tubes for the Dynaco; EH 6CA7, Genelex KT77 reissue, JJ E34L, etc... Or even just some old stock Tesla EL34. Try those first, they really are very good! But of course its your money and you have to go at this hobby as you see fit. :)
That is the first I have ever heard a 5U4 thought to be significantly more stable than a 5AR4. Could be, I won't argue but I would want some real evidence to support that claim. I would be interested to know actually.
That can cap is a ticking time bomb. I can understand why you would accept a man's opinion who services these with regularity. But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject. Anyway, don't want to worry you too much. Listen and enjoy! You made a good decision getting a Dynaco. You will enjoy it.
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Not at all worried about it. Sometimes on forums, a very few control the discussions and a bad experience can outweigh all the good ones. I've been around vintage tube guitar amps for the last 20+ years and I truly believe in fixing only what is in need of fixing. I've seem 50+ year old electrolytic caps that are still right and new amps less than 5 years old with bad ones. Most of the old Guitar Tube Amps have selenium rectifier circuits that have presented no problems. My point is simple, each amp has to be taken on a case by case basis.
One of my friends is Aspen Pittman who is/was Groove Tubes and he (as do I) would disagree with you regarding vintage Vacuum Tubes versus new ones. The metallurgy is far superior in vintage tubes, as is the overall construction and tolerances. Find me a EL34/6CA7 with true welding that will last several thousands of hours in normal usage.
I do agree that it would be a little foolish to spend $500.00 for a Dynaco ST-60 and then spend around 1K for tubes for it. There are some nice EL34's from the late 70's to mid 80's that I will try to find, but these actually are more scarce.
Thanks,
Jim
I can't speak from real experience, but man I don't think a matched quad of Mullard xf2 are even remotely worth it! Output tubes today are, by and large, considered every bit as good as most of the historic counterparts. There are so many really good output tubes for the Dynaco; EH 6CA7, Genelex KT77 reissue, JJ E34L, etc... Or even just some old stock Tesla EL34. Try those first, they really are very good! But of course its your money and you have to go at this hobby as you see fit. :)
That is the first I have ever heard a 5U4 thought to be significantly more stable than a 5AR4. Could be, I won't argue but I would want some real evidence to support that claim. I would be interested to know actually.
That can cap is a ticking time bomb. I can understand why you would accept a man's opinion who services these with regularity. But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject. Anyway, don't want to worry you too much. Listen and enjoy! You made a good decision getting a Dynaco. You will enjoy it.
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I won't come back with condescension to match, but I would hate for someone reading this to think that preventive maintenance on a Dynaco ST-70 is a non-issue and that 50 year old electrical parts should just be left until they present a problem. I can't think of anyone on the Dynaco board who has ever suggested otherwise. But perhaps that isn't really the point, we simply disagree on what "merits fixing" up-front and what can be left alone. I do tend to err on the side of caution but then I get that from Grainger! Regardless, enjoy!
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I would tend to agree with that assessment. BTW, please tell your friend that I keep his book (The Tube Amp Book) close at hand!
I ran my Stereo 70 for many, many hours before I did anything to it beyond replacing the tubes, because one of the 7199's was defective and the EL-34s were a mismatched bunch of questionable lineage.
I eventually replaced the can cap, not because it wasn't working, but because a drop-in replacement of new manufacture became available and I really had no way of testing the actual capacitance of the sections on the original one at operating Voltages. IIRC, the original was an AeroVox, and as far as I know, it is still a functional part.
The Selenium rectifier was somewhat of a moral dilemma once I read that one that gives up often spews toxins into the air. Since I would be using the amp around others, I wasn't comfortable exposing them to an unknown risk. So, as a compromise, I left the Selenium in place, but simply took it out of service. Again, I had played the amp "as is" for hours at this point.
My decision to retain as much of the original amp as possible centered on its heritage. When I first acquired it, I naturally wanted to inspect it "under the hood". When I turned it over and removed the bottom plate, I discovered a name and date scratched into the underside of the chassis with an engraver. The date put it at Christmas of 1959, but the name was of an old acquaintance since passed who had owned the first music store in my town to sell drums, guitars, and amps!
At that point, I knew that fancy new driver and PS boards were out, and hunted up the NOS and new tubes I needed, set the bias, and put it into service!
Yours looks to be in very good shape, physically better than mine was.
The link should get you to a .pdf of the original manual, and the site has lots of good stuff:
www.curcioaudio.com/st7_mnl.pdf (http://www.curcioaudio.com/st7_mnl.pdf)
Happy New Year!
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I won't come back with condescension to match, but I would hate for someone reading this to think that preventive maintenance on a Dynaco ST-70 is a non-issue and that 50 year old electrical parts should just be left until they present a problem. I can't think of anyone on the Dynaco board who has ever suggested otherwise. But perhaps that isn't really the point, we simply disagree on what "merits fixing" up-front and what can be left alone. I do tend to err on the side of caution but then I get that from Grainger! Regardless, enjoy!
Paully, I wasn't being condescending, merely an observation. Sure, eventually everything will go bad, but since its all well within proper specs why mess with it? I personally agree with the way Carl tastefully serviced the ST-70 he's shipping to me. I could have easy opted for one that he fully restored, but that wasn't what I was seeking. Our differences are of philosophy and personal choice and I tend to err on the side of originality on vintage gear when possible. Besides, I have one of the best amp guys if something goes wayward on it to put it back right.
Who knows, I might get it and not like the way it sounds and then have to do some tweaking and upgrades.
On Audiogon right now is a couple of original ST-70's that I was considering...would you get one and then fix what ain't broke?
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JC:
Pretty cool having one from 1959! I will have to check the EIA coding on the Tranny's on mine to see when the kit was assembled. I may opt to do the same thing to bypass the Selenium and go with a new "Can" down the road. Too bad Aspen sold GT to Fender as I used to be able to search the back room for tubes he had. I'm sure that is where I got the EL34's that I hope will match nicely for ma output... It is a conundrum to spend twice and much for tubes than the unit itself, but at the end of the day, tone is king.
For the record, my McIntosh MX-110 is from 1961 and is very complete and original and was recently serviced and aligned and only needed a couple of caps and tubes...it would be real cool if the ST-70 I'm getting is from 1961!
Jim
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"On Audiogon right now is a couple of original ST-70's that I was considering...would you get one and then fix what ain't broke?"
Yes, I have a severly overstated tendency to do just that... I do love upgrading parts and tinkering. I have some Altecs I am getting ready to "fix" that play quite well. Can't help it. But I understand the tendency to keep it stock to a degree as I kept the stock 7199 circuit even if I did upgrade the board and its parts. I wanted the orginal sound, just more of it! Sorry if I misinterpreted condescension however, just how I read it. But there are multiple issues. The cap doesn't even have to fail to be taxing to the power tranny and rectifier as it ages. Like you said, it is just which side we want to err on. For me we buy the Dynaco for the output transformers mainly. Leaving it stock is good, but not when the part in question most likely detracts from the sound as it was used as an economy and when its failure can cause siginificant damage I start upgrading. My nature. Maybe I get something better, maybe I have nothing like the designer intended. But anyway, I have had two Dynacos, the can cap failed on one, the other I never gave it a chance. I may be overeacting as others may be doing as well. It doesn't matter, we have said our piece and its your amp and as you have made clear you have your own experience with components over a long period of time with guitar amps. So again I say congratulations, I would love to know how you like the sound after you have listened for a while. This is all a side issue and it really does come down to personal experience and opinion, I don't know of any hard data to back either of us up one way or the other. So again, enjoy and please do post back on your assessment of the sound. I think I will be plugging mine back in for a while when I get back home.
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I wouldn't be surprised if you learn that it is in that same neighborhood regarding its age.
The one I got had been serviced at some point by someone who learned about servicing those new-fangled printed circuit boards in military training: When in doubt, cut the trace! I discovered that the ones Dyna used were remarkably tough, as I neatened up his work!
Yours, I'm happy to say, looks like it avoided such "service"!
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IThat can cap is a ticking time bomb. I can understand why you would accept a man's opinion who services these with regularity. But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject.
Not to flog a dead horse but back in the mid 1970s to early 1980s I fussed about a number of Dynaco MKIII, ST-70s, PAS preamps and even a couple ST35s.
I agree whole heartedly with Paully here. Based on my experiance with a number of Dynaco (and Scott..and Fisher tube units) the quad caps and original selenium rectifier have a high probablity of failure.
One of the issues - at least for the quad cap - is that common domestic AC voltage has risen in most places in the USA from the 110 to 115 volts in use when the ST-70 was designed to 125 volts or even 130 volts now. The safety margin for over-voltage at start up has been reduced because of this. Keep in mind we are talking about a power supply that features a step-up high voltage transformer that only magnifies the change.
I have also seen bad connections in orignal old tube sockets cause a thermal runway and distruction of output tubes. Given the relative cost of tubes it seems odd economy to not replace some less expensive old parts.
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I love good passionate discussions...
Don't you think that if the Selenium/Quad Cap made it this far that it did so for a good reason? As I said in one of my posts, I probably will do a selenium bypass and drop a new can in it, but only after listening to it. I have full faith (right now) in the guy who did the work on it as he has good rep...if it was out of spec, he would have dealt with it.
Jim
Note: The Quad Cap Can was replaced for one that was tested and was in spec, the Black Cats tested in spec, bias caps were replaced. The only thing that has not been addressed (per the concerns) is the Selenium Rectifier and I will consider replacing it with a silcon diode say a 1N4004 or of higher value.
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Just to put it to rest, I'm having Carl replace the Selenium Rectifier with a Silicon Diode before he ships it to me. As I posted already, the Quad Cap Can is in proper spec as are the Black Cat's.
I already have a space made for it and will be using the Meadowlark Swifts as they have the sweetest mids and highs. They have Vifa 5.5" drivers and 1" Soft Dome Tweeters that can reach down to 35hz (with a 4db rolloff) and being 8ohm and 89db should be able to get to pretty impressive sound levels with the ST-70.
Jim
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Glad to hear that you are going with the diode. 0 down side and it may save you an expensive quad of outputs someday...John
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2wo:
I agree, it is a smart move. It may or may not change the tone, but as you say, if it goes it will do damage, especially to the output tubes. As you may have already surmised, I fancy nice tubes.
It will be interesting to compare it to my Marantz 9's. It should be no contest, but that's what I thought when I A/B'd my Upgraded FP2 against my serviced and aligned McIntosh MX-110 and the FP was the clear winner.
It's nice that I still get thrills and excitement over this stuff and hope I will always! I admire you guys that actually understand the insides and can work and design upgrades. Some day I might attempt building a kit or two....but I need to practice on my soldering skills first!
Jim
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Okay, one of my good friends has a great collection of NOS Vacuum Tubes and sold me a tightly matched quad of EL34/6CA7's Two are branded Haltron and have welded plates and two small halo getters. The other pair are RCA (Mullards) from 1970 that also have welded plates with a single large halo getter. They tested 7400 7600 7700 7700, well above New. They are also very tight ma wise 25.5 to 27ma...I believe they were tested on a Hickok 539A.
This is the same guy who I got an octet for my Marantz 9's. My justification is that if I ever need a tube for the Marantz's, I will have some to pull from the Dynaco and put something back into the Dynaco. I have mixed double/single getter Mullards in my Marantz's and they sound great. The plan for the Dynaco is to have one of each on each side.
I should have it some time next week and really am getting excited about it...
Jim
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I've had the Dynaco ST-70 for several days now and all the adjustments and set-up tweaks have been done.
I rolled in a NOS matched quad of Mullard made xf2 EL34's, a NOS set of Sylvania 7199's and used a tests very stong 1957 metal base Amperex GZ34. The bias point is 1.50v (sounds the best there). I had to adjust the new powered subwoofer several times to get it to a place where it was perfectly integrated with the Meadowlark Swifts. It takes several hours for a new speaker to break in and do its thing...
Last night was the pinnacle (so far), so it all came together and wonderful music was coming out of my new system. The beauty is that as the tubes burn in, it will only get better! Right now the Soundstage is every bit as nice as I get from my Marantz 9's. There are subtle differences in instrument placement and depth.
I love my Marantz 9's and they sound great and deserve their place in history as being one of the best amps ever. At the same time, I also have to say that the Dynaco ST-70 is truly a great amp in its own rights.
The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.
Jim
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The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.
I think you might be pleasantly surprised to find that some current single ended designs can show improved resolution over the classic push pull amps.
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I would also recommend the Dynaco Doctor as an excellent reference. I've rebuilt an ST70 with Joe Curcio's boards and a new PT from Dynakitparts and it turned into a very seductive amp.
Last year I finally finished building a brand spanking new pair of Mark IV's. Dynakitparts makes a really nice looking chassis.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco-Curcio%2520Mk%2520IV%2FIMG_2424.jpg&hash=33fbdca6b0099061549b90d1f0a2472b46026a3d)
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The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.
I think you might be pleasantly surprised to find that some current single ended designs can show improved resolution over the classic push pull amps.
Doc B.:
I have no doubt that SET amps have improved dramatically. However, I do run my Marantz 9's in triode mode and they are pretty awesome.
My next system will be SET.
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I would like to address the comments regarding the Quad Can as being a ticking time bomb. There will tell tale things such as hum or white powder residue leaking from the bottom of can. If your Dynaco ST-70 is quiet and has a nice soundfloor, it is fine. Moreover, there's a 3A Slo Blo fuse to protect the Iron and Tubes.
Jim
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Yes, indeed. I may have already mentioned, I replaced the quad cap in mine primarily because a new one was available with slightly improved specs, and I really didn't have a way to test the existing one at working Voltage. As far as I know, the original is still a working part and had exhibited no sign of failure after 40+ years.
I would hazard a guess that the longevity may depend on its lifetime: If it got regular use, it probably stands a better chance than one which sat for years without a charge. I had no idea which might describe mine, so I took the precaution of bringing it up very slowly over a period of many days with a Variac and a solid-state replacement in the rectifier socket so that it started getting Voltage at a very low level relative to a vacuum tube rectifier.
I have no idea if this was necessary, but the cap appeared to work very well.
I am happy to hear you are enjoying your ST-70; mine has brought me many hours of pleasure.
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Grainger:
It behooves anyone with a nice system to some sort of power management system as well as a spike filter. I have a Furman that keep my current coming in at 117v...I can adjust it to 112v which is the standard back when most of my gear was made. Then I have a Belkin Surge Protector to assure no nasty spikes.
Jim
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The Stereo 70 instruction manual describes the transformer as a 120V model. Makes the incoming voltage point moot.
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The Stereo 70 instruction manual describes the transformer as a 120V model. Makes the incoming voltage point moot. Did you check with Terry?
Grainger, if my memory serves me well, current flowed at about 112v in the 60's, while the specifications for the transformers was 120v, 112v was what was being drawn back in the day. Just for the record, I have my voltage set at 120v on my Furman.
Jim
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When I was running my ST-70 up on the Variac after just acquiring it, I kept a DC Volt meter clipped to the first cap section on the can capacitor; even at 130 VAC input to the amp, I did not exceed the WVDC rating of the first section on the original cap. Nor, did I ever exceed the WVDC rating on the subsequent sections of the cap.
When I spoke of not being equipped to check the cap at working Voltage, I should have been more explicit in indicating that I was referring to checking it for capacitance values at working Voltage. Again, I had no evidence that the capacitance values weren't up to spec, I just had no way to check them.
We all think of AC line Voltage back in the day as being lower than what we receive today, and perhaps, generally speaking, that may be true. In reality, though, they could also run considerably higher, especially out in the sticks. I'm guessing that designers of the era took that into account. IIRC, DeWalt designed their radial arm saws to run properly on a range of input Voltages from below 100 VAC to upwards of 150 VAC at the top end, precisely because they had them in the city and the countryside, and were aware that the actual Voltages delivered at various times of the day in various venues could be all over the map; so, they designed for an approximation of worst case, based on the available data.
I suspect Dyna built in similar safety margins for their designs, since they intended them to run anywhere as well.
Again, after bringing it up over a period of about 10 days on the Variac, mine ran perfectly on my line Voltage of 125 VAC for many weeks before I replaced the can cap.
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Bear in mind guys, that the legend is that those 500V rated quad cans were really just 450V cans that were custom stamped 500V for Dynaco.
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Well, I imagine legends get started for a lot of reasons. In the end, though, I either have to trust AeroVox, or a legend about AeroVox.
Since the cap didn't show any signs of failure after 40+ years, I'm inclined to trust AeroVox.
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While there is not much gear that will measure accurately the capacitance of an electrolytic cap, it's also my understanding that the best measurement is the equivalent series resistance (ESR). This will increase by something like a factor of four over the life of a cap, and is a good measure of remaining lifetime - at least, if you know the ESR of a fresh new one. A decade or two ago there was a DIY circuit to make this measurement, possibly in Wireless World(?) - if I recall correctly there was even a kit, and there may be commercial meters that exist.
Here's an article from the CDE web site, with more than anyone really wants to know about electrolytic caps:
http://www.cde.com/catalogs/AEappGUIDE.pdf
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Thanks! I have that filed away for future reference, for sure.
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The fact still remains that in just about every instance there will be tell tale signs of problems in the Can Cap before it fails. The primary sign will be audible hum at idle. The second will be white powder residue on or about the bottom of the Can itself.
I make a living in the vintage guitar world and that involves vintage tube amplifiers that have filter/electrolytic capacitors. Yes, they do go bad for whatever reason, but I have never in my years seen one that has suddenly failed that didn't hum like a banshee or have visual signs of leakage.
Look, if changing the Quad Cap Can gives you peace of mind, then by all means go for it and I respect you for doing so. But at the same time respect the choice of those who chose to use their senses to determine when to change it.
Jim
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The fact still remains that in just about every instance there will be tell tale signs of problems in the Can Cap before it fails. The primary sign will be audible hum at idle. The second will be white powder residue on or about the bottom of the Can itself.
I am not going out of my way to be unduly contradictory. My experiance does not match the above. I have had old multi-section caps fail as shorts in a number pieces of gear. I recall off the top of my head a Dyna Mk3, ST-70 and a Scott tube integrated (a 222 or a 229, can't recall which). No white powder, no out of ordinary hum, no phone calls at 1 am and not even a tersely worded note, just POP!, SNAP and a blown fuse. One section of the cap now tested as a dead short.
Perhaps unique to me but that has been my experiance.
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I had a Heathkit integrated suddenly start hissing and piss its innards all over the chassis after bringing it up slowly on a variac and then having it run for quite a few hours of quiet operation.
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Maybe I'm the luckiest guy in the world! Doc B., so how many times have you restored an old tube amp? I'm going to go out on a limb and guess well into the hundreds over your years and to have only one fart and sputter. Wanderer, I have no doubt that what your telling me is true. Maybe I will live and die by the sword that I wield, maybe not.
I find it ironic that is all but one instance mentioned by both Doc B. and Wanderer, they were on DIY's where the integrity of the original build possibly may be called into question! If we were talking McIntosh, Marantz, Harmon Kardon or other high-end amps, then maybe an argument could be made. In the mean time, I'm going to enjoy my humble 1963 Dynaco ST-70 with an original Quad Can.
Jim
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And, sometimes they don't.
I wonder how many people here change their light bulbs before they burn out?
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they were on DIY's where the integrity of the original build possibly may be called into question!
The Scott was factory wired as I believe the Mk III was also (so long ago....such fading memory).
My thinking is more along the lines of HI-FI amps running wth voltages nearer the limit in an attempt to get lower distortion. Aren't music amps run cooler and distortion is viewed as a plus? I keep hearing about guitar amps having "nice tone" and "good crunch".
....oh, and when my light bulbs burn out they don't take $100 of matched output tubes with them.
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Guitar amps are generally run at higher Voltages and less accommodating environments. Many models met or even exceeded maximum recommended Voltages on plates and screens, even at the legendary lower line Voltages back in the day.
If you have had an amp take out a set of output tubes due to a power supply filter failure, that would indicate that the tubes must have sacrificed themselves to protect a 50 cent fuse. Not to mention several other parts in the power supply. I would have several questions about such a design.
I have replaced many a power supply filter cap in many vintage guitar amps because they showed visible signs of leaking electrolyte, bulging outer containers, etc. Many caps have not, and have continued in service; I suspect they may fail someday, just like a brand new part may fail someday.
In all those amps, though, the only one that burned new-manufacture output tubes was one that was "legendary" for having a narrow range of negative bias adjustment. Which it had 37 years ago when it left the factory with Mullard EL-34s that evidently tolerated it better, or required a smidgeon less negative bias to set up properly.
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I certainly bow before those with greater experiance with musicial instrument amps - not something I have much backgound with and my comments about them were uninformed.
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Jim has chosen not to head the warnings that are all through this thread.
We shouldn't beat this dead horse further.
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Jim has chosen not to head the warnings that are all through this thread.
We shouldn't beat this dead horse further.
Grainger:
Not about listening to or taking heed of the said warnings, just that no tangible evidence has been brought to the table to convince me to change what is not broken. JC is dead on right about Guitar Amps and their operating ranges. As JC aptly noted, new parts fail, old parts fail. I find comfort in the fact that in my ST-70, the Quad Cap is 47+ years old and still works as designed. Trust me, if it starts to hum, or show signs of leakage, it will be replaced, but not until then.
As far as tubes go, I have a NOS quad of Mullards in her that cost $100.00 per tube...Let's take the other road and replace the Quad Can...what's the going price for a good one, $70.00, and Wanderer indicated that a matched quad of new EL34's is around $100.00. My point is that the changes of a catastrophic Can Failure is rather minimal and even if you change the Can, it still can suddenly fail if you buy-into the rhetoric. Let's extrapolate the numbers...if the assumption is one in a hundred will fail and have collateral damage....
100x $70.00 (cost of a new can)= $7000.00
Under this assumption, those who replace the can will spend $7000.00 to avoid a potential loss of say $100 to $400.00.
Look, I understand and get it that the advice to change the Can was made in good faith, but I feel based on false premises.
Jim
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I don't really like the look and feel of where this thread has headed, so now that everyone has expressed their opinion I am going to lock it.