Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: syncro on April 16, 2010, 12:04:31 PM
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Just completed the build and beginning the resistance checks. Power transformer terminals 1 and 2 read "0". This is per spec, but the others that should read "0" do not. Terminals 6, 7, 9 and 10 all are reading with variable values.
I'm about half way through the other resistance checks and they are looking good. What should I do to get help - finalize my resistance checks, post them all...?
Thanks in anticipation,
David
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The measurements of the power transformer terminals vary a lot between meters. If everything else looks OK, go ahead and move onto the voltage checks.
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Thank you very much for the direction - exactly what I was hoping to hear. I am using an Amprobe 35 XP-A DMM. I am new to using a DMM.
I like the continuity of a needle because that's what I'm used to, so I was about to put my trusty 1970's Micronta (Radio Shack) analog multi-tester to work for verification. I will post any further discrepancies, or listening notes as the case may be, here.
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All resistance and voltage tests checked out okay, and the sonics are wonderful! Thanks again for the support. After I listen for a while and get multiple inputs connected I'll be looking for advice on how to best adjust output for use with my Dynaco ST-70.
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... After I listen for a while and get multiple inputs connected I'll be looking for advice on how to best adjust output for use with my Dynaco ST-70.
Congrats on a successful build!
While you are listening and getting other things attached, read my paper of signals and noise (linked from the community page). The ST-70 has more gain than you need, unless your speaker are extremely inefficient - you'll probably want to attenuate at the ST-70 input.
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The first bit of advice with a ST-70 is to remove the two 10 ohm resistors that are between the input common and the signal common. Replace them with a short. These very often cause hum.
The output of a FP III is pretty compatible with a ST-70. You may find you want additional padding so you can use more of the volume control. Otherwise they are a good match.
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Thanks, and thanks. Yes, I'm happy to have a successful build, and I do want to adjust something so I can get the first position listening level to be very low. Now it is about where I would think a 3rd or 4th position should be.
Mr. Joppa, I will read your paper again and hope it helps me decide which place to "pad" things. I saw in the "Easy volume control" thread a suggestion for padding the FPIII with resistors between selector and the attenuators in place of the red wire. So, it sounds like I could either do that, or something similar in the ST-70, correct?
Mr. Morrison, does the removal of that pair of resistors in the ST-70 "attenuate its inputs"? I need to go into it anyway to replace a the tube sockets on the PC board. (The amp is stock but with some upgraded capacitors.) I'll move this inquiry to the ST-70 discussion forum to find out their recommendations for lowering the gain in the Dynaco. Unless you all think I should keep asking questions here. I'll need some hand-holding, as I'm a lay-person in this world of audio electronics. (between input common and the ?"signal common"? I'll have to take a look and read up on how to drain capacitors...)
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Mr. Joppa, I will read your paper again and hope it helps me decide which place to "pad" things. ...
Well, the paper is pretty dense. I tried my best but the subject is inherently complex and confusing, and I don't claim to be the most articulate writer either. If you have further trouble, just post - or you can ping me through the forum if you like. Meanwhile I'll try to outline the options. There are three:
1) Pad the preamp input. This is what's in the Foreplay manual. This is the right place if the source device has a large voltage output.
2) Pad the power amp input. This is the right place if the amp and/or speaker is too sensitive.
3) Pad the preamp output. This is not a good idea unless all the stars are aligned - it makes sense if the desired attenuation is much greater than 20dB, and the preamp input is not too high a voltage.
If you need help identifying the best option or combination of options, you must first locate the necessary specs. I'll put them here, in case you or other people need to refer to them through the search function:
* source output level. CD players are usually specified by their full scale (FS) voltage, while vinyl and tape are specified by their average level, e.g. vinyl is referred to 5cm/sec groove velocity. In the case of vinyl, the cartridge sensitivity and the phono preamp gain are both necessary specs.
* Amp power (35 watts per channel for a ST-70).
* Amp sensitivity. This is usually the volts input required to generate the rated output power, though sometimes it is the voltage needed to generate 1 watt. For example, a quick Google search told me that the stock ST-70 is specified to produce 35 watts with 1.3vRMS input.
* Speaker sensitivity, usually dB at 1 watt at 1 meter. Sometimes this is given as dB at 2.83 volts, which would be 1 watt if the speaker were 8 ohms exactly (very few are!)
Good luck, and call on your buds at the forum (here!) if you have questions.
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David,
The resistors do not attenuate the input but was there because equipment "of the day" had grounding problems. Today with a FP III, and I know this from personal experience, it causes hum.
Go here: http://www.curcioaudio.com/st7_mnl.pdf
You can save this file onto your computer. The schematic is 8 pages down and it shows 2 - 10 ohm resistors on the circuit board all the way to the left. What is contained on the circuit board is inside the dashed lines. All the way to the left and outside of the dashed lines are the stereo/mono switch and two other 10 ohm resistors (and the two 470k ohm input load resistors). These two 470k ohm resistors are where PJ suggests you can pad your ST-70's input.
If you PM me with your email address I will send you a picture of my ST-70 in this area. I have removed the stereo/mono switch and some other unneeded wiring. I think I can find where on the board the 2 - 10 ohm resistors are. Or you can just measure across it. The color code should be Brown Black Black.
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Thanks for the further details. So, unrelated to the volume attenuation issue, I should remove these two 10 ohm resistors (the ones ON the pc board of the stock ST-70) because they cause hum. In doing so will I need to short the two terminals of the resistor, or just leave the terminals disconnected? (Thankfully, I'm not hearing any hum, hiss or noise of any kind, BTW!)
Related to the attenuation issue, again on the ST-70, the two 470Kohm resistors at the inputs can be padded with an "L-pad" circuit in place of the existing 470Kohm resistors? (I found some variable L-pads at Parts-*xpress - stereo, 15, 50 or 100w versions. Not sure if this is what you all have in mind should that location be chosen for attenuation.)
At this time I am trying all my inputs and gathering the data suggested. While the CD and Phono (w/ Hagerman Ripper as phono preamp) seem loud, the Ripper as DAC source is very, very low. I'm going to check with J. Hagerman about that next. Then I'll bring the summary back to this thread.
Thanks again, David
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Yes, the two 10 ohm resistors on the PC board are what I'm saying to remove. Replace them with a piece of solid wire or bring the wire from the input RCA jack outer conductor to a common/ground connection on the circuit board. An easy way to do this is at eyelet 10 for one channel, eyelet 8 for the other. I'm not sure which channel is which from the schematic. Like I said in modern equipment these resistors are not needed.
The L pads you found are probably like potentiometers, if I am thinking of the right thing. That isn't what you need. You could just unsolder the 470k resistors at the center conductor of the input RCA jacks. Also remove the input wire from the center conductor. It is easy to identify the input wires, they go to eyelet 7 and 17 on the circuit board. Then put in a series resistor from the center conductor to the end of the 470k that you just removed. This leaves two resistors in series going to ground. Then the input is taken from the "junction" between these two resistors; the solder joint you just made. The input wires should reach since this point is closer to the circuit board. The value of the series resistor will be determined later.
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I would advise not getting overly-focused on those two 10 Ohm resistors, particularly if you don't have any hum issues. In my experience, they just don't matter all that much.
You can always try jumpering them with test leads to see if you can tell the difference. Taking them out involves de-soldering from an old phenolic PCB, which is always to be avoided if possible, in my opinion. If you decide the hum level is lower with them jumpered, it is usually easy enough to make that condition more permanent by using a bit of wire attached at each end of the resistor on the "part side" of the circuit board. Solder lightly, and you're done.
Every case is different, but I have tried a few different pre-amps with my ST-70, and found no difference with a scope whether those resistors were there or not.
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JC,
That is good advice. Paully and I found the problem using a pair of jumpers. His FP III and ST-70 had plagued him with hum. Here at my house his ST-70 and my FP 2 had no problems. My ST-70 and FP 2 never had a problem. But finally his rig here hummed. And I tried two jumpers and it was gone.
I have worked on the old circuit boards for a long time so I have skill getting things off and back on. The inexperienced shouldn't try it in their main system.
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Yeah, that's the thing about hum, isn't it? Something that works perfectly in one situation hums the instant you try it in another situation.
You will recall that safety ground and circuit ground on the FP2 were separate, but with the option to connect them. In my particular situation at the time, I found that actually using a ten Ohm resistor between the two resulted in less hum, in that particular set-up.
Of course, that was before I installed a three-wire AC cable on the ST-70 it was connected to, so that may no longer be the best solution!
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Okay, I'm back with my data. Please help me determine where and how to attenuate. I do have a little hum, but I have not done enough observation to provide any details on it yet. The levels are high from all inputs and CD seems the highest, so this individual input might take it's own resistor after I get in the ballpark with the whole set up.
By the way, I'm really enjoying the sound qualities of this preamp. Thanks for the kit and support.
Input 1: Phono - LP12 w/ Basik Plus tonearm
Phono Cartridge: Linn K5 (Moving Magnet)
Sensitivity: 4.5 mv Output at 5 cm/sec at 1 kHz
Phono Preamplifier: Hagerman Ripper (SS, has volume pot - P2T3503)
Gain: 48dB (Output Impedance: 320 ohms)
Input 2: CD Player
Rotel RCD-975
Output Level: 2.0 Volts rms (Output Impedance: 200 ohms)
Input 3: PC > Foobar2000 > Hagerman Ripper (as DAC & no volume control)
Amplifier: Dynaco ST-70 (stock)
Power output: 35 watts/channel continuous, 80 watts/channel peak
Sensitivity: 1.3 volts RMS input for 35 watts output
Speakers: Linn Index I
Sensitivity is 86SPL, 1w/1m (impedance is 8 Ohms nominal, 6 Ohms minimum)
RE: digital input. I have to correct my previous statement that the digital input was very low - I found a volume in my computer software that was lowered, and Foobar2000 (my digital player software) has dB annotations on it's volume control. Is that possibly accurate enough to determine the amount of attenuation I might try?
Thanks, David
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How many clicks on the volume controls before you have too much output?
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That depends on who you ask, but since I get to answer and not my family, I'll say I've had it up to 3 with the CD (loudest input). It is really rocking the house.
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At first look, you are not that bad off with the stock build. Your speakers are pretty inefficient, so you will probably use all 35 watts on musical peaks. You should be able to get that with a volume setting four clicks below maximum, using the stock 33K resistors.
The other sources have their own level controls, and you can set them to match the CD player.
If you find this is too loud, you can change the resistor on the CD player inputs. The supplied resistors come in 6dB increments, and the Whispers are 3dB per step except for the lowest few, so it should be easy to decide which resistor to use. I base my levels on statistical averages for musical genre, personal preference, room acoustics, etc. so it's not unusual for your situation to differ by as much as 6dB either way.
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Thank you for the recommendation based on my input data. Seems like I have a lot more gain than the numbers may be indicating. I'd like to have the CD be able to play with a full volume control range, from low-level, background-to-family-dinner music to I'm-in-the-kitchen-and-still-want-to-hear loud. I cannot approach four-below-max, but rather two above silent. It is just too loud. One above mute is louder than I often need, and two above is driving out the family. So, I'm inclined to attenuate the ST-70 with the series resistor technique suggested above by Granger49 who said:
"....Then the input is taken from the "junction" between these two resistors; the solder joint you just made. The input wires should reach since this point is closer to the circuit board. The value of the series resistor will be determined later."
Can someone suggest a resistor value, type and/or part no? I have a few floating around (like from the FP III kit I have pairs of 80.6 K ohm, 182 K ohm and 360K ohm.) Are these the correct type to try? Are the attenuation levels from the FP3 instructions for these input resistors going to hold if they are installed in series in the ST-70 inputs? I need to open up the ST-70 to replace some tube sockets anyway. Another option would be to rewire the ST-70 with one of the upgrade kits to operate at a lower wattage. Suggestions are welcome, but this will not be an option for me immediately.
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OK, now we have some data to work with! Two above silent is -27dB, so a 27dB reduction will give you a full range. A good split would be -20dB in the ST-70 and -6dB in the Foreplay. So replace the 33K in the Foreplay with 80.6K; that will reduce the Foreplay sensitivity by 6dB and keep its signal levels well above the noise floor.
Then reduce the ST-70 by 20dB, which is a 10:1 resistance ratio. Grainger is probably better suited to describe this since he has an ST-70. I'd suggest you keep the stock 470K resistor at the center of the RCA, and from the other end (the junction) use a 56K or something near that value to ground, i.e. the outer connector of the RCA jack. Then take signal from the junction to the circuit board input. The outer connector of the RCA jack is connected to ground, I gather this is through the 10 ohm resistor that Grainger says you may want to replace with a piece of wire, but this is the part of the circuit I'm having a hard time visualizing.
I like quality metal film such as the Vishay/Dale RN series resistors, but in this application with no DC current carbon film can be an excellent performer as well. Most listeners seem to think it is a bit warmer sounding.
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David,
Radio Shack, now just called "The Shack," sells (I almost said "stocks" but that is up to the local store) a metal film resistor assortment. It lists the contents on the back of the blister pack. At least they used to list them. You will likely find what you need in this assortment.
Paul has described the installation very well. If you need more I might be able to cobble a picture into a post. (I have previously had problems with pictures. The new board hosts them so I'm doing much better.)
I might have a couple of 56k ohm resistors, or something near the value I can mail you.
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OK, now we have some data .... So replace the 33K in the Foreplay with 80.6K....
Thank you both, again. I like the notion of splitting the attenuation (and the assurance of keeping off the "noise floor" of the FP.) Regarding the recommendation quoted, above: Would that be the 33K ohm resistor pair on the input I am using for the CD? (Input #1 looks the easiest terminals to access to make this change.)
Or is the intent to attenuate all the FP inputs by 6dB? I know I have volume control on my phono preamp and my digital music player, but I'd rather have them wide open for ease of control and best sound. Another recent thread was discussing attenuating all the FP inputs with a resistor between each Sweetest Whispers and the selector switches. This seems a simple mod, but
Even if so, I think I should try the series resistors in the ST-70 first and listen again for a while.
Granger,
Thank you for the offer of resistors. I have some leftover from the HagermanRipper build, so I'll check around for what I have and get back to you via the forum's p-message feature, if that's best.
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I was suggesting you change the CD input only, as in the manual.
Once that is done, you can use the level controls on the other sources to match all levels. It may take several albums to find the best-matching compromise, since album signal levels are not exactly constant (!)
Once THAT is done, you can (if you like) maximize one of them, and use the Foreplay controls to bring it back in line. Since the Foreplay controls are 3dB per step (except for the first 2 or 3) you can then estimate how much attenuation you want on that source, and implement it in the Foreplay. Repeat as necessary. Yes, it;'s a lot of work, but you only have to do it once. :^)
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David,
The volume controls on your phono stage and digital music player are the equivalent of padding each input on the FP III between the input and the selector switch. But using the volume controls would be much faster.
I'll assume that dropping the level of all inputs has already been explained since you mention that in your second paragraph.
Just send me a PM if you need the resistors and I will look to see if I have some.
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I'm attaching two diagrams of the suggested modification to attenuate my ST-70 amplifier (one channel only is diagrammed.) I have the blank diagrams I can post if someone wants to start from scratch. If I'm close, please let me know, or if I got something wrong.(http://)
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Schematic version... and here I have circled the 10 ohm resistor spoken of earlier in the thread as something to possibly eliminate if a hum develops.
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David,
In both diagrams you need to swap the positions of the two resistors. Edit: I looked at it backward. Both sketches are right. That is, the input feeds one end of the 470k ohm resistor, the other end is soldered to one end of the 47.5k ohm resistor and the other end of the 47.5k ohm resistor goes to ground. The input wire that feeds the circuit board at eyelet 17 is attached to that eyelet and to the solder joint that connects the two resistors.
Yes, that is the 10 ohm resistor that you can jumper out and see if it eliminates noise or causes it. It seems to be a crap shoot as to whether it will help in an ungrounded house. I bet it has a lot to do with whether you use a grounded power cord too.
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The sketches look correct to me, though a little hard to unscramble mentally. Grainger's description is of course correct, it just looks to me like you have in fact sketched what he wanted.
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Paul, I was looking back at this post, maybe I should take another look at the two sketches:
OK, now we have some data to work with! Two above silent is -27dB, so a 27dB reduction will give you a full range. A good split would be -20dB in the ST-70 and -6dB in the Foreplay. So replace the 33K in the Foreplay with 80.6K; that will reduce the Foreplay sensitivity by 6dB and keep its signal levels well above the noise floor.
Then reduce the ST-70 by 20dB, which is a 10:1 resistance ratio. Grainger is probably better suited to describe this since he has an ST-70. I'd suggest you keep the stock 470K resistor at the center of the RCA, and from the other end (the junction) use a 56K or something near that value to ground, i.e. the outer connector of the RCA jack. Then take signal from the junction to the circuit board input. The outer connector of the RCA jack is connected to ground, I gather this is through the 10 ohm resistor that Grainger says you may want to replace with a piece of wire, but this is the part of the circuit I'm having a hard time visualizing.
I like quality metal film such as the Vishay/Dale RN series resistors, but in this application with no DC current carbon film can be an excellent performer as well. Most listeners seem to think it is a bit warmer sounding.
Yup, both sketches are right. My brain was in reverse.
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Thanks again, and again! I'm following this, scratching my head, and seeing what we all see, now. I'll let you know once I've completed the addition. New RCA's should arrive tomorrow.
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When I rebuilt my ST-70 I removed the stock RCA jacks and the Stereo/Mono switch. I put one input where the stock ones went and one where the switch went. Good ones will not fit where the stock inputs are. I just used the screw holes for the switch to anchor the plate that held both inputs.
Doesn't really look elegant, but works.
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The padded "CD" input has balanced the inputs to the preamp nicely. I used, as recommended by P. Joppa, the 80.6K resistors provided with the kit for this. Phono and digital levels are very close to the CD now.
Regarding padding input of my ST-70:
I have installed a 47.5 K ohm metal film resistor on each input of the ST-70 as in the diagrams I posted above in this thread (where the 56K is shown.) The result is that I have attenuated the input beyond what I would like with this modification.
Can someone explain to a lay person how these two series resistors work with the input wire taking off from the joint between them? Is the 470 K between the RCA input and the wire doing the attenuation? If previously the stock wiring had no resistor there where there is now a 470K could I reverse the two resistors so I lessen the attenuation? Suggestions are welcome, or if I should take this to an ST-70 board just let me know. (I am having an issue with my ST-70 that I have posted to the dynacotubeaudio forum at http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm (http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm) )
Suggestions for driver board updates are welcome, as it may come to that soon.
Thanks!
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In looking at your diagram, you are basically making what is known as a "Voltage divider" where the signal Voltage gets "divided" proportionally between the two resistors in series. Then, you are extracting the signal at the dividing point to feed the input to the amplifier.
If I understand the manner in which you have constructed your divider correctly, the lion's share of the signal Voltage is being dropped by the larger resistor with the leftover Voltage being developed across the smaller of the two. This ratio follows the ratio of the two resistance values, so, very roughly speaking, you have created a 10:1 divider with the smaller part of the signal being input to the amp.
So, if the result is that the signal attenuation is too great for you, the solution is to make that ratio smaller; this means, of course, either replacing the smaller resistor with something larger, or replacing the larger resistor with something smaller.
If you are able to, I would suggest trying to rig a way to clip-lead various values into place before making one permanent.
In reading the linked thread about your ST-70, since the issue is a temporary imbalance between left and right, the first question that comes to mind is whether you have swapped tubes from left to right to see if the issue stays on one side or follows the tubes. Swapping the 7199s from left to right would be the first step, I think.
I also note that you have mentioned the tube sockets, and I can only say that in my 1959 vintage ST-70, that is the second thing I replaced, after the bias diode. I can certainly imagine one of those old funky 50s vintage plastic sockets being the cause of many an issue. "Freeze-mist" of some sort or another is often helpful if you think that there is a thermal issue with some part or solder-joint, but you will need to be careful with it when working around hot tubes; you don't want to spray a coolant on hot glass!
I really can't say much about the replacement driver boards available for the amp; I have only ever heard one of the variety available, and I really wasn't sufficiently impressed to call it any kind of "improvement" in sonics.
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Thank you J.C. for the description of how these resistors are working. It has some intuitive logic, which helps. Does that mean that before modification, when there was only one resistor (470 K ohm) to ground after the input wire in the schematic (both soldered to the center conductor of the RCA in fact) that essentially 100% of voltage went to the input wire? If that is the case, and now I have approximately 10% of the voltage then I have an idea of where to go.
I might try a 50-50 split by two series resistors of the same value in the "voltage divider". I'm also wondering about your suggestion on using clip leads. Can I clip lead a smaller resistor across an in-place larger resistor to simulate replacement? Or should I remove the resistors (all) and use clip leads and resistors on both legs of the divider?
I'm also wondering about creating a mute switch to lower the levels when desired for fine adjustment at lower levels. I enjoyed that feature on a previous integrated amp. Maybe I should first do some research on the pros and cons of a mute circuit.
I will try swapping tubes across the channels again, but I did this some time ago and it did not change things at that time. I replaced only the driver tube sockets. I have not tried probing with a pencil eraser yet. Thanks again for the input.
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"...Can I clip lead a smaller resistor across an in-place larger resistor to simulate replacement? "
That's what I would recommend. Parallel the existing 470K with leftover resistors from the Foreplay (360K, 180K I guess?) and see what works best. You can in fact just solder the winner in place, which might be easier than replacing the 470K completely.
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Yes, you are on to it! When only the 470 kOhm resistor was there from "signal hot" to ground, then the entire signal voltage being fed into the amp was developed across it and thus fed the control grid of the pentode. When you added the smaller resistor from the previously grounded end of the 470 k, you created the divider and the Voltage across the smaller resistor became the one fed to the grid.
Here's an easy experiment which may give you a ballpark to work from: If you obtain another 470k resistor and clip it across the existing one (parallel to it), you will essentially cut its value exactly in half to ~235k. This will change your Voltage divider ratio from roughly 10:1 to roughly 5:1. In other words, it will reduce the attenuation to about half of what it is now. If that moves things closer to where you want to be, then you can measure the resultant paralleled resistors with your Ohmeter (approx. 235k), and clip a similar value across the combination. The typically available values of 220k, 240k, or even 270k will be close enough for this third parallel. Anyway, you will have reduced the Voltage divider ratio (and, the attenuation) by approximately half again.
Once you find the combination you like, then you can measure everything with your Ohmeter again and easily calculate the Voltage divider ratio you like. Once you've done that, you can decide what you want to do about making the divider permanent.
Two things to keep in mind are: First, the total resistance of this Voltage divider to ground is essentially the input impedance of your ST-70. While the original spec calls for 470k, you can get away with quite a bit of deviation around that value, providing that you don't lower it so much that you overload the output of your preamp. IIRC, the output of the Foreplay is around 600 Ohms, and the general rule of thumb is that you want to drive at a minimum 10 times that, so 6000 Ohms is your lower limit for input impedance, and I seem to recall that Mr. Joppa wouldn't mind something higher. The upper limit is pretty arbitrary, but you can go higher than 470k for sure.
The second thing is that this input impedance is where the signal is developed for your amp, so it never hurts to make this a nice-quality resistor or combination of resistors. Metal-films are my favorite for this duty, but carbon-film could work very well, too.
As far as your other issue is concerned, I wonder if this gain imbalance is a condition you can create on demand? Can you make it happen and keep it that way long enough to take measurements? That will make it a lot easier to hunt down, I'm thinking.
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JC and PJ have given very good advice. If you use jumpers across a larger in place resistor you are paralleling the two values. The resulting equivalent value is the product of the two divided by the sum of the two. It is always smaller than the smalest resistor.
R1*R2/R1+R2
Right now as of the Reply #31 above, are you not able to get enough volume at the highest volume setting on the FP III? It is fine to use all of your volume control, probably best to use it all.
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More problems have arisen in the ST-70 after installing the voltage dividers to attenuate the inputs and the new tube sockets. I have been posting to http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm#1944 (http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm#1944) where I got (along with, of course, advice to get a new driver board) a suggestion to replace the 1.5 MOhm resistors on the PC board, and checking them I find they have drifted high.
I found some encouragement to stick with the stock board on the Old Bottlehead Forum site from a Gary Kaufman who has some good dynaco resources on his site http://www.the-planet.org/ (http://www.the-planet.org/). Since the original PC board is in pretty good shape, even after I have re-soldered most traces and connections and replaced the 7199 sockets, I am getting new parts to rebuild it following the Van Alstine recommendations from 1982 now posted at http://www.avahifi.com/index.php?option=com_content&view=article&id=149&Itemid=172&74a0ad6b5f7a1df0ef4ab98b8fffbb41=6ba9f832ffb94d9aa84ef8d686e656ca (http://www.avahifi.com/index.php?option=com_content&view=article&id=149&Itemid=172&74a0ad6b5f7a1df0ef4ab98b8fffbb41=6ba9f832ffb94d9aa84ef8d686e656ca)
Any advice from this forum is most welcome.
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The problem you describe on the Dynaco Tube Audio board sounds like a problem with the preamp. That is easy to figure out just by swapping your input cables at the input of the ST-70. If it moves then it is not in the ST-70. But you have probably done that.
A component going thermal makes no sense. But a solder joint that is "ohmic" is more likely. I suggest you first inspect every solder joint you made under a magnifying glass. If you find nothing obvious, like one with no solder on it, then rewet all that you have gone over. If this is every solder joint on the driver board, don't do all of them in the bad channel at the same time. Give the board a rest. Do some on the board, do some at the inputs, drink a Coke, then proceed.
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Today, now that I have ordered new components to the tune of $40 - the system sounds normal - good! I appreciate your opinion of the "component going thermal" idea. Well, when I complete the rebuild I will know it is all it can be. The new FP3 preamplifier is still working correctly as the switching of inputs from that have no effect. There still may be some imbalance between channels in the ST-70 since the old 470K resistors padding the ST-70 inputs have some variation between them, likely directly affecting this? While I wait for the new parts I think I'll continue experimenting with the input voltage divider to allow more signal through.
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Took some official readings under the hood of the ST-70 while powered and playing music, and I found that most voltages were close to the design levels. The AC voltage on EL-34 pins 2 and 7 should be 6.4 but it measures 3.3 VAC. Same anomaly on 7199 pins 4 & 5, both channels, should be 6.5 but reads 3.3 VAC.
Anyone have a clue as to why this would be, and why it matters? (I'm double posting this again to the dynacotubeaudio forums in a new thread.) Thanks in advance.
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This sounds as if you were measuring from each tube pin to ground. I think if you measure AC Volts between the specified heater pins, you will get closer to what you're looking for.
BTW, it is not uncommon for Voltages to be somewhat higher than when the amp was new; in those days, the Voltage coming out of the outlet was supposed to be 117 VAC. Now, 120 VAC is the goal, and 125 VAC is common. With higher Voltages going in to the power transformer, you end up with higher Voltages coming out.
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That is correct, Jim. I was measuring to the grounding terminal near the quad capacitor. Looking at the schematic, I cannot figure out which pin on each tube I would use as ground to measure otherwise.
In the mean time I am going to temporarily solder the 180 KOhm resistors into the voltage divider parallel to the 470 KOhm. Sounded about right with the clip leads, but I might err to the louder side when I finalize the resistors to replace this little cluster. The Van Alstine input mod I am interested in trying will complicate things here also, but I'll cross that bridge when I get to it (with the help of the good people of this forum!)
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David,
As Jim points out you have a good voltage for the output tube heaters. The measurements you made add to 6.6V AC, that is fine considering the higher incoming voltage. As it is AC you can measure with either lead on pin 2 or 7.
Looking at the voltage divider, paralleling a 180k with the existing 470k gives you a 130k. The resulting voltage divider will be a 2.7:1 divider. Very different than the 10:1 you have now. Is this what you are looking for?
If you have a target I can give you an idea of what to parallel with the 470k to get there.
How far are you going on the FP III volume control? The goal is to get as loud as you ever want with a click or three left.
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Perhaps better to look at the Heater circuit as a separate entity from the rest of the amp. The heaters are powered by a separate secondary winding from the power transformer (or, maybe a separate winding for the tubes in each channel). And, while the center tap of that heater winding is referenced to circuit ground for a couple of reasons, if you want to measure the AC Heater Voltage, you will need to measure across the entire winding; if you use circuit ground for one meter lead, you will only be measuring from the center-tap to one end of the winding, which will give you only ~ half of the total Voltage you are looking for.
If the Heater connections on a given tube are pins 2 & 7, for instance, one meter lead will go on pin 2 and the other lead on pin 7, with the meter set to AC Volts. Since you are expecting ~ 6.3 - 6.4 VAC, the meter range should be set to something higher than that, if it is not an auto-ranging meter.
Again, I would expect the Voltages to be somewhat higher than specified due to an increase in line Voltage delivered to your home, as compared to when the amp was new. Tubes were usually designed to operate well within + or - 15% or so of nominal line Voltage, since it could vary quite a lot from town to country and place to place back in the day.
What will be interesting to see is what Voltages you are able to read if the amp is acting up; they may tell you something more about what is going on. BTW, if you haven't heard it already, it is always a good idea to keep one hand in your pocket while measuring Voltages. This usually means that at least one of your meter leads will need to be clipped into place. This is to minimize the possibility that you would accidentally shock yourself up one arm and down the other.
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The problem you describe on the Dynaco Tube Audio board sounds like a problem with the preamp. That is easy to figure out just by swapping your input cables at the input of the ST-70. If it moves then it is not in the ST-70. But you have probably done that.
Well, I thought I had done this. Today I bypassed the new how-could-the-problem-be-with-my-newly-built Foreplay III preamplifier. Connecting the Hagerman Ripper directly to the ST-70 there is no imbalance in volume between channels. I must have done something incorrectly in the changing of the input resistors operation. Guessing I should test resistances across these input resistors and compare to opposite channel/specs. Seems like a one-notch variation, but not exactly so. Other ideas for troubleshooting this? Guess I need to switch the 12AT7 tubes first and see if it follows the tubes.
In the mean time, the ST-70 has a new voltage divider configuration with the 470K + 360K + 180K (all three) parallel on the input side and the 47.5K to ground. I measured 90.6K across the former (from memory) and the volume is about right. I will wait to sort out the FPIII preamplifier issue before thinking about changing this in the ST-70.
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. . . Today I bypassed the new how-could-the-problem-be-with-my-newly-built Foreplay III preamplifier. . . I must have done something incorrectly in the changing of the input resistors operation. Guessing I should test resistances across these input resistors and compare to opposite channel/specs. Seems like a one-notch variation, but not exactly so. Other ideas for troubleshooting this? Guess I need to switch the 12AT7 tubes first and see if it follows the tubes.
In the mean time, the ST-70 has a new voltage divider configuration with the 470K + 360K + 180K (all three) parallel on the input side and the 47.5K to ground. I measured 90.6K across the former (from memory) and the volume is about right. I will wait to sort out the FPIII preamplifier issue before thinking about changing this in the ST-70.
I chuckle because this sounds like something I would have done. The best laid plans... and all.
Sounds like you have the input of the ST-70 sorted out at about a 2:1 ratio.
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Okay, I finally sorted out my mistaken placement of the input resistors in the Foreplay III. Naively following only my intuition I had installed the resistors symmetrically, which looked good but padded the left channel of input 1 and the right of input 3. Since I kept changing inputs and wires, thinking it was the ST-70, it was a moving target!
So, back to the ST-70, yes, I have about a 2:1 voltage divider padding the inputs now and I'm pretty happy with that. I'm looking at modifying the inputs per the Van Alstine page mentioned above, so I'm not sure how this divider will work with that, but in the mean time I would appreciate a recommendation to optimize or simplify this voltage divider with just two resistors on each channel.
I am also curious, having bypassed the preamp in the troubleshooting, why my phono preamp (Hagerman Ripper) when connected directly to the amp is, or can be, very loud compared to the Foreplay. It seems that the Ripper is designed to do double duty as phono + pre-amp. It appears about the lower half of the volume steps are attenuating the inputs, and above that they're amplifying. Is this happening with my CD input, and is this normal?
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I looked at the AVA page. The two filters on the input of the amp will attenuate the overall sound as well as the frequencies that they don't intend to become part of the feedback loop. I don't know how much these filters will attenuate the midrange which is untouched by the filters. Most likely more than you have right now.
According to the AVA site the assumption is that, "the driving source has a reasonably low output impedance." Certainly the FP III is "reasonable" in that it includes a cathode follower stage so you can use longer than average cables. Three feet is average.
With their suggested circuit you shouldn't need the 2.7:1 attenuator you have now. A two resistor per channel attenuator for you now would be 33 k where the paralleled resistors are now and 10 k where the 47.5 k resistor is now.
As for the Ripper, it has full gain with no attenuated inputs, the FP III now has attenuated inputs. Remember back to your post #16, you were rocking the house at three clicks. With the stock 33 k ohm resistors it is considered unattenuated. Now that you have attenuated the inputs and you use more of the volume control.
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Thanks again for the advice. So, I don't need to approach the 470K ohm resistor-to-ground of the original spec input of the ST-70? If that's the case, that lower resistances are suitable and the ratio is the design requirement, then I'll sort through my stock of resistors and come up with something - or just implement the AVA modifications and take it from there.
On the voltage divider I now have 96k and 47.5k to ground (= 2.0:1 ratio.) Your suggestion of 33k and 10k to ground (= 3.3:1) would yield more volume, correct?
BTW, I'm happy with the sound of my system, thanks to the bottlehead designs/products and approach to sharing.
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No, 470k is not necessary. The spec for the FP III says a load of 10 k ohms or greater.
The 2:1 ratio (you are at 2.7:1 now). The 3.3:1 would yield a higher volume. But not by much, not discernible. I was looking for a load under 100 k ohms.