Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: rlyach on March 28, 2013, 07:19:22 AM
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I was wondering how much effort it would be to convert the Stereomour to use 300B tubes and up the output to 8.5 Watts. I am so impressed with the 2A3 version in my bedroom I wanted to replace my downstairs system. I love the parafeed design but I wanted to stick with an integrated system. I plan on purchasing (or building) some 97dB+ speakers to match. I sort of have an idea on the changes necessary, including transformers and chokes. I just don't want mono-blocks or a separate pre-amp. Is this even possible?
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You'd need to change the power transformer, plate chokes, output transformers, parafeed caps (most likely), cathode resistors, cathode bypass capacitors, power supply, chassis plate, ....
You could try the JJ 2A3-40 in the amp and see what PJ says could potentially be a higher power operating point, but I'm not the one to speak regarding the power handling of the iron in that amplifier.
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Randy, I've been looking at ways to do - or approximate - that for a couple years. It will likely come about, but not soon - there are several other things ahead of it in the queue, and we're not going to talk about all of them.
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Paul,
I would be willing to prototype something on my own. I have already run through many calculations using the Stereomour typology. My initial estimates are to run the 300B a little hot at 80mA with a ~900 ohm cathode resistor and ~700 ohm choke. I think I would use two 6AN4 triodes for the pre-amp stage. I would use between a 2.25K and 2.5K equivalent output impedance. I would also probably run with a 480V B+. AT these conditions you get to 8.3Watts at 7.8% distortion. The 6AN7's are perfect as they have a mu of 70.
Update: If you push it a little more (90mA) and use a cathode resistor equivalent with humpot of 770 ohms you can get 8.8 watts at 6.2% distortion. You would also have to run a B+ of 500 volts.
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That would be quite a reasonable design, and should sound quite good , though I haven't heard or measured the 6AN4 so that would be a question to answer.
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Paul,
If I use 488 for B+ I can use the same plate choke that comes in the Stereomour kit as long as it can handle 150mA or so. The output transformer is a little more difficult. Does Bottlehead have a 2.5K output transformer suitable for a parafeed configuration? I can use two power transformers, one per channel. Each should have ~370V rms at 150mA for a center tapped transformer. If I use the voltage doubler it will take ~200V rms. Each transformer should also have both 5V and 6.3V filament windings. I can use a C4S board mounted between the 6AN4s to bias them for maximum gain. Once I find a suitable power transformer the rest of the component values can be calculated. I can use your rule of thumb for the parafeed cap (3uF to 6uF) assuming a 15H to 20H output inductance and a 2500 ohm impedance. if I can find the transformers I should be able to lay this out with the power transformers in the center and the chokes/output transformers on the outside rotated 90 degrees from the power transformers in either the X-Y or X-Z plane.
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I just looked at the pre-amp design. The 6AN4 can be biased at 0v and run positive. In fact, this is the preferred condition as it will give the maximum amplification factor. The shunt regulator can be removed and the constant current source can be set from 8mA to 15mA. The input will have to be modified to be capacitively coupled with a 1Meg termination to ground.
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Now you pinpoint the issues that make me take my time!
The Stereomour plate choke can handle 50mA, no more. The Paramount choke can handle about 75mADC maximum. Power - and hence size - go as inductance times current squared, so you can easily see why a greater current is expensive! Neither is currently available as a retail product - these are custom designs, optimized for the Paramount.
Bottlehead has only two parafeed output transformers at present, the OT-1 rated 3K ohms and 8 watts at 30Hz, and the OT-2 rated 4K-8K ohms at 4-2 watts respectively, at 25 Hz. Neither is currently available as a retail product.
The PT-4 used in the Paramount has a 200v winding, along with a 6.3v/1.2A and an 8-vCT winding designed for use with the FC-1 filament choke to power 2A3s or 300Bs. The 200v winding can deliver a maximum 80mADC with a voltage doubler. Only the FC-1 is currently available as a retail product.
As you can see, this is a very small corner of the audiophile world, and parts selection is somewhat limited!
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I just looked at the pre-amp design. The 6AN4 can be biased at 0v and run positive. In fact, this is the preferred condition as it will give the maximum amplification factor. The shunt regulator can be removed and the constant current source can be set from 8mA to 15mA. The input will have to be modified to be capacitively coupled with a 1Meg termination to ground.
At more than -0.5v or so of grid bias, most tubes will draw grid current which will create nonlinearities (distortion) and rectification (bias shift) unless the source impedance is extremely low. Look up "grid-leak bias" for another aspect of the issue. Don't forget to look at the performance over the full range of grid voltage!
Shunt regulators (there is none in the Stereomour, but it is present in the Paramount) do improve the sound. The C4S has a high source impedance, around 5 megohms, which you would think sufficient to isolate the stage completely from the power supply - but the shunt reg still makes a clearly audible improvement to my ear, and that of almost everyone who has heard it.
And the fewer capacitors, the better. That's why we have no caps at the input of anything, and we use direct coupling where it is possible. Try it yourself!
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I thought the LM431 in the C4S board was a shunt regulator. The Stereomour schematic I have shows it on the cathode of the 12AT7. Anyway, here are the plate curves of the 6AN4. It is a very interesting triode. It appears the best place to bias it is 10mA.
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Regarding the added caps. I think if you bias the 6AN7 at 1V on the cathode and 140V on the plate (7mA) you can direct drive the 300B stage and eliminate the coupling cap between stages. Therefore there will still only be two caps in the signal path.
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The added voltage and direct coupling of such an amp makes some other problems, such as:
Allowable voltage and power across the C4S
Allowable voltage across available power supply capacitors
Soft-start warmup in the directly coupled DHT circuit
It's worth the investment, it's just not all that easy.
For iron, I would recommend contacting Sowter and asking if they can produce a 2.5K/10 Watt parallel feed transformer (SE05 with no gap?) and appropriate plate loading choke (they have a 50H/100mA choke, sweet!!).
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Thanks, I think I may actually attempt this challenge. I really want a parafeed 300B integrated solution. I will contact sowter. There is also electra-print. They make custom transformers as well. Incidentally, I got the voltages reversed in my last post. The cathode of the 300B needs to move up to 140V and the plate of the 6AN7 needs to be set at 70V. This may make things easier in that I don't need as much resistance in the choke, but I will need additional B+, probably around 600V. Actually building this may take me a while. I still have to finish my Tode (1 more day), and then I have to work on the Stereomour for my pastor. I am building a base to match the Hoyt Bedfords I ordered. Thanks for all the help. Paul, Sorry for taking up so much of your time. You guys are awesome and I love your products.
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Here is another suggestion. Would it be possible to simplify the paramount enough to fit it in one chassis? I could have a plate custom laser cut and build my own box. This seems more doable and has the added advantage of going with a proven design. Let me know what you think.
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Yes, you could pack two Paramounts into a single chassis. The design won't need to be simplified any, but I'd suggest using 1/8" thick aluminum for the chassis plate at minimum. If you wanted to, you could have a single IEC inlet and single power switch. The parts layout can be mirrored without a whole ton of trouble either.
If you're going completely custom, I would suggest a 10" x 20" chassis to allow for flexibility and easier wiring around parts that would otherwise be on the edge of the chassis plate.
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Here is another suggestion. Since I have JJ 40W tubes, I started with the Stereomour, moved the loadline using a 3K output transformer that BH can get, and moved the cathode resistor to 800 ohms. The result is a 6W amp with 4.33% distortion. I have not run the power supply simulation yet but I suspect I am going to be a little short. Is this something that I might be able to do?
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You are suggesting taking more current from the power supply. Since heat goes as power which is current squared, that is always a risk! Fortunately in the present case, the PT-6 is able to handle 75mA per channel.
You will get less plate-to-cathode voltage when you draw more current - not a lot less, but maybe 5-10v less(?). You have shown 325 volts, but the design point (at nomimal power line voltage) is 300v, so you can probably count on 290v.
You'll need a new plate choke as well, to handle more than 50mA. The PC-2 that we use in Paramount, and the BH-7 that Magnequest offers, both have exposed terminals with plate voltage on them, and are unsuitable for mounting topside for obvious safety reasons. When looking for an available choke, remember that as a plate choke it must handle a large AC signal as well as the direct current - most power supply chokes are rated for max current near saturation, so look for a rated current about 50% higher than the actual DC current.
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In order to compensate for the drop in plat voltage I was planning on raising B+. I ran a simulation and the most I could get was 395V using diodes to maintain channel separation in the power supply in place of the 270 ohm resistors and increasing the supply caps. Based on this data, in order to keep the 2A3 biased at 60V, the most plate current I can get with the current power supply is 60ma and not 75ma. This hardly seems worth it. The Power calculated at this operating point is 5W, which would only increase the volume by 1db. If I had a different power supply I could raise the B+ to 450 and thus run a 325 plate to cathode voltage. This would get the target 6W but too many things will have to change. I did find a 150ma 15H 256 ohm plate choke, but I think the inductance is too low. I did check into custom iron but that gets expensive very quickly. I am currently re-evaluating the Paramount with BeePre.....
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I did check into custom iron but that gets expensive very quickly. I am currently re-evaluating the Paramount with BeePre.....
Yeah, custom iron can get pretty scary in a hurry!
I know I'll annoy PJ a little bit by saying this, but the Paramount is an outstanding test bed for experiments.
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Hey Randy - There are Paramounts in my future too. Is there any way to use the Stereomour as a pre-amp, beyond just passive control?
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Hey Randy - There are Paramounts in my future too. Is there any way to use the Stereomour as a pre-amp, beyond just passive control?
Theoretically there could be a way to wire it for 45's with a single shunt regulated B+ supply using a completely different power supply, and some kind of DC on the 2A3 filaments. This would turn the circuit into a preamp, but I'd expect the parts to be very expensive (maybe more than the Stereomour was in the first place), and there's a good possibility that a lot of R&D work would be required to get good results.
(Remember, a Quickie is only $99)
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Well I think knew the answer to that question anyway
I will likely build the BeePre next. I have been wanting to use my expensive balanced interconnects anyway. All of my current gear is balanced and I got suckered into buying these things! Then I'll build the the Paramounts (after I let Randy figure everything out)
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Balanced has piqued my curiosity lately as well. I was previously a skeptic.
On a lark I bought some Blue Jeans balanced interconnects about 6 mo. ago since all of my gear has balanced outputs. I was pleasantly surprised. Thats why I retired my quickie there for a spell. The quickie is back in now with the 'new' digital front end. Of course the proposition of going full balanced with beepree is gnawing. It would be cool if someone could give their impressions of the prebee sound over balanced vs. unbalanced.
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I used to be a musician and so was used to always using balanced cables - I kind of always thought it was the best way to go.
My Ayre equipment is balanced all the way through - I do imagine? I notice a difference with the balanced cables. I think that is specific to the Ayre gear and isn't necessarily strictly because of the balanced interconnects. having said that, I like the sound of the Stereomour better than the Ayre with my DIY magnet wire single ended interconnects so that says something . . .
I think we hijacked Randy's thread :)
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No problem hijacking the thread. I have literally run into a wall anyway. The more I dig the more I find out how little I know about amp design. I attempted to do my own simple parafeed circuit. I actually designed it around iron that I could get easily. Then I submitted it to a friend of a friend who builds custom iron and designs. He thought my circuit was a good attempt "for someone who is new to tubes," but then proceeded to point out several issues. I now know I don't really understand reactive plate loads, and I am finding I don't have time or the resources to find out. If I really wanted to design my own amp I will need to build, test, change, test, change, test... I guess that is why BottleHead amps sound so good. They have put in the time and effort, and they have built the knowledge to design something that works and sounds amazing to boot. I have resigned myself to purchasing an integrated kit based on the 300B just to satisfy my need to build something else. Besides, I need to replace my solid state amp (Linn LK140 if anyone is interested :) )
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I hear ya Randy - Feelin your dilemna - thinking about selling my Ayre but need it right now for videos. I definitely don't have enough juice with the stereomour for videos. I guess the output is much lower from my DVD player.
I liked your idea about building the Paramounts on one platform. I am used to an integrated amp and struggling with whether to have all the different components needed for the monoblocks. Sounded like a reasonable compromise.
Let us know what you build. BTW, i am lovin having a remote for the Stereomour, now I can sit back and relax . . .
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Drew,
Your woodworking skills have inspired me. I am getting ready to build another Stereomour kit for my Pastor. We purchased a pair of Hoyt Bedford monitors for his amp, so I purchased this nice piece of Sycamore to match. I plan on finishing with tong oil and wax, as per a woodworker I met in the Woodworkers Source. This should look cool, especially with the dark edge. I plan on doing a 1/4 or 3/8 round over around the top outside edge as well. Plus I am making it 1/2 inch taller than the supplied kit base.
Incidentally, I have chosen my next amp project but it is not a BottleHead kit. I really wanted an integrated stereo amp based on 300B's
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that is a beautiful piece of wood - and rounding the edges will add a great finished look. Good call to make it deeper - I am glad I made my base a bit deeper (4"). it easily accomodates the stepper motor I added.
Post pictures when you are done!
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Drew,
Here is the finished base. This is the first time I have used wax and I must say that I like using it. Much better for the wood and less messy. Plus I was able to use a buffing pad on a drill to give it a nice sheen. Now I am waiting for the kit to come so I can start building.
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that's gorgeous, I really like the rounded top edge. I also think the extra depth has an elegance too it, as well as allowing more room for large expensive caps!
very nice work - be careful, you might have to try making your own tubes soon :) Still awaiting my new "improved" crossovers. I almost bought a couple of Clarity MR caps for the coupling but am saving my money for the paramounts.
Oh, I picked up a matched pair of Telefunken NOS tubes, don't need two, just happened to be a good deal. Let me know if you want to try one in your Stereomour, I'll send you one so you can compare. So far I like the stock tube alot though.
drew.
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Randy,
I KNEW that was sycamore! It is one of my alltime favorite grains. The grain seems to interlock.
Now, I get the idea that you want to customize an amp, but my question is, since you will be building speakers that are 97db sensitive, why would you need more than a couple of watts?
This has become a fascinating thread by the way.
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Hey Greg,
You might have Randy and I confused - I was going to build some FR Metronomes but am holding off now. Everything sounds too damn good. The GMA's are sounding great and loud enough. Randy is using Orca's and is super pleased with them.
I just want to build more stuff, and I can afford it if can just sell this other stuff . . .
drew.
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Yes... I wanted to build some speakers but I was having trouble finding a full range driver and I am afraid of veneer. I was trying to design my own integrated stereo 300B system (using a parafeed topology) but I realized I don't know enough yet. I also don't have the funds to experiment with different components. So I ended up purchasing a 300B kit which uses a direct drive topology and a two stage pre-amp circuit. I also ordered a pair of Omega Super 3Ss to go with it. The Omega's are 94 or 94.5 db efficient so they should be a good match for the 300B's. By the way, the Orcas with the Stereomour keep getting better. The more the system breaks in the better the response. The bass response is around 75 Hz even though the driver is only rated at 100 Hz. The ported cabinets are very well tuned.
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Yah, at least I got part of it right. The Sycamore really looks good for that base. I still have some in my stash. I have had it too long for it to just sit there any longer.
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Drew,
Thanks for the offer of the tube. To tell you the truth, I have not even listened to the kit tubes yet. I plugged them in to do the final voltage checks and then immediately switch to the JJ 40W tubes I purchased. I have only listened to the JJ's. They also have a larger bottle than the standard 2A3. I think it is the same size as the 300B bottle. I think they sound very good and now that they are getting broken in, I don't want to switch them out. I will keep the Sovteks for a future use.
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FYI - it is a driver tube - I am playing around with driver tubes because it is fairly inexpensive! Can't afford those big guys and still really thinking about the next build.
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Sorry, I was confused on terminology. I assumed driver tubes and power tubes were synonyms. My bad ;D. I always called this the pre-amp tube or the gain stage. I guess I have never heard it called a driver tube before. Anyway, in reference to the driver tube I am in the same boat. I am using a JJ gold pin 12AT7 and have never listened to the kit tube. Maybe I should! I am so satisfied with the current configuration that it never occurred to me to change it.
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I had a bad 12at7 tube shipped and replaced it with the JJ Goldpin too. When I got the stock tube last week, I dropped it in just to see if I could hear a difference. I really like the stock tube better than the JJ. I noticed a lot more clarity and separation, better all around. So I have the JJ sitting in a box now. My experience anyway.
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I have to agree about stock tubes. It's not like a so-so product was sent. Probably had an evaluation before it was chosen to begin with. I have been rolling in my Quickie, and the stock tube rates quite high. I was just lucky enough to get a couple of NOS Telefunkins that have the edge over some others that I have tried.
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I am trying a NOS Telefunken next, I would also like to try the Mullard (I hear that is what Doc uses!). I will have a spare Telefunken if anyone wants to trade for a Mullard (just to try them out).
Those about the only ones I am interested in right now. For me it is just to try some different things to hear them, not really feeling the need to improve necessarily.
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FWIW The Sylvanias that I have used in my Quickie were promising, but honestly I don't think that they were broken in yet. They still had a bite to them. So may upgrades you can do, but not many are as easy to hear and do as rollin'
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You guys have got me wondering so.... I just ordered an NOS Mullard CV tube. I will let you know after it has broken in a bit.
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well I am glad to hear that Greg likes the Telefunken as I am not so patiently waiting to get mine. Seems they shipped from Siberia or something.
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I got capacitors from Canada and tubes from Ukraine this week. The USSR military surplus electronics sometimes come from Siberia. It isn't unusual.
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Ha, I was attempting sarcasm! but you have a good point Grainger.
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Sorry, Drew, I was going for tongue-in-cheek. I did get the sarcasm but wanted to say Siberia is a good place to get some stuff from.
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"Spring comes late to the Ural Mountains" 8)...John
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Gotta say I am LOL'in back here in Sunny California! and boy has this thread gone awry!
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To whoever is the OP, I'm very sorry for the skids.
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I got my Mullard NOS 4024CV today. They were running a special at TubeDepot. I have to say that right out of the box I liked it much better than the JJ gold pin. The sound was more defined. More clarity as well. The JJ sounded good to me until I tried the Mullard. It seems that all frequencies sound better. The Bass is more forward if than makes any sense. I think the JJ is a little edgy as well. Perhaps the JJ is meant for guitar amp applications. Thanks so much for spurring me on to try new tubes. I am in you debt.
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I like Grangers burnout with posi rear end. I miss those days. Maybe when I'm as old as Granger. I'll get a C7 Corvette. Lay some rubber.....
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Glad to hear the Mullard worked out. I am enjoying the Telefunken, pretty similar experience. will try the Mullard soon. I also converted my silly and expensive liquid polymer cables to single ended. This stuff is a real b(&) to work with and really didn't do anything noticeable.
maybe we should convince Randy to roll 2a3's next :)
The vet is nice, but I still want a Porsche Carrera S. Can't really burn out in an all wheel drive though . . .