Somehow way back in 2011, I went down the "home theater" rabbit hole only to find out that it wasn't so great after all. I never should have given up my 2-channel tube amp in favor of a 5-channel solid state monster. About 10 years later I discovered that I had simply stopped listening to music - period. That's sad. Very sad. I tried listeing to stereo recordings with the solid state amp and preamp, and it just wasn't right. I hated it. Then I remembered why I dumped solid state in 1993 and went all tube.
This time last year, I was between jobs and fed up with trying to listen to the expensive - not cheap - solid state amplifier that I hated. With no job and not wanting to spend any money, I somehow found an inexpensive little Chinese tube amp kit built by XrayTonyB on YouTube:
It looked like a nice little amplifier, and it seemed to perform ok in his tests, especially given the sub-$400 price tag. I found out that the kit had been updated to remove the automatic "loudness" compensation circuit that he spent time removing in the videos. So I bought the kit from China on AliExpress. I hadn't fiddled with electronics since the early 1990's, but with his video tutorials, I figured that I had a decent chance of success. I sold my solid state amp and preamp to pay for the project once it was complete. The build wasn't without trials and tribulations. I first had to learn to use PSUdesigner to determine that changing the resistor in the C-R-C filter could very easily and inexpensively reduce the overvoltage caused by putting 120V from the wall through a 110V power transformer, and without using a bucking transformer. The problems started when I broke a secondary lead from the power transformer and had to do a tiny, tiny repair of that fine wire. Then I broke one of the output transformers the same way without knowing it. A power tube then blew, and I was left very frustrated. With patience and help from a number of forum members here, we found the problem, I fixed the hair-like wires that I broke on the OPT, and I ended up with a very nice sounding little amplifier for only $400. I will never, ever use a solid state amplifier again.
The first build thread is at the link below, with a lot of boring newbie questions, and of course the problems that I caused by breaking those tiny transformer wires.
https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
I spent a great deal of time translating the 67 pages of directions from Chinese to English. My translated instruction manual was available on a Google drive link in the first build thread. I have them if anyone needs them. I don't know if those Google drive links still work.
Why am I doing this same amp kit again? Maybe curiosity killed the cat as the saying goes, but I want to see what happens if I change a few things, like using larger output transformers. I'm really happy with the way the amplifier sounds, and anyone who builds this kit shouldn't hesistate to build it just as it comes in the box. Just be sure to adjust the voltage as I described if you are using USA 120V instead of 110V. It can be done simply by changing the power supply resistor and using the included lower voltage Zener diode as described in my first built thread. Then curiosity sets in ... what would happen if I change this ... or this ... and so on.
Why this particular amplifier? 1) It's a kit so ... no fabrication that I hate, I don't have to spend a bunch of time and effort ordering individual parts, and I only have to replace/upgrade any parts I choose like the OPTs, 2) It's not expensive at $312.81 with shipping and tax included (it was on sale plus I used a promo code), 3) I know that I like the sound because I already built one, 4) The compact left-right footprint is perfect for me, 5) I already built one so there is little chance of me making mistakes this time. I had to raise my confidence by building the first one, breaking it twice, blowing a tube, repairing it, and finally ending up with a great amp. So those all are valid reasons for me, despite the circuit not being "ideal." I know there are other designs available for people who really want to build their own amps from scratch, but I don't want to do that, at least not anytime soon. Maybe someday but not now. In an ideal world I'd find a proven circuit and scratch build a custom pair of EL84 monoblocks. We don't live in a perfect world.
I am not against making simple modifications to this amp, and I would like to look at those before I proceed. Suggestions welcome. However, it is a PCB amp with a very crowded chassis, so modifications have to be limited. I also do not want to seriously alter the sound as I love it the way it is. It sounds "right" to me unlike so many amplifiers. I don't know what makes the amp sound so good, but it does. It gets the timing of the music right, and so many amps get that wrong. I'm not sure how else to put it. I do want to see what bigger better OPTs bring to the table. There also may be parts of the circuit that haven't been properly optimized, so suggestions welcome. I'd like to find a way to use 4 individual bias resistors, but it can't be done on the PCB. I'll have to see if they can be relocated to a terminal strip screwed to the side of the chassis I guess.
I will be using the excellent sounding long-life 6P14P-ER (Russian 6П14П-EP) power tubes and the equally excellent sounding 6N1P-EV (Russian 6Н1П-ЕВ) driver tubes. I already have these tubes. Spec sheets are attached.
The schematic is attached. Values in green are actual values measured in my current amplifier. When I say that modifications must be limited, I mean very seriously limited as there is precious little space inside:
The first step is to change the output transformers to see if bigger better ones will make a significant difference. Their larger size will require leaving off the metal box chassis cover to open up additional room on top. That would leave high voltage exposed, and I wanted a 120VAC power transformer instead of 110VAC anyway, so I had one custom made. It has end bells and wire leads, so there will be nothing dangerous left exposed. Since the output transformers also will be exposed, they also require covers. I prefer the look of end bells rather than boxy square covers, so I made traditional end bells a requirement. I like their "industrial" look. A thread looking at various output transformer options for this build is at the link below:
https://www.diyaudio.com/community/threads/el84-pp-output-transformer-recommendations.405967/
I ordered an inexpensive, small SMD-based stepped attenuator for the volume control that seems to be quite popular. SMD stepped attenuator
This time I'm going to spend a few dollars on known quality power supply caps. The coupling and bypass caps that say "WIMA" on them I would bet are fakes, so I'll spend a few dollars getting genuine ones.
I did the first build in all black because the silver "hurts" my eyes 👀 and this one will be all black too, so I have a lot of paint work to do first. I'll start whenever it quits raining. ⛈️🌩️
If I could relocate the adjustment pots for the two magic eye tubes outside the chassis, I'd be tempted to build the kit with them and just leave them out of the sockets when I don't want them flashing at me. They are purely for entertainment and it's just 2 more tubes to worry about. I don't think I can relocate the pots without a major hassle though. Having to disconnect the amp, remove the bottom, reconnect it upside down, adjust the magic eye pots inside, and then put the amp back in the system is ridiculous. Last time I just plugged the holes for the magic eye tube sockets and left them out. If I'm going to leave them off again, I'd like to find someplace that can fabricate a new top plate without those holes instead of just plugging them. Is there someplace online that does small fabrication jobs like making a new top plate for me? The holes have to be very precise as they have to align with the tube sockets and PCB.
And here we are almost 1 year later.
Useful links:
Resistor power calculator: https://www.amplifiedparts.com/tech-articles/resistor-power-rating
Tube bias calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
One of many sellers on AliExpress selling this kit: https://www.aliexpress.us/item/3256802532801472.html
Whenever it quits raining and we get some pleasant weather, I can start the painting. I stripped the silkscreening off of the faceplate last night. It requires a 4 hour soak in acetone and then scrubbing hard with a Scotchbrite pad to get that cleaned off. No other solvent or stripper that I tried last time worked. A soak in acetone does the trick. I do custom decal work, so that's how the printing gets put on the new black paint. After drying, the decals are sealed with multiple layers of satin clear overcoat with multiple wet sandings to get a perfect painted-on look and permanent protection from discoloration.
This time last year, I was between jobs and fed up with trying to listen to the expensive - not cheap - solid state amplifier that I hated. With no job and not wanting to spend any money, I somehow found an inexpensive little Chinese tube amp kit built by XrayTonyB on YouTube:
It looked like a nice little amplifier, and it seemed to perform ok in his tests, especially given the sub-$400 price tag. I found out that the kit had been updated to remove the automatic "loudness" compensation circuit that he spent time removing in the videos. So I bought the kit from China on AliExpress. I hadn't fiddled with electronics since the early 1990's, but with his video tutorials, I figured that I had a decent chance of success. I sold my solid state amp and preamp to pay for the project once it was complete. The build wasn't without trials and tribulations. I first had to learn to use PSUdesigner to determine that changing the resistor in the C-R-C filter could very easily and inexpensively reduce the overvoltage caused by putting 120V from the wall through a 110V power transformer, and without using a bucking transformer. The problems started when I broke a secondary lead from the power transformer and had to do a tiny, tiny repair of that fine wire. Then I broke one of the output transformers the same way without knowing it. A power tube then blew, and I was left very frustrated. With patience and help from a number of forum members here, we found the problem, I fixed the hair-like wires that I broke on the OPT, and I ended up with a very nice sounding little amplifier for only $400. I will never, ever use a solid state amplifier again.
The first build thread is at the link below, with a lot of boring newbie questions, and of course the problems that I caused by breaking those tiny transformer wires.
https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
I spent a great deal of time translating the 67 pages of directions from Chinese to English. My translated instruction manual was available on a Google drive link in the first build thread. I have them if anyone needs them. I don't know if those Google drive links still work.
Why am I doing this same amp kit again? Maybe curiosity killed the cat as the saying goes, but I want to see what happens if I change a few things, like using larger output transformers. I'm really happy with the way the amplifier sounds, and anyone who builds this kit shouldn't hesistate to build it just as it comes in the box. Just be sure to adjust the voltage as I described if you are using USA 120V instead of 110V. It can be done simply by changing the power supply resistor and using the included lower voltage Zener diode as described in my first built thread. Then curiosity sets in ... what would happen if I change this ... or this ... and so on.
Why this particular amplifier? 1) It's a kit so ... no fabrication that I hate, I don't have to spend a bunch of time and effort ordering individual parts, and I only have to replace/upgrade any parts I choose like the OPTs, 2) It's not expensive at $312.81 with shipping and tax included (it was on sale plus I used a promo code), 3) I know that I like the sound because I already built one, 4) The compact left-right footprint is perfect for me, 5) I already built one so there is little chance of me making mistakes this time. I had to raise my confidence by building the first one, breaking it twice, blowing a tube, repairing it, and finally ending up with a great amp. So those all are valid reasons for me, despite the circuit not being "ideal." I know there are other designs available for people who really want to build their own amps from scratch, but I don't want to do that, at least not anytime soon. Maybe someday but not now. In an ideal world I'd find a proven circuit and scratch build a custom pair of EL84 monoblocks. We don't live in a perfect world.
I am not against making simple modifications to this amp, and I would like to look at those before I proceed. Suggestions welcome. However, it is a PCB amp with a very crowded chassis, so modifications have to be limited. I also do not want to seriously alter the sound as I love it the way it is. It sounds "right" to me unlike so many amplifiers. I don't know what makes the amp sound so good, but it does. It gets the timing of the music right, and so many amps get that wrong. I'm not sure how else to put it. I do want to see what bigger better OPTs bring to the table. There also may be parts of the circuit that haven't been properly optimized, so suggestions welcome. I'd like to find a way to use 4 individual bias resistors, but it can't be done on the PCB. I'll have to see if they can be relocated to a terminal strip screwed to the side of the chassis I guess.
I will be using the excellent sounding long-life 6P14P-ER (Russian 6П14П-EP) power tubes and the equally excellent sounding 6N1P-EV (Russian 6Н1П-ЕВ) driver tubes. I already have these tubes. Spec sheets are attached.
The schematic is attached. Values in green are actual values measured in my current amplifier. When I say that modifications must be limited, I mean very seriously limited as there is precious little space inside:
The first step is to change the output transformers to see if bigger better ones will make a significant difference. Their larger size will require leaving off the metal box chassis cover to open up additional room on top. That would leave high voltage exposed, and I wanted a 120VAC power transformer instead of 110VAC anyway, so I had one custom made. It has end bells and wire leads, so there will be nothing dangerous left exposed. Since the output transformers also will be exposed, they also require covers. I prefer the look of end bells rather than boxy square covers, so I made traditional end bells a requirement. I like their "industrial" look. A thread looking at various output transformer options for this build is at the link below:
https://www.diyaudio.com/community/threads/el84-pp-output-transformer-recommendations.405967/
I ordered an inexpensive, small SMD-based stepped attenuator for the volume control that seems to be quite popular. SMD stepped attenuator
This time I'm going to spend a few dollars on known quality power supply caps. The coupling and bypass caps that say "WIMA" on them I would bet are fakes, so I'll spend a few dollars getting genuine ones.
I did the first build in all black because the silver "hurts" my eyes 👀 and this one will be all black too, so I have a lot of paint work to do first. I'll start whenever it quits raining. ⛈️🌩️
If I could relocate the adjustment pots for the two magic eye tubes outside the chassis, I'd be tempted to build the kit with them and just leave them out of the sockets when I don't want them flashing at me. They are purely for entertainment and it's just 2 more tubes to worry about. I don't think I can relocate the pots without a major hassle though. Having to disconnect the amp, remove the bottom, reconnect it upside down, adjust the magic eye pots inside, and then put the amp back in the system is ridiculous. Last time I just plugged the holes for the magic eye tube sockets and left them out. If I'm going to leave them off again, I'd like to find someplace that can fabricate a new top plate without those holes instead of just plugging them. Is there someplace online that does small fabrication jobs like making a new top plate for me? The holes have to be very precise as they have to align with the tube sockets and PCB.
And here we are almost 1 year later.
Useful links:
Resistor power calculator: https://www.amplifiedparts.com/tech-articles/resistor-power-rating
Tube bias calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
One of many sellers on AliExpress selling this kit: https://www.aliexpress.us/item/3256802532801472.html
Whenever it quits raining and we get some pleasant weather, I can start the painting. I stripped the silkscreening off of the faceplate last night. It requires a 4 hour soak in acetone and then scrubbing hard with a Scotchbrite pad to get that cleaned off. No other solvent or stripper that I tried last time worked. A soak in acetone does the trick. I do custom decal work, so that's how the printing gets put on the new black paint. After drying, the decals are sealed with multiple layers of satin clear overcoat with multiple wet sandings to get a perfect painted-on look and permanent protection from discoloration.
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Christmas came early this year!
I should be able to measure for OPTs on Monday when the new power transformer arrives. Apparently US Customs opened my package, which doesn't usually happen.
And this is why I like kits. No shopping and no fabrication, just paint work because I hate silver. I don't have to replace anything if I don't want to. I'm sure the WIMA coupling and bypass caps are not genuine. I'll get genuine this time. I don't trust the power supply caps so I'll order some good ones this time.
Any suggestions for PCB-mount tube sockets? The Micalex ones (or whatever they are) in the ST-35 kit that I built are awful. Their death grip on the tubes is going to break the PCBs one day. It's ridiculous. Cheap porcelain ones come in the kit, and one is starting to loosen a bit in the other amp. A snug fit is great, but a death grip for PCB-mounted sockets is not good. I need PCB-mount sockets with low friction but with a firm grip that won't loosen too much over time.
I even got a free Chinese newspaper. I'll run OCR and translation on it and get up to date on current events.
I should be able to measure for OPTs on Monday when the new power transformer arrives. Apparently US Customs opened my package, which doesn't usually happen.
And this is why I like kits. No shopping and no fabrication, just paint work because I hate silver. I don't have to replace anything if I don't want to. I'm sure the WIMA coupling and bypass caps are not genuine. I'll get genuine this time. I don't trust the power supply caps so I'll order some good ones this time.
Any suggestions for PCB-mount tube sockets? The Micalex ones (or whatever they are) in the ST-35 kit that I built are awful. Their death grip on the tubes is going to break the PCBs one day. It's ridiculous. Cheap porcelain ones come in the kit, and one is starting to loosen a bit in the other amp. A snug fit is great, but a death grip for PCB-mounted sockets is not good. I need PCB-mount sockets with low friction but with a firm grip that won't loosen too much over time.
I even got a free Chinese newspaper. I'll run OCR and translation on it and get up to date on current events.
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That looks great. I'll get a quote. A plate without the holes would be better than plugging them this time. Thanks! 👍
They also do really beautiful engraving - anything you want. One possible negative is that they start with anodized panels, and the holes, engraving, etc. aren't anodized in the holes, etc. If you're painting post-machining that wouldn't matter. The only place who did anodizing post-machining was Sescom, and AFAIK they're not still in that biz. Would love to be wrong of course.
All good fortune,
Chris
All good fortune,
Chris
Moving this here for further discussion as it's a little over my head right now. Attachments from the post are included here as well.
Step 1, remember how math works and all of these notations. 🤔
I haven't used LHS, RHS, >> in a long time. Left side of equation much greater than right hand side.
formula: (mu + 1)*Rk >> rp + Rl
Is there an article or thread, 1) Explaining the formula with examples, and, 2) How the change makes this improvement? The device looks simple enough to implement physically.
That's me.
I am surprised that 1.45% is not sufficient. Is it that the two halves of the split signal come out "lopsided" in relation to one another due to this difference?
What does this mean?
I didn't know what OD3 and OA2 were so I looked it up. Gas-filled voltage regulator tubes. Interesting. No room for them though.
Perhaps you can help me figure out how to implement this on this build. I'd like to understand this modification, and then I would need help implementing it on my schematic with my tubes - specific values, connection points, etc.
This is very interesting. 🤓
Step 1, remember how math works and all of these notations. 🤔
I haven't used LHS, RHS, >> in a long time. Left side of equation much greater than right hand side.
formula: (mu + 1)*Rk >> rp + Rl
Is there an article or thread, 1) Explaining the formula with examples, and, 2) How the change makes this improvement? The device looks simple enough to implement physically.
That's me.
I am surprised that 1.45% is not sufficient. Is it that the two halves of the split signal come out "lopsided" in relation to one another due to this difference?
What does this mean?
I didn't know what OD3 and OA2 were so I looked it up. Gas-filled voltage regulator tubes. Interesting. No room for them though.
Perhaps you can help me figure out how to implement this on this build. I'd like to understand this modification, and then I would need help implementing it on my schematic with my tubes - specific values, connection points, etc.
This is very interesting. 🤓
Attachments
I used this to my advantage in the early 1990s when I had a few heavy faceplates machined from black anodized aluminum. I took the faceplates to a trophy shop that did engraving. They engraved my name and things like "ON" and "OFF" for me. The result was a nice black panel with shiny silver lettering since the engraving scratches away the black anodizing. It looked great.
The top plate for this amp needs to be steel for strength and thinness.
I thought about having a second hole put in the faceplate for a volume control bypass switch. However, it may not be worth it since the last step of the stepped attenuator should essentially bypass itself. I'll ask the seller if the final step is 0Ω or some other value.
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I don't think FPE does steel, but worth checking. Aluminum can bear weight well with internal ribs of L or C channel, but may be more prep than you seem to want. If you live somewhere that has an old industrial base (not here, fersure) you could get an outfit with a water-jet to cut you pretty much any sheet steel you want. You'll need to do some CADCAM. And lots of folk do powder-coat.
All good fortune,
Chris
All good fortune,
Chris
I have little room inside the chassis for reinforcement as the PCBs are mounted underneath at a distance equal to the height of the tube sockets. I might be able to find a local shop to make a new top front plate from steel, but most places here have a minimum order, like $250 because they have to set up their machines. I already have removed the rear top plate that was under the transformers, which is going to weaken the chassis a little. I might have to do reinforcement there or make another steel plate to replace it, which isn't difficult because it's not a precision piece. The new transformers are going to be heavy, so I have to watch the stength of the chassis. Steel bars across the bolts underneath the power transformer on the inside looks like a possibility. It's on the very back of the chasis (top of the picture). The chassis is pretty thick steel fortunately. It's not thin, weak sheet metal.
Powder coating is nice, but I didn't bother last time. Ordinary black paint sealed with satin clearcoats is durable enough. The clearcoats help prevent scratches. It does need to spend a month fully drying though.
Edit: Looking at it, the gap between the circuit boards looks like a prime place for an L channel reinforcement from side to side. Notice the now-clean faceplate and two new quads of tubes that arrived from Khazakstan yesterday.
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Ah, so you don't need a whole top panel, just a partial panel to hold the transformers? If so, and if this sub-panel reaches out near the chassis sides, you might be happy with a normal 1/16" aluminum sheet. Everything bows some, and how much is too much?, that argument. I like adding ribs using the transformer mounting bolts passing through the ribs as stiffening, to take advantage of aluminum's non-magnetically-conducting property. Not a huge deal, but removes a difficult variable. But you may not even really need ribs in your design - easy enough to test - sit your transformers on a 1/16" aluminum sheet (buy a Hammond or Bud chassis bottom) and observe,
My building partner is like you, averse to metalwork, but I think you guys are caca - that's the purest part of building. OTOH, it's maybe the hardest to make pretty.
All good fortune,
Chris
My building partner is like you, averse to metalwork, but I think you guys are caca - that's the purest part of building. OTOH, it's maybe the hardest to make pretty.
All good fortune,
Chris
How fricking exotic is that? Khazakstan sounds, to a guy who's never been anywhere that needed a passport (except in the Army, where we didn't need a passport), like some place from a Bruce Sterling or William F. Gibson novel, but they have vacuum valves, which are actually a very high tech device, not even manufacturable in the Good Old USA today (and don't try to include the Georgia nonsense - smells like scam on ice). The Soviet and just-post-Soviet valves are excellent. Have had excellent decades long life, and still going in my use, with "Sovtek" brand from the 1990s, in severe use (1960s SCA-35, identical conditions to ST-35) with modern line voltages. Tough duty, look like two packs a day inside, but still flying high.
Sorry to ramble, all good fortune,
Chris
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I didn't know anything about Khazkstan as it was part of the USSR growing up, and I always lumped it in with the rest of the USSR. It looks like Almaty, Khazakstan is a nice place. I do like the Soviet-era 6P14P-ER and 6N1P-EV that I am using. I have no desire to change them, and you can't beat the lifespan. They aren't too expensive either so long as supply holds out. My policy always has been to find tubes I like for an amplifier and then buy a lifetime supply. When sold or given to someone, the spare tubes go with the amplifier.
Good tubes are like gold. They almost always can be sold for more than you paid years later!
The chassis goes together like this:
The chassis is heavy steel, not thin sheet metal. It has two top plates.
The front top plate covers the amplifier PCB with holes for the tube sockets. The PCB is mounted underneath and the tops of the tube sockets are flush with the plate, for a clean appearance. The holes with the red Xs are for the magic eye tubes that I just leave out. I plugged the empty holes with 1" black plastic plugs from the hardware store last time, but a clean top plate without the holes would be nice. Aluminum would have to be significantly thicker than the steel to have any strength at all. It has to withstand the force of tube insertion and removal as the PCB standoffs are mounted to it underneath.
The rear top plate serves to reinforce the chassis, and it holds the boxy transformer cover in place. That plate and the transformer cover will not be used this time in order to get the extra space required for much larger OPTs. I think a steel reinforcing bar inside, across the bolts of the power transformer, will be fine. The power supply PCB underneath has standoffs that should leave enough clearance for the reinforcing bar.
Beauty is in the eye of the beholder I guess. I have seen some b*tt ugly DIY amplifiers over the past year! Some people don't care. I do. I want things to look like finished, professionally made retail products, not homebuilt jobs. So I always strive for that, and I'm not good at doing it for myself. I hate doing metal work, have no space and no tools, and never end up with perfect results. With nice-looking exposed transformers on top with end bells painted satin black, it will have a traditional "industrial" look to it like my ST-70 and ST-35.
I never should have said this. I put ideas into my own head. I need to focus on this project, which was very carefully selected to meet my expectations, abilities, and needs now.
@jhstewart9 See post #6 when you have time. Thanks.
Good tubes are like gold. They almost always can be sold for more than you paid years later!
The chassis goes together like this:
The chassis is heavy steel, not thin sheet metal. It has two top plates.
The front top plate covers the amplifier PCB with holes for the tube sockets. The PCB is mounted underneath and the tops of the tube sockets are flush with the plate, for a clean appearance. The holes with the red Xs are for the magic eye tubes that I just leave out. I plugged the empty holes with 1" black plastic plugs from the hardware store last time, but a clean top plate without the holes would be nice. Aluminum would have to be significantly thicker than the steel to have any strength at all. It has to withstand the force of tube insertion and removal as the PCB standoffs are mounted to it underneath.
The rear top plate serves to reinforce the chassis, and it holds the boxy transformer cover in place. That plate and the transformer cover will not be used this time in order to get the extra space required for much larger OPTs. I think a steel reinforcing bar inside, across the bolts of the power transformer, will be fine. The power supply PCB underneath has standoffs that should leave enough clearance for the reinforcing bar.
Beauty is in the eye of the beholder I guess. I have seen some b*tt ugly DIY amplifiers over the past year! Some people don't care. I do. I want things to look like finished, professionally made retail products, not homebuilt jobs. So I always strive for that, and I'm not good at doing it for myself. I hate doing metal work, have no space and no tools, and never end up with perfect results. With nice-looking exposed transformers on top with end bells painted satin black, it will have a traditional "industrial" look to it like my ST-70 and ST-35.
I never should have said this. I put ideas into my own head. I need to focus on this project, which was very carefully selected to meet my expectations, abilities, and needs now.
@jhstewart9 See post #6 when you have time. Thanks.
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They don't do steel. 🙁
I may have to resort to plugging the magic eye tube holes again. Years ago I had a new steel top plate like this made to fit my upgrades to the ST-70 series ii, but I slipped it into an order for 5 large custom chassis to meet their minimum order. I'm not fabricating one myself. The holes for the tube sockets have to be exact to look right.
You may wish to take a look at SendCutSend
I've used them for a few builds & have been quite satisfied
I've used them for a few builds & have been quite satisfied
Great! They accept standard graphics file formats like .eps, and I certainly can do that. It's also nice to know that they can accept 3D STEP files, which I can generate in Blender. I don't need that right now though.
Anyone have any suggestions for PCB-mount tube sockets? The Micalex ones (or whatever they are) in the ST-35 kit that I built are awful. Their death grip on the tubes is going to break the PCBs one day. It's ridiculous. Cheap porcelain ones come in the kit, and one is starting to loosen a bit in the other amp. A snug fit is great, but a death grip for PCB-mounted sockets is not good. I need PCB-mount sockets with low friction, but with a firm grip that won't loosen too much over time.
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Belton sockets are well regarded. I use the chassis-mount versions, but they also have PCB-mount ones.
I may be paranoid about the tube sockets, but I'm afraid not to use really good ones because it would be impossible to replace one later. It seems like someone should make a "low friction" tube socket for use on PC boards. I'm surprised Mouser doesn't carry any tube sockets. I thought they had everything.
I drew a new top plate in my graphics program and uploaded it to https://sendcutsend.com/ and I can order one anytime I want. They have a $29 minimum and it's only an $11 item though. I'll wait to see if I need anything else. If not, I'll just plug the two unused tube holes for the magic eye tubes.
I also bought a bunch of new empty tube boxes because most of the old Russian tubes don't have boxes. The exception is the quad below, which came in original boxes and with original Russian spec sheets. I need to run OCR and translation on the spec sheets.
The power transformer arrived, and it looks like the Primary Windings OPTs will just fit with it. It would be a tight fit, but no tighter than the stock transformers from China. I'm not sure why the new power transformer (made in USA) uses fewer laminations than the one from China. The laminations are the same dimensions but there are fewer of them. Maybe it's better quality iron.
It concerns me that Primary Windings isn't answering email, but I'll give them a few more days. Their OPTs would be about the same size as the power transformer.
I drew a new top plate in my graphics program and uploaded it to https://sendcutsend.com/ and I can order one anytime I want. They have a $29 minimum and it's only an $11 item though. I'll wait to see if I need anything else. If not, I'll just plug the two unused tube holes for the magic eye tubes.
I also bought a bunch of new empty tube boxes because most of the old Russian tubes don't have boxes. The exception is the quad below, which came in original boxes and with original Russian spec sheets. I need to run OCR and translation on the spec sheets.
The power transformer arrived, and it looks like the Primary Windings OPTs will just fit with it. It would be a tight fit, but no tighter than the stock transformers from China. I'm not sure why the new power transformer (made in USA) uses fewer laminations than the one from China. The laminations are the same dimensions but there are fewer of them. Maybe it's better quality iron.
It concerns me that Primary Windings isn't answering email, but I'll give them a few more days. Their OPTs would be about the same size as the power transformer.
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That segment of a circuit was used in the very successful Mullard PP amplifiers of the 1950s,
I had thought I'd seen reference to it by someone early on in one of your threads, So that piece I did is just a filler.
Read on for some quick history,
David Hafler’s company was primarily in the transformer winding business. He & Walter Keroes came up with the unique idea of the tapped OPTs, something that had come out of the 1930s. But had seen no serious use. The damping factor of such an amplifier, even without full loop NFB would be about One. Some folks labeled this Unity Damping. That worked well with various but not all loudspeakers. And higher damping could be easily got with some full loop NFB. All this was possible with the new high G power tubes. And they called it Ultralinear.
Out of this the Dynakit was developed & sold very well. But some thought the very simple pentode-triode front end phase splitter was a little lacking in performance. As time went on many circuits were devised for an improved front end, the Mullard solution was one of these. And circuit boards were built to be sold, in many cases grafting right onto the Dynakit chassis.
Some of the folks here on DIY are probably familiar with these boards & who produced them.
Something for people to keep in mind for future builds.👍
I'm approaching the day, about 10 days or 2 weeks from now, when I will start putting components on the PCB. Suggestions for simple circuit improvements welcome. I may be able to find room for 4 bias resistors instead of 2. There isn't room for bias adjustment pots.
How/why are the cathode bias caps 220uF?
From another thread I got some formulas. Let's see if I did this right:
cathode bypass cap calculation
-----------------------------------
fc = (gm + 1/R)/(2πC)
C = (gm + 1/R)/(2πfc)
Want fc to be far (two octaves or more) below the audio band. 20Hz/2/2 = 5Hz
gm of 6P14P is ~11.3 mA/V
C=(gm + 1/R)/(2πfc)
C=(.0113A/V + (1/150))/(2*3.14*5Hz))
C=(.0113 + .00666666)/(31.4)
C=0.0005721866242038217F=572.18uF
Why use 220uF?
C=(gm + 1/R)/(2πfc)
C*(2πfc)=(gm + (1/R))
fc=(gm + (1/R))/(C*2π)
fc=.0113+(1/150)/(.000220*6.28)
fc=.0179666/.0013816
fc=13Hz
How/why are the cathode bias caps 220uF?
From another thread I got some formulas. Let's see if I did this right:
cathode bypass cap calculation
-----------------------------------
fc = (gm + 1/R)/(2πC)
C = (gm + 1/R)/(2πfc)
Want fc to be far (two octaves or more) below the audio band. 20Hz/2/2 = 5Hz
gm of 6P14P is ~11.3 mA/V
C=(gm + 1/R)/(2πfc)
C=(.0113A/V + (1/150))/(2*3.14*5Hz))
C=(.0113 + .00666666)/(31.4)
C=0.0005721866242038217F=572.18uF
Why use 220uF?
C=(gm + 1/R)/(2πfc)
C*(2πfc)=(gm + (1/R))
fc=(gm + (1/R))/(C*2π)
fc=.0113+(1/150)/(.000220*6.28)
fc=.0179666/.0013816
fc=13Hz