Hello all! I'm in the planning stages for my first stereo (hifi) tube amp build. I've scratch-built a bunch of guitar amps, but this is the first time I'm shooting for LESS rather than MORE distortion. I have dumb questions.
I'll be using the transformers and power tubes from a scrapped Nutone intercom/stereo, and an Alixpress 6DJ8 preamp board. I've attached both schematics (just the power section of the Nutone. The plan so far is to use the board, but do the rest point-to-point. FYI I chose the 6DJ8 as I have a large collection left over from tube rolling my Counterpoint pre.
I'm uncertain about a few things. The Nutone had two 12AX7 preamp stages plus a 12AU7 PI feeding the four PP 6AQ6 power tubes. Do really I need two stages? I was hoping to feed the PI directly from the 6DJ8 -- is that practical/possible? Just less gain?
I'd also very much like your opinions on the Nutone power section. Are the values given still workable? It wants 0.1uf coupling caps. Should I go higher, like 1uf? Given modern sources and speakers, would a greater value increase bass response? The caps on the 6DJ8 board seem to be 1uf, though the schematic lists 0.1uf. The amps I've built have two coupling caps, but the Nutone seems to have more. Is that true? Why?
Any other suggestions for modernizing or improving the circuit would be most welcome! I'm more used to de-modernizing circuits for guitar amps. Thanks!
I'll be using the transformers and power tubes from a scrapped Nutone intercom/stereo, and an Alixpress 6DJ8 preamp board. I've attached both schematics (just the power section of the Nutone. The plan so far is to use the board, but do the rest point-to-point. FYI I chose the 6DJ8 as I have a large collection left over from tube rolling my Counterpoint pre.
I'm uncertain about a few things. The Nutone had two 12AX7 preamp stages plus a 12AU7 PI feeding the four PP 6AQ6 power tubes. Do really I need two stages? I was hoping to feed the PI directly from the 6DJ8 -- is that practical/possible? Just less gain?
I'd also very much like your opinions on the Nutone power section. Are the values given still workable? It wants 0.1uf coupling caps. Should I go higher, like 1uf? Given modern sources and speakers, would a greater value increase bass response? The caps on the 6DJ8 board seem to be 1uf, though the schematic lists 0.1uf. The amps I've built have two coupling caps, but the Nutone seems to have more. Is that true? Why?
Any other suggestions for modernizing or improving the circuit would be most welcome! I'm more used to de-modernizing circuits for guitar amps. Thanks!
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I recognize the term, but I don't understand how it works or how it would be applied here. Just because I've built a bunch of amps doesn't mean I know anything about electronics! 🙂 Not quite true, but there are def. areas of theory that I just don't know. I've attached a full version of the schematic too if that helps.
What do you suggest? I'd love to see other circuits that might inspire me too.
FYI my primary motivation on this is cost -- no income due to Covid, but boredom never sleeps.
What do you suggest? I'd love to see other circuits that might inspire me too.
FYI my primary motivation on this is cost -- no income due to Covid, but boredom never sleeps.
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I doubt any Newcomb unit would have "hifi" credibility, including their output transformers.
Their schematic shows a conventional design, but what are the transformers? - size?
If undersized, forget about any "hifi" type of sound, including bass response.
We'd need pics to determine more.
Their schematic shows a conventional design, but what are the transformers? - size?
If undersized, forget about any "hifi" type of sound, including bass response.
We'd need pics to determine more.
I used the term hifi very loosely. I'm committed to using the parts I have, but I'd like to make it as good as it can be given the (many) constraints. I've attached a bad pic from the service manual but it gives an idea how small the OTs are.
I'm not looking for great sound here. I have a big tube system downstairs for that. This will be a bedroom amp or maybe a gift.
On the plus side, the Nutone came with a matched quad of RCA 6AQ5s in good shape, and the 6DJ8s will be 2 Amperexes I've had sitting around.
I'm not looking for great sound here. I have a big tube system downstairs for that. This will be a bedroom amp or maybe a gift.
On the plus side, the Nutone came with a matched quad of RCA 6AQ5s in good shape, and the 6DJ8s will be 2 Amperexes I've had sitting around.
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Well, with 6DJ8 and 6AQ5 you have countless options. Good choices are Morgan Jones' Bevois Valley, SY's Red Light District, gingertube's Baby Huey, tubelab's SPP.
I'd forget about the preamp PCB, not worth the complication. 1 6DJ8 in front of a 6AQ5 pair will drive it to clipping with a CDP source.
I'd forget about the preamp PCB, not worth the complication. 1 6DJ8 in front of a 6AQ5 pair will drive it to clipping with a CDP source.
Apply your line level signal to pin 2 of the AF amp on what you already have. You could try moving the ground connection on the OT from the yellow wire to the black wire. That should improve performance a bit and increase feedback a little.
I do not understand why you want to use a crap board off AliExpress. How does that fit in?
I do not understand why you want to use a crap board off AliExpress. How does that fit in?
Dang, that bad? It was so cheap I thought it would save money on parts. What's bad about it, the circuit or the parts? I had planned to replace the 1uf caps with better ones. Think I should toss it?
I wanted to use 6DJ8s instead of the 12A*7 tubes in the Nutone because I have them, and all the preamp tubes were missing in the Nutone. FYI all I kept were the iron and the 4 6AQ5s. The rest was not worth keeping.
I wanted to use 6DJ8s instead of the 12A*7 tubes in the Nutone because I have them, and all the preamp tubes were missing in the Nutone. FYI all I kept were the iron and the 4 6AQ5s. The rest was not worth keeping.
If you haven't scrapped the Nutone, get some used tubes and use it.
What you're planning to do makes zero sense.
What you're planning to do makes zero sense.
The Nutone is gone. Only the transformers and power tubes remain. I will look at the suggested designs. Thank you.
From the looks of those (tiny) output transformers, they'd be lucky to get down to 100Hz.
They're geared more towards small radio-type speakers like in table radios, which is what Nutone used in their intercom systems to pipe music around the house.
They're geared more towards small radio-type speakers like in table radios, which is what Nutone used in their intercom systems to pipe music around the house.
The previous warning about undersized O/P transformers is almost certainly correct. Select circuit topology that employs short loop NFB and eschews global NFB. That sort of arrangement allows maximum power transfer of bass info., within the limitations of the "iron". A global NFB loop requires the presence of magnetic headroom, lest core saturation (VERY bad) occur.
Max. bass extension will be squeezed out by triode wiring those push/pull 6AQ5 "finals". Handling reduced power delays core saturation's onset. Tie the 6AQ5 screen grids to the corresponding plates with 1 Kohm resistors. The 6AQ5 is a member of the 6V6 family whose g2 is fragile, when compared to its Octal forebear.
Guys, should a differential cascode splitter/driver made from 6DJ8s, to allow for "Schade" style NFB, be considered?
Max. bass extension will be squeezed out by triode wiring those push/pull 6AQ5 "finals". Handling reduced power delays core saturation's onset. Tie the 6AQ5 screen grids to the corresponding plates with 1 Kohm resistors. The 6AQ5 is a member of the 6V6 family whose g2 is fragile, when compared to its Octal forebear.
Guys, should a differential cascode splitter/driver made from 6DJ8s, to allow for "Schade" style NFB, be considered?
I believe that when a transformer core saturates, that the primary inductance also goes down.
When the primary inductance goes down, negative feedback taken from the primary during core saturation (Local), will increase the current in the primary in an attempt to correct the problem. But that is only going to make the saturation even worse.
That does not fix the problem.
So using that kind of local negative feedback is almost as bad as taking the negative feedback from the secondary (Global).
Eliminate the Negative feedback from the primary
Eliminate the Negative feedback from the secondary
Or . . .
Get a better transformer that has lots more core, and now you can apply negative feedback from the transformer.
Negative Feedback does not fix some types of bad selection of parts.
When the primary inductance goes down, negative feedback taken from the primary during core saturation (Local), will increase the current in the primary in an attempt to correct the problem. But that is only going to make the saturation even worse.
That does not fix the problem.
So using that kind of local negative feedback is almost as bad as taking the negative feedback from the secondary (Global).
Eliminate the Negative feedback from the primary
Eliminate the Negative feedback from the secondary
Or . . .
Get a better transformer that has lots more core, and now you can apply negative feedback from the transformer.
Negative Feedback does not fix some types of bad selection of parts.
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Sometimes manure can be turned into fertilizer. However, no such luck with crappy O/P "iron".
IMO, short loop holds up a little longer, but lack of capability always becomes evident.
IMO, short loop holds up a little longer, but lack of capability always becomes evident.
Everybody's different, naturally.
And everybody has different goals.
However, I am not one to waste time puttering with sub-standard stuff, in an attempt at making a "silk purse".
That's why, when I decided to invest time and money into a tube amp, I went for something that would "fit in" with my present system, quality-wise.
Yes, the "iron" wasn't cheap, but I knew after some investigating, and along with my experiences with tube equipment, that it should satisfy my wants.
Besides, the cash outlay was only a "one time thing".
And so, I chose the Dyna Clone Z565 transformers, driven by some nice EL84s.
Glad I did.
Carefully designed circuitry, they easily compete with those "online amps" retailing for thousands more than I spent.
But it's all up to the individual to decide, of course.
And everybody has different goals.
However, I am not one to waste time puttering with sub-standard stuff, in an attempt at making a "silk purse".
That's why, when I decided to invest time and money into a tube amp, I went for something that would "fit in" with my present system, quality-wise.
Yes, the "iron" wasn't cheap, but I knew after some investigating, and along with my experiences with tube equipment, that it should satisfy my wants.
Besides, the cash outlay was only a "one time thing".
And so, I chose the Dyna Clone Z565 transformers, driven by some nice EL84s.
Glad I did.
Carefully designed circuitry, they easily compete with those "online amps" retailing for thousands more than I spent.
But it's all up to the individual to decide, of course.
The Z565 is a price/performance champion. It does fine in a ST-35, the "El Cheapo" I'm associated with, and L_RD knows what else. Yes, you can do better, but be prepared to spend lots of $ for a minimal ROI.
JMO, the valuable item in what the OP has salvaged is the power trafo. That had to feed a fair amount of circuitry. Therefore, feeding a decent power amp well should be feasible.
Where that undersized O/P "iron" could be useful, in a true HIFI rig, is in the treble amp of a multi-amped configuration. Don't ask it to do what it can't. If bass info. is not present, a GNFB loop to ensure full extension to 20 KHz. becomes feasible. 😉
JMO, the valuable item in what the OP has salvaged is the power trafo. That had to feed a fair amount of circuitry. Therefore, feeding a decent power amp well should be feasible.
Where that undersized O/P "iron" could be useful, in a true HIFI rig, is in the treble amp of a multi-amped configuration. Don't ask it to do what it can't. If bass info. is not present, a GNFB loop to ensure full extension to 20 KHz. becomes feasible. 😉
The original poster said they were a student and only had a couple of hundred bucks to invest.
From this starting point, wouldn't an old console amp from Craiglist or eBuy be a better gamble? Something that has a chassis and had reasonable musical aspirations?
Over here in Sweden I have been collecting old Tandberg reel-to-reels. I get them for about $30 and they have ECL86 or EL84 single ended stereo outputs, inbuilt speakers, and the basis of a reasonable preamplifier.
The hard part is canibalising them, in their nice teak cases, which so far I have not been able to do :-(
From this starting point, wouldn't an old console amp from Craiglist or eBuy be a better gamble? Something that has a chassis and had reasonable musical aspirations?
Over here in Sweden I have been collecting old Tandberg reel-to-reels. I get them for about $30 and they have ECL86 or EL84 single ended stereo outputs, inbuilt speakers, and the basis of a reasonable preamplifier.
The hard part is canibalising them, in their nice teak cases, which so far I have not been able to do :-(
OP here, not a student, just unemployed and in need of distraction. And I have less than $100 to invest. I'm also out of my technical depth on this forum.
I think perhaps a stereo guitar amp might be a better use for these transformers. Undersized OTs can be a good thing for guitar amps. Hector, I've had a few nice old Tandberg reel to reels. So pretty! Anyway, thanks for your suggestions everyone.
I think perhaps a stereo guitar amp might be a better use for these transformers. Undersized OTs can be a good thing for guitar amps. Hector, I've had a few nice old Tandberg reel to reels. So pretty! Anyway, thanks for your suggestions everyone.
I have to include the "Classic Tone" brand of transformers also.
Less costly than the Dyna transformers (no UL tap) but capable of respectable performance.
I've used them in a console stereo rebuild, and they are able to reach down and growl nicely in the bass region.
If one can get their hands on an old Magnavox 93 Series amp chassis, that's a great foundation to get started on building a superior amp.
Even the stock OT's are decent, however that's where I spent the money on the Z565's to kick up the quality a few notches more.
Was well worth the effort, and over a decade later, I'm still quite pleased and impressed with the results.
I've used Classictone transformers in all my guitar amps, except for a Heyboer OT in one, and the odd vintage piece. Very nice, and an excellent deal. Plenty respectable for a guitar amp, happily.