Hi new here and also new to tube amps. I got a Nobsound 6P1 Amp a few months ago off Amazon and am looking to mod it a little.
Reading up it seems there are a couple of issues with this amp right out of the box. The first issue is the power transformer primary is rated for 110V, which throws off all the internal voltages when used with the 122V my house has, plus makes everything run hot! I setup a 12V bucking transformer to deal with that issue. But also the first cap after the rectifier is WAY too big at 150uf, so replaced it with a 22uf. Then I rolled in some matched pairs of the Russian 6P1P-EB tubes, and it sounds pretty sweet now for such a cheap amp. Seems the early versions had an undersized rectifier tube as well, mine came with a Russian 5Ц4C in place of the Chinese 5Z4, they both have a 2ma filament but this Russian tube has a higher load rating while keeping the B+ the same.
Anyway, I'm interested in trying a mod to replace the output transformers with something a little better, that can also be setup for UL mode. I would probably set it up with a switch for triode/UL and also want to play with a switchable schade feedback loop while I'm modding it to see what that sounds like. I'm listening through some high efficiency Klipsch KB-15 Icon Bookshelf speakers so it does pretty well for a low watt amp. Given the power supply had some design flaws, I'm not sure the supplied output transformers are an ideal Z anyway + they aren't marked what they even are.
I've attached the original schematic (with issues noted) along with the one with my proposed mods. My question is what impedance transformers will I need? I've read that parallel tube designs like 1/2 the normal rated impedance for SE use. I've also read this tube, and it's US electrical equiv the 6AQ5, like 5K in an SE amp, so does a 2.5K transformer sound right for this use? I'm also open to suggestions for the resistance for the shade loop, most seem to be 180k-220k so I picked something in the middle.
I don't need blasting loud sound, but I don't want to kill the output either. It sounds really sweet with jazzy type music setup in the factory triode mode, but my hope is rock type music will sound punchier in UL mode. I'm not running any preamp or tone controls, just straight out of a Yamaha S300BL CD deck.
I am thinking about a pair of Edcor GXSE10-2.5k at 8ohm as they physically will fit without hanging off the chassis, 10W each should be plenty (I get bigger is better, but this is a sub $300 amp lol), and they have a 40% UL tap to play with. A pair would be less than $80 + shipping.
Again, new to working on tube amps so let me know if I'm off on my thinking here. I'm seriously hooked on how great tube amps sound and have a SEUL KT88 build planned for the future.
Steph
Reading up it seems there are a couple of issues with this amp right out of the box. The first issue is the power transformer primary is rated for 110V, which throws off all the internal voltages when used with the 122V my house has, plus makes everything run hot! I setup a 12V bucking transformer to deal with that issue. But also the first cap after the rectifier is WAY too big at 150uf, so replaced it with a 22uf. Then I rolled in some matched pairs of the Russian 6P1P-EB tubes, and it sounds pretty sweet now for such a cheap amp. Seems the early versions had an undersized rectifier tube as well, mine came with a Russian 5Ц4C in place of the Chinese 5Z4, they both have a 2ma filament but this Russian tube has a higher load rating while keeping the B+ the same.
Anyway, I'm interested in trying a mod to replace the output transformers with something a little better, that can also be setup for UL mode. I would probably set it up with a switch for triode/UL and also want to play with a switchable schade feedback loop while I'm modding it to see what that sounds like. I'm listening through some high efficiency Klipsch KB-15 Icon Bookshelf speakers so it does pretty well for a low watt amp. Given the power supply had some design flaws, I'm not sure the supplied output transformers are an ideal Z anyway + they aren't marked what they even are.
I've attached the original schematic (with issues noted) along with the one with my proposed mods. My question is what impedance transformers will I need? I've read that parallel tube designs like 1/2 the normal rated impedance for SE use. I've also read this tube, and it's US electrical equiv the 6AQ5, like 5K in an SE amp, so does a 2.5K transformer sound right for this use? I'm also open to suggestions for the resistance for the shade loop, most seem to be 180k-220k so I picked something in the middle.
I don't need blasting loud sound, but I don't want to kill the output either. It sounds really sweet with jazzy type music setup in the factory triode mode, but my hope is rock type music will sound punchier in UL mode. I'm not running any preamp or tone controls, just straight out of a Yamaha S300BL CD deck.
I am thinking about a pair of Edcor GXSE10-2.5k at 8ohm as they physically will fit without hanging off the chassis, 10W each should be plenty (I get bigger is better, but this is a sub $300 amp lol), and they have a 40% UL tap to play with. A pair would be less than $80 + shipping.
Again, new to working on tube amps so let me know if I'm off on my thinking here. I'm seriously hooked on how great tube amps sound and have a SEUL KT88 build planned for the future.
Steph
Attachments
I've built a SET stage with 6P1P and 6P6S, which are broadly alike.
Both like about 5 or 6k primary. In parallel then, 2k5 to 3k would make sense.
Caveat being a parallel pair doesnt behave quite like double the valve; so if it were me, I would probably just try the parallel pair into a 4k primary. Less power out, probably lower distortion, perhaps a safe bet.
A member here KodaBMX has far more experience with 6P1P though, maybe he will post a reply?
Both like about 5 or 6k primary. In parallel then, 2k5 to 3k would make sense.
Caveat being a parallel pair doesnt behave quite like double the valve; so if it were me, I would probably just try the parallel pair into a 4k primary. Less power out, probably lower distortion, perhaps a safe bet.
A member here KodaBMX has far more experience with 6P1P though, maybe he will post a reply?
Just to be able to be sure of the operation of the 6P1 tubes, both when new, and then over a couple of years of tube aging:
Use separate 100 Ohm screen resistors
Individual 540 Ohm self bias resistors, and individual 220uF bypass caps.
What?
No Bleeder resistor across the B+?
Get a couple of 25k 5 Watt wire-wound resistors,
and wire them in series across the 300V B+ to ground.
Safety First! Prevent "Surviving Spouse Syndrome".
Be sure to power down the amp, and wait for the B+ caps to discharge before switching from Triode to UL, or UL to Triode. Bleeder resistors help with this.
(Save your tweeters and ears from a nasty switching transient).
Have fun building and listening!
Use separate 100 Ohm screen resistors
Individual 540 Ohm self bias resistors, and individual 220uF bypass caps.
What?
No Bleeder resistor across the B+?
Get a couple of 25k 5 Watt wire-wound resistors,
and wire them in series across the 300V B+ to ground.
Safety First! Prevent "Surviving Spouse Syndrome".
Be sure to power down the amp, and wait for the B+ caps to discharge before switching from Triode to UL, or UL to Triode. Bleeder resistors help with this.
(Save your tweeters and ears from a nasty switching transient).
Have fun building and listening!
They do make a 3.5k version, which might make sense from what you posted. I'm not in a huge hurry to do anything, I'll wait to see what some other responses suggest! Right now I rarely go past 1/2 volume now (but have no idea what the current Z is), so if it lost 10% and had less distortion, that would be a OK trade off.
I might need to remove one to do the "Apply voltage" technique to determine the turns ratio, and then multiply by 8 ohms technique to really make an informed choice by knowing what is currently being used. It's possible there are some markings under the bells?
Separate cathodes first of all, and use 560 Ohm cathode resistors shunted by their individual caps. Such a way you spread load to both tubes more equally.
If the original amp was supplied with 3K5 transformers, you still can use them, since for 1 tube it was suboptimal.
If the original amp was supplied with 3K5 transformers, you still can use them, since for 1 tube it was suboptimal.
I have no idea what the original transformers are nor, given how sub-optimally the circuit is, would I trust what they spec'd. Clearly it was built to a price point. Plus they are tiny little cheap transformers with no UL tap.
I do appreciate the tips on seperate cathodes/screen + bleeders, I will deal with that when I get inside to do these other mods.
I was hoping someone would know what a pair of these tubes would optimally need as far as Z for the OPT, or maybe had some experience with these in pairs or the similar 6AQ5.
I do appreciate the tips on seperate cathodes/screen + bleeders, I will deal with that when I get inside to do these other mods.
I was hoping someone would know what a pair of these tubes would optimally need as far as Z for the OPT, or maybe had some experience with these in pairs or the similar 6AQ5.
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A simple general statement:
Use a well known, successful, and proven single output tube design.
Start with that single output tube, and operating conditions of plate current, plate voltage, screen I and V, bias, and output transformer primary impedance, inductance, and DC current rating; such combination that gives a good mix (compromise) of output power, harmonic and intermodulation distortions, damping factor, etc.
Then, use the same B+, but with double the current capability, same bias (but now individual bias per tube), an output transformer that is rated at 1/2 the primary impedance, 1/2 the primary inductance, and 2x the DC quiescent current, and 2x the laminations.
Use the same rg1 resistance, but now 2 of them for individual rg1 resistors, and individual coupling caps (or 1/2 the original rg1 resistance, and 2 times the capacitance of the original coupling capacitor.
Then change the driver to be able to drive the same voltage peak to peak, but that has 1/2 the output impedance and 2x the current drive capability.
If you do it all correctly, you get the same frequency response, same distortion, same damping factor, but . . .
you get the advantage of 2X the output power.
Well, an exception to the same frequency response . . .
The new more robust output transformer has to have 1/2 of the leakage reactance, 1/2 the primary DCR, and no more than 2x the distributed capacitance.
Just one way to do it.
Higher primary impedance, has lower output power, lower distortion, and higher damping factor.
(damping depends on the tube and circuit configuaration including Pentode/Beam Power, UL, Triode wired, Negative feedback, and also on the ratio of the primary DCR to the primary impedance, and the ratio of secondary DCR to the secondary tap rating (such as 8 Ohm).
I hope I did not miss anything, and that I did not miscalculate.
I am sure there are other ways, maybe better to do it for Parallel Single Ended.
And do not forget, with 2 output tubes, you can go to push pull, but that is a completely different design.
Use a well known, successful, and proven single output tube design.
Start with that single output tube, and operating conditions of plate current, plate voltage, screen I and V, bias, and output transformer primary impedance, inductance, and DC current rating; such combination that gives a good mix (compromise) of output power, harmonic and intermodulation distortions, damping factor, etc.
Then, use the same B+, but with double the current capability, same bias (but now individual bias per tube), an output transformer that is rated at 1/2 the primary impedance, 1/2 the primary inductance, and 2x the DC quiescent current, and 2x the laminations.
Use the same rg1 resistance, but now 2 of them for individual rg1 resistors, and individual coupling caps (or 1/2 the original rg1 resistance, and 2 times the capacitance of the original coupling capacitor.
Then change the driver to be able to drive the same voltage peak to peak, but that has 1/2 the output impedance and 2x the current drive capability.
If you do it all correctly, you get the same frequency response, same distortion, same damping factor, but . . .
you get the advantage of 2X the output power.
Well, an exception to the same frequency response . . .
The new more robust output transformer has to have 1/2 of the leakage reactance, 1/2 the primary DCR, and no more than 2x the distributed capacitance.
Just one way to do it.
Higher primary impedance, has lower output power, lower distortion, and higher damping factor.
(damping depends on the tube and circuit configuaration including Pentode/Beam Power, UL, Triode wired, Negative feedback, and also on the ratio of the primary DCR to the primary impedance, and the ratio of secondary DCR to the secondary tap rating (such as 8 Ohm).
I hope I did not miss anything, and that I did not miscalculate.
I am sure there are other ways, maybe better to do it for Parallel Single Ended.
And do not forget, with 2 output tubes, you can go to push pull, but that is a completely different design.
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Different transformers have different frequency response, and different phase response. That can make using negative feedback tricky.
Those parallel 6P3 are in Triode Wired Mode, so that should make it easy to double the output power, and not have any special problems doing that (there is not any global negative feedback wrapped around the output transformer, so there is no feedback adjustment required; what is not there does not need adjusting).
And . . . if the new transformer is of good quality, and has the same UL tap percent (%), using Ultra Linear operation should not be a problem either (no adjustment necessary).
Those parallel 6P3 are in Triode Wired Mode, so that should make it easy to double the output power, and not have any special problems doing that (there is not any global negative feedback wrapped around the output transformer, so there is no feedback adjustment required; what is not there does not need adjusting).
And . . . if the new transformer is of good quality, and has the same UL tap percent (%), using Ultra Linear operation should not be a problem either (no adjustment necessary).
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