Idea for a 2 tube 6080 PP amp.

[rcavictim]

Shoog,

You have to derate the voltage on that 60Hz transformer you are planning for an interstage if you want response below 60Hz. Dropping the winding voltages to 1/2 will get you to 30 Hz. Dropping to 1/3rd gives 20 Hz.

I think you can do this with a 6SN7GTB on a separate HV supply using the first stage as input gain and the second stage as a concertina splitter and driver.

[Shoog]

The transformers will be 50hz rated and are 50VA. Since I will be driving them with about an 18V signal off the 5687's with only about 15-20mA per transformer I don't think there will be issues with saturation. There may be issues with HF roll of due to the high inductance of the driver side - though I doubt it.
I don't think anyone has documented trying this approach so I am voyaging into uncharted territory. It may or may not work, but I don't think I will lose much by trying (apart from the cost of 2 transformers). It has an elegant simplicity which appeals to me, though of course it may be to simple. I am more concerned with resonant tanks than anything else. sizing the coupling cap is a real suck it and see scenario.

Shoog

[rcavictim]

OK, sounds like you have this covered. I think the Z of a 50 volt winding will be totally too low for use as an IT if plate coupled but you are right, using an IT does end up with a simpler looking schematic than other methods.

The beauty of DIY is that you can find this stuff out yourself and get a practical and valuable education along with the fun, all at the same time. In rare instances you even save money. .)

[Shoog]

I am going RC coupled to the interstage so the 5687 will have its own load ala parafeed.

Shoog

[rcavictim]

That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore.

If it was my house, I'd be trying the 6SN7 as I suggested above without iron.

[Shoog]

Point taken.

Shoog

[Shoog]

"That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore."

Excuse my ignorance if I am wrong, but is my thinking on this correct.
If I go for the maximum grid circuit resistance of 1meg ohm for a 6080, and I am using a 1:2 ratio I get a reflected resistance of 250K. If this is in parallel with a CCS I should be loading the 5687 by about 240K which is a good load is it not.
If my logic on this one is wrong can you explain to me how, so that I can learn.

Shoog

[rcavictim]

Originally posted by Shoog
"That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore."

Excuse my ignorance if I am wrong, but is my thinking on this correct.
If I go for the maximum grid circuit resistance of 1meg ohm for a 6080, and I am using a 1:2 ratio I get a reflected resistance of 250K. If this is in parallel with a CCS I should be loading the 5687 by about 240K which is a good load is it not.
If my logic on this one is wrong can you explain to me how, so that I can learn.

Shoog

If you need a mathemagical explanation I'm not your best tutor. I do a lot of thinking like this with my hands which I can use to pick up my handy reactance chart. It works for me. For any given frequency as you increase the number of turns and increase L, you also increase the AC impedance. For a fixed number of turns on a given core, as you lower frequency the Z will drop. The plate circuit of your driver stage wants a pretty high plate load Z, otherwise the dinky amount of plate current available won't be capable of creating a sufficient voltage drop across the coil.

Forgetting about frequency or any loading, if you have a plate swing of 100 volts at 10 mA, that primary winding has to look like 10K ohms. Now consider bass frequencies which require the most L for a given current. Looking at my handy reactance chart I see that 10K Z= a 30 Hy coil at 60 Hz. To swing 100 volts at 20 Hz from a 10 mA source would require about 85 Hy's.

As a power xfmer, if your 50 volt winding is rated at 50 watts at 50 Hz that is 1 amp. According to Ohms Law (not R's Law) that is a Z of 50 ohms. According to my handy reactance chart that coil is about 0.16 Henries. You are way on the short side of not enough L.

By cap coupling this winding to the driver tube plate you will get whatever voltage across it that can be developed with 10 mA. At 50Hz that would be about 0.5 volts. The waveform will be extremely distorted. Now add secondary windings and loading and this gets even worse. Try to reduce the frequency to 20 Hz and it gets even worse again.

You might be able to use this xfmer as an interstage but it would have to be cathode driven by a tube with a low output Z. Maybe another 6080 with sections parallelled.

[Shoog]

Good explanation. I have learn't. Does the same thinking apply to output transformers. I have successfully used toroidals as SE parafeed output transformers, and the bass response is satisfactory.

Shoog

[rcavictim]

Absolutely the same applies to output xfmers, both directly in the plate circuit DC path or cap coupled only to the AC component. With a power stage however the plate Z is much lower, so lower L may be tolerated.

My PSET 6080 amp design has a really low plate Z with 8 triodes in parallel. I discovered that a certain PS toroid I found at a surplus store was great with two (120 volt primaries in series, low voltage secondaries in parallel) as an output xfmer for this amp, so I bought up as many as I could, enough to make a few of these amps eventually. I use two ~80 VA cores (~160 VA) at 60 Hz to handle 20 Hz at about 20+ watts with negligible waveform distortion.