New PT question for Champ Clone

[Firestorm]

I love tube rectifiers, but Phil is right that they make almost no sense in this amp. Current changes very little in Class A so there's essentially no sag to play with. Better to save the cost of the tube, socket and 5V winding and put it toward the speaker or a nice NOS 6V6.

[surfsup]

I had never done a recto before so I was thinking I'd do it anyway. $25 for the tube and socket isn't a big deal. But, if it causes a problem trying to find a pre-drilled chassis, and chassis cost skyrockets, I'll skip it since I'm doing one on the rocket build too. I just figured it would be good to get my feet wet on one first before doing the more challenging rocket.

I wanna say thanks again to all you guys helping answer my relentless Qs...!

[martin manning]

Current changes very little in Class A so there's essentially no sag to play with.

It's true that the average current doesn't vary much in a class A power stage, but there is considerable variation with the waveform being amplified. I would think that this would cause some second-order harmonic distortion in the output.

[Phil_S]

I think I should amend what I said about 50mA as the requirement for the HV secondary. This is probably a hair too low. We should allow 5mA for the screen current on the 6V6 and consider that a SE amp never cycles "off" meaning that a PT with the minimum requirement will run hot. SE amps are not very efficient and some de-rating is appropriate. I'm not sure how much to de-rate, but I'd imagine 70mA is plenty of juice.

[Firestorm]

It's true that the average current doesn't vary much in a class A power stage, but there is considerable variation with the waveform being amplified. I would think that this would cause some second-order harmonic distortion in the output.

Yeah. SE operation has significantly higher harmonic distortion, probably predominantly even order. But I always assumed this was due more to it being single-ended rather than class of operation. In either case, how would this interact with a tube rectifier versus SS?

[martin manning]

I was thinking that when passing signal the instantaneous current draw can vary plus or minus almost 100% from the idle current in a class A amp. So, during the high current part of the waveform there would be a reduction in supply voltage due to the internal resistance of a vacuum rectifier which would alter the way the waveform is clipped vs. a solid state rectifier.

[Firestorm]

I was thinking that when passing signal the instantaneous current draw can vary plus or minus almost 100% from the idle current in a class A amp. So, during the high current part of the waveform there would be a reduction in supply voltage due to the internal resistance of a vacuum rectifier which would alter the way the waveform is clipped vs. a solid state rectifier.

Not sure. With current demand fluctuating at the frequency of the signal and the rectifier "topping off" the main filter at only 120Hz, the instantaneous current demand would be handled by however many joules are stored in the cap and the rectifier's resistance would come into play only with respect to the sustained demand. Seems like it would be the same as running the amp at a slightly lower B+. With a very small cap, this might happen for notes below 120Hz, but then I'd think the 120Hz rectifier hum would become unacceptable.

[martin manning]

Nevermind... I think since the rectifier is on the other side of the OT primary it doesn't see the changing current. I tried a quick sim with a resistor to simulate a tube rectifier (25V drop at idle) and increased B+ to get the same plate voltage. No change to the harmonic content in the FFT that I could see.

[Zippy]

Rather than look at "sag" happening, adding a resistor of reasonable value in that position when using a SS rectifier drops the voltage to the range one might get with a tube rectifier. To me, it is all about dialing in the appropriate voltages.