Pure class A guitar amp design help

[Spent Electrons]

This is my first attempt at designing truly from scratch. I currently have it assembled and "working", although it's in more of a breadboard state. I was hoping a kind soul might be willing to look over my design, and offer feedback, criticism, or helpful guidance.

 

[rayma]

The output stage power should come from the C tap, and the driver stage should use the B tap.
Also the secondary common should be grounded, to avoid shock hazard.

 

[Spent Electrons]

Thank you for the reply. It is wired in that configuration to allow the phase inverter b+ 650v, in order for it to output 150v of swing from idle

 

[jcalvarez]

I would change from Concertina to an LTP, reducing the need for such high voltage for the phase splitter.
Also the transformer seems to have a very high secondary voltage. Not sure of you are using a centre-tapped one end-to-end, with a bridge. Maybe changing to choke-input can save you from those 50W resistors.

 

[Spent Electrons]

I agree about the LTP. The transformer is very high voltage. I suspect it was powering a field coil, and possibly running on 110vac. The iron is what I'm trying to avoid replacing. It's intended to be a learning project. I previously had it through a tube rectifier, and choke but still ended up with too high of voltage. I decided to roll with it and utilize the diodes to up the capacitance, and reduce noise. Also the concertina configured utilizes the excess voltage. At this point I'm sort of doing it. Just because I can, and I'm learning a lot. The output transformer has also been a struggle at about 45:1. It hogs a lot of power, for a low output

 

[rayma]

The 6SN7 has a max 500V, so it could be damaged under some conditions.
Agree that the LTP is preferable.

 

[Spent Electrons]

I've been wondering about the voltage on that tube. I was operating under the assumption that the cathode resistors are dropping 200v, so it's actually seeing 450v. 200 of which is dropped on the plate load resistor, idling at "250v". I may be to be educated on that

 

[jcalvarez]

@rayma good point, and I just notice 200V on the PI cathode, Ukf max seems to be 100V, needs heater elevation.

 

[20to20]

I'd like to see a load line for the outputs and the tranny specs.

 

[Spent Electrons]

Elevating the heater is next on my to do list. I overlooked that. The transformers have no specs. I'm pretty sure it was a push pull AB 6V6 setup. The only info I have to go on is measuring the winding ratio, abs calculating the reflected impedance. I can provide load lines asap. I'm currently at work.

 

[rayma]

I've been wondering about the voltage on that tube. I was operating under the assumption that the cathode resistors are dropping 200v, so it's actually seeing 450v. 200 of which is dropped on the plate load resistor, idling at "250v". I may be to be educated on that

The supply DC voltage should never be higher than the maximum rated tube DC plate voltage.
That's poor design practice. During warmup the tube sees the entire DC power supply voltage.

Same conditions exist for the coupling capacitor DC voltage ratings.
There always will be voltage transients higher than those during quiescent conditions.

 

[Spent Electrons]

The supply DC voltage should never be higher than the maximum rated tube DC plate voltage.
That's poor design practice. During warmup the tube sees the entire DC power supply voltage.

The only thing I can say in regard to warmup, is that the 6AS7 likes to see a 30 second plus heater warmup before b+ is applied. So I am currently doing that via a switch

 

[kevinkr]

Moved to Instrument & Amps per forum policy.

Hopefully you will get additional help here.

 

[Spent Electrons]

Moved to Instrument & Amps per forum policy

My bad. TY

 

[Spent Electrons]

I notice now, I originally had the 6As7 at 400v, not 450. I just learned how to do load lines so I don't have a lot to go off besides plugging one onto my bench supply and seeing where it operates at DC. This is another area I'm not confident in.

 

[Spent Electrons]

Thank you all. This is very helpful.

 

[kevinkr]

I think you are OK as long as B+ stays below 250VDC, 200VDC is probably safer. Watch out for dissipation. I have attached some data sheets, suspect you probably have them.

Interesting project!

 

[Spent Electrons]

I notice another discrepancy in the schematic. All cathodes have a bypass capacitor.

 

[20to20]

I notice now, I originally had the 6As7 at 400v, not 450. I just learned how to do load lines so I don't have a lot to go off besides plugging one onto my bench supply and seeing where it operates at DC. This is another area I'm not confident in.

I don't see your sim loadline numbers matching the schematic. You have 200v on the K, not 91v.

 

[Spent Electrons]

I don't see your sim loadline numbers matching the schematic. You have 200v on the K, not 91v.

From my understanding the grid is referenced between the two cathode resistors, which is 91v negative of the cathode