another option would be to follow the schematic I've got that uses fixed bias - i had thought it would be more complicated but since I've got all the details already worked out for me maybe it makes more sense?
Thanks guys, I'm off to look at the Dynaco ST-70 schematic!While C3 isn't necessary, you can add a resistor where the ? is to reduce dissipation through the CCS a little bit.
If you look at the Dynaco ST-70 schematic, you'll see a diode (UF4007 is fine) connected to the red/black power transformer wire, then some caps and resistors. If you make the 47uF caps 220uF/160V caps instead, make the 1000 ohm resistor 220 ohms, and make the 18K resistor 22K, that supply will work for what you're doing. You will notice that the negative bias voltage goes to the junction of the grid leak resistors in the ST-70. In your circuit, this will be connected to the center tap of the output side of the interstage transformer.
If you want to put a meter on your amp to measure current, it will go between the wiper of your hum pot and ground.
Thanks Paul, I really appreciate you taking the time to explain this to me. What rating for the resistors should I be looking for? I have some 100uf 500v jensen electrolytics that might do?Yeah, you'll want R6 to be a small value to keep the available voltage high, so 220 ohms. R8 can be increased significantly since you're not going to need much of the bias range. 22K-47K would be reasonable.
C4 and C5 aren't super critical, but I find that it's just easier to build with slightly larger parts that have fatter/longer leads, and to some degree with R6 being 220 ohms, bigger caps will reduce the increase in ripple that going to a smaller R6 will bring about.
He's going to need the bias pot almost all the way fully negative, hence my recommendation to have R8 very high.This will give you the full range of the pot to dial in the proper bias.
Since you asked, I am skeptical of the choke input. If I were building it, I would use cap input, which means a different power transformer, but the rest of the supply design looks fine. In the several projects where I tried choke input, I thought the sound was too soft and lacked impact. The one exception is my 46 SE amps where my PT allows me to use either choke input or cap input. With the 46 I prefer choke input.Guys can I ask what you think about the Psu in general? Does it seem like a good idea to proceed with it? I don't have the chokes and power transformers yet so It could be changed if its really less than ideal
Thanks for your thoughts and schematics, very interesting.I'm a little late to this discussion, but I went down a similar path, starting a couple of years ago. One inspiration was a Dr Gizmo design, which looks similar to your starting point:
Interstage Coupled PP 6B4G Amplifier
A friend advised against the 417a because it can be prone to oscillation, so I decided to go old-school instead.
Page 44 of the pdf here, http://www.tubebooks.org/Books/Atwood/Robin & Lipman 1947 Practical Amplifier Diagrams.pdf
Page 149 here. 12 watts, class A.
Both of those had cathode bias, but my available transformer was a little low on B+ so I had to decide between cap loading and cathode bias, or choke loading and fixed bias (so you don't lose the voltage across the bias resistor). I chose the latter (one version of the schematic attached). Notice that the original 2a3 spec sheets specify 3k for fixed bias but 5k for self bias; I ended up with 5k and fixed bias, which I think costs me some power, if I remember my load lines correctly.
Ultimately, I decided that the amp didn't have enough LF or gain for my needs, but better IT's might have fixed that. But I couldn't afford the iron to do that (running unbalanced current through the IT makes it difficult to wind one with 20-20khz response) so I changed my topology. (27-->6a6 LTP phase splitter-->IT-->PP2a3-->5k PP OPT). It delivers about 14w before clipping, which is adequate for Altec speakers.
I will say that even with AC filaments, and only small capacitors in the PS, hum is negligible. The required chokes make it heavy to move, though! No balance pot, just four bias pots. I'm cheating and using a diode with electrolytic capacitors to provide the bias voltage; I might change it to a tube bias rectifier at some point.
mhardy6647 has heard both the original and new versions; Redboy has heard the new version. I've been listening to it in a breadboarded format since the spring, so no photos, but I'm hoping to make a proper chassis this winter.