So I have this old Voice of Music amp. Model 8809-8810. I see these on eBay for $100 all the time. I have only the amp itself, no record player/speaker. I’m going to use the transformers to build a single ended 6v6 guitar amplifier. The amp is very interesting because it has a fixed bias output but apparently no diode rectification for the negative voltage. How does this work??
It also has the cathode of the 6v6 attached to a secondary on the output transformer. My understanding is that this provides negative feedback. Please correct me if I’m wrong.
My plan is to convert this to a more usual, cathode bias, with negative feedback applied to the cathode of the second gain stage. I’ll also toss out the tone stack and put the volume control after the input stage. Do any of you have any suggestions considering the fixed bias and the cathode/output transformer feedback? Thanks in advance for any input and I hope you all find this schematic interesting.
It also has the cathode of the 6v6 attached to a secondary on the output transformer. My understanding is that this provides negative feedback. Please correct me if I’m wrong.
My plan is to convert this to a more usual, cathode bias, with negative feedback applied to the cathode of the second gain stage. I’ll also toss out the tone stack and put the volume control after the input stage. Do any of you have any suggestions considering the fixed bias and the cathode/output transformer feedback? Thanks in advance for any input and I hope you all find this schematic interesting.
Attachments
"Fixed" how?
No, it is biased by the 300 Ohm resistor in B-. If the 6V6 were alone, this would clearly be cathode-resistor bias, just wired different. In fact the 300r carries the 12AX7 units also, but they amount to a very small fraction of the 6V6 current, so 6V6 bias depends 97% on 6V6 current.
Have you played guitar through it as-is? It's not a classic guitar amp and it is a little light on gain and distortion, but should play fine. *After* you fix the leaky and dead caps.
Moved to the Instrument & Amps forum per forum policy.
I think you are pointing to a circular reference. (Hard to be sure because of the age.)
opacheco: I think this is all there is.
I apologize for commenting on such an old post, but since it popped up in the last few days, and I have not seen this one before...
We don't see back-biasing very often. The voltage developed across that 300 ohm resistor is negative with respect to the circuit ground at the grid of the 6V6. It's a poor-mans fixed bias (sort of) because the load of the entire power supply tends to be pretty stable in a single-ended power amp. This approach eliminates the need for an extra bias rectifier. The cathode feedback through the output transformer is what makes this somewhat uncommon because that extra d.c. resistance adds a cathode-bias component to the mix plus the signal feedback in the cathode.
It's a strange and clever little design.
Stph
This is very interesting. I wonder if there is a downside to using the output transformer minus the 32ohm tap?
Just taping it off , not using it.
I don't get a lot of hours on my home built guitar amps but , haven't noticed any overheating , just taping it off. Building simple Champ or Kalamazoo SE amps.
I've bought a few VoM transformers on eBay , all the VoM OT's seem to have this tap. Maybe I've seen this on a few other OT's from organ amps.
Not to be cheap or too lazy to build the circuit but , a Voice of Music VM730 reel to reel is a bit more complicated than the schematic here.
I have just bought another VM730
Just taping it off , not using it.
I don't get a lot of hours on my home built guitar amps but , haven't noticed any overheating , just taping it off. Building simple Champ or Kalamazoo SE amps.
I've bought a few VoM transformers on eBay , all the VoM OT's seem to have this tap. Maybe I've seen this on a few other OT's from organ amps.
Not to be cheap or too lazy to build the circuit but , a Voice of Music VM730 reel to reel is a bit more complicated than the schematic here.
I have just bought another VM730