Voltage on single channel ab763

[deuce42]

My apologies if this has been asked before but I've done a search and can't come up with results.

I want to make a single channel version of an ab763 super reverb using only the 2nd channel and also removing the tremolo circuit. My thinking was to simply remove the first channel parts and tremolo circuit parts and increase the second cathode resistor to 1500 rather than 820ohms.

The difficulty I have however is that the 12ax7 for the second channel would now theoretically have an increased voltage since the first channels 12ax7 has been removed. Also, perhaps the power tube voltage and reverb transformer voltages might be messed up since the tremelo tube has been removed.

How does one deal with this voltage issue? I think its about changing the 1k or 4.7k resistors between the power supply nodes but what do I change them to? This is where I'm stuck.

Thanks

[Leo_Gnardo]

Good question, and by coincidence just yesterday I worked on a Vibrolux Reverb for a jazz player for whom I recommended, leave out the Norm channel pre tube, just to reduce the hiss level from the unused channel. On his amp, I simply removed the tube and didn't worry much about the rise in hi voltage for the power supply node for the preamps. Sounds and works just fine.

However, if you want to preserve the exact or nearly so operating voltage for the pre tubes you could increase the 4K7 in the power supply. With a back-of-the-envelope calculation, I'd go for 6K8. However, you CAN use your ears, try a few different values, pick the one you like best. Some stock Fender models used 10K, and I have no problem as to how they sound.

[deuce42]

Great I'll give a 6.8k a go and thanks Leo

[Roe]

try the silverface dropping string values rather than the blackface values. start with 2k2 instead of 1k etc

[Firestorm]

The stock voltages are +/- 20%. Removing a couple of preamp tubes won't take you outside that range. But if you're specifically trying to make it either cleaner or "browner" you can fiddle with things to go in that direction.

[fusionbear]

I built one like this before. I left the voltage wherever it went to. But I did use a 270K resistor to ground between the .047 and .001 coupling caps to the PI to simulate the drag of the Trem channel.

But, use your ears ultimately...