Fender-esqe

[ricach]

Try as I might, I just can't figure out filter caps and how to set values for them. Here is the schematic of a Merrel High-Fidelity Tube Amp that I heavily modified for guitar use. The PI and power section is how it came. I removed everything before that and installed a general Fender tone stack and pre-amp section. However, I left the B+ Rail as they were on the original amp. The amp sounds absolutely fantastic now - I can't get over how much it sounds like my friends BFProVerb. (Even more unbelievable is that's what I was going for and I nailed it first time.)

Ok, my question. I'm going to add another channel identical to the first minus the tone stack - I plan on using this as an effects loop. As much as I don't want to alter the sound I have now, I really would like to set the B+ rail up as if by design - and not by accident, considering I'll be adding another 12AX7. I took voltage readings at various places and they're noted on the schematic - I hope is attached. Where do I start with the caps?

[Old Tele man]

...measure the cathode voltage (Vk) on each preamp tube, then divide that value by the 'actual' (measure it) cathode-resistor value, so you get a plate current value. Sum those values, and you have your CURRENT preamp current load.

...*if* you're replicating the "old" circuits in the "new" circuit, then simply double the above current...then, using Ohm-Law, backsolve to estimate the NEW power-supply resistor value needed to obtain the SAME preamp plate voltage at each "node" (ie: [A], [B], [C], etc.) using the assumed 2X currents.

...should put you in the 90-95% accuracy range of values.

[ricach]

I'm assuming you're neglecting the grid current because its negligible in the preamp? If so, what about when determining values for the power tubes?

I also assume that with the new resistor values I then determine the new cap values so as to filter out 60hz and below?

And thanks for reminding me it really is not so difficult to understand - I graduated BSEE in '84 and within years was already in the computer field. I figured I've forgotten too much by now.

[Old Tele man]

....correct me if I'm wrong, but I'm assuming you're only "adding" a second preamp string (stage?) and not a whole additional PI and output stage?

...since the preamp tubes are typically "downstream" from where the PI and output voltages are taken from the power supply, *if* the above is true, then you're only "adding" increased current load to the filter stage(s) that supply the preamp tubes...*if* you don't change the PI and output stage, there'll be almost NO current to worry about (notice I wrote "almost").

...for a very "quick-n-dirty" estimation of preamp currents, you can usually 'assume' 1mA per triode section...thus if you already have 2x12AX7, that'd be about 4mA...and assuming addition of additional 2x12AX7, that'd just up the current to about 8mA...from there, it's Ohms-Law to determine what size SMALLER power supply resistor you need at each (if more than one) stage.

...always start at the "furthest" stage down the power supply string, which conversely will the be the "first" preamp stage(s) which commonly use the most well filtered B+ voltage(s), and work back toward the PI & output stage (typically before/after choke).

...back to the earlier "...almost..."--here's why you often can ignore small-to-moderate increases in downstream current: it's so much smaller than the output tubes idle (ie: 8mA vs. 2 x 22-40mA at idle, and 8mA vs. 100-300mA at maximum power)...it's a "too small to be significant...yes, it's there, but in the BIG picture you can often ignore it using the 1/10th and 10X "rule-of-thumb."