adding a presence control to a bogen cha33 question

[Regis]

"but I'm not clear whether r23 should be replaced with a jumper or gone completely?" R23 should be gone completely, no jumper.

Thanks Enzo, I've adjusted the schem again and listed it here.

I have some other questions about this circuit but for now I'll get the presence running and then we'll see.

Thanks again for all the help, you guys are great.

regis

[Regis]

I started to wire it up last night but realized I had the wrong pot, I went to Radio Shaft today to get it.

I measured the voltage at pin 2 of the phase inverter, zero volts at pin 2 and about 5 millivolts on the other side of C14. Should I still remove C14 and R24?

I'm also curious why the pot should not be grounded like other presence controls I've seen.

Thanks

regis

[MWJB]

5mV shouldn't be a problem, must be some leakage via the PI coupling caps, as long as they don't leak more than 0.25v I wouldn't worry.

You don't want to ground the pot because it's in parallel with the 2.2K cathode resistor, if you turned the pot so that the wiper was grounded, you would simultaneously rebias the PI triode. Not all presence controls are grounded at the pot, see 5E# Fenders.

[Regis]

Well dang... It worked!!!

Got it wired up as instructed last night and it worked on the first try. Sounded pretty good too. Spanky and crisp dialed down and really thick overdrive when pushed. The presence control allows me to go between crunchy and creamy.

The wiring was easy because I put the pot in the old phono pot hole and the phase inverter was right there.

I forgot to add the second 82k FB resistor, will try that tonight. Actually it was a 100k in there rather than an 82k, should I tack on a 68k to get the same impedence as the two 82k's?

A couple of other questions:

The inputs have a .033 cap and a 3.3meg resistor input, should I change those to something more traditional like a 68k/1meg setup? What would be the result tone wise?

I actually like the tonestack in this amp except of course the bass control is overwhelming much past three or four, even when playin clean. What would I change to reduce that?

The amp is sounding really good, I really appreciate all your help on this. A buddy helped me build a head cab for it and I'll post some pics when it's all together.

[MWJB]

Well if the pot gives a range of sound that you like, you might not benefit from going to a lower value NFB resistor? Doing so will generally soften & smooth the amp. You could try a 68K or a 47K to 56K in parallel with the 100K and snip it off if it has too adverse an effect on the amp's response.

the inputs are "grid leak biased", you'll notice that there are no cathode resistors on the input stages. Grid leak traditionally doesn't like very large signals...I would try modding one input triode first to see how you like it compared to stock. Remove the 0.033 cap, the 3.3Meg resistor and the ground connection for the cathode.

Run a 1meg accross the input jack, with the switch grounded. Run a 33K resistor in series from input jack tip to tube grid. Install a 2.2K resistor, bypassed with a 25uf/25v cap (observe correct polarity), between cathode & ground. This input will now be a more traditional cathode-biased input stage. Voltage on the 12AX7 plate will also rise a little, this may be beneficial to your excessive bass problem. Voltage at the 12AX7 plates can be raised further by reducing R37 (maybe down to 10K?) if you feel this is a direction you might like (improves highs & fidelity).