Mixing resistors

[Toxophilite]

If I want to get a bit more preamp volume into the PA of a basic 2 channel non-reverb deluxe Ab763 build, can I lower the value of the mixing resistors, say to 180k each? Is the 220k used by fender absolutely necessary?

[pdf64]

It wouldn’t help.
As long as they’re high enough not to excessively load the preceding stage, there’s a 6dB (signal level halved) across them, whatever their particular value (provided) they’re both the same).
As their value is reduced much, eg below 100k, the preceding stage will become loaded, reducing their gain and max signal swing.
If made a lot larger, eg above 470k, they will start to lose more than 6dB, due to the input impedance of the LTP / power amp (>2M).

[OrganicEffects]

What about lowering the grid stoppers on the PA, or hanging a larger resistor off the bass pot?

[sluckey]

You can disconnect the top side of the INTENSITY pot to increase signal into the PI. Some people like to replace the INT pot with s pot with pull switch to use as a gain boost.

[jjasilli]

"more preamp volume into the PA of a basic 2 channel non-reverb deluxe Ab763 build."   Some ideas:

* Double value of Power Tube Grid Leak Resistors (verify fixed bias afterward)
* Increase Voltage to PI  (smaller B+ dropping R)
* Add bypass cap to tone recovery stage (if not there already)
* Use 220K plate resistors in one or more preamp stages.  Maybe increase K R accordingly.  Maybe increase supply voltage (smaller B+ dropping R), if larger plate R is/are used.

[Toxophilite]

Hey thanks.


This is actually my stereo deluxe reverb. The echo channel is a plain AB763 deluxe circuit (no trem or reverb sorry for not specifying.) Otherwise it's basically the same as two similar preamps being fed into the same PI via 220k mixing resistors.


 I was keeping it simple for the question but didn't realize people might offer other solutions. I wish I could put the schematic up but I don't have at this location.

[sluckey]

Hey thanks.


This is actually my stereo deluxe reverb. The echo channel is a plain AB763 deluxe circuit (no trem or reverb sorry for not specifying.) Otherwise it's basically the same as two similar preamps being fed into the same PI via 220k mixing resistors.


 I was keeping it simple for the question but didn't realize people might offer other solutions. I wish I could put the schematic up but I don't have at this location.

Seems you left out some very important information. I feel like I've just been led down a dead end rabbit hole! THX

[Tony Bones]

How are the output tubes biased? Sometimes just running them a little hotter will get them involved a little sooner.

[Toxophilite]

I actually did apologize, I hope you didn't lose too much sleep over it.


Seriously though this was meant to be a simple and specific question, I am away from my home and don't have access to the schematic however I really just wanted to know about the mixing resistors.

I have the information I wanted, thanks, regardless

Hey thanks.


This is actually my stereo deluxe reverb. The echo channel is a plain AB763 deluxe circuit (no trem or reverb sorry for not specifying.) Otherwise it's basically the same as two similar preamps being fed into the same PI via 220k mixing resistors.


 I was keeping it simple for the question but didn't realize people might offer other solutions. I wish I could put the schematic up but I don't have at this location.

Seems you left out some very important information. I feel like I've just been led down a dead end rabbit hole! THX

[pdf64]

Consider using a bit less nfb on the power amp.