Valveking mh 20 negative feedback question

[Rod]

Need some help here.

If i lower or raise the R38 resistor in the schematic bellow, wich i think is the presence resistor, what results will i have?

[Steve A.]

Need some help here.

If i lower or raise the R38 resistor in the schematic bellow, wich i think is the presence resistor, what results will i have?

http://music-electronics-forum.com/a...60-vk-20mh.png

[ATTACH=CONFIG]49005[/ATTACH]

It would help if you posted link to full schematic. In any case the negative feedback circuit shown in your drawing is fairly complicated with the signal going through R61 and R23 (bypassed w/ C59) before reaching C23 & R38 going to ground.

What exactly are you trying to accomplish?

Steve A.

[Rod]

Attached is the full schematic, i think that the amp has a little to much presence, i want to improve this, and im triyng to see if changing the R38 resistor value in the negative feedback loop will increase or lower the presence.

[kmensik]

Raise the value to lower the presence. Try a 2k or 5k pot. Or you can raise the C23 cap to make it bypass more mids.
R78, R38 and C23 form a cathode bypass for highs, whereas R77+C5 form a full range bypass.

[Rod]

Raise the value to lower the presence. Try a 2k or 5k pot. Or you can raise the C23 cap to make it bypass more mids.
R78, R38 and C23 form a cathode bypass for highs, whereas R77+C5 form a full range bypass.

HI!

I raised the vale of the R38 to 1.5K in the negative feedback loop, and it raised the presence, it did the oposite of what i wanted.

[Chuck H]

HI!

I raised the vale of the R38 to 1.5K in the negative feedback loop, and it raised the presence, it did the oposite of what i wanted.

Looking at the circuit I think that your result is impossible. There must be an error in perception or the circuit alteration.

[dstrat]

change c23 to .1 or 100n will lower the frequency?

[Rod]

OK I'm going to check it, I think that the presence could be also of the El84 tubes, if I change something in the power section, could I get less highs?

[dstrat]

"I think that the presence could be also of the El84 tubes"

Not sure what you mean.

when you say too much presence, you are talking about high frequency response right?
may not be what you think, depends on the tube health mabe they are worn and loosing lows? just asking because its not clear what your trying to accomplish.

[Rod]

I was thinking for example if I change the bias of the El84 tubes, maybe I could get less highs.

[dstrat]

I would not mess with the bias, if it were me, I would try some values around the c23 .1 should lower the freq a bit.
if r38 at 1.5k "went wrong way" try 1k ohms and see? ( that should brighten things even more ) as its a 180 now.
according to the schematic i am looking at.

[Rod]

What kind of potenciometers should i put in the place of the R23 and R38 negative feedback resistors, Logaritmic or linear?

[Jazz P Bass]

I would use a linear taper.

Easier to control.

[Rod]

If change the location of the negative feedback loop resistor R78 just to after R23//C59, will this change the presence of the poweramp?, in what way?


ValveKing2_20MH.pdf

[Rod]

If I change the location of the negative feedback loop resistor R78 just to after R23//C59, will this change the presence of the poweramp?, in what way?

There must be someone that can help me with this question.


ValveKing2_20MH.pdf[/QUOTE]