Best location for a plate resistor bypass cap?

[harddriver]

Where's the best location to locate a plate resistor bypass cap to attenuate inaudible high frequencies and possible oscillations in a 4 gain stage amp that retains the cathode follower like an SLO?

V1 V2 or V3 gain stages?

I had a 1000pf cap on V3 I figured that would be the best spot since it was last in line of the gain stages. I removed the 1000pf to see what would happen and the hiss increased quite a bit and things seemed a bit harsher. Would a 2000pf cap be better here or what about 500pf I've seen people use that value.

Lead dress is good and I am running DC elevated heaters on the amp already.

[ryu]

you have a bypass here mostly because of the big 39k cathode resistor, it's clipping a lot and makes the high end ear bleeding (along with 220k's plate and (too) high voltage on the preamp IMO)
if you're after smoother try 33k here
FYI, it's not the last gain stage, the best place to use a plate bypass is where you feel you need one
this circuit doesn't sufer from oscillation when done properly, if you really want to add another plate bypass, choose either the 220k on the first or the fourth stage(1k8//1u before the cf), 220pf should be fine
if your layout allows it connect the bypass between plate and cathode ground, it's a tad quieter this way

[joey]

One thing to keep in mind is that if you go overboard, and kill too much treble in the preamp you can affect the usefull range of the treble control, in other words if the frequencies are not present that the treble control adjusts, than nothing gets adjusted.

[harddriver]

Thanks for the suggestions, I had a 1000pf cap on my V3 plate resistor which is 100K.

My V1 plate resistor is 220k V2 is 100K and V3 is 100K. My setup is :V1 is 1.5K/1uf, V2 is 1.5K/1uf and V3 is like a 3K or 5K I I can't remember exactly,it been awhile since I've been in the amp) that I shunt down for a lead boost here.

The house where this amp is located the AC power (dirty AC) seems to generate noise, my splawn modded marshall also is noisey and hissy at his house so that is primarily is what I believe I'm dealing with. My brother got used to absolutely no noise or hiss when the 1000pf plate resistor cap was installed but it tends to smooth and compress the lowend spank of the low E string and overall tone. He liked the more rawness of the tone and what seemed to be the immediateness of the low end but hates the hiss and noise that has reappeared since I removed the cap. I guess the best way to describe it to me it seemed less rounded by effects the 1000pf but I read the the 1000pf should pretty much transparent to the tone and that is only bleeds(dampens) the inaudible frequencies that might cause oscillations and or noise/hiss. I have not heard any oscillations just more noise and hiss.

My problem is the extra gain stage while retaining the cathode follower setup seems destined for extra noise. The amp is very close to SLO specs on how is configured.

Would a 2000pf cap be the way to go to have less effect on the rawness versus the 1000pf cap?

[harddriver]

I've seen in alot of Jose style mods a .022uf cap( I believe) bleeding off to ground in conjunction with the V1 .002uf coupling cap on the V1 stages. Would this be a better way to bleed/attenuate high frequencies/oscillations versus using the plate resistor bypass cap setup? I've seen these caps in place on alot of techs Jose Mods but have never used it.

Are there better values to use in this application? .0022uf , .00047uf, .22uf.

[joey]

100pf maybe mostly inaudible, but 1000pf, unless you are using a low ra valve like a 12at or au7, is quite audible! In the slo it rolls off at around 2.2k, and at 3-4k on something that has a typical output impedance of 40-50k. So yeah 1000pf most definitely Non transparently audible

[ryu]

+1
you might try 220pf over the first stage's plate
unless you have a big bright cap over the gain pot (it also depends on what you have between the stages) you shouldn't have that much hiss with those values (I mean not to the point of being annoying)
do you have some carbon comp or 1/4W resistors in there?
for a cap to ground after the coupling cap, if you have .022 as coupling try .001 or .0022 but I doubt you'll like the "effect"

[SteadyEddie]

I never cared for the plate resistor bypass cap on the v1-v2 stages, but on the 82k and 100k phase inverter plates, it smooths out the sound without affecting response. I used 100pf caps here but I was later informed that Mesa amps use the same technique, and go with 120pf caps.

[harddriver]

Thanks for all the suggestions there are some good ones to try. I am really convinced it's an AC interference issue at his house not helping things. So I have to try to clamp the noise to an acceptable level without sacrificing the tone.

He has a two tage plexi that does not amplify any noise but it is not a 4 gain stage amp like what we are talking about here.

I want to try the Jose .002 cap from the V1 coupling cap to ground first then move on to trying the Mesa setup on the PI ,then we'll backtrack to my V3 plate resitors values. We know the amps noise level is fine with the 1000pf cap but the tone is affected, so we just ahve to experiment.


Any other suggestions?

[harddriver]

I just wanted to give you guys an update I just got done working on the amp tonite. We tried a bunch of different things that were suggested in the thread, we found a great combination of caps that worked great for suppressing the noise(hiss) that was there at his house down to nothing and we were able to corner some great R/C constants focus the tone better!

Thanks alot for all the suggestions the amp's owner couldn't be happier!

HD

[erikm5150]

What did you end up with?

[harddriver]

What did you end up with?

We settled on the 470pf. They shelved the frequencies we liked while still attenuating the high end noise of all the extra gain pushing through. We tried above and below the 470pf and liked them best in this amp. YMMV in you amp but we are pleased with the results.

The main purpose was to clamp the extra noise that is present in his AC at his house only. An amp that is quiet at my house hisses at his, go figure! We got it all straightened out thank god!

Hope this update helps your build out!

[erikm5150]

Thanks HD... good info to know