cap in parallel with gain stage plate R?

[12AX7]

Can anyone tell me in layman's terms what this does ? I have done a lot of experimenting with caps in the small pf range (usually 100pf or less) across the PI plates and other things like this and i find they tend to change the tone in a similar way no matter how i implement them, and it seems to be different from what i should expect by what i have read. It's usually said they are used to remove that very top end tinniness, but i seem to get the opposite result, tho in a good way. What i hear is a considerable bit of added high harmonics and a change in the feel and tone that i can only describe as spongy. Very nice actually, tho not at all what i've always been led to expect. I have one on the PI from plate to plate, and one in parallel with the plate resistor on the second gain stage in my cascaded ala marshall preamp. Just wondering what these do technically, tho hopefully like i said in layman's terms that i can understand.

[HotBluePlates]

Cap across plate load resistor (from plate to B+ node), or cap from plate to cathode, reduces treble in the stage.

I have done a lot of experimenting with caps in the small pf range (usually 100pf or less) across the PI plates ...

This is different than your topic title, but also reduces treble by some amount.

[kagliostro]

I ask confirm about this

but cap in parallel with plate resistor isn't (also) a way to avoid oscillations ?

K

[sluckey]

but cap in parallel with plate resistor isn't (also) a way to avoid oscillations ?

Yes. Remember, B+ is at signal ground due to the big filter caps. So, a cap across the plate resistor (plate to B+) acts just like a cap from plate to ground.

[kagliostro]

Ciao Steve

Come al solito debbo rigraziarti per le tue utili spiegazioni

Many Thanks

Franco

[12AX7]

Cap across plate load resistor (from plate to B+ node), or cap from plate to cathode, reduces treble in the stage.

I have done a lot of experimenting with caps in the small pf range (usually 100pf or less) across the PI plates ...

This is different than your topic title, but also reduces treble by some amount.

Yes, i know. What i am getting at is i have done both and both seem to do the same thing tonally. Yet from all i've read they both are to tame top end issues, be it harshness, too much of that ultra top end sizzle, or oscillations. In any case, whether or not any of those are supposed to be the result, why is it the result i hear with BOTH scenarios (parallel to plate R or across the PI plates) is the same and different than what i have read ? If anything to me it sounds like theres MORE high end even if it's harmonics. And the tone sounds a bit thinner. But it's tonally an improvement due to the feel and complexity.

[HotBluePlates]

... why is it the result i hear with BOTH scenarios (parallel to plate R or across the PI plates) is the same and different than what i have read? ...

I don't know... could be a lot of things.

- Wiring goof on your part, so it's not working the way it should?
- Cap value too small to have an audible effect? (I ran into this trying to tweak my implementation of a  Vox-style Cut control on my Standel build)
- Cap working, but effect too subtle for you to notice (see "too small" above; this is the step between that and "obviously working")?
- Cap working but not the effect you envisioned?
- Cap working, but you're also cranking the amp more to hear what the change will be, boosting remaining highs by introducing more distortion?
- Cap working, but the way it places an additional load on the circuit at highs introducing unexpected distortion?
- Cap working but being used on a stage inside a feedback loop? (Things generally don't work as you'd expect inside a NFB loop)

If you're tinkering the long-tail phase inverter on an amp with NFB, remove the cap from that inverter. Try the cap earlier in the preamp before the NFB loop.

It occurs to me the amps that typically use these caps at the inverter (except 5F6-A, JTM-45) typically don't have a feedback loop. If everything starts to work like it should when the cap is moved outside the loop, then I'll explain what changed.

[12AX7]

I didn't mean to give the impression i want to get them to do anything differently....i LIKE the result. I'm just trying to understand whats going on. But the results are similar regardless of whether i'm talking about the one on the PI between the 2 plates or the one in the pre paralleling the V1B plate resistor. I was hoping someone else had similar results and could explain it. The values i'm using according to my meter (which reads caps pretty accurately according to the stamped value) are around 100pf.

[Geezer]

The values i'm using according to my meter (which reads caps pretty accurately according to the stamped value) are around 100pf.

I refer to them as "smoothing" caps..........use something bigger (250p to .001) and you'll hear the change. I've used as large as .00125 on some builds. The trick is to find the right balance for each particular section of your build. I also use them in conjunction with caps from plate to cathode, and bleeder caps to ground to remove harshness, especially in OD.

On a side note (not directed to anyone here), some call them "band aides", but they are just tools to use to get the best tone (to YOUR ears) out of your amp. I build amps for myself to use, to my ears, and to my guitars....these are useful tools to me and I couldn't care less if they aren't "the way Leo did it"....the results are pleasing to my ears and inspire me to play more and more creatively, which I enjoy. Enjoyment of this hobby and the satisfaction that it brings is the end goal for me, not impressing someone else.  

G

[tubeswell]

Plate bypass caps work the opposite to cathode bypass caps. Whereas in the case of cathode bypass caps, smaller caps boost higher frequencies, in the case of plate bypass caps, smaller caps roll-off higher frequencies.  Beware of the voltage ratings of plate bypass caps, especially where you are bypassing the plate to the ground return.

[12AX7]

Beware of the voltage ratings of plate bypass caps, especially where you are bypassing the plate to the ground return.

Yeah, i had thought of that but it seems most of the caps i have in that range don't have a V rating on them. They are mica so i think they are ok but you're right...i should be sure and find some others i know are high enough.

[12AX7]

Just wanted to bring this thread back because i now realize what i am hearing more than i did before. And this seems to be the case whether i put a small (100pf or less) pf value across the plate resistor in the gain stages or one either PI plate resistor or from plate to plate on the PI. As i initially described i felt it seemed to increase high end more than remove it like thy are supposed to. Now i realize that what i was hearing was more the low end changing. Without the caps it was wooly. Thats the best way i can describe it in a word, but what i mean is the lows were sorta fuzzy and not focused at all. And i did NOT realize this till the caps went in. With the caps the lows get much more focused and seem to sound more harmonically complex and thinner, tho thinner in a good way. It also gets more bouncy feeling in all ranges. So the highs are not accentuated as i first thought, it was the lows being sculpted and much of the unwanted artifacts in it removed which apparently makes the highs more apparent. The highs still aren't reduced tho a they are suppose to with this tweak, however i also now know why....the small values i'm using. I tried a 250pf and the highs did then start to be attenuated a bit. But under somewhere around 100pf and it seems not to remove  any highs i can detect, but it does cause the entire tone to be sweeter and gives the feel that sponginess we all love and makes the entire sound much more focused. But with small cap values i hear none of what thier said purpose is, removing excessive top.

I think these caps are a big part of the magic in a good cascaded preamp type of amp. I can't imagine this amp without them. The one at the PI I have been using for a long time now, but i started this thread after putting a second one in the pre.

[jim]

I have placed 330-500pf mica caps "smoothing"caps from cathode to plate to remove stubborn hiss problems with minimal effect on tone. It does impart a tiny, tiny bit of nasal tone but in an OD pre-amp its called "British".  Jim