is there any reason i can't...

[daz]

....use a resistor in series with a PI cap from plate to plate? I tried a 270 pf to tame the ultra highs, but it tames a bit too much. so i tried some other values, but between trying what i had both seperately and in parallel or series with each other, i couldn't get quite the right degree of taming. i was thinking if i could put a pot in series with a cap that tames a bit too much i could dial it in then replace with a resistor. besides the fact that i need to be sure to put a knob on the pot, keep one hand away, and be extremely careful, are there any other reasons i shouldn't do this, and any reason it won't work as i am looking for it to work?

[Steve Conner]

Hey Daz

Sure, it'll work. Provided the amp was stable with no cap there in the first place.

The three terminals on a pot are not connected to the shaft or the case. So you can protect yourself from a zapping by grounding the case.

[daz]

Thanks. I tried it with a resistor and it got somewhat close, but with a pot i should be able to dial it in perfectly. I'll give it a go today.....as carefully as possible ! I recall getting a taste of B+ about 20 years ago and i involuntarily threw a pair of pliers across the room and nearly embedded them in the wall. So i know what it feels like ! (and at my age now i probably wouldn't live thru it)

[J M Fahey]

Rather than use a pot or a resistor in series, which would defeat the stabilising purpose (that's why Steve said

Provided the amp was stable with no cap there in the first place.

)
, I'd just lower step by step the value of that cap.
270pF *does* sound too large to me, why don't you try 100pF or the classic 47pF?
I doubt this last one takes much sparkle, if at all.

[Robert M. Martinelli]

Hi daz,
What Steve and JM said.
If you don't get the desired results by simply lowering the capacitor's value (and I agree with JM that 270 pF looks way too much, you'll probably find that something in the 100 pF range will do just fine), I'd use a pot only as a means to search for that "sweet spot", and, once that "sweet spot" has been found, I'd throw in a fixed resistor of the same value in its place. Too many pots to adjust can be a hindrance in the search for the "perfect tone" IMHO.

Hope this helps

Best regards

Bob

[Chuck H]

OK... Listen to what others are saying and experiment on that basis. But also...

You may be trying to cut highs in an area that isn't the problem. Extra current at any frequency can create voltage spikes in the power tubes that manifest as (what I can best describe as) high frequency peaks that sound beamy on high end single note attack. Perhaps a "conjunctive filter" may help. A conjunctive filter would tame voltage spikes relative to power tube clipping and leave all the high end reactive mojo in the preamp. It CAN smooth things out without losing the chaos of overdrive that you've created with the rest of your circuit.

You need big strappin' parts for this circuit. IIRC you are running a pair of el34 tubes. So your primary impedance is probably between 4k and 6.6k. So try an 8.2k 25W resistor in series with a 1000pf to 2200pf 1000V cap right across the OT primary leads. The 1000pf to 2200pf cap should only really target the spikes and remove the beamy-ness from the attack of single notes.

Keep a 47pf across the plates of the PI as it's just good practice and shouldn't really detriment top end, and experiment with the cap value for the new filter. I'm using 1500pf for my circuit right now. This value is just starting to be audible but I like what it does to the square wave power tube OD on my scope.

JM2C

Chuck

[daz]

Thanks all. But yes, i WAS trying to cut highs. But the problem was that any notable difference took away something good too. however, it doesn't seem to be much of an issue anymore because i want to fixed bias and somehow the issue isn't quite as bad. But Chuck, can you elaborate on the 47pf? I know a lot of amps use this, and i know it's not for taming highs but for oscillations or such. But what if your amp seems to have no issues like that? Would you still want it there and why? In other words, if you can't hear any difference w/o it, whats the reason to have it?