I need help understanding a Tone Stack

[deepmojoman]

I have an amp circuit I'm trying to understand, to help me get rid of an unappealing high-end sizzle that occurs with increasing prominence the more gain you dial in from the Gain knob.

I changed what I thought to be the coupling cap-which initially was a 10n-, coming right off the plate of the only 12ax7 in the amp (single ended, with 1 EL84 power tube), exchanging that cap with a .1uf, and that does seem to have increased the bass, but that high-end sizzle is still there.

I have included a schematic of the amp, along with inquiries about the Tone Stack. I am intuiting that here is where the problem-and the solution-ultimately lies. The amp being a single Tone knob affair makes knowing how to tweak Lo Pass a little bit more challenging for me than if it had 2 or more Tone knobs-simply because there are so many other Tone Stacks out there based on this format, and it would be what, for me, would be a much simpler matter of adjusting the various caps for each Pot. But I'm not as clear about how to manipulate various frequencies with a one knob stack. I am aware that it is intrinsically more limited, but then again, in that I'm not trying to effect middle frequencies at all, but rather just the top or bottom of the spectrum, I should think what I'm trying to do will be possible.

But I need help understanding how to construct a Lo Pass Filter on the circuit path to the Gain Knob-seeing as the high-end sizzle increases with the Gain level. Or do I? Is this a correct assumption?

Please check out the schematic and you can see what I'm talking about. Thanks for all your help.

[Stevem]

Try placing a .005uf cap across either The V2A plate resistor, or the V2B plate resistor.
This cap needs to have a voltage rating that's higher that the B+ voltage that is coming into those plate load resistors.
Using two .01uf caps in series will do the same thing and may also be easy to to find, even two disc type .01 caps will do, but since disc caps can be quite microphonic be sure to glue them down and to each other with hot glue or silicone glue if they do the trick!
You can try out / audition other small value caps also and you can not harm the circuit with these, any where from a 250 pf cap to the .01 uf can be stuck in there to try out!
I just did this to a guys Vintage Highwatt 50 head that was just too brite with his Strat to the point thst he had to put two layers of a moving blanket over the 4-12 Fane loaded cabinet to temper the high end.
I placed two 01uf caps in series across the third gain stage,and dropped back the first gain stage cathode bypass to 22 down from the stock 250uf.
Now he is able to open up the preamp controls more and the amp sings like mad!

[tele_player]

Try placing a .005uf cap across either The V2A plate resistor, or the V2B plate resistor.
This cap needs to have a voltage rating that's higher that the B+ voltage that is coming into those plate load resistors.

I don't think that's strictly correct. The capacitor needs to be rated to handle the max voltage drop across the resistor, which will always be less than the B+ voltage.

Suppose, for example, we have a typical 12ax7 setup, supply voltage about 300v, with about 100v across the plate resistor, for 200v at the plate. Even if we're driving the tube at 25v p-p output, the drop across the plate resistor ranges from 87.5 to 112.5. Way less than 300v supply voltage.

Or I'm missing something?

-Robert

[Stevem]

Trust me, the cap must be rated for at least 25 volts more than power supply voltage on the non tube side of the plate load resistor!

[tele_player]

Trust me, the cap must be rated for at least 25 volts more than power supply voltage on the non tube side of the plate load resistor!

I don't usually trust things which don't have an explanation.

-Robert

[matt h]

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[deepmojoman]

Thanks for all the technical information-I appreciate it all. But I do really just want to re-direct to my question: how to Lo Pass within my tone stack, so my thread doesn't become a meandering postings that can't really help me.

[matt h]

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[deepmojoman]

Point taken, Matt H. Thank you for the confirmation. And now I'd like to say something else: the way you speak to me isn't nice. As, I am gathering, you should be able to tell, I am new at this. I clearly don't know near as much about electrical circuits as you do. But to call my question-a question coming out of clear ignorance-a "bad question" does no one any good. I just simply "don't know", right now. Telling anyone-who is trying to learn-that the question they ask is bad will be extremely discouraging for them to do anything else to learn how to do it right.

I called the direction of the thread in danger of meandering because 3 people had started to debate technical points amongst themselves. I wanted to re-direct back to me-regardless of whether or not my question actually asks what I should ask-because if one does not do so in a forum, in such a situation, you can easily lose the ability to actually gain assistance for your problem because several individuals have started to discuss-or even debate-newly cropped up issues amongst themselves, rather than speak to the individual whom has come to the forum for a solution to a specific problem.

The short of what I'm saying is: I saw there people who starting to talk to each other, about a certain issue, rather than talking to me: the person who came to them with a specific question-whether worded good or bad, ignorant or in the know-and asking for assistance.

I don't know how many forum users you have spoken to about my post, enough to garner a consensus that the overall reason why no one was replying to my satisfaction was because of my "mostly-bad question", but, in the event that you do have such an incredible sense of why your fellow forum members are not answering me, then let me say this: there is a solution to the problem I'm asking about (you and SteveM have already suggested one). Rather than dealing with the medium that introduced this problem to you (i.e.: my skill as a questioner), I'm sure it would be much more helpful to me if someone like you just simply gave the answer that came to you rather than issue out criticism as well-criticism that just doesn't help anyone to absorb whatever they're wanting to absorb.

You, once upon a time, didn't know about electrical circuits, just like me, and asked, I'm sure, a bunch of "bad questions" yourself, before you were fortunate enough to run across a teacher who was patient enough with you to listen-and to answer to-the valid line of inquiry within your question, regardless of all it's "badness".

Sorry for the long reply, but this has been the second time in two weeks that I have been treated not particularly well on this forum-a forum which was suggested to me by a friend, as a place where a new person could get friendly help as he struggles to learn.

[matt h]

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