tone stack question - p2p, ceramic caps

[antichef]

About two years ago, I went on an ebay spree and wound up with several 1960's era practice amps (most for under $100). Recently, as I'm getting bolder with the soldering iron, I'm picking at one brandless single ended 6V6 one. Today I cleaned up some connections, and now it's really quiet -- I can dime it, and it makes so little noise you might not think it's on -- not extremely loud when I play, but that's kind of nice. By "p2", I mean point-to-point. Not really relevant for my question, but that's what it is.

Anyway, the sound is OK, but it's a little reedy/woody/kermit-the-froggy. The amp has two controls - a volume/on-off knob, and a tone knob, which is no doubt a low pass filter of some sort. Here's what the tone knob (& inputs) looks like from behind:

rather than ask intelligent questions about the circuit (which I can't claim to understand), let me ask this -- could I expect a change/improvement in tone by changing those old RMC ceramic disk capacitors with orange drop equivalents? I suppose those are carbon resistors? Any sense in replacing them with metal film?

 

[Milnoque]

Alot of people make alot of noise about the superiority of this or that type of capacitor in the tone of a circuit. My experience is that it doesn't make as much difference as they claim. I would say you would be able to hear the difference using orange drops in certain places in your circuit, but it wouldn't fix your problem.

You might try swapping out the speaker with one of your other project amps. I suspect that is where your problem is. If one of the other amps has a speaker that registers about the same ohms it should be safe to use. The amp needs an specific impedance load to operate safely. The resistance and impedance of the speaker coil track each other enough to use the resistance to know you won't harm the amp.

 

[Light]

rather than ask intelligent questions about the circuit (which I can't claim to understand), let me ask this -- could I expect a change/improvement in tone by changing those old RMC ceramic disk capacitors with orange drop equivalents? I suppose those are carbon resistors? Any sense in replacing them with metal film?

No.

Which is not to say you can't change the tone by changing certain caps (or, more likely, a certain cap), but the change in tone comes from a change in VALUE, not brand. If you can figure out which one is the tone cap, you could try changing the value of that cap and you might like the change, but you don't want to change the caps between gain stages, as you probably won't hear a big difference, and they are there to block DC from going places you don't want it. The one you can try is one of the caps hanging off the tone control there, but I'm not sure which one - if one of them is going to ground, that's probably it.

You could also likely improve things by changing out the filter caps - use the same value or slightly higher, and the same voltage rating or slightly higher. The filter caps are the big ones, and will probably be in the 8-20μF 300-500 volt range. Don't buy the Sprauge Atom bullshit (the current TVA caps on the market are just modern caps housed in a big empty tube so they LOOK like the old TVA's, but they are mostly hollow inside - I've seen them sawn in half - they are a scam). I like Illinois, but use whatever you can find that works - including TVAs, but they are awfully expensive. The reason for changing these is that they are probably bad.

And since you haven't already killed yourself, you probably know, but make sure to drain the filter caps before you go poking around in there.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[antichef]

Thanks guys! I'll focus on finding the tone cap, and think about the value. I'll look into the filter cap issue, and thanks for the warning about the scammy ones (jeez ). Yes, I'm still here, and I haven't yet earned the nickname 'sparky'

edit: and oh, yes -- I'll try a different speaker!

 

[dementedchord]

that cap at the top of the picture looks like it could be the one you want to play with... difference due to a change in material will be minimal... but try doubling and halfing the value and it may give you ideas on what you can do as far as changing the freq of the filter...

 

[Light]

that cap at the top of the picture looks like it could be the one you want to play with... difference due to a change in material will be minimal... but try doubling and halfing the value and it may give you ideas on what you can do as far as changing the freq of the filter...

You know, I think you may be right.


Light

"Cowards can never be moral."
M.K. Gandhi