filter caps and tone

[daz]

I built 2 amps that are almost exact (same trannys, choke, signal caps, etc) except the layout and the PI and preamp filters. Tho the 32/32uf can is the same in both. I gave the first to a friend and i have the second. Mine sounds amazing and very very marshally with pristine highs and lots of complexity. It's a very lively amp. the other is much darker and lifeless by comparison and doesn't sound like a marshall. I've tried playing around with his and changing the layout to reflect mine. i've checked voltages and tried to make them match closer. Checked component values, etc. The only thing i haven't done if to replace the sprague filers with F&T's as i have in mine. It's pretty much the only difference in components.

I always read about filter caps no making any notable tone difference, but i can't help but wonder. They ARE different values, but i don't think 20uf verses 22uf would make any notable difference. Could the difference in brands make that kind of difference?

[CarlZ]

I built 2 amps that are almost exact (same trannys, choke, signal caps, etc) except the layout ...

Nuff said? Yes Virginia, layout can make quite a difference in the tone and feel of an amp even if it's completely stable!

Carl Z

[6267]

Different cap values and ESR can have an effect on powersupply voltage & impedance, so yes, it could happen. Chances are though there's something more to it. Have you tried swapping all the tubes into his? Even "identical" OPTs could be wound differently. I'd probably put my money on a tube difference or OPT difference (let alone voltages/bias points) before chasing the filter caps...

[Chuck H]

If you have two different layouts and both are stable and benign, they will sound very similar. Hence the preference for certain circuits though the layouts vary. But if the layout is prone to coupling in any way, negative or psitive, it can have a profound effect on tone. I experimented once with the OT position in one of my builds. Not only was there a change in hum, which was the reason for the experiment, but there was also a change in tone. In some places it seemed to dull the tone and in others it seemed to exite the tone. This was a real eye opener for me. Though the use of negative feedback is commonplace in tube guitar amps, I don't know of any examples that intentionally use positive feedback as a means of boosting gain or harmonics. I'd be willing to bet that in some Marshalls it is an accidental benefit to the tone. Not just OT placement, but any circuit that may be only slightly coupled to another due to proximity will effect tone.

Chuck

[daz]

If you have two different layouts and both are stable and benign

Yes, very stable with no issues on either amp. Both are quiet and have no oscillation or such even at high volumes.

The PT, OT and choke are all the same exact models and placed physically in the exact same place and relation to each other on both amps. The layout is indeed different, but i did try changing that to match as much as i could and it made no difference. the circuit itself is identical except for the filter cap brand and values.

i really think that somehow this is happening in the preamp, and i DID find the biggest voltage difference was at V1A. So i changed the 1st amp's cathode R value to bring the voltage on that plate to what my amp is, and it did indeed help. So i think it may be there. But again the only real difference is in the PSU caps, so i can't help but think thats it, especially with that voltage difference.

[Chuck H]

Plate voltage on preamp tubes can make a huge difference in tone. If both amps are using the same gain structure and PT rhen I would suggest duplicating the resistance values of your amp in the B+ rail of his. If they are the same now, but his has significantly lower plate voltages, then I'm not sure what the cause could be. Increasing the cathode R will raise plate voltage, but it will also alter the bias point of that tube. A better MO would be to decrease the B+ rail R to that tube. You shouldn't notice any increase in coupling in the power supply.

FWIW, different filters can have significantly different impedance values. THAT could mean more coupling in the power supply and a decrease in gain. Unless your B+ rail couples stages of like phase and then too high an impedance can result in instability...And extra noise.

If you can duplicate your plate voltages and tubes in your friends amp and it has the same gain as yours, but still sounds dull by comparison, I would suspect proximity coupling in the layout. Even the kind of shielded input lead you use can make a difference.

Chuck

[daz]

That was the first thing i did when i heard how different they were. I measured all voltages. there was only one off enough to matter, or so i figured. And i changed that which helped but that was before i even asked this....it's still way different. So what i have here are 2 amps with the exact same circuit and components except the 3 filters and the layout is different. weird. What seems so odd is that the kinds of things that are different as far as layout are the kinds of things i have changed around while tweaking amps that never seem to affect tone. So if we exclude that as a possible reason 2 things are left....component tolerance and those caps. I guess i'll have to carefully check every component and look for any big variance. then of course the OT's may be off by a lot i suppose.

[loudthud]

I would try cross connecting the preamps to see if the difference is in the preamp or the power amp. Connect the preamp from one amp to the power amp of the other amp and vise versa. Does the bad tone stay with your friends preamp or is it his power amp?

[daz]

Thats a good idea. I've done that before to see how one pre sounds thru another amp's PA. I may have to give that a shot.