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atmars 14th November 2008 01:03 PM

Snubber caps
 
I have guitar amp that uses a 5E3 circuit built into a donor chassis. It's a lot like a 5E3 with some changes ( switch to parallel the first triodes, different iron, etc). Generally this amp is too bright. I have tried several solutions, but the one that works the best seems to be "snubber" caps over across the plate resistors on the first stage (.001 or similar) without trying everything, are there any guidelines to this technique? Value limits? What is the effect of adding snubber caps to the second stage instead of the first? Or somewhere else?

Thanks

m6tt 14th November 2008 02:55 PM

some of the older Fenders would put the cap between the two signal lines of opposite phase after the inverter. The later you put the HF bleed-off caps in the chain, the more you should be affecting the total response of the amp. Personally when an amp is too bright I start by removing the bypass caps and increasing the coupling caps slightly. Make sure it's not just that you don't like the tone stack. Try bypassing it with a wire and see if the bad sound persists (the bass may be boomy this way).

Removing the cathode bypass caps raises the output impedance, thus reducing high frequency response into the same load (as far as I can tell).

Increasing the coupling caps will change the bandwidth of the following stage, allowing more bass through. If the valve is amplifying more bass, it has less total bandwidth available for highs.

If that doesn't work, you can use larger grid stoppers in the input, like 220k or something fairly high. You can also add these to the outputs, for normal operation they're like 1.5-2k, you could make them 5 or 6k to really start rolling things off.

Finally, sometimes amps will sound shrill when they are oscillating at a few khz...I'd scope it for oscillations, especially if it sounds that way with the tone stack bypassed.

atmars 15th November 2008 01:10 PM

Thanks for all the great info - a couple of clarifications:

Quote:

some of the older Fenders would put the cap between the two signal lines of opposite phase after the inverter.
Are there any values you have tried for the cap between the sides of the phase inverter?

Quote:

Personally when an amp is too bright I start by removing the bypass caps and increasing the coupling caps slightly.
Removing the bypass caps is going to cost me gain, right? If that's the case I probably don't want to do that.



Quote:

Try bypassing it with a wire and see if the bad sound persists (the bass may be boomy this way).
My tone stack is the standard "tweed" tone with lower values on high side. I thought taking the tone stack out increases gain and thus treble? No?

Quote:

Increasing the coupling caps will change the bandwidth of the following stage, allowing more bass through. If the valve is amplifying more bass, it has less total bandwidth available for highs.

Is this principle true globally? I mean, does increasing bass not just mask treble but actually reduce it?

Quote:

If that doesn't work, you can use larger grid stoppers in the input, like 220k or something fairly high. You can also add these to the outputs, for normal operation they're like 1.5-2k, you could make them 5 or 6k to really start rolling things off.
I have tried this technique, actually increasing the grid stoppers up to 1 meg. It definitely reduces the overall ice pick, but i worried that it was reducing gain and break up so I returned to the original values. Is there any evidence that high grid stopper values will reduce perceived gain and volume?

Thanks for any light you can shed.


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