12ax7a cathode bias resistor variance discussion

[hewo]

ten microfarad cathode bypass, nonstandard

Thx DH. I agree. On a second reading that is what is meant. A combination of punctuation and phrasing threw me off, I guess.

I remain skeptical on the difference as there are just too many other variables. I would only trust a statistically significant properly conducted ABX test.

Could it be the nonstandard value 10uf cathode bypass ?
Historically, its suppose to be 25uf fender, 22uf boogie, 15uf peavey, 0.68 marshal

So SLM chose ten uf, maybe it's cheaper, they're tiny tiny electrolytic axial 16wvdc.
I swapped tantalums, then os-con, then bipolar electrolytic.
But the cheapy factory ten uf is best, dunno why.
On the otherhand swapping the power supply electrolytic did make improvement.
I purchased appliance "running" electrolytic capacitor, gigantic size for onehundred uf, gigantic size for 47uf. The vc3112 has enough chassis space for these gigantic caps. These improve the throughput, like "bow and arrow" impetus, tubes breathe through energy fed from feed resistors without hindrance. I paid thirtyfive at granger but got the exact one from communist china six dollars, onehundred microfarads. The 47uf communist china costs three dollars. Must umbilical shortest wiring runs and safeguard crosstalking high tension AC.

BTW, the American tele pups installed, push out much much much more than ten mv.
I fluke79 rms vac strumming lightly tele strings, generates eighty mv, single note plucked 35mv.
Thus the vc3112 is a bit oversensitized to input size, unless vc3112 designers compatibled yesteryear pups

[nickb]

Could it be the nonstandard value 10uf cathode bypass ?
Historically, its suppose to be 25uf fender, 22uf boogie, 15uf peavey, 0.68 marshal

So SLM chose ten uf, maybe it's cheaper, they're tiny tiny electrolytic axial 16wvdc.
I swapped tantalums, then os-con, then bipolar electrolytic.
But the cheapy factory ten uf is best, dunno why.

The -3dB frequency is determined by 1/2/3.14/R/C so 10uF + 1.5K => 10.6 Hz - too low to make it through out of your guitar, through the amp, speaker or your ear. Bigger caps will have no effect as they shift the frequency even lower.

0.68uF + 1.5K => 156Hz. Now that is smack in the interesting range and will have a very audible effect.

There is no difference in this application between these different types of capacitor, other than the tolerance except I would prefer to use a film cap only for the 0.68uF case due to the sensitivity of the ear around this frequency and their low distortion.

It's really easy to fool yourself when doing these AB tests. There's a pretty good chance the difference in the speakers between the two amps has a bigger effect. Just standing in a different relative position has a dramatic effect. Setting of controls is another. To make a fair comparison you have to eliminate all the variables.

On the otherhand swapping the power supply electrolytic did make improvement.
I purchased appliance "running" electrolytic capacitor, gigantic size for onehundred uf, gigantic size for 47uf. The vc3112 has enough chassis space for these gigantic caps. These improve the throughput, like "bow and arrow" impetus, tubes breathe through energy fed from feed resistors without hindrance. I paid thirtyfive at granger but got the exact one from communist china six dollars, onehundred microfarads. The 47uf communist china costs three dollars. Must umbilical shortest wiring runs and safeguard crosstalking high tension AC.

The bigger value caps will make the power supply 'stiffer' and so change the sound on transients, especially when cranked. Again, film vs electrolytic will have no significant difference in this application, other than perhaps lifetime. The tolerance is the most important difference. They will also reduce hum and intermodulation products thereof.

Good wiring of the power supply is crucial to avoid noise due to hum and it's harmonics.

BTW, the American tele pups installed, push out much much much more than ten mv.
I fluke79 rms vac strumming lightly tele strings, generates eighty mv, single note plucked 35mv.
Thus the vc3112 is a bit oversensitized to input size, unless vc3112 designers compatibled yesteryear pups

Agreed. That's why I said "at least 200mV". The 10mV is just is reference point used when testing the amp. They make the inputs sensitive to give you nice overdrive. You can't really measure the pickup output very well with a Fluke as they take an average and sample slowly. Use your scope to get a better idea. You can monitor the signal on the plate of that first stage to see how big it actually is right where it matters.

[Enzo]

Don't try to make it more complicated than it is. The cap isn't "supposed to be" anything. it is simply the designers choice. Some stages don't even have the cap. it has nothing to do with a 10uf being cheaper than a 22uf. When companies like SLM or Peavey buy such caps, they buy them by the million, so cost is negligible. If a designer arbitrarily decides 10Hz is low enough to ensure all USEFUL frequencies are covered, and another company choses 22uf so 5Hz is the bottom end, the difference is minimal, those freqs will neither one come out of the amp or speaker.


If I needed a 50mm piece of wire, I might pull about 60mm off the roll then trim it to length, and the next guy over might reel off 70mm and trim it to length. it is just a choice, and ultimately either way 50mm will wind up in the circuit. Just so your 10uf and 22uf caps.