input Vac vs power tube grid Vac

[pjd3]

Hello,

First of all, I want to thank you all for sharing your valuable experience with me as I navigate through my amp builds and currently attempt to finalize this Fender 6L6WGB upgrade (now with reverb but without tremelo).

I was curious if anyone puts any weight or value on say, input voltage (AC sine wave) vs the input to the grids of the power tubes (same thing, the amplified AC sine wave).

Is there any utility in meeting any amplification factor in terms of voltage inputting into the grids? This is a part of me looking to learn a batch of new things regarding these amps. My first two were 95% straight from schematics and layout diagrams but now, this 3rd project is going a bit more deeply into having to look at B+, power node voltages and perhaps taking a more empirical approach to getting the amp where it will be the best for my purposes.

Also, regarding these voltages when inputted with a sine wave, is there any accepted standard to look at these with, whether we assume P-P, peak, or RMS? I just want to be communicating properly, and documenting properly while changing component values, measuring and communicating.

Thank you very much!
Best,
Phil D.

[martin manning]

For a start, you'd like to be able to fully drive the output tube(s), meaning take the grid to 0V Va-k. That means the peak input voltage needs to be equal to the bias voltage. There it's convenient to refer to the peak, but in other cases peak-to-peak or RMS might be more useful. Do a search on RMS voltage and you will find lots of discussion about the what, where, and how of it.

[pjd3]

Thank you Martin,

That makes sense to me - having the inputted grid voltage bring negative bias voltage to "0" volts. I take it that you would get the greatest voltage swing at the plates without pushing the grid positive and causing things that are not necessary, well, at least for my application (Clean, never terribly overdriven, pedal platform).

I do understand fairly well about peak, P-P and RMS, how they relate and the math and numbers to navigate them. But sometimes builders will be talking about those voltages present in the preamps and power stage while not specifying those voltage units, or descriptions, and I will wonder what aspect of the ac voltage they are referring to, and if there is some understood rule to assume one or the other (P-P, peak, RMS,etc..).

Thank you for this insight and I certainly will be looking for this at my next "home bench session".

Oh, one more thing - is there any "Fender standard" used for an input voltage, perhaps the one Fender used when testing their amps? (Super, Twin, Pro).

Best,
Phil D.

[martin manning]

...having the inputted grid voltage bring negative bias voltage to "0" volts. I take it that you would get the greatest voltage swing at the plates without pushing the grid positive and causing things that are not necessary, well, at least for my application (Clean, never terribly overdriven, pedal platform).

In fact you won't be able to drive the grids positive (venturing into Class AB2 operation) unless the PI has more current sinking capability than the typical LTP or cathodyne. CF drivers as seen on Hi-Fi amps and here on the Dumble SSS can just manage it.

Oh, one more thing - is there any "Fender standard" used for an input voltage, perhaps the one Fender used when testing their amps? (Super, Twin, Pro).

I don't know if there was any standard when Leo was the chief engineer, but if you look at current Fender schematics they will show a test signal level at the input, typically 20-40 mV RMS. Guitar pickups can produce a couple of hundred mV RMS.