How Low Can I Go? (cathode bias resistor)

[Fraggy0117]

I'm tuning a SE amp that has a switch to select a cathode bias resistor for a 6V6 and a 6L6. I started with 670r & 750r, respectively. Didn't get high enough plate dissipation, so I went to 400r and 225r but it's still a little bit low.

Basically, how low can I go before there are negative side effects? Playing with a bias calculator I think my next steps are 300r (6v6) and 120r (6L6), but I know from the last time that reducing the resistor value won't exactly give me the plate dissipation that the calculator says (since other voltages will change).

I know it'll never be perfect and it's a compromise; I have an undersized PT, two different power tubes using the same power section, etc. Just curious for any insight on when to call it.

If it helps, here's my voltages (and I'm using this calculator https://robrobinette.com/Tube_Bias_Calculator.htm):

6V6 (JJ6v6)
Plate-to-Cathode: 318v
Kr voltage: 14.6v
Kr value: 400r
78% dissipation

6L6GC (Russian mullard)
Plate-to-Cathode: 292v
Kr voltage: 13.13v
Kr value: 225r
48% dissipation

[shooter]

Plate-to-Cathode: 318v

the lower that value goes, the less "headroom" you have, the earlier the distortion.


post the R value so we can do some oldschool math, just to verify your calculator
Vk/Vr = Tube current
Vp - Vk * Itube is a smidge less than Pdiss, but close enough

[pdf64]

Dissipation is just a limit, work out a suitable operating point (see link) then check it idles within the design centre limit.
http://www.valvewizard.co.uk/se.html

[sluckey]

Cathode biased amps typically run 90% to 100% plate dissipation, some even higher. I would not exceed 100%.

[tubeswell]

Is your amp using a tube rectifier? Got a schematic?

[PRR]

It is fine the way it is. If you actually wanted the MOST power, you would not be dinking with an inefficient one-tube design.

[Fraggy0117]

Took me a while to update my schematic — I've tweaked it a lot since the original. Thank you all for the replies

Is your amp using a tube rectifier? Got a schematic?

I uploaded it and attached it: https://imgur.com/a/AKU6ah2

Solid state rectifier, and light OT/PTs. Partially as an adventure in using in-production parts and to see if there was any mojo in using undersized OTs like I've heard about in Silvertones.

It is fine the way it is. If you actually wanted the MOST power, you would not be dinking with an inefficient one-tube design.

Very fair. I wasn't looking for more power, necessarily, just seeing if the tone would be any better. It sounds great as it is currently, but I'm hoping to squeeze every bit I can.

post the R value so we can do some oldschool math, just to verify your calculator
Vk/Vr = Tube current
Vp - Vk * Itube is a smidge less than Pdiss, but close enough

6v6 cathode resistor is 400 ohms and 6L6 is currently 225 ohms.

Dissipation is just a limit, work out a suitable operating point (see link) then check it idles within the design centre limit.
http://www.valvewizard.co.uk/se.html

This is probably the best method. I should've known there's always more math I can do!

[shooter]

there's always more math

the fuzzy math is my favorite

[Williamblake]

Always nice to see new ideas about driving a reverb tank. How happy are you about that?

[Fraggy0117]

Always nice to see new ideas about driving a reverb tank. How happy are you about that?

I haven't actually hooked up that portion of the circuit - I wanted to tweak the rest of it first. As drawn it is a copy of what was traced from the Benson Monarch and I think it shares some similarities with some of the Ampeg amps (if I'm remembering the discussion correctly). I'm hoping it'll drop in with minimal tweaking but I'll be sure to update once I've got it hooked up.