Out Of Curiosity...

[zozoe]

The builder of my guitar amp said "The bias pot is on the left hand side of the board. the beige square with black dial. Set it for 25mV across the 1Ω resistors on each power tube cathode."

Now, going either way, above or below 25mV would do what?? Would going up to, let's say, 35mV, be what's known as 'running it hot', and I'm biasing hotter? More OD/gain?
thx

[cdemike]

Biasing hot means running more current through the tube, which would mean less negative voltage at the grid. The trimmer pot adjusts that grid voltage. At its extreme, there would be zero volts from the bias circuit on the grid, which could destroy the output tubes and/or the output transformer. So the bias circuit reduces the grid volts from zero to some negative value. For reference, my last Hiwatt build has about -45V at the grids for about 65% plate dissipation. Plate dissipation is what's being indirectly measured at the 1-ohm resistors mentioned in the quote from your builder and is essentially a measure of power. Electrical power is generally calculated as volts * current, and the 1-ohm resistors make the math easy to figure out current using Ohm's law (Voltage=Current*Resistance, or V=IR). With the resistor equal to 1 ohm, Voltage = Current, so you get 25mA or 35mA in the examples you listed. If you multiply that by your plate voltage, you have the plate dissipation of the individual tube. Most tubes used for guitar amps generally work best at around 50-70% plate dissipation, or 50%-70% the tube's maximum power rating of that tube. This is assuming your amp is running class AB, which isn't a given -- we'd need more information about your amp.

This is a complicated topic in terms of the sound. I actually tend to prefer a slightly cooler bias, but many people prefer hotter biases. There are many videos demonstrating the effect of bias on the amp's sound, but generally speaking the hotter the bias, the less headroom, which is not to say that more overdrive is available. I thought this is a pretty good demonstration of the effect of bias on sound: https://www.youtube.com/watch?v=VWEsYIg_1_o

[zozoe]

ok,, so is that trim pot the way to 'run or bias hotter' & perhaps going from a 25mV to a 35mV reading is in affect doing just that? again, I'm not going to start twisting & turning stuff,,, I just seek clarification of what turning that pot does.... thnx

[cdemike]

The mV rating you're seeing across the 1 ohm resistor is a result of the bias setting, which is controlled from that pot your builder alluded to. Bias setting is the amount of negative bias voltage reaching tube grids. So yes, turning the pot will make your bias hotter or colder. But it's an indirect effect, since what you're actually changing is the amount of negative voltage on the power tube control grids.

[zozoe]

so I'm not truly in affect changing any hot or cool biasing on the power tubes... right? A tuslly the amp is a Victoria Ivy League Harvard clone, & I want some more of that 50/60's Tweed OD out of it, & of course I'd have someone else do the work, cuz I know next to nothing, but I know what I want to hear, & it ain't there,
yet~ What should I really be looking to do to get more if that Tweed goodness? thnx

[pdf64]

The builder of my guitar amp said "The bias pot is on the left hand side of the board. the beige square with black dial. Set it for 25mV across the 1Ω resistors on each power tube cathode." …

so I'm not truly in affect changing any hot or cool biasing on the power tubes... right?

No, not right. From what you write in post 1, the bias pot changes the bias applied to the output valve grids.

The bias is adjusted to achieve a suitable idle anode current, that’s what the 25mV measurement is used to assess.

If you set the bias so that you measured 30mV, the output valves would be passing 20% more idle anode current, so they’d run hotter and wear out sooner.

Have you explained your issue to the amp builder? If not, I suggest that would be a really good idea.

[sluckey]

What should I really be looking to do to get more if that Tweed goodness? thnx

I have a real Harvard 5F10 and it sounds very tweedy to me. You can probably get a bit more "Tweed goodness" by converting to cathode bias for the output tubes.

[zozoe]

SLUCKEY,,, Talk to me,,,, So, as it sits, it's fixed bias, right? I have a 'guy' that's not really an amp tech so much as an electronics buff, but when i give him a directive, he follows.... What's the deal with the cathode bias you speak of?? I would do it only if tweedy goodness TRULY awaits me at the end.... Big job, with major surgery? DM me if you prefer... I want details~
kennyslewitt@gmail.com

thnx

[sluckey]

Small job. Just make the output stage look like the 5E3. Even better, make it switchable. Look at page 11 of this pdf...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[zozoe]

SLUCKEY,, could you kindly send me & circle the complete section from the 5E3 you refer to.... thnx

[sluckey]

...

[zozoe]

thnx, SLUCKEY!! I love the 2-10 Vicky, but it needs a kick in the pants.... Sounds like an amp on 8 that wants to go to 10,, or even 12~ I'll keep you advised...ॐ