Princeton Reverb - the immortal hum

[bnwitt]

I believe I am in the ballpark with a 21k resistor.

I would assume so, but I still don't know your plate voltage at the power tubes. Also remember that a different set of 6V6 power tubes could run hotter or colder and even that set could change over time. I think the question here is "How does it sound?" If the answer is "Great!", you're done.

[Enzo]

Amen.

[tubeswell]

Let me make sure I understand it. I need to lower the value of the 27K resistor in order to increase the bias current over the tubes.

I effectively need to double the tube bias current from 16ma to 32ma or adjust the bias voltage from -39vdc to -30vdc. Can I parallel a 100K pot (about 4 times higher to start) with the 27K resistor, set the bias - then measure the pot and do the math to get the resistance?

Hiya Sir Cuitous

You need to raise the value of the ground leg resistor in the divider to get more negative bias voltage. If you lower the value of the ground leg resistor, you end up with more current going to ground there and consequently less voltage at the 'knee' of the divider. As one of the other posters already said, if you wire a trimpot as a variable resistor in series with another fixed resistor, and use the whole thing in place of the ground leg resistor in your bias supply voltage divider then you can do instant micro-tweaks and get it just right.

[Sir Cuitous]

Howdy, tubeswell.

I want less negative voltage at the 'knee' of the divider. Since I needed less, my method of clipping on a parallel pot worked in a similar way with "micro-tweaks". It would not work exactly like that if I have needed more negative voltage. My goal was to simply change the resistor - once - those old Fender eyelets always seem fragile to me and I wanted to avoid trial-and-error.

Just to round off this discussion, bnwitt asked about voltages:

427 plate
422 screen
(both 6V6s)

They are a little high, but I think it's OK. Remember this thread was about AC coming through the bias voltage circuit which may have been the cause of a loud hum that was coming through. It seems as if this repair requires re-biasing and I appreciate the input as always. Anyway, I changed the resistor to a 21K and my bias probe reads 28ma on each tube. The amp sounds great and the hum is long gone.

[bnwitt]

Ok, well just so you know, 427 plate and 28ma for each tube puts them at about 85% of max dissipation for 6V6GTA tubes taking out average screen current. That is a little warm but not much and I can imagine that amp sounds mighty fine. Nice work SC. the owner will be thrilled I'm sure.
Barry

[Sir Cuitous]

Thanks Barry. It does sound great. The amp belongs to my friend, but I will trade him for some guitar lessons as he is a monster player and I play more like a tech. It was at 32ma before fixing the bias circuit, so I believe it will sound closer to what he expects.

I can tell that if I keep this up I will have to do math. I did use parallel reisistor calculations and now I see that knowing how to calculate the dissipation is important. I can do it, but like many I seem to avoid it.

Now my boss wants me to fix his Vibrolux Reverb. Does it ever end?

My appreciation to my fellow forum members. Thanks.

[bnwitt]

Now my boss wants me to fix his Vibrolux Reverb. Does it ever end?

Hopefully not. It's way too much fun.