bias trim pots

[cminor9]

I have been pondering for quite some time where to put the bias trim pot (I assume I'd want them, correct?) on my 6g3 build and which value to use.

Looking at the layout diagram http://ampedia.redbeartrading.com/da..._6g3_schem.gif, I am guessing the trimpot would replace the 100k resistor right after the HV Secondary on the PT and right before the diode. Is this correct?

The value would be something like 120k, right, so that I'd have some room for adjustment? Or would I be better off with say a 68k resistor + a ~50k trimpot (so that I couldn't go too low or too high)?

I am really trying to figure this out, but I think I just need some help. I have looked at modern schematics to find where they put the trimpots, and haven't had any success.

[scole]

try this
http://www.el34world.com/charts/Biascircuits.htm

[tubeswell]

Hi cminor9

In the 6G3, the bias is set by the 22k resistor-to-ground (which is in parallel with the 25uF cap) on the bias supply rail (after the reverse-biased diode).

When you decrease the value of this resistor the bias will be hotter, and vice-versa.

So I'd put a 10k-15k resistor in series with the ground side of a 20k trimpot wired as a variable resistor.

Wire the wiper of the trimpot to the input side of the pot (as per the suggestion on the Hoffman site) to protect the whole she-bang from going into meltdown if your wiper contact fails).

[cminor9]

Thanks for the responses. I don't know why I didn't go to the hoffman site!

Tubeswell, did you put a bias pot in the 5g9 you built? I read on the hoffman site that Fender didn't put a bias pot on some of the amps with bias-vary trem. like the 5g9 and the 6g3.

I wonder if it's necessary. Thoughts?

[tubeswell]

Tubeswell, did you put a bias pot in the 5g9 you built?

Yep I put one on the 5G9 because I wanted to be able to adjust it. Similar sorta thing as the 6G3 setup (except that in the 5G9, its a 56k resistor to ground and I used a 39k resistor in series with a 50k pot. I set the whole thing for 56k when I first installed it, but that resulted in the tubes idling at 31.5mA! - Even tho' that was tube current - and you need to knock off a couple of mA for each screen - which would make the plate current 29.5 - I decided to back it off a bit to 28mA tube current, which still sounds good).

I understand that the way Fender built the original tweeds (i.e.; with a fixed bias resistor in that position), was the old-fashioned way of setting bias - by trying to rely a particular -ve grid voltage value (instead of more accurately setting it according to the tube or plate current). Given that most of the tubes I can get are modern manufactured ones, I didn't want to risk it.

Don't forget to put a 1% cathode resistor on each cathode so that you can measure the mV and hence determine the tube current (by dividing the mV by the resistor value). (I use a 0.6W 1% 1R resistor at each cathode for this purpose. Some guys use 10R).