I don't wanna be forced to buy the overpriced Mesa tubes, also do I want to be more flexible with my bias settings, so I'm usually installing bias pots in my Rectifiers and 5150s/6505s to make the bias adjustable.
it's a very easy mod to do (for the 6505 it's even quite a bit easier than for the recto, cause you have more space in the 6505).
anyways, what you wanna do is remove the 22k resistor in the bias section (it's the one in the middle, next to the two caps, number R255)
in its place you have to install a combination of R and a mini pot, technically you don't need that R, but it keeps you from fucking up the amp by accidentally turning the pot to zero.
for that "safety-R" I chose a 10kΩ in this case, but anything between 5k and 20k will do really.
the trimpot I used in this case was a 47k (usually I prefer smaller ones, like 25k or so, but the 47k was the only one I had, so I used it, it makes a wider range (lower idle currents possible), but it's also a bit more difficult to fine-adjust of course).
the higher the combined resistance of the R and the pot, the lower the idle current - so with a 10K R (which would then also be the lowest possible combined value) the tubes will run quite a bit hotter than with the stock 22k resistor. the combined maximum resistance in my case now is 57KΩ (35KΩ for a 25K pot) enabling you to dial in colder settings.
After you removed the stock 22k R you wanna solder two wires into the holes, one end to the safety-R and from there to one of the outer legs of the pot, the other wire to the other outer leg AND the middle leg (wiper).
I like to run my rectos at 50% plate dissipation, which is on the cold side (32mA for Ua=460V).
now you'll have to find a place for the pot inside the amp, usually I'm hot-glueing it on top of the bigger caps, in this case I wanted the bias to be adjustable from outside the amp, but I did of course not want a pot shaft to stick out.
What I did is use a mini pot, drilled a little 2-3mm hole into the chassis between the tubes and superglued the pot onto the inside of the chassis over that hole.
here are some pics (the 10KR is a bit difficult to spot cause it's under the white electric tape, very close to the board)
and the pot from the outside:
if anyone is interested in the bias mod for the 5150/6505 I'd do that soon
it's a very easy mod to do (for the 6505 it's even quite a bit easier than for the recto, cause you have more space in the 6505).
anyways, what you wanna do is remove the 22k resistor in the bias section (it's the one in the middle, next to the two caps, number R255)
in its place you have to install a combination of R and a mini pot, technically you don't need that R, but it keeps you from fucking up the amp by accidentally turning the pot to zero.
for that "safety-R" I chose a 10kΩ in this case, but anything between 5k and 20k will do really.
the trimpot I used in this case was a 47k (usually I prefer smaller ones, like 25k or so, but the 47k was the only one I had, so I used it, it makes a wider range (lower idle currents possible), but it's also a bit more difficult to fine-adjust of course).
the higher the combined resistance of the R and the pot, the lower the idle current - so with a 10K R (which would then also be the lowest possible combined value) the tubes will run quite a bit hotter than with the stock 22k resistor. the combined maximum resistance in my case now is 57KΩ (35KΩ for a 25K pot) enabling you to dial in colder settings.
After you removed the stock 22k R you wanna solder two wires into the holes, one end to the safety-R and from there to one of the outer legs of the pot, the other wire to the other outer leg AND the middle leg (wiper).
I like to run my rectos at 50% plate dissipation, which is on the cold side (32mA for Ua=460V).
now you'll have to find a place for the pot inside the amp, usually I'm hot-glueing it on top of the bigger caps, in this case I wanted the bias to be adjustable from outside the amp, but I did of course not want a pot shaft to stick out.
What I did is use a mini pot, drilled a little 2-3mm hole into the chassis between the tubes and superglued the pot onto the inside of the chassis over that hole.
here are some pics (the 10KR is a bit difficult to spot cause it's under the white electric tape, very close to the board)
and the pot from the outside:
if anyone is interested in the bias mod for the 5150/6505 I'd do that soon
