Bias Mod Mesa boogie Express+ 5:50 6l6 help? add pics help

[jamme61]

Anyone have info how to do this mod? I have some NOS 6l6's which won't run, with the current bias (run to hot) I have attached a schematic I have for the older non plus version- only one I could find
Thanks for any help on this

[xtian]

This is the circuit you want to mess with. Essentially, you want to increase the amount of resistance between point F and ground in order to get more negative bias voltage. For example, replace R54 with a 50K pot. Start off with the pot dialed to 50K resistance for the greatest negative bias, and dial it warmer from there.

What method are you using to measure tube current?

[jamme61]

Thanks! I'm using a weber bias rite with v1 option to measure plate volts. Thanks for the help on this

[jamme61]

hey just looking at the schematic and noticed a 39k cathode resistor on V2A just curious what that does with the 5k6 resistor? is that switched in and out? thanks again I know I'm off subject sorry thanks

[tele_player]

hey just looking at the schematic and noticed a 39k cathode resistor on V2A just curious what that does with the 5k6 resistor? is that switched in and out? thanks again I know I'm off subject sorry thanks

Look at J175A. When it's open, the cathode sees 39k to ground, when it's closed, cathode sees 4.9k to ground.

Robert

[roberto]

replace R54 with a 50K pot.

I'd suggest 10k plus 25k in series for two reasons: the first one is that being a newbie (don't be offended), with a 50k he can simply fry the tubes setting the resistance at zero; the second one is that with the 25k pot he can be more accurate in regulating the bias.

What are the values of B+ and bias current you have now?

[jamme61]

hey just looking at the schematic and noticed a 39k cathode resistor on V2A just curious what that does with the 5k6 resistor? is that switched in and out? thanks again I know I'm off subject sorry thanks

Look at J175A. When it's open, the cathode sees 39k to ground, when it's closed, cathode sees 4.9k to ground.

Robert

thank you that is so cool to know this stuff I wasn't sure what that j175a was - now I know thanks so much.

[jamme61]

replace R54 with a 50K pot.

I'd suggest 10k plus 25k in series for two reasons: the first one is that being a newbie (don't be offended), with a 50k he can simply fry the tubes setting the resistance at zero; the second one is that with the 25k pot he can be more accurate in regulating the bias.

What are the values of B+ and bias current you have now?

I'm not sure what the B+ was last time I checked maybe around 450 volts? not sure I think the 6l6's in there are running around 30-32 ma I want to put some NOS 6l6wgb's -The short ones philips. but I think they were were runing around 45 ma so I thought that was to hot. I'll check out this bias mod as soon as I get a pot.you guys are the best I appreciate all your help thanks much.

[jamme61]

can't find a R54 tried just clipping a 50k pot (pot adjusted to 50K) in parallel on r55 and i can only make the tubes run hotter which is not what I want. tubes are running at 40ma now do I need to take out r55 and just go with the pot by itself? or go to a higher value pot? attaching pics thanks for the help on this.

[xtian]

can't find a R54 tried just clipping a 50k pot (pot adjusted to 50K) in parallel on r55 and i can only make the tubes run hotter

Right. Because you need to increase the resistance, not decrease it.

[jamme61]

right so should I take out r55 and just go with the pot? they're in parallel so the pot by itself would give me a lot resistance right? don't understand where r54 is ? I can measure the bias in standby mode right now its at -48 volts so as long as I increase that number im good right? don't want to fry anything thanks sorry I'm such a hack at these things

[xtian]

You've got the right idea. Just note, as Roberto mentioned, that if you replace the resistor with the pot, it will enable you to dial the bias far closer to zero volts than you want to go, making it possible for you to redplate your tubes very easily.

[jamme61]

I hear you thanks- I just took out the 22k resistor replaced with a bourn 50k pot- adjusted the pot to about 35k which = -52 bias vs stk -48 dropped my tubes from 40ma to 30ma success! now I can try the nos tubes I have. thanks for all your help.

[dano-rator]

...don't understand where r54 is ? ...

It's in the 5:25 only , that's what the single asterisk means on the schem.

Glad you got it right!
Dan

[jamme61]

thanks learning a lot- thinking of messing with that 39k cathode resistor next