Bias Mod Update

[mansfieldguitars]

I just thought everyone here would like to know I took the time to contact Mesa Boogie regarding their bias supply. The tech I spoke with informed me that these amps definatly do run hot and reccomend their coolest running tubes (red). The method they use for cooling the bias down is the same method that Bob posted with only one differenceand that is to change the 120k which comes directly after the diode to a smaller value like a 100k or so. Although this will affect the 2.3 volts on the phase inverter they claim it is not an issue. I would suppose the 15k could be altered to bring the phase inverter back to stock voltages but again they felt it is not necessary.

Another suggestion was to possible string some zener diodes in series on the high voltage wires to knock the 400 volts on the plates down some. This is a contributing factor to the reason these amps run so hot. This would involve more work and adjusting the bias supply is a better option but I just wanted to post all the options Mesa Boogie suggested.

To anyone interested you can source the parts yourself or I have modified my kits and instructions to work for anyone still interested in the kits.

Thanks
John

 

[BobR]

I just thought everyone here would like to know I took the time to contact Mesa Boogie regarding their bias supply. The tech I spoke with informed me that these amps definatly do run hot and reccomend their coolest running tubes (red). The method they use for cooling the bias down is the same method that Bob posted with only one differenceand that is to change the 120k which comes directly after the diode to a smaller value like a 100k or so. Although this will affect the 2.3 volts on the phase inverter they claim it is not an issue. I would suppose the 15k could be altered to bring the phase inverter back to stock voltages but again they felt it is not necessary.

Another suggestion was to possible string some zener diodes in series on the high voltage wires to knock the 400 volts on the plates down some. This is a contributing factor to the reason these amps run so hot. This would involve more work and adjusting the bias supply is a better option but I just wanted to post all the options Mesa Boogie suggested.

To anyone interested you can source the parts yourself or I have modified my kits and instructions to work for anyone still interested in the kits.

Thanks
John

Actually I came up with the same fix mesa suggested! The 120K with a 470K I piggybacked on it effectively becomes a 100k resistor! The change in the PI I too estimated is is neglegible at .3 or less vdc. Can I get a job there? lol!
A 50k in the 33k resistor I used isnt very effective but it does have some range -limited though.

The 'quick fix" is simply piggybacking a 470k on the 120k.
John I imagine a 76k with a 50k pot would would well in place of the 120k. I might br interested in something like that as it would give more range. Bob

 

[mansfieldguitars]

Bob,

I have 100k trimpots that fit the board as well. Since you bought the kit I can send you out a 100k trimpot and a 22k resistor free of charge. This way you can swap it out and put the 33k back in and use your kit in place of the 120k.

Just let me know what you want to do.

Thanks
John

 

[BobR]

Bob,

I have 100k trimpots that fit the board as well. Since you bought the kit I can send you out a 100k trimpot and a 22k resistor free of charge. This way you can swap it out and put the 33k back in and use your kit in place of the 120k.

Just let me know what you want to do.

Thanks
John

Thats very gratious of you! I'll accept that offer! I guess the 100K is better suited to the position. Thanks Bob

 

[mansfieldguitars]

Bob,

I decided to just send out a new kit already done. This way all you have to do is install and not worry about swapping components. It's my way of saying thanks for your willingness to experiment. As per our phone conversation you were aware that the kit may or may not work without some modifiactions and you took a chance anyway. I feel it is my obligation to provide a working kit now that the issue is cleared up. With sooo many different model amps it's almost impossible for me to get my hands on everyone to experiment and greatly appreciate your help in this. Hopefully between us we have been able to clear up an issue that seems to have plagued many boogie owners. Now they can correct thier amp either on their own or with my kits. This is an interesting bias circuit that was also used on a few other Mesa amps and this mod also applies to those models as well.

Thanks again Bob and your kit will ship tomorrow

Thanks
John