Mesa Rectoverb Adjustable Bias Mod Question

[IzzyB]

I'd like to mod my Rectoverb from fixed to adjustable bias. Mesa won't provide the schematic for amps in production but I did find a posting for the Solo 50/Recto Series I on free society:
http://www.freeinfosociety.com/elect...page.php?cat=1

I know they change board layouts all the time in amps. My rectoverb circuit board is not exactly user friendly. I believe I found the resistor in the bias supply. Its a 22K resister (red red brown orange) at the bottom of the board under the right corner of the 30uf filter cap. The schematic for the bias supply shows three resistors in series: 68K, 22K and 33K but my circuit board only has the 22k resistor in place. The 22K resistor leads to a 1k resistor then goes across the bottom of the board to a white 220 ohm resistor before it goes on to a series of rectofiers and gets lost.

Questions:
1) is that 22K resistor the right one to mod?
2) what do you think of a 15k resistor in series with a 15K pot?
3) Or maybe a higher value pot?

Thanks for the help.

[stokes]

http://www.tubefreak.com/mk2pwr.gif Check this link.It is a MKII power supply that shows the bias supply.In my experience most Mesa amps I've seen are similar.This one shows a bias pot,which I have never seen in any Mesa I've worked on,but that shows how you would put a pot in there.Just replace the fixed resistor that is in your amp in that position.If you are replacing a 22k resistor,I would use a 50k pot.I dont use the fixed resistor in series as you describe,just make sure the pot is set in the middle before you install it.I like to use multi-turn cermet pots,they stand up to the dc voltage better than a standard volume type pot.The multi-turn feature makes it easier to dial in.Always use a pot of twice the value of the resistor you are replacing,that way you have a fairly wide latitude to dial in for different tubes.

[sixstring63]

http://www.tubefreak.com/mk2pwr.gif Check this link.It is a MKII power supply that shows the bias supply.In my experience most Mesa amps I've seen are similar.This one shows a bias pot,which I have never seen in any Mesa I've worked on,but that shows how you would put a pot in there.Just replace the fixed resistor that is in your amp in that position.If you are replacing a 22k resistor,I would use a 50k pot.I dont use the fixed resistor in series as you describe,just make sure the pot is set in the middle before you install it.I like to use multi-turn cermet pots,they stand up to the dc voltage better than a standard volume type pot.The multi-turn feature makes it easier to dial in.Always use a pot of twice the value of the resistor you are replacing,that way you have a fairly wide latitude to dial in for different tubes.

I'm sorry but I do not see the bias pot you described in that shematic I only see the presence pot and the effects send level pot. If you could point it out I'd like to know because I want to put a bias pot on my old Mark 3.

[stokes]

Look at the bias supply.There is a 10k pot in parallel with a 10k fixed resistor.You've found 2 out of the only 3 pots in that schem.If you cant find it,maybe it would be better to bring it to someone who can.If you arent familiar with working on amps,they can be dangerous and you could get yourself hurt or worse.Those PCB's are kind of fragile,too much heat and you could burn or lift a trace.

[Pedro Vecino]

The Rect-overb does not have adjustable pot in the bias circuit. Here I put the scheme of both versions:

I never use adjustable circuits in M_esa. Simply I adapt the value by means of a resistor in parallel to raise the bias regime, but you can withdraw this resistor and replace it with a line in series of a 10K resistor and 22K adjustable pot, for example :



Regards

[stokes]

Pedro,it has already been established that there is no pot in a stock Mesa,although that first schem shows one.Not a good idea to add and remove resistors each time you want to re-bias.Better to add the pot and be done with it.Those PCB's can be fragile and damaged from to much heat.

[sixstring63]

Look at the bias supply.There is a 10k pot in parallel with a 10k fixed resistor.You've found 2 out of the only 3 pots in that schem.If you cant find it,maybe it would be better to bring it to someone who can.If you arent familiar with working on amps,they can be dangerous and you could get yourself hurt or worse.Those PCB's are kind of fragile,too much heat and you could burn or lift a trace.

Duh !! saw it first thing on the second look. Kinda tired when I was looking at it the other night. Thanks for the warning on getting bit. I'm familiar with working on amps especially this one as I've had ripped apart and recapped it a year ago. Just was curious on putting in an adjustable bias control. Read some ideas from Dan Torres but just looking at different points of view.

[Pedro Vecino]

Pedro,it has already been established that there is no pot in a stock Mesa,although that first schem shows one.Not a good idea to add and remove resistors each time you want to re-bias.Better to add the pot and be done with it.Those PCB's can be fragile and damaged from to much heat.

Sorry, I did not understand it well.
Roadking takes a pot to fit the bias in the EL34 pair but it looks like an exception.
I prefer the fixed adjustment in Mesa and using tubes of the same model and degree (across TAD normally). This avoids somehow his manipulation, and when it takes automated bias adjustments with complex circuits (minor number of tubes, tube rectifiers...) I consider it more suitable still.
It helps to support the reference and to 800 miles the adjustments are kept better
Regards

[stokes]

Pedro,I think you are missing something here.In either case,with a set resistor or an adjustable pot,you still have "fixed" bias.The term fixed does not mean non-adjustable.It refers to a fixed amount of negative voltage applied to the control grid to set the operating parameter of the tube.In either case you are accomplishing the same thing,just making your life easier with the pot,and insuring you dont burn/weaken the trace on the PCB by repeatedly applying heat to it.