Mesa studio 22 bias issue

[phsyconoodler]

Hi there,
   I've got a Mesa Studio 22 combo with a strange bias issue.
   I replaced filter caps and bias cap and did the mods to improve footswitch function and while checking voltages before buttoning it up I have weird bias voltage.
  It measures -122vdc. I did not disturb any other components. It should be - 15 or so for a pair of EL84s. No overheating of any components while doing the job but this strange bias voltage shows up. All caps oriented correctly. All grounds good.
   Other voltages are fine with no tubes.
This amp does not have the EQ sliders on it.
  It's not that complicated but where is that huge negative number coming from?
  Any possible insight would be appreciated

[Latole]

What can I say other than revise your work because if everything was correct before, you must have made a mistake.

 Check the entire bias circuit from point D to the transformer.

[sluckey]

...I have weird bias voltage. It measures -122vdc.

Where did you have the meter probe connected. If you really have -122v on pin 2 of the EL84s they will likely be biased to hard cutoff and there will be no sound. The bias cap will likely be damaged too.

BTW, I believe there is an error in the bias circuit on the schematic drawing.

[PRR]

The bias circuit depends on the load of the transistor EQ to drop its voltage. Other wise the unloaded voltage want to rise to inverse of B+. If you have changed that, or found a factory variation, it needs re-consideration.

[phsyconoodler]

This doesn't have the transistor for the eq slider system on this amp. There are no eq sliders.
   Yes I do believe the schematic provided isn't correct for the model but there really isn't any way I could have damaged the bias supply because the components I changed are nowhere near the bias supply. I tested the bias supply cap I replaced and subbed in the old one tontest. No change.
   So strange??? Guess I'll change all the bias supply components and see if that does anything.

[PRR]

Did it ever work or is this a new build?

What components did you change?

[phsyconoodler]

It did work. I am kind of mystified at that crazy nefmgstive number. I'm going to replace everything in that bias circuit in the hope that something is amiss.

[Latole]

No need to replace if every parts are good , check your work

[phsyconoodler]

I've checked my work. My soldering and desoldering doesn't heat any component of the bias supply except the 220uf/63v cap. I tested tye old and the new one with my capacitance meter. Both test good.
   I'm going to change the diode today but that's grasping I'm sure. I may have to get the proper schematic for this amp with no eq sliders cause the only one I have is for the parametric eq

[JB]

There are two schematics out there. One for the Studio 22 and other for the + version that has the Lead Master. From memory the + schematic is easier to read. Mesa have or had a reputation for inaccurate schematics, deliberate or otherwise.

[phsyconoodler]

I did find the + version. I'm going to follow this bias supply routing and see what I can come up with.
  It's a pain in the butt. Lol.

[Latole]

   I'm going to change the diode today but that's grasping I'm sure.

Diode is easy to test with a voltmeter, no need to change it if it is good

[phsyconoodler]

I did check it with my diode setting. Tests good. The bias voltage is derived with voltage dividers from resistor placement and cap orientation cancels the ac noise.  I'm assuming I'm correct and it has to be one of the resistors in the negative supply that's giving me this wonky reading.
  I'm going to try another nlmeter first and see if it's the same. I'm not giving up. Lol

[phsyconoodler]

No luck at all with this silly thing. Guess I'll call Mesa boogie and see if they will offer some help. Maybe an accurate schematic at least

[phsyconoodler]

Well I figured it out! I looked it over every spare moment I had a deduced finally that the bias supply can't work unless the phase inverter tube is installed. I just tried it and low and behold it worked!
  It has this tap that goes to the Pi Tube that absolutely has to complete the circuit through tye tube before the correct negative voltage happens.
  I've never seen this before but I'll sure never forget it. I went over my work and concluded I didn't make an error so I looked over that schematic( really poor drawing at that) and thought it HAS to be that.
  I was right. Just thought I'd share that.
Cheers

[PRR]

> can't work unless the phase inverter tube is installed.

Well spotted. I agree. Even re-drawn it is hard to see.

[nandrewjackson]

That's silly. Thank you for the update.