Mesa Simul 295 bias issue

[Wes]

I've got one of these I'm looking at and the problem I'm having is that the bias voltage
seems too low. It basically has two amps in one that share the same bias supply and
each amp has a pair of 6L6s and a pair of EL34s. The bias voltage is -70 and checking
just the EL34s shows that they're idling at about 15mA with 500V on the plates.

The schematics can be found here:

http://music-electronics-forum.com/t25389/

They aren't entirely accurate to what I've got in front of me, but close enough.

Does this seem strange? If it's not right, what are your thoughts on what could be
causing this?

[Alex R]

On the first 290 schemo there are various bias voltages given at various points. Two of the 6L6 seem to want -67v on the grids. If those are the points you're looking at then -70v is fine.

15mA per tube is going to sound and work ok, though obviously it could be higher.

You don't say what problems you're having with the amp though. if it's just the fact that a measured voltage looks wrong whilst the amp sounds ok then I'd just assume it is in fact ok!

[Wes]

Sorry, the problem is that there is no sound coming from the amp. -70 on the EL34s as well.

[Jazz P Bass]

Do you have a test setup where you can send a signal into the amp.
If so, see if the signal is reaching the grids.

[Wes]

I have a signal generator, but no scope. Would my meter be able to pick it up?

This is my meter:



Click to enlarge.

It has a setting for frequency, but I've never used it. Should I just buy a scope?
I'm gonna need one eventually...

[Jazz P Bass]

Your DVM will do just fine.
Set it to read volts/ac.
With the controls dimed & a 100 mv signal in, you should see a really healthy Vac signal at the grids.
Use a dummy load on the speaker connections to save your hearing.(and the output transformer)

[Wes]

Cool, sounds good. Now to build a dummy load!

[Wes]

Well, I got back around to working on this amp a couple of weeks ago and I noticed that
there were abnormal voltages on the grids of the 12AX7s. Come to find out- the coupling
caps had shorted. What are the odds that both went bad between V1 and V2 on one
side of this amp and one went bad on the otherside (B amp)?

Replaced them and all is working well.