Biasing on Crate VC120H

[Kindly Killer]

Got this 100W American-made head as a tack-on in a trade LOL.

The dirty channel sounds okay for a 100w Marshall type sound, but the manual says it will run on 6l6's too and I am curious to try that.

http://www.crateamps.com/pdf/manuals/VC120H.pdf

I don't understand how to use the test point or what I'm measuring. With 4xel34 in it as it came, the range of the adjustment pot is about 42v to about 60v (I have my DMM set to DCV). It was nearer to 42v as it came.

I want to stress that it is reading Volts NOT Millivolts. Even if I set to manual range or use another meter, I get those same numbers.

I want to try 2x6l6 in the middle two spots (16ohm speaker with 8ohm selected on amp) and see what the amp can sound like that way.

What number am I looking for to bias tubes on this amp? What steps should I take?

[NickC]

...... What steps should I take?

Take it to a qualified amp tech.

The innards of a typical tube amp, like yours, can kill you. Is there intentional irony that this is your first post and you chose the handle "Kindly Killer"?

Seriously, messing about with tube amps ...... even for experienced techs ...... is deadly dangerous. Incidentally, the manual you posted is of no practical value in actually setting bias on that amp. However, it does provide some EXCELLENT counsel ..... Page 5, Item #23 ...... Bias Adjust / Bias Test Point For qualified service personnel only All others please just ignore these internal Bias adjustment potentiometers.

[Kindly Killer]

Did a little reading and I think I figured out what the "test point" is actually showing - it's just the bias voltage that sets the current draw. I still need a bias probe to see what's actually going on. Is that right?

If you look at the illustrations in the linked manual, it really gives the impression that this is a convenient test point that you can use with just a DMM to swap tubes, like on my Hot Rod Deville, for which no special tools are needed....

Hmm...

Any info is appreciated.

re safety: I turn off power with the standby still in "play" position. If I have any doubts I meter the caps. If I still have any doubts I discharge with pin 1 of V1 for 1 minute.

A tech is not needed to bias an amp. I know how to do it on my HRD, just not on this amp.

[NickC]

Did a little reading and I think I figured out what the "test point" is actually showing - it's just the bias voltage that sets the current draw. I still need a bias probe to see what's actually going on. Is that right?

If you look at the illustrations in the linked manual, it really gives the impression that this is a convenient test point that you can use with just a DMM to swap tubes, like on my Hot Rod Deville, for which no special tools are needed....

Hmm...

Any info is appreciated.

re safety: I turn off power with the standby still in "play" position. If I have any doubts I meter the caps. If I still have any doubts I discharge with pin 1 of V1 for 1 minute.

A tech is not needed to bias an amp. I know how to do it on my HRD, just not on this amp.

Okay, no offense intended. It was your first post here, and I have no idea your experience, credentials, abilities.

You know about discharging the e-caps. Good! Those guys can store enough charge to kill you, a long time after the amp has been disconnected from wall power.

Hot Rod Deluxe has a pair of output tubes, yes? The Crate has four.

Here is a link to an online Bias Calculator:

http://www.webervst.com/tubes1/calcbias.htm

Choose Class AB (push-pull), tube type, feed it your plate voltage (taken with DMM at test point presumably), and it will give you the mA Bias setting you want based on 70% plate dissipation.

I don't know the amp, never seen a schematic, and don't know the configuration of the test point(s) or the Bias adjust pot(s). Maybe someone here can advise. That said, I'm guessing there are two bias adjust pots ...... one for each pair of output tubes.

A Bias probe would be a handy tool to have for this if you can justify the expense. I use the Weber Bias Rite .... works fine.