Bias adjustment in a 72' vibrosonic- how to use?

Steve Waltz · Post by **Steve Waltz** » 5 Aug 2005 8:24 am

I just bought a vibrosonic from a guy in LA. It was shipped and I'm nosey so I took the head out to look for any signs of burnt parts, lose conections and all that. Everything looks perfect. I noticed what I think is a Bias balance pot. I guess you use a flat screwdriver from under the amp to adjust.

How should this be used? There is a whole new quad of JJ power tubes in it that seem to work fine. I was just curious.

The inside of my head ( I'm using the word head because I don't know what to call it when it is part of a combo) was stamped with a copule of ink stamps of R.espinosa and CAbarillo something as well as a date of July 24th 1972. I thought these amps were always a mistery for dating, do many amps have these stamps?

Steve

Ken Fox · Post by **Ken Fox** » 5 Aug 2005 9:20 am

Adjust for minimum hum. This balances the voltage, not the currrent! I prefer to use a bias meter and balance for idle current. Amp hums more but it is a true tube current balance. Some of the later 70's had another balance pot on the back as well for the heaters. It balanced the 6.3VAC heater voltage to ground via that pot. Handy, but often the pot is blown. If you have one and it does nothing, that is likely the case. They usually blow when there is an old tube that gets a heater shorted to other tube elements. Typically a lot of smoke and carbon are realeased and you will see a lot of black on the chassis by the pot. I replace the pot with two 100 ohm resistors, like they did it on the earlier amps that did not have a transformer center tap for the 6.3 VAC heater supply.

Jay Fagerlie · Post by **Jay Fagerlie** » 5 Aug 2005 1:27 pm

Um...Ken, isn't that for adjusting a hum balance pot? (for lowest hum)
I think he might have a bias balance pot. If so, adjust so both sets of tubes draw equal current.

If I'm totally off here, please disregard and continue with normal Forum activities

Jay

[This message was edited by Jay Fagerlie on 05 August 2005 at 02:28 PM.]

Steve Waltz · Post by **Steve Waltz** » 5 Aug 2005 1:53 pm

I should have been more clear. The pot that I mentioned is under the amp back behind the output tubes inbetween the transformers. I think the hum balance is usually on the back of the amp where the sockets for the foot pedal are.

I think the quad of JJ's are mathced, and if they are, is a bias balance needed?

Steve Waltz · Post by **Steve Waltz** » 5 Aug 2005 1:54 pm

I should have been more clear. The pot that I mentioned is under the amp back behind the output tubes inbetween the transformers. I think the hum balance is usually on the back of the amp where the sockets for the foot pedal are.

I think the quad of JJ's are matched, and if they are, is a bias balance needed?

Bill Hatcher · Post by **Bill Hatcher** » 5 Aug 2005 5:07 pm

That IS the bias adjustment pot and yes it does need to be set. You have to have a meter to do it.

Joseph De Feo · Post by **Joseph De Feo** » 6 Aug 2005 4:33 am

Steve...
Some later model Fenders had holes near the
bias adjustment screw. This would be for
+ and - probes from your test meter, others, with just the set screw, you need
a bias meter. It has tube scockets with
leads to the meter. The scockets go in
between the tubes and the scockets on the
amp. If your not sure about this procedure,
take it to an amp tech! It's just a small
fee, but well worth it not to fry your
tubes from a bad bias setting

Ken Fox · Post by **Ken Fox** » 6 Aug 2005 10:19 am

Later Silverfave amp have a bias balance pot underneath, not a bias adjust. When set for minimum hum the voltage is equal not the current. Been there and done it many times and confirmed that with my Bias King dual socket bias tester. The other pot on the back is for minimum hum for tube heaters.

PS: A Vibrasonic never had a bias adjust, only a bias balance.[This message was edited by Ken Fox on 06 August 2005 at 11:20 AM.]

Jim Sliff · Post by **Jim Sliff** » 7 Aug 2005 6:00 am

To best thing to do is change the bias and phase inverter circuit to BF specs. This is the essential part of a blackface conversion, and is easy and cheap to do.

Then to bias the amp all you really need is a Bias King, Bias Rite or another tool that plugs into the tube sockets between the socket and the tube. Some have bui8lt-in meters, some you hook your own meter to.

Caveat - they aren't 100% accurate unless you've checked your plate voltage anyway. The Bias Rite from WeberVST has an option that does that, making it the most versatile unit.

When changing to a matched set of tubes, yes, you need to rebias. the tubes are matched to each other, not the amp. Alwayys check the bias whne replacing power tubes and phase inverter (driver) tube - which should always be replaced along with the power tubes.

I'd have tech rewire the bias pot so it actually adjusts the bias and get a Bias Rite. While your tech is inside, he should look at the filter caps and bias cap - if they have never been replaced thhats a must-do job at this point. those caps will be WAY beyond their service life.

Mark Herrick · Post by **Mark Herrick** » 7 Aug 2005 2:02 pm

Why is replacing the PI tube always recommended when replacing power tubes?