Vibrosonic Bias Question

TRAP TRULY · Post by **TRAP TRULY** » 4 Dec 2011 5:55 pm

This amp is a '72 MV with no pull volume boost. I'm sure it's a bias balance. Each tube is balanced at or around 18ma with the plate voltage running around 445v.Is this amp able to be converted to a blackface adjustable bias instead of the balance?
Any help would be greatly appreciated.
Thanks,
Trap

Tim Marcus · Post by **Tim Marcus** » 4 Dec 2011 6:05 pm

yup - thats definitely bias balance. Also, the phase inverter is the weird silver face kind with the 330k's

that probably also means that the power supply is different too

usually blackfacing an amp involves 3-4 steps:

cutting out the ceramic caps that are not on the blackface design
rewiring the bias supply
replacing resistors in the power supply
changing the phase inverter

possibly addressing lead dress if its unstable

That amp is nice and clean on the inside - but it does appear that some work has been done. Those 470's on the octal sockets are not CC's!

Every Fender amp (excluding the reissues I suppose) can be rewired back to Blackface spec. I HIGHLY recommend doing it too - the amp will really come alive!

Jason Hull · Post by **Jason Hull** » 5 Dec 2011 3:03 am

TRAP TRULY · Post by **TRAP TRULY** » 5 Dec 2011 6:03 am

Does anyone have a schematic or reference of how to do the bias conversion for this particular '72 circuit? I can't find one with the master volume not being a pull boost. What circuit is this?
Thanks so much for your replies.
Trap

Jason Hull · Post by **Jason Hull** » 5 Dec 2011 5:39 pm

TRAP TRULY · Post by **TRAP TRULY** » 5 Dec 2011 6:00 pm

Thanks Jason and Tim. I'm going to pick up that book and one of Gerald Weber's soon. I can't find a schematic of this circuit anywhere. Was the '72 a one off? Did Fender just make the non pull boost master volume that one year? I don't know what other circuits to search for. I have a bunch of fender schematics I have found, but none like this one.

Tim Marcus · Post by **Tim Marcus** » 5 Dec 2011 7:13 pm

Jason Hull wrote:Also, that looks like a bad solder joint on the bias pot, where the resistor is grounded.

not necessarily, thats just how the solder sticks to some of the CTS pots. Its been working for 40 years so I think its ok

@ Trap - as long as you take it circuit by circuit you can figure out how to blackface it using any BF era schematic and layout. They are all very similar. Maybe use a Twin Reverb as a reference - it has the same bias and phase inverter

Jason Hull · Post by **Jason Hull** » 6 Dec 2011 3:33 am

Larry Robinson · Post by **Larry Robinson** » 6 Dec 2011 6:16 am

If the static current is 18ma then something is definitely wrong. Static current should be around 35-38 ma for each tube. Are you sure you measured the values correctly? I personally would not hack this amp. Just make sure the components and voltages are in tolerance. I make my recommendations based on my 40+ years servicing these amps. However, there are those who disagree which is OK.

J Fletcher · Post by **J Fletcher** » 6 Dec 2011 9:04 am

You should be able to turn up schematics and layouts for similar Twin Revs on the net. The layouts are easier to follow than schematics, they look like the inside of the amp. To change your amp to the non-balance circuit is a pretty staight forward job for an experienced tech.
My advice would be to skip buying the books you mentioned, all that info contained within them is free on the web.

Murnel Babineaux · Post by **Murnel Babineaux** » 6 Dec 2011 9:44 am

Set your bias at -52 volts and forget about it !!!!

Rich Hlaves · Post by **Rich Hlaves** » 6 Dec 2011 2:13 pm

I have a set of step by step instructions with illustrations to convert a SF Twin bias balance to fix adjustable bias. The blackface set up, step by step, AB763.

The bias current you quoted in your post Trap is low for the plate voltage you mentioned. This amp is quite cold if your numbers are good. It needs a bias resistor at present so best to blackface that section and be done with it.

Email me through the forum so I have an email address for you and I'll send you the scans.

Best,
Rich

TRAP TRULY · Post by **TRAP TRULY** » 6 Dec 2011 3:09 pm

Thanks guys. I thought it was too low according to what I have read.

Rich, I'd love the instructions.I'll email you and send you my email address.
Thanks so much!
Trap

Tim Marcus · Post by **Tim Marcus** » 6 Dec 2011 5:53 pm

Another option is to convert the amp to cathode bias. Then you really can set it and forget it. And switch tubes without biasing! (within reason)