Peavey Utra Plus bias question

[redneckgeek]

I have read about a bias mod for the Utra Plus Peavey amp. Were any of these amps produced with adjustable bias? Where might I find specs for the bias mod?

[Enzo]

These amps did not have any adjustment.

Modification of most any amp to make bias adjustable is done in similar manner. ANy competent amp tech can figure out what a particular model needs. I don;t know if anyone has gone so far as to produce a "put this here, move that there" sort of descritpion for this model.

[Enzo]

These amps did not have any adjustment.

Modification of most any amp to make bias adjustable is done in similar manner. ANy competent amp tech can figure out what a particular model needs. I don;t know if anyone has gone so far as to produce a "put this here, move that there" sort of descritpion for this model.

[tedmich]

Double double post post Enzo Enzo!

The JSX bias circuit with a switch for EL34/6L6GC can be fitted to the Ultra amps pretty easily, and fits on a small board. The SGRs should also be beefed up to about 750-1k ~5w

[Enzo]

Not my fault, since the most recent forum meltdown, my browser (and those of others here as well) has ben doing that here.

The JSX bias circuit has the adjustment as well as the EL/6L switch, but it also has a higher raw voltage for bias and an upper resistance of 5.62k for the bias voltage divider. (Lower leg is 12k-22k variable.) The UltraPLus has only a 470 ohm resistor coming out of the doubler - the raw supply voltage is essentially the same as the bias. Making the lower resistance variable will not make the bias adjustable. Well it would be adjustable over a half a volt range maybe. So that upper resistor must be changed as well as changing the lower to variable

[tedmich]

quite right Enzo, I just bypassed the whole Ultra bias circuit and used the JSX tripler off the C59/C64 node, which is the same AC voltage as the JSX, then inject the bias at the R73/R72 voltage divider
something like this:

Test points and SGRs need to be modified too, and the ground scheme has to be tested out as the JSX has a few more ground planes.

[Gtr_tech]

The bias circuit in the Ultra series sucks pretty bad. Even after making it adjustable there's only so much voltage available, so you'll play hell trying to get a set of 6L's to bias into correct range. I got tired of this with my Ultra 120 and converted it to EL34s. Problem solved. I'm sure a voltage doubler circuit could be added, but shame on PV for not making more voltage available.

[tedmich]

Agreed G_T! minor quibble: Ultra has voltage doubler, JSX has tripler which can provide needed higher voltage

[Gtr_tech]

K. Didn't remember what game they were play'n....been a while since I had to look at the schemo.