Fender Rivera 80's Concert Bias Question

Riley Hart · From: South Carolina, USA

Hello all...I've got a weird one here. Can't seem to obtain enuf bias voltage to get my plate current under control. My supply seems to top out at about 56 volts, and changing the 33k resistor on the pot doesn't seem to help to any degree, except to lower it. My screen resistors are good, and the bias is making it to both tubes, but at 500 volts on the plates, they are drawing in the 40ma range. What's weird is the plates are not getting red. Also prior experience(which I no longer count on being accurate) tells me this is pretty much enuf, and things should be okay.

Also, i might add that I installed some 1 ohm resistors on the cathodes of the power tubes to make it easier to read the current. I checked the current via the transformer shunt method, and my readings are the same basically as read across the resistor, so I am discounting any resistor discrepancies.

I notice there is a zener diode(6.2 volts) connected across the bias supply cap, and I can read that same amount across it. What is the purpose of this diode, and can I get more voltage out of the bias supply by changing this?

Also, the supply is running through a balance pot, not a voltage adjust pot if this matters.

Thanks and hope someone can point me somewhere. I may be able to obtain some more info tomro, but my whole weekend is busy and can't hardly work anymore until Monday, so take your time folks lol...

Sorry abt the quality of my schematic...a better quality one is here;

http://www.stratopastor.org.uk/strato/amps/twoseriesfenders/concertschematicwiringdiagram.pdf

Tim Marcus · From: San Francisco, CA

I would rip out the balance circuit and rewire the whole thing to blackface fender spec for the bias adjustment control. You will see a lot more range that way. It does not require many additional parts - just a few resistors to get the voltages right
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Jerry Kippola · From: UP Michigan, USA

Don't mess w/the zener, it sets the v for the switching. 40 ma ain't bad, but u might change the 3.9k to 1k, see if it gets u something,

Riley Hart · From: South Carolina, USA

Thanks everybody who has come in so far...

Changing the amp to blackface may not be all that difficult, if I can live with the bias voltage pot instead of the balance. The resistors in the PI are the same(220k's and 82 and 100 k i think).

Had my twin setup with the older PI and liked it .

Thanks very much for the tips abt not messing with the zener. I don't need to further complicate things lol...

I have a set of matched output tubes on order, and hopefully they will not draw as much current as my old 6l6gc's or my 6l6wgb's.

I guess i'll post back after I try the new dropping resistor in place of that cheesy little half-watt 3.9k. I gotta help baby sit tomoro night, so will have lots of time to post!

Riley Hart · From: South Carolina, USA

Thanks everybody who has come in so far...

Changing the amp to blackface may not be all that difficult, if I can live with the bias voltage pot instead of the balance. The resistors in the PI are the same(220k's and 82 and 100 k i think).

Had my twin setup with the older PI and liked it .

Thanks very much for the tips abt not messing with the zener. I don't need to further complicate things lol...

I have a set of matched output tubes on order, and hopefully they will not draw as much current as my old 6l6gc's or my 6l6wgb's.

I guess i'll post back after I try the new dropping resistor in place of that cheesy little half-watt 3.9k. I gotta help baby sit tomoro night, so will have lots of time to post!

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