Twin Reverb bias mod

Mark Herrick · From: Bakersfield, CA

As you may recall, I have been working on the addition of an adjustment pot to the bias circuit of my ’74 Twin Reverb. I made some clip-on jumpers so I could put the pot in and see how it worked without soldering it all up. I tried a 10K cermet pot and a full sized 10K carbon pot (the only other type they had in 10K at the store) and a 10K resistor.

Here are a couple of pics. The first is a portion of the original schematic. The second is my drawing of the added pot. (Hope I did it right!)

Here are my observations:

Without the adjustment pot (original circuit) the bias voltage can vary from –43VDC to –57VDC, in a reciprocal manner, on either side of the output, but balances at –53VDC.

With the adjustment pot added (modified circuit), and the balance pot set to the balanced position, the bias voltage can vary from –46VDC to –56VDC on both sides of the output simultaneously.

I then tried setting the bias for two different sets of power tubes; Sovtek and Harma. (I’ve been told the Harmas may actually be JJ Teslas). With the Sovteks the maximum current I can get them to draw is 35.5mA; at the upper end of the settings recommended by several sources. But with the Harma tubes the maximum is only 25.9mA; well below the recommended settings.

Here are my questions:

Does the range of bias voltage adjustment on this amp seem correct?

Is it possible to extend the low end of the bias voltage range so I am able to set the current on both sets of power tubes within the 30mA – 35mA range. (And is this an advisable thing to do!)

Does the type of pot I use (cermet, carbon, wirewound, etc.) make a difference?

I should also point out that I noticed a single resistor in the rectifier circuit that appears to have been replaced. The schematic:

calls for a 1K resistor in this position, but the one that is there now is only 410 Ohm. I don't know if this has any bearing on the bias voltage questions...

Any advice will be greatly appreciated! (Basically, I don’t want to proceed in ignorance and make any modification to this amp that may turn out to be a time bomb waiting to do serious damage in the future…)

Thanks!

Ken Fox · From: Nashville GA USA

The 10K resistor should be on the middle pot contact (wiper). One end should be open on the pot, the other end tied to ground (case).

In retrospect, looks like that is an equivalent circuit!

[This message was edited by Ken Fox on 09 February 2004 at 06:15 PM.]

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