SFDR Unexpected Bias Readings..Advice Please

[bobbyk]

This past weekend I upgraded the OT on a SFDR that had been recently re-capped. After the install of the new OT I checked the bias of the 6V6's just to make sure that it was okay. Without adjusting anything I read -35v at the junction of the two (2) 220k reisistors and 402v and 403v on the 6V6's respectively. When I check the bias on the individual 6V6's (transformer shunt method) I'm getting readings of approx. 18ma and 26ma respectively. I've re-tubed and re-biased amps before but never have seen such a wide variance in the bias. I replaced the tubes twice...both times with sets of brand-new 6V6's and get the same readings. As the DR has a bias adjust but not bias balance does anyone have any thoughts on way the difference in the bias of the two tubes or how I can go about getting them more in-line with each other? Adjusting the bias control does change the ma reading but the spread (8ma or so) between 6V6's remains constant no matter which why the bias pot is adjusted.

Any help/advice would be greatly appreciated.

Thanks in advance,

Bob

[maxvintage]

Maybe this is no help, but I thought the later silverface amps got bias balance rather than bias level. I know my 74 Princeton reverb had a bias balance which i converted to bias level. What year is the deluxe

On the other hand, I have a silverface 69 deluxe which has a bias level control

[bobbyk]

...with bias adjust rather than bias balance.

[Tim Swartz]

... a Princeton w/ a bias pot?

As for the DR, you say all tubes have about an 8mA difference? Is the same socket always higher? If so, and the problem was not there prior to the OT change, then I suspect an OT problem (like someone wasn't paying attention to the winding machine). This would also explain the plate voltage difference. Measure the DC resistance between the center tap and each plate connection.

[Teleologist]

If you haven't already, also check for a fried 1500 ohm grid stopper resistor. Check the screen resistors too. These normally get baked pretty good and soldering right at the socket may have done one in.