Guys got a bias issue (Express VVR)

[Gargoyle]

Noticed my Express getting kinda warm sounding over the last few days. Thought I would check the bias .... 46mv hmm thats getting up there. When the tech put the VVR in for me it was set to around 34mv (checked out on the external pots) and the VVR imeg bias pot was maxed out as per Dana's instructions.
Problem I have is turning the external bias pot down to minimum only drops it to 43mv... Ok had another back up set of el34's (same brand Winged "C") not new But have had little use, put em in warm her up and at minimum setting , 50mv.. hmmm
Ok took her out of the cab checked the mini bias pot on the VVR (was on max fully clockwise) turned back and forwards a bit, put it back at maximum, had VVR fully off ,warmed her up, set ext bias pot at minumum, tested bias 52mv........

Guys am I doing something fundamentally wrong here....... I had checked the bias myself around a month ago and it was around 36mv from memory.
Cheers Mick

[sliberty]

Of course, make sure that your bias circuit is setup correctly first. But assuming that it is....

Measure the voltage you are seeing before the bias circuit - on the trannie side of the 220K resistor. It should be approximately 290-300V AC.

If that is OK, check the voltage at the wiper of your bias pot. You should see perhaps up to -55V DC (approx).

If that is OK, try replacing your bias pot. if you are seeing much more than -55V DC, then try replacing the 220K, 15K and 47K resistors (cheap enough to not do them one at a time).

Note - If all of this still doesn't help, some tubes are just wacky. You may have to change the value of your bias pot or one of the dropping resistors. You could increase the value of the 15K for example. But, I haven't seen the need for this in an Express before. If you do replace the 15k with a higher value, you might also increase its wattage rating for safety sincer it will eat more voltage.

[Gargoyle]

Sliberty, thanks for the input..........
I was thinking that maybe one of the bias pots may be the culprit. The main one or the one on the VVR.
I just turned the bias pot on the VVR (fully Anti - clockwise) thinking that must be the minimum setting, and had the external bias on minimum also.... flicked her on , instant redplating on the tubes.. oops???? The VVR must have already have been on minimum, not on maximum as per Dana's VVR instruction.....

[UR12]

I assume you have external bias points installed and that
is what you are using to set the bias. You have to use either
one bias pot or the other. Set either one to max and use the
other only to set the bias. Before installing any tube, set the power
level vvr control to it's highest power setting. Never try adjusting
the bias without that control on max power. With the bias trimmer
one the vvr board set to max ( arrow on pot pointing towards the
back of the vvr board) adjust you normal bias pot from one
side to the other to see what kind of range you have. If the
measured range is not within the required range you need to fix that
first. On an express idealy you shoud be able to measure -25 - -40v.
If that all works, then set the bias voltage on the tube sockets to about
-35 volts. Monitor this voltage with you meter at the tube socket
with your meter while turning the vvr power control. You should
be able to watch the bias go from -35v down to about -3.5v. If that
works the vvr is functioning properly. Also check your b+ the same
way. It should read say 400v at full power and 40v at the lowest setting.

[Gargoyle]

Dana,

Thanks for responding mate.

I haven't yet had a chance to check the voltages, I did have the VVR Bias at the correct maximum setting.
Will see how I go........
Cheers
Michael