new build problems -- help, please!

[jimipage]

some good news -- I finally got my Express done today!
Now the bad news: most of my voltages seem fine, except for the output plates (6v6's). I've got 410v on one, and 29v(?!?) on the other. There is quite a loud hum coming through, as well. What could this be a symptom of? (I've tried swapping tubes, to no avail)

Next is what I originally feared would happen: I'm having bias issues. (this is my first build with fixed bias) The bias circuit is based upon the one found on the A1 schematic, but with three differences:
1) the 220k resistor is paralleled with a 150k, yielding about 89k
2) I am using a 10k linear trim pot instead of the specified 20kL
3) I have a 22k resistor going to ground from the ground side of the bias pot, whereas in the schematic there is no resistor

So my first question regarding bias is: what is the best/safest method for me to measure my bias? (I do not have the 1ohm resistors)
I'm also getting the very faintest of redplating with the bias pot all the way down, which brings me to my next question: does anybody have any suggestions as to what I can do to actually get more range out of my bias circuit? I'm thinking maybe I should change the 22k bias pot ground resistor to a 47k -- am I on the right track? Other than that, I have no clue where to go from here.
I appreciate any and all help -- thanks!

[keithrick]

I would check your output tube wiring first. Did you use the layouts posted?

[joe6v6]

I've got 410v on one, and 29v(?!?) on the other

If thats an acurate reading it may be you output trans is shorted, Did you install the diodes from plate to ground? If so try removing them just to rule out anything screwy there, I didnt use them on either of my builds.

the 220k resistor is paralleled with a 150k, yielding about 89k

Did you do this to get your bias voltage right? you should have
-30V on pin 5, the 10k pot is ok as long as your bias voltage is ok. I would loose the 22K to ground. . . The plate voltage needs to be addressed first , maybe those diodes are backwards ( if you used them) the banded end goes towards the PT plate.

JOE

[jimipage]

thanks for responding, fellas.
Output tube wiring is as it should be, although I did tie both cathode pins together and then run them to ground. And, no, I did not use the diodes on the output tubes.
Joe, are you sure I should remove the 22k to ground? I put it in there for safety, as explained from this article:
http://www.el34world.com/charts/Biascircuits.htm

* The 27K (22k for me) resistor on the bias pot ensures that the path to ground can never be zero ohms. If the bias pot was turned to zero ohms, you still have the 27K resistor in the path to ground. If the 27K resistor was not there and you turned the bias pot to zero ohms, that would short the negative bias voltage directly to ground and probably harm the power transformer or hopefully it would blow a fuse and save your power transformer.

If my plate voltage problem resides within the OT, how would I test/measure it to make sure?

again, thanks guys -- I really appreciate any and all help.

[Darkbluemurder]

Now the bad news: most of my voltages seem fine, except for the output plates (6v6's). I've got 410v on one, and 29v(?!?) on the other.
I appreciate any and all help -- thanks!

Have you tried swapping the tubes in their sockets? If the 29v is on the other socket after the change the tube is faulty and you should be fine with a new set. If the same socket shows the 29v after the swap then either the socket is miswired or something else is out of whack. Check the debugging page on www.geofex.com for advice on how to check power transformers.

Good luck!

[jimipage]

OK, I've gone back and remeasured the plate voltages. It seems that one is reading about 410, and the other is reading about 270. I don't know why I've gotten a different reading on the 'deficient' plate, but I tested it for quite a while and that's what it settled at.
I did measure each output primary resistance(with the amp off) and both sides read about 147ohms. I have not noticed any redplating. I did plug in to at least get an idea of the sound -- it's horrible. Flat and "cut-offy" is how I think I can best describe it. And I can tell that it's the output and not the preamp that sounds like crap.
Any idea what my next steps are?

[jimipage]

I just made another observation: measuring the plate voltages of each 6v6 with the volume turned all the way down to zero(and tone controls turned up just a little) I read about 403v on each plate. As I turned up the volume, the 'deficient' plate began to settle down to about 270v. The other tube that seems to reading fine stays at 410v until the volume is turned up about 1/3. After that, the voltages just jump up and down all crazy. I'm beginning to think the OT might be the problem -- it is a used one, BTW.
Any ideas?
thanks!

[jimipage]

I think I may have solved the problems
I began tracing each component and lead, and when I got to the end of the PI, I noticed that I had the 100k and 82k plate resistors reversed so that they were each feeding the wrong plates I switched the plate wires and switched the OT primaries to the 6v6 plates. Now the amp sounds great! I think I may be running the 6v6's with a little too much current, but I will attempt to adjust the bias when the amp cools down. Of course, I will probably have more questions, but for now I can breathe a sigh of relief.
Pics and clips to come...