Biasing help needed

[Lance]

Hello, I have finished my Mainline build and have checked resistance values and voltages and all are fine.
Side "B" will bias up fine, but side "A" only has a 77-81V range.

Double checked the bias reg board, both C4S boards and all three tube sockets to make sure everything is wired up correctly.
(I did see a post with a resistor in the wrong spot on a tube, and checked for this also!)

I do see that when turned on the voltage starts high and drops slowly into place with "C" side
"A" side starts high holds around 209V, then drops quickly down to the 80V range!
I also measured the voltage drop on the 90.9Ohm res with C side at 1.53V (16.8ma) and "A" side at 1.54V (16.9ma), so the current seems OK when warmed up.
I did pull off the 10mF output cap to see if it might be leaky and its fine.
All tubes and boards appear to be wired correctly.

Any thoughts???

[Paul Birkeland]

Hello Lance,

Issues like these seem to be almost always an error with the center PC board.  What do you see on OA and OB of the center C4S board?  On side A of the amplifier, what do you see at OA on the large C4S board?

Another possibility is that you have a PN2907 in backwards.

-PB

[Lance]

Hi Paul,
thanks for the fast reply!
So, reg board;
AO is 1.51V
OB is .69V

C4S "A" side is 220V

[Paul Birkeland]

OB is .69V

That is definitely the issue there.  There is very likely a wiring issue on that center PC board, I would look for inconsistencies between sides or solder pads that have been accidentally bridged with solder.

[Lance]

Hi Paul,
I have removed the Regulator PCB and carefully checked it visually and with an Ohm meter to make sure it is built correctly and all parts are in proper orientation. I even removed the suspect 2n222 and checked it on a curve tracer to make sure it was good.
I also removed both C4S boards to carefully inspect them as well. At the same time I carefully verified all connections to each tube socket.

My final step was to carefully assemble everything. I did swap the C4S boards to see if the biasing problem would remain constant.
I am seeing the same "A" side tube start up at high voltage, stabilize at about 200V for a short period and then drop to about 80V!!!!


Not sure where to go from here! Any help would be greatly appreciated!

[Paul Birkeland]

Can you post photos of your build? 

[Lance]

Hi Paul,
here is a picture of my Reg board. Let me know if you see anything funny. (besides the solder that needs to be cleaned up!)
I wired with my own silver wire with the same color insulator, so I did not include a pic of my own build.
I did however carefully make sure that the tube sockets and boards are all wired correctly including the resistors and caps on the tube sockets.

The "B" tube contacts did look pretty tarnished. If they are not silver plated, could that be a failure point?

[Paul Birkeland]

How about the bottom?

-PB

[Lance]

Here is the back side of the reg board

[Paul Birkeland]

I would be extra sure that the two legs of the trim pot on the B side are well connected together (they look great on the A side).

You seem to be stuck really close to the base-emitter voltage, try turning the bias pot on the offending side 5 turns one way, then measure the DC resistance on the bottom of the board between the two pins soldered together and the third pin on the pot, then rotate 5 turns the other way and recheck.  I think if your pot was somehow stuck at 0 ohms that you might run into this issue.

-PB

[Lance]

Hi Paul,
the trim pot is soldered well and has full value range.

I did clean the "C" tube pins a bit and gave the circuit another try.
What was odd was that now the "C" side bias could only be adjusted up to 125V!!!! Still the exact same issue with the "A" side.
I powered down the amp and gave the "C" tube a few more inserts and a good wiggle to scrub in the pin contact points and powered back up.
"C" side was able to be  re-biased correctly. SAME WITH "A" SIDE!!!!! Whoooo, success!!!!!

Could I have a faulty "C" tube, or should I possibly make a better effort to clean the tube pins to remove further oxidation?
If I should clean them, is fine sand paper OK?? (New to the tube world, only played with fets!!!!!!)

[Paul Birkeland]

I wouldn't worry about sanding the pins, the socket should clean them off well enough.  I'm glad you got it sorted.

-PB

[Lance]

Thanks for your help Paul.
Much appreciated!