Mk III Bias issues

Have a pair of Mk III amps that I recently restored with all NOS carbon comp resistors , mil spec teflon coated wire , teflon sockets , new quad cap ,ClarityCap capacitors. Tubea are Gold Lion KT88 , Sovtek 5AR4 , & RCA 6AN8A.
Everything is run through a Variac at 117V.
For the first 3 months all went well. This morning one of the amps showed a low bias reading. Both amps were set to 1.56V and ended up near the middle of the pot adjustment. Now one of the amps will only go up to .88/.90 with the pot maxed out.
Can anyone point me in the right direction to troubleshoot this amp ?
I have already switched the tubes from amp to amp and the problem remains with the original amp so I do not believe the problem is tube related.
Thanks

stevenally wrote:Have a pair of Mk III amps that I recently restored with all NOS carbon comp resistors , mil spec teflon coated wire , teflon sockets , new quad cap ,ClarityCap capacitors. Tubea are Gold Lion KT88 , Sovtek 5AR4 , & RCA 6AN8A.
Everything is run through a Variac at 117V.
For the first 3 months all went well. This morning one of the amps showed a low bias reading. Both amps were set to 1.56V and ended up near the middle of the pot adjustment. Now one of the amps will only go up to .88/.90 with the pot maxed out.
Can anyone point me in the right direction to troubleshoot this amp ?
I have already switched the tubes from amp to amp and the problem remains with the original amp so I do not believe the problem is tube related.
Thanks

Start with checking voltages as per the dynaco manual ( p.14) Also check the voltage
around the bias adjustment pot.
In addition , double-check all soldering where components are changed.
Are the selenium rectifier replaced ?

As the 'other Peter' suggests, wandering bias is the first sign of a selenium rectifier on its way out. When you replace it with a silicon device, you will need to add resistance. Dynakit parts sells a package if you do not want to hunt the parts down for yourself.

http://www.dynakitparts.com/dynakit-products/bias-kit-parts/MARK-III-BIAS-CIRCUIT-KIT

Hard to imagine rebuilding a MkIII and leaving the Selenium stack in place. Use the same type of 1n4007 for the bias circuit as you used to do the "yellow sheet" mod. You did do the yellow sheet mod... right?

Also wouldn't an aging selinium rectifier reduce the -ve voltage and cause the valves to conduct harder creating a higher bias voltage?

The OP has a low standing current which would indicate a larger -ve voltage on the grids, or a low HT. Could it be that only one of the OP valves is conducting? Assuming there is no red plating.
Best get the DMM out and compare voltages especially the HT and Grids between amps.

Have replaced selenium stack with kit from Dynakit parts. Also included "yellow sheet" mod at time of rebuild. Put on a huge pot of coffee and will start checking voltages this morning.

Installed cap board and checked all connections. Amps now working fine.

Just finished a rebuild of a pair of mark III amps.
used new components but no modifications to the original Dynaco circuitry other than replacement of the selenium rectifier.
Amp 1 is fine. Powered up, bias set, all good.
Amp 2 arced in the rectifier tube and blew the fuse.
Checked all wiring point to point and both amps are identical.
All tubes check good in other amps.
Where should I start ?

stevenally wrote:Just finished a rebuild of a pair of mark III amps.
used new components but no modifications to the original Dynaco circuitry other than replacement of the selenium rectifier.
Amp 1 is fine. Powered up, bias set, all good.
Amp 2 arced in the rectifier tube and blew the fuse.
Checked all wiring point to point and both amps are identical.
All tubes check good in other amps.
Where should I start ?

Start with removing gz34 and power tubes. Power up and measure pin 5 ( g1 ) of the power tubes. See
if it is negative and can be adjusted with the pot. It should be -30 to -50 or so.

just checked voltage on both amps at pin 5, no power tubes or rectifier.
reading 48/49 and the reading adjusts with the bias pot

Sorry , make that -48 or-49

stevenally wrote:Sorry , make that -48 or-49

Good. Adjust to most negative before installing tubes.

Then check if NFB is backwards making the amp to an oscillator.

If an external adjustable DC is available use that instead of the GZ34 slowly increasing voltage
while observing voltages and possible oscillation with a scope.