1960 5E3 Tweed Deluxe on the bench - plate resistor weirdness

[topbrent]

Hi all,

I have a beautiful, bone stock '60 5E3 Deluxe on the bench tonight.  Performing the first maintenance this amp has ever received.  Minor stuff, photo documenting, cleaning pots & sockets, new filter caps, cathode bypass caps, and a grounded power cord.

However, this amp is throwing me a curve ball and am scratching my head a bit on this one. 

One side of the plate resistors of the triodes of the first stage 12A_7 is reading about 100v lower than the other.  The low reading happens directly at the resistor.  When measuring on both sides of the resistor, one side shows the B+ node, and the other side measures low compared to the exact same point on the opposite plate resistor.

To investigate, I replaced the 100K plate resistor with another 100K resistor and still the same low reading.  I swapped several tubes in to check for a bad tube, but the results are the same every time. 

Why would one side read 100 volts low when they are fed by the same node in the B+ supply and the resistor values are the same when branched from that node (ie, 2 x 100k)? 
    

[VMS]

What are the cathode voltages?

Is the jumper from pin3 to pin8 good?

[sluckey]

Also check the plate coupling caps. If leaky, turning the volume pot 'may' change the plate voltage. And be sure that the grids (pins 2 and 7) read some resistance to ground.

[topbrent]

Unsoldered the leads at the tube socket (pins 1 & 6) to the plates and the DC reading is equal and proper on both sides, reading a solid 200+. 

When I re-connected the plate leads to the socket pins, the missing 100v difference is present once again. 

I also unsoldered the leads to the grids (pins 2 & 7) and it makes no difference, still minus 100v on the opposite plate.

As an experiment, I also swapped out the old Astron coupling caps for new orange drops just to check for DC leakage. 
There is no difference and no leakage.  The old caps will be put back in.
 

[sluckey]

Connect pins 2 and 7 to ground via gator clip leads. Does that make the plate voltages equal?

[Inca Roads]

It seems to me that the problem must be either a faulty tube socket or a conductive board.  If there is a parallel resistance to the plate resistor that exists in the board material itself, this wouldn't show up by just measuring the voltage when no load is present, but would result in a larger voltage drop once the tube is in the circuit.  You could test this out by lifting the offending plate resistor/plate wire off of it's connection to the eyelet board ( but keep them connected ) and measuring then with the tube in the circuit.  If the problem is at the socket then switching the plate connections at the board should result in the higher voltage drop occurring at the other resistor. 

[phsyconoodler]

Not necessarily. When Sluckey recommended grounding the grids,that would isolate the issue.The tubes don't conduct that way and it would eliminate the tubes.then looking at the volume controls and the 500pf cap would be wise.Aalso a bad ground will cause you grief looking for a possible answer.