LA-2A 6AQ5W screen voltage and 12AX7 plate voltage

[Clbraddock]

Recently built a point to point LA-2A using the attached 1967 schematic. It works great and everything tests correctly, except the plate voltage to V3 -12AX7 - pin 1 and screen voltage to V4 - 6AQ5W - pin 6.

The plate voltage to V3 should be about 105, but I am measuring around 85 VCD.

The screen voltage to V4 should be 100, but I am measuring about 80 VCD.

Both tubes are fed from a 275 VDC B+, each across a separate 220K resistor to the relevant pin. The B+ measures correct at 275, but I am getting too much voltage drop across the 220K resistors. The resistors measure within 5% of spec.

These are both sidechain tubes - i.e. not in the signal path.

Edit: The first time I fired the unit up the cathodes for V3 and V4 were not connected to ground (wired wrong). I guess this could have damaged both tubes? Still seems like some other cause though since the unit works overall.

1) Should I worry about this/does it make any difference
2) What is causing this? Im assuming to the tubes are drawing more current than expected, thereby causing a larger voltage drop with ohms law. If it was just one tube I would think it was a bad tube, but since two different tubes show too low VDC I would guess its something else.
3) Should I swap the 220K resistors for a lower value? Or should I not worry about it? (Sorry kind of a repeat of question 1)

 

[Rob Flinn]

Have you tried a different 6AQ5 ?

 

[beatnik]

What are the cathode voltages ?
I think this is probably ok and just due to tube variations

 

[Clbraddock]

Have you tried a different 6AQ5 ?

I don't have another one to try. Looks like NOS ones are only like $10-$15 though. So, if it is possibly the tube I could order one.

 

[Clbraddock]

What are the cathode voltages ?
I think this is probably ok and just due to tube variations

Ill check tonight and report back

 

[Clbraddock]

Readings:

V3 (12AX7) Plates 85.2 VDC (should be 105)
V3 (12AX7) Cathodes 0.87 VDC

V4 (6AQ5) Screen 78.5 VDC (should be 100)
V4 (6AQ5) Plate 184.5 VDC
V4 (6AQ5) Cathode 4.42 VDC

 

[Clbraddock]

I tried swapping the tubes, but that gives the following results:
V3 swapped with an electro-harmonix 12AX7 I read 92.7 at plate. So there's a slight improvement, but still 12% off.

V4 swapped with a strong testing GE 6AQ5W (sale stated 3400gm where 2320gm is minimum, although I don't 100% know what that means) and that one reads 114 VDC at screen (so too high now). I'm surprised switching tubes would cause a 35.5 VDC swing on the screen. That doesn't seem right.