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.

 

[Clbraddock]

Did a third tube swap for an RCA 6AQ5A and that solved the problem (reads 102 VDC now). So, if someone has this problem in the future, and all of your component values are correct, you might have to just start trying different tubes. Luckily, NOS 6AQ5 tubes are really cheap.