Odd voltage drop readings on Deluxe reverb

[72Blazer]

I wanted to check my bias and when I did I got these readings:

OT Resistance on A is 119ohms
OT Resistance on B is 135.7ohms

Voltage drop on Tube A is -0.87 and the plate voltage is 432
Voltage drop on Tube B is -5.5 and the plate voltage is 427

I was not ready for such a drastic difference between A and B.  Is this normal or not?

Not knowing if I had a bad/weak tube  did swap them out but pretty much got the same results.

[pdf64]

…
Not knowing if I had a bad/weak tube  did swap them out but pretty much got the same results.

So if the valves are swapped around, does the high current draw stay with the socket or follow the valve?

How about if the valve in V6 phase splitter is removed?

[72Blazer]

high current draw stays with the socket when tubes swapped

Removing V6 had no effect

[AlNewman]

Maybe you just have a dirty socket or a bad solder connection.  Maybe a grid leak is open from your bias?  Start measuring all the resistors within a 10 mile radius.

[tubeswell]

high current draw stays with the socket when tubes swapped

Removing V6 had no effect

If that’s the case then suspect a leaky coupling cap on the high current socket side, and/or, if it’s a SF amp, suspect a leaky grid-to-cathode cap on that side (if your amp is one that has those HF snubbing caps on the output tube grids).

[72Blazer]

I swapped out the coupling caps but that did not fix anything.

Cleaned the Power Tube sockets and check resistors as well.

FYI:  This amp has the mod for tremolo and reverb in both channels

[glass54]

Silly Question 72 but where are you measuring the "voltage Drops" ?

[72Blazer]

measuring the voltage readings between OT center tap and plate of the power tube

[sluckey]

I would put a 1Ω resistor between cathode and ground of each power tube. Then measure the mV on each cathode and compare to your OT method.

[72Blazer]

installed the 1ohm resistors.

Tube A = 8mA
Tube B = 43mA

Not good.

[AlNewman]

Do you have 6v AC between pins 2 and 7?  We're running out of pins.

[72Blazer]

I measured 5VAC

[AlNewman]

You probably just have a bad connection.  Likely to ground.  Besides a bad tube, there's only a handful of parts that can sway bias between sides like that, and you've already measured them and swapped tubes.  Just hit it with some heat, it'll all be good.

[72Blazer]

one of the heater wires cam loose.  I resoldered and have 7VAC now.  Still have huge disparity on neg voltage drop as reported in the beginning.

I'll check more grounds as you suggest

[72Blazer]

Still no luck here.  Hitting the rack. 

[glass54]

Can we assume that if you swap output tubes, the "low" current tube reading will follow?
I think you know where this is going 

[PRR]

> Tube A = 8mA
> Tube B = 43mA

Yeah, that's what you had in the original post, it just wasn't mathed-out.

Check the Screen G2 voltages. If one of those has fallen-off, you can have "good" voltages at Plate, G1, and Cathode, yet get very low current.

[72Blazer]

I hope this is what PRR was wanting me to measure...

Tube A pin 6:  ~465V   Pin 4: 290V
Tube B pin 6: ~465V   Pin 4: 465V

Screen resistor issue?

[tubeswell]

I hope this is what PRR was wanting me to measure...

Tube A pin 6:  ~465V   Pin 4: 290V
Tube B pin 6: ~465V   Pin 4: 465V

Screen resistor issue?

Use your R-meter to check that you have got the correct screen resistor value for Tube A

[72Blazer]

Modified this post:  I did have the wrong resistor (470K) on Tube A. 

I've had this amp for at least 12 years and don't know how this happened or when...I seem to recall (maybe), changing those resistors from 1 watt to 3 watters-and that must have been when the mistake happened-the amp developed noise and subsequently started using other amps.

I replaced it and now I have 44.5 mA on tube A and 48 mA on tube B.  I dialed em back to 23mA on Tube B

Feeling dumb now and sorry for the wild goose chase but very much thanks-again-to all.

Vr
J

[glass54]

We enjoyed the braintease 
...and on the positive side, (Not HT  ) you fixed a poor quality joint in your Heater Wiring.
Enjoy your amp.
Kind regards
Mirek

[72Blazer]

NP.  I'll be back soon.  That resistor got me and this is all about attention to detail which the military tried to instill.  That's why I always admired our maintenance techs-and anyone else in this type of work with electronics etc.

I've got more of my amps/collection to fine tune.

Y'all are the best.

Vr
J

[JPK]

Did you notice an improvement in sound after your two fixes?

[72Blazer]

Thanks for asking JPK.  So there was noise that I would describe as sounding like an electrical storm.  At first power up the amp was reactively quiet and sounded good-but as the amp warmed up after (5-10 min) the noises started.  This noise was unacceptable obviously and I shelved the amp for a long time.  It was time to address the problem now that I had learned a little more about bias.  This amp is very quiet now and a joy to play again.