OC3 screen regulator in Plush 3000B

[LarBal]

I've got one these on the bench that i've revived. It's got got tubes in it and I biased it up, and one side of this (6) 6L6GC amp, was pulling waaaay more current than the other.
I pulled the board to find burnt trcaes, and a burnt resistor that went across the OC3 regulator tube, for the screens.
My problem is that I can't read the resistor, so I'm not sure how I should go about with a replacement.
I've attached a schematic I found on the net, but all of these amps are inconsistent.
The schematic shows R44 as 10K, across the OC3, but the burnt resistor I'm looking at looks like black, white, red, then some space, then some tolerance band I can't read....but I've never seen a resistor start with a black band
Any ideas on what this resistor should be?
Plush3000B.pdf

[The Dude]

Often when resistors get hot the colors fade/bake/change. If the schematic says 10K, I see no reason not to trust it.

[Helmholtz]

^^^^That.

With a working regulator tube, the 10k resistor will dissipate 1W. If the regulator fails (open) dissipation will increase considerably. You may find out by quickly measuring the voltage across the resistor without tube.
What is the size of the burnt resistor?

[mozz]

Burnt black, white, red,
Unburnt brown white orange

19k?

I'd start with a high value and see what it does to that voltage, adjust accordingly. Screen supply i think?

[Helmholtz]

Burnt black, white, red,
Unburnt brown white orange

19k?

I'd start with a high value and see what it does to that voltage, adjust accordingly. Screen supply i think?

The regulator tube acts like a series wired zener and its dynamic impedance is much lower than the resistor. The influence of the parallel resistor on voltage is negligible, it does not adjust voltage. The current through the resistor stays constant at 10.5mA. The current through the tube adjusts according to current demand. Once ignited, voltage across the tube will always be close to 105V with or without resistor.
The resistor may be required to protect the regulator in case of peak current demands >>40mA, e.g. caps charging.

[Enzo]

19k? Oh come on. How about not trying to decipher what burnt color stripes become and just stick a 10k resistor in there and move on?

[Gingertube]

10K looks right.
The OC3 is a 105V gass regulator tube.
As soon as the total screen current from 6 off 6L6GCs goes above 10.5mA then the voltage drop across 10K reaches 105V and the OC3 tube will conduct.

Lets see - each 6L6GC screen current is 5.6mA at idle and 22mA at max output.
Well that is ALWAYS.

That resistor will dissipate 1.1Watt (from V^2/R) when the OC3 tube is good and 11.2Watts at idle (from 6x 0.0056 ^2 x 10K) if the OC3 goes bad.

Replace the resistor with a 5W (minimum) type and I would be looking for a replacement OC3.

Cheers,
Ian

[mozz]

19k? Oh come on. How about not trying to decipher what burnt color stripes become and just stick a 10k resistor in there and move on?

Just trying to be helpful, your response is? If you see a burnt resistor, and the schematics are known to have many changes or be outright wrong, what method do you use?

[Enzo]

The schematic shows R44 as 10K

From your post, it seems you have a schematic. I see no reason to doubt the schematic. Other amps may have other values of this resistor, but we can presume the schematic you have works.

I have seen tons of burnt resistors in my years, and indeed, the colors often change appearance after going crispy. And yes, a resistor can heat right in a confined area and toast just one color band among the rest, it does happen.


I am not trying to be snarky, I just hate to see people bog themselves down in the tinyest details.

[LarBal]

Allrighty thanks everyone for the responses. I didn't entirely trust the schematic, given some errors, so was looking for a bit of a lesson there. 10K seems to be working fine. IIRC the screens were right around 100V lower than the plates, which were 560V with all the tubes in. In addition to the burnt rsistor(looked like a 2-3W), the B+ trace was burnt up, so had to deal with that as well, then rebias. I settled on about 34ma per tube. Amp sounds really great too, with a massive bottom end, imagine that