Cathode Bias Red Plating Discussion

[Mr_bibbles]

Hey,

Was working on a Krank Rev Jr. recently, tubes are red plating. I would post a schematic but I can't find one anywhere. It is a cathode biased amp, seems pretty simple in terms of the OT tube stage, Cathode biased 6L6, can be subbed with 6v6 according to Krank.

Increased the Cathode resistor from 500 Ohms to 1k (quick and dirty test...) no change there. Tried with (several) known working tubes, they all red plated. (These were crummy test tubes FYI).

The Plate / Grid voltages are:

WITH TUBES
Pin 5 Control - 30mv
Pin 4 - Screen - 267VDC
Pin 3 Plate -243VDC

WITHOUT TUBES:
Pin 5 Control 0mv
pin 4 Screen 411 VDC
pin 3 Plate 411


From my limited knowledge, I am under the impression that Screen voltage is kept slightly below plate voltage to help control the current in the output tubes. if this is true, could that be the source of redplating for the tubes?

And further, can someone help explain the relationship between the screen and it's control circuit? Without a schematic (and with an untraceable, unremovable PCB) I'm at a bit of a loss.

Thanks in advance,

Mr. B

[The Dude]

You didn't report the cathode voltage which, IMO, is the important thing. Measure voltage across the cathode resistor and calculate bias. Is there a cathode bypass cap? Maybe it's shorted. Measure the in circuit resistance across the cathode resistor. If you measure less than your cathode resistor, my bet would be a shorted cathode cap. Of course all conjecture without a schematic.

[Mr_bibbles]

Thanks Dude,


Good guess! Measured 345 ohms across the resistor in circuit, measured 500 ohms with one leg up. There is no bypass cap connected to the resistor or near it, I did however bridge the resistor with a 100uf Cap in parallel to see if that would help, and it did not. The traces are on the underside and the sockets prevent me from removing the board without significant work. Bonus aggravation is polyester block caps/radial electrolytics who's leads I can't measure for continuity or anything else. I also already contacted Krank and they cannot get me a schematic...

Cathode resistor voltage drop (Measuring at both sides of resistor with tubes in) 1.985VDC, which according to Weber Bias Calc puts the tubes at 5% of 6v6 dissipation at 290 Plate to ground voltage??? This is either crazy low or I am doing it wrong. Without access to a schematic, would there be a way to hook up a Bypass cap to correct the short? Or is there some way I could make an educated guess on the electrolytics around the OT pins??

TL/DR
Cathode resistor measures low in circuit, fine when lifted. Cathode bias seems low but I'm not 100% sure I calculated it correctly. Bypass cap possibly shorted, difficult to find out which one it is.

[The Dude]

Adding another cap won't remove the short (or low resistance). You'll have to hunt it down. Try removing the output tubes and checking the resistance again. You could have a tube with an H/K short. If that doesn't change things, you'll have to go deep diving. When you calculated bias current, did you use the measured resistor value or the value stamped on the resistor? The cathode cap, if there is one, would likely be in the 25-50V range. It wouldn't be one of the larger 350-500V caps. Since you don't have a schematic, can you upload a picture?

[Mr_bibbles]

Makes sense for the cap not making a difference, duh! 0 ohms (short) parallelled with anything will be 0.

Measured, meaning in circuit value? 258 Ohms, plugged it in with same voltage drop and P --> C voltage, 9% per tube?

I've tried multiple tubes, so I feel like I can rule out the 6v6's. What is an H / K short though?

here are some pics:



There are really only 3 appropriately sized polarized electrolytic caps for the bypass, I could just snip / top mount them all.... guessing it's one of the two on the right though. The one by the FX loop (left side) is 100uf, the other two are 22uf.

[eschertron]

my hypothesis: something (I can't say what) is shorting out the cathode resistor, or seriously reducing its ability to control tube current. You've tried replacing the cathode resistor? Good. Now unsolder it and remove it from circuit. Do the tubes still redplate or show the B+ being pulled down?

edit: could a lead from the cathode resistor be touching the chassis?

[Chuck H]

Well you said the cathode resistor measured 345 ohms in circuit and then you said it measured 258 ohms in circuit. If both of these are accurate then something other than the cathode resistor is kludging the resistance. I like escher's idea of lifting the resistor and rechecking. That would tell you if you need to hunt down a short real fast.

[Enzo]

Pull the tube, power up, any voltage on cathode pin?

Lift 500 ohm resistor, now measure resistance to ground from cathode.

Is cathode pin touching heater pin?

[Mr_bibbles]

my hypothesis: something (I can't say what) is shorting out the cathode resistor, or seriously reducing its ability to control tube current. You've tried replacing the cathode resistor? Good. Now unsolder it and remove it from circuit. Do the tubes still redplate or show the B+ being pulled down?

edit: could a lead from the cathode resistor be touching the chassis?

Pulled the resistor, replated still. The B+ on pin 3 became 290, about 50 VDC higher than previous measurement. Does not look like the lead could touch the chassis on the pin. side.

Well you said the cathode resistor measured 345 ohms in circuit and then you said it measured 258 ohms in circuit. If both of these are accurate then something other than the cathode resistor is kludging the resistance. I like escher's idea of lifting the resistor and rechecking. That would tell you if you need to hunt down a short real fast.

I'm sorry, that 3 was supposed to be a 2! measured again, cathode resistor always around 250... When I removed it, i found 500 Ohms across cathode to ground.

I poked around the circuit and found another 500 Ohm 5w Cement resistor near the filter caps, My guess is that the designer (or some previous tech) had used 2x 500 Ohm resistors in parallel, creating a combined 250 Ohm cathode resistor that would jump to 500 if one fails?

Pull the tube, power up, any voltage on cathode pin?

Lift 500 ohm resistor, now measure resistance to ground from cathode.

Is cathode pin touching heater pin?

No Voltage on Cathode pin without tube.

Resistance from cathode to ground becomes 500 when unsoldered, see above about two parallel resistors?

Cathode shows OL to all pins except 1 ---> 8


So, based on this information, we can rule out a short from Cathode to Ground? Leaky Coupling caps from the previous stage maybe?

As always, thanks boys.

[Enzo]

Nothing to rule on, you found a parallel resistor.

An amp designer would not use two different 500 ohm resistors in different places around the amp but wired together.

[Chuck H]

Nothing to rule on, you found a parallel resistor.

An amp designer would not use two different 500 ohm resistors in different places around the amp but wired together.

Maybe. We don't have a schematic and we've seen a few "things". I thought it was implied that the resistors were NOT together. So who knows why it was done. FWIW 250 ohms would be in keeping with most vintage Fender designs of the tweed era. Most of those amps ran pretty hot. Close to class A from what I've read here. I've never opened one.

So... If 250R is a typical cathode resistor in similar conditions, why the red plating? And with the two 500R resistors located remote from each other, just what's really going on with the design? A schematic would really help and may be necessary.

[Enzo]

They were not placed together, but he said they were wired in parallel, that kind of together. Wired together in their separated placces.

[Chuck H]

They were not placed together, but he said they were wired in parallel, that kind of together. Wired together in their separated placces.

Ok. Of course. Probably a heat thing.

Still... 243Vp and 267Vg2? That's a wider than typical discrepancy and with an unloaded B+ of 411V along with red plating and the the cathode resistance (pretty much) checking out I think it's reasonable to suspect the plate loading. It could be a board short, the OT, a bad speaker cable, speaker, jack, etc. But I think something is loading the plates into too low an impedance rather than this being a bias issue.

[dstrat]

assuming this is a single tube amp... its no wonder tube is glowing, bias too hot!! should have 500 ohms R
suggest remove one of those 500 ohm resistors... 250 ohms works for Two Tubes!

the cathode voltage on pin8 would answer the question.. methinks

[Chuck H]

assuming this is a single tube amp... its no wonder tube is glowing, bias too hot!! should have 500 ohms R
suggest remove one of those 500 ohm resistors... 250 ohms works for Two Tubes!

the cathode voltage on pin8 would answer the question.. methinks

Oh, hey, right! I was thinking two tubes. Indeed 250R for a single tube seems really hot. Voltage across the cathode resistor is reported at less than 2V??? I think there must be a mistake or a problem there. So the mystery continues.