Runaway current draw from single 6L6 tube

[HaroldBrooks]

Just got an old RCA amp the other day, no schematic looked everywhere including here...

Things were going well tonight testing, and I actually got to play through it for about a half hour, but with one caveat, the whole time the amp was running through my light bulb limiter, so the wall voltage was lower. I decided to run it straight to the wall and measure the plate voltages and current through the output transformer and I was shocked to find one tube with a 406vdc plate voltage, while the other was down around 354vdc. I checked the voltage drop between the center tap and each tube, and lets just say there was a HUGE difference, so I quickly got out my infrared temp gun and the tube with the low plate voltage and big voltage drop was 538 degrees Fahrenheit, while the other was 200 degrees.

Now here's the weird thing, I ran the amp once again through the Light bulb AC voltage short detector and both plate voltages were identical and both tubes close in temperature.

I substituted another tube in place of the "hot" one, same thing. The output transformer is a brand new Classic Industries one, and it wasn't hot.

No schematic, but here's a pic of the underbelly of the amp. I couldn't measure the ohms on those big violet/blue (resistors ?) that were connected to the screens, so I cut them out, and just ran voltage to the screens on a second node coming after the first 40uf cap from the rectifier (replaced that cap), and then another 30uf cap to ground. That yields around 292vdc for the screens, at least through the light bulb short protection.



Any ideas ? Thanks.

EDIT #1 : Put a JJ 6L6 in place of the one super heating tube socket, and now it appears to be behaving even at the 125vac wall voltage, but... I think the 180 ohm cathode bias resistor is way to low, as one tube has a plate dissipation of 33.2 watts ! while the other is a more acceptable but still high 26.6 watts. I will straighten this out and see if it helps.

EDIT #2 : Put a larger resistance between the B+ and screen nodes (now 2 x 5.6k = 11.2k) and screen voltage is back down to 33.2vdc. All seems well with the one JJ tube. I think I was just pushing the RCA tubes too hard, as the wall voltages are a lot more than when this amp was first built, around 1948.

Still if anyone has an idea about anything else that may be an issue, I am all ears ! and thanks once again !!!

[The Dude]

There would have to be quite a large difference in OT primary resistance from each side to CT for plate voltages to be that different. If you measure resistance each side of the primary to CT what do you get? Were these plate voltage measurements taken with no signal at idle?

As an aside, I wouldn't actually use an amp hooked to the limiter. That is for testing purposes only and initial fire up.

[Randall]

"I couldn't measure the ohms on those big violet/blue (resistors ?) that were connected to the screens"

Why not? That seems easy enough.

[eschertron]

If you have questions about the amp, even - or especially - with questions about the component function, start drawing out a schem for us and yourself. Input circuitry, power supply, output section, etc., can be broken out from the rest of the design and handled (schem-wise) independently. I can't see under the mess of wires running over the tube sockets. Is this fixed bias? Cathode bias?

Get some dissipation readings from each tube. One may not be 'hot', the other may be not conducting and the hot tube is normal. If there is a shared cathode resistor, however, a single operating tube will draw half the idle current and so the bias voltage will be very low. Possibly low enough to put the tube over dissipation limits?

What exactly is "same thing" with a replacement tube. The running hot, I presume?

[HaroldBrooks]

There would have to be quite a large difference in OT primary resistance from each side to CT for plate voltages to be that different. If you measure resistance each side of the primary to CT what do you get? Were these plate voltage measurements taken with no signal at idle?

As an aside, I wouldn't actually use an amp hooked to the limiter. That is for testing purposes only and initial fire up.

224 ohms each side spot on. I just added some more resistance to the cathode resistor from 180 ohms to 230 ohms. I think I should have gone a bit further, as now one of the tubes is still at around 26.6 watts of plate dissipation. Ran the amp playing for about 15 minutes and nothing crazy happened yet... I think I need add a bit more resistance to the cathode resistor and use another new JJ tube, as the one I put it seems calm, while the old RCA metal can tubes are running hot. I won't be using the limiter anymore from this point on. I had to correct the shorted filter caps, so in the beginning it saved the amp, but you are correct, time to run wall voltage and fix what is left to be fixed.

Thanks for pointing out the ohms measurement on the OT, I am new to all this, and still learning a lot as I go along (low on the learning curve).

[HaroldBrooks]

If you have questions about the amp, even - or especially - with questions about the component function, start drawing out a schem for us and yourself. Input circuitry, power supply, output section, etc., can be broken out from the rest of the design and handled (schem-wise) independently. I can't see under the mess of wires running over the tube sockets. Is this fixed bias? Cathode bias?

Get some dissipation readings from each tube. One may not be 'hot', the other may be not conducting and the hot tube is normal. If there is a shared cathode resistor, however, a single operating tube will draw half the idle current and so the bias voltage will be very low. Possibly low enough to put the tube over dissipation limits?

What exactly is "same thing" with a replacement tube. The running hot, I presume?

I will try to do just that. I actually got a better view of everything just tonight. I am not quite as up on this stuff as I might seem, LOL, but maybe that would help. One thing I did was pull the 6SK7 tube out all together, and ran a cap from the plate of the 6J7 to the 6SN7 pin #1 grid. That worked out quite well, as the gain jumped considerably, and the distortion is now sweet sounding, whereas before it was a bit nasty. The 6J7 tube has a very high amplification factor, and I tied the screen grid to the screen supply for the 6L6 tubes using a 1 meg resistor and a .047uf cap to the cathode. I got this arrangement from one of my old National (Valco) model 50 schematics. Seems to work well here.

Replacement JJ tube runs OK, and seems to take everything (unfortunately I only have one JJ). Only issue now seems to be reeling in the power tubes, specifically the old RCAs I am still using. For now the one cooler running JJ will stay put, and I will most likely buy another one to match. I like the "Chunky" and "Beefy" sound of this "New" amp ! different than my 6973, 6V6, or EL84 amps. I think it might have something at least in part do to the 6L6s, but that is probably only part of the picture, i.e. Power and OT transformers, etc...

[HaroldBrooks]

"I couldn't measure the ohms on those big violet/blue (resistors ?) that were connected to the screens"

Why not? That seems easy enough.

Tried again, this time using alligator clips attached to the leads so I could get a really good "Bite" on the wires of the two resistors, and they are out of the amp entirely... Nothing, not using any scale on my multi-meter, as if they were open, and perhaps they are or something else I am not seeing.

One of them ran up the scale at the 20M ohm setting, and then just showed nothing, so there appears initially to be some continuity, just nothing I can measure ohms wise, so either they are shot, or not resistors, but they don't look like any type of capacitor, so there you have it.

[Enzo]

Don't we already have a thread on this amp?

[HaroldBrooks]

Do you have the schematic for this amp ?

That's what I was looking for in my other post.

If you have the schematic, please post it, and I will be eternally grateful !

And once again, Thanks for all your help Enzo !

[Enzo]

If I had one I would have posted it, I suggested you draw one from the circuit.

[Mick Bailey]

I always suspect leaking caps in older amps and these can be a source of trouble if ignored. The leakage isn't always evident at lower voltages and an amp operated at reduced voltage (as with a limiter) can behave fairly normally. The leakage can jump right up when under full voltage and cause an excessive bias voltage to appear on the tube grids. Caps that have a small leakage usually get worse so for longer-term reliability it's worth checking. I install 470R screen resistors with 6L6 tubes by default and I like the flameproof fusible resistors that Fender uses in some amps.

[pdf64]

I guess a 1948 PT would be intended for 110V; feeding it 125V will probably result in all secondaries running too high, resulting in limiting values being exceeded. eg what are the heater voltages?
Bear in mind that the amp was designed around early 6L6, which had much lower voltage and dissipation limits on plate and screen grids.

[eschertron]

I just added some more resistance to the cathode resistor from 180 ohms to 230 ohms. I think I should have gone a bit further, as now one of the tubes is still at around 26.6 watts of plate dissipation.

What was the other tube's calculated dissipation? Also, posting the voltage across your 230R cathode resistor will help me get a better idea of what you're facing. And I'm assuming the plate voltages are different now with different tubes and a different Rk. Please share.

[eschertron]

Tried again, this time using alligator clips attached to the leads so I could get a really good "Bite" on the wires of the two resistors, and they are out of the amp entirely... Nothing, not using any scale on my multi-meter, as if they were open, and perhaps they are or something else I am not seeing.

One of them ran up the scale at the 20M ohm setting, and then just showed nothing, so there appears initially to be some continuity, just nothing I can measure ohms wise, so either they are shot, or not resistors, but they don't look like any type of capacitor, so there you have it.

Drawing and posting at least the section of the circuit where these devices were, showing them in circuit (identified with a question mark?) will help us answer those questions. Really, take a half-hour and sketch it out on a piece of paper. Don't worry about how bad it looks, it will jump start your understanding of this amp tremendously.

[HaroldBrooks]

I always suspect leaking caps in older amps and these can be a source of trouble if ignored. The leakage isn't always evident at lower voltages and an amp operated at reduced voltage (as with a limiter) can behave fairly normally. The leakage can jump right up when under full voltage and cause an excessive bias voltage to appear on the tube grids. Caps that have a small leakage usually get worse so for longer-term reliability it's worth checking. I install 470R screen resistors with 6L6 tubes by default and I like the flameproof fusible resistors that Fender uses in some amps.

I checked the 6L6 coupling caps and no DC voltage leaking from one end to the next. Those are the best looking caps of the lot, so I left them in, but I will take your advice under advisement just the same. There are a couple of other caps I have not changed that visually are a wreck, and today I will swap them out. It's possible they were causing an imbalance that I did not understand, so they need to go.

Thanks !