SVT CL Bias issue: Red LED behavior

[Enzo]

Did you verify the health of each cathode resistor?

[glebert]

Did you verify the health of each cathode resistor?

This appears to be the problem. I checked the resistors on the power tube board but at the time didn't look at the schematic to realize the cathode resistors were on the other board. Before I took anything back apart I measured from the socket pin 8s to chassis ground. All of the other sockets read <20 ohms to chassis but that one is more like 20k. I'm not sure how I am getting 25mV reading across my 1 ohm bias probe (implying 25mA of cathode current) if the cathode is essentially open though...

Thanks for all the help.

[Enzo]

My theory was a bad tube burnt out the cathode resistor.


10 ohm, 2W

I almost suggested just a measure from the socket to ground, but you got there already.

[glebert]

Cathode resistor replaced, can now bias it up with green on both sides and no red.Thanks all! Still don't understand how that tube was conducting 20mA of current with such high resistance in the cathode path, but that isn't my problem for today...

[g1]

Still don't understand how that tube was conducting 20mA of current with such high resistance in the cathode path, but that isn't my problem for today...

Could your bias probe somehow bypass it? I don't think so but thought I'd throw it out there.

[glebert]

Could your bias probe somehow bypass it? I don't think so but thought I'd throw it out there.

I was thinking the same thing, but that seems like it would also keep the red light from coming on. I'm sure there was some sort of operator error involved, but I usually drink AFTER I work on the tough amps.

[nevetslab]

Did you check that Cathode resistor you removed, or did it hit the trash bin already? Like you, I can't see how you were measuring the voltage drop. Now, because the plate voltage is so high on SVT's and similar amps, I measure the voltage drop across the cathode resistor directly (on SVT-CL's, AV's, SVT2-Pros. On SVT-VR's, you're now measuring at Plate Potential, but still small voltage drop so you can still get the plate current figures per tube. My Bias Probe set...Compu-Bias isn't rated for anything greater than 600VDC. And, with your bias probes, it's all in-series between the tube and tube socket, so not like you have a path to ground in your probe. I suspect that 10 ohm resistor was fine.

Reading between Pin 8's will always yield 20 ohms between tube sockets, so I do find that valid. I do the same to quickly verify the screen resistors from Pin 4 to Pin 4, yielding 440 ohms or very near that. Should be similar for the grid resistors at Pin 5. 2 x 47k.

[glebert]

Did you check that Cathode resistor you removed, or did it hit the trash bin already? Like you, I can't see how you were measuring the voltage drop. Now, because the plate voltage is so high on SVT's and similar amps, I measure the voltage drop across the cathode resistor directly (on SVT-CL's, AV's, SVT2-Pros. On SVT-VR's, you're now measuring at Plate Potential, but still small voltage drop so you can still get the plate current figures per tube. My Bias Probe set...Compu-Bias isn't rated for anything greater than 600VDC. And, with your bias probes, it's all in-series between the tube and tube socket, so not like you have a path to ground in your probe. I suspect that 10 ohm resistor was fine.

Reading between Pin 8's will always yield 20 ohms between tube sockets, so I do find that valid. I do the same to quickly verify the screen resistors from Pin 4 to Pin 4, yielding 440 ohms or very near that. Should be similar for the grid resistors at Pin 5. 2 x 47k.

The resistor was definitely bad. All the other resistors measured at 10 ohm when I used a good DMM, For some reason one of my DMMs is giving strange measurements on the 200 ohm scale, they were reading about 15 instead of 10, The bad resistor was open though.

[nevetslab]

While we're on the subject of getting tubes biased up and running, I have an SVT-CL over from our rental inventory, having the problem of it shutting down (goes into protect mode...Flashing RED LED). I tracked that down to the tube plugged into V3. The upper set was biased to around 14-16mA, while the lower set biased to 21-25mA. Then, I watched V3 start conducting, with it's plate current climbing past 40mA, RED bias LED came on quickly, then it hit Protect mode, pilot light began flashing as it turned the HT supply xfmr off. I did have to find which tube it was, not knowing right off until I monitored the six tubes with the open-chassis set up I use.



This new matched Sextet of J/J KT-88's just came in from Antique Electronic Supply in Arizona. so I set the amp up like this, only no power tubes installed. I checked the driver bias voltage for the upper and lower sets of tubes, both around -51.7VDC. Set both to -52VDC for a start, then plugged in the first two tubes into V3 & V4 positions to see what voltage I read across the cathode resistors. Both read around 110mV (11mA), so I decreased the bias to hit 220mV for both tubes, then checked the bias voltage change. Now both set at -48.7VDC. I let that first pair idle a couple minutes to see how much change there was. Increased to around 0.226VDC, so I marked the first to tubes per their position and moved on going thru the rest of the tubes from the matched Sextet. Got similar readings, spaning from 0.215V to 0.226V, until the last tube....I got a reading of 0.115V. Sigh.........

Let it idle for a while to see if there was any increase. It got to 0.120V and stayed there. Swapped the two tubes around (I tend to use V3 and V4 tube positions for all of the readings in single pairs for consistency sake. I had previously checked the readings of the Cathode resistors and screen resistors. The cathode resistors ranged from 10.00 to 10.26 ohms, so tight tolerance. Similar readings for the 220 ohm screen resistors. When i swapped the tubes V1 & V6 with each other, the readings moved with them, so I have one tube reading 10mA low in this new matched Sextet. RATS! I'll have to call Antique to see what else they have....if there's another tube from a similar reading set that could offer a better match, as this is not acceptable for a matched set. I've previously read the currents of a set having such an imbalance, and under load, there's also similar imbalance in the current sharing of the triad of tubes under drive conditions. I haven't tried that with this set, but having had high success with the J/J KT-88's in Matched Sextets from Antique, I'd like to continue that. Each tube has the Transconductance Value, used as the Matched Set code, along with the Plate Current used. Each tube's label has those to numbers, along with a seven-digit ID and a four-digit alphanumeric code, so there's info in the database associated with the Alpha Tube Matching system they use.

After sending that matched set back to Antique (Tubes and More), they verified that tube that had been a match to the other five when they had tested them on their Apex test set, it now measured defective. So, someplace between Arizona and Burbank, CA something happened to it, I guess. Slowed down the wheels of progress for about a week, but I received another matched Sextet, and measured those to get the best two sets of three. Even in that set, there was one a little weak, but it still yielded a min/max range of 3mA....the majority +/.- 1.5mA for starters. It does pay to work with a supplier who stands by their tubes in cases like this.