Extremely low bias reading, blown fuse/tube in a Valveking after bias pot install

[Jason43]

Hey guys. I have a Peavey Valveking 212 that I recently installed a bias pot in. Before installing anything, I checked the bias reading with a bias probe and a multimeter, getting an average reading of 40ma. After installing the pot, I got a bias reading ranging roughly from .20ma to .55ma(note the decimal point). After about a minute, the first power tube blew as well as the main fuse on the back of the amp. During that minute, the amp seemed to work properly with a guitar plugged into it. I had used an old set of 6l6's when I did this and thought it may have just been a bad tube. Tried a different set of tubes and a new fuse and got the same results. I checked and rechecked the soldering as well as the resistance of the pot I installed. Today, I sat down and started pulling and testing the diodes and resistors in and around the bias circuit, as well as the screen resistors. Everything I tested checks out fine with the multimeter.
Here is the bias mod I did(scroll down to see it):
ultimate-guitar-valveking - Amp Mods - Electronics
VK schematic:
http://www.schematicheaven.com/newam...ng_100_212.pdf

I'm at a loss here. If any of you guys could point me in the right direction, I'd appreciate it.

[txstrat]

Welcome to the forum.
.20ma to .55ma is so low a current the power tubes would not blow.
If it was .2 A or .55 A that would cause the tubes to redplate and eventually blow.
How'd you measure the bias current? What settings on your DMM?

[Jason43]

Welcome to the forum.
.20ma to .55ma is so low a current the power tubes would not blow.
If it was .2 A or .55 A that would cause the tubes to redplate and eventually blow.
How'd you measure the bias current? What settings on your DMM?

Thanks for the welcome. I was reading it with an auto-ranging DMM, set to DC mA.

[Robert M. Martinelli]

..... I was reading it with an auto-ranging DMM, set to DC mA.

Hi Jason,
I'm with Matt, you stated the DMM to be an "auto-ranging" one, are you sure the "auto-range" doesn't switch to Amps while taking the reading?

.2 or .5 mAmps is not enough for the fuse(s) or the tube(s) to blow, while .2 to .5 Amps is overkill, so I think you should re-examine your measuring methodology and the readings you' re getting.

Are you measuring the bias current at the cathode(s) or at the plate(s)? Can you describe which measuring methodology are you deploying?

After installing the "bias pot", have you checked the bias voltage to be still present on pin 5 ( control grid ) of each power tube? You should get a reading in the -40 to -50 VDC range between each power tube's pin 5 and GND, if this negative voltage is less ( negative ) than that or not present, this could cause your tube(s) to redplate and die, so should this be the case, re-examine the mod you performed as well.

( The above "bias voltage" measurement can be performed with the power tubes out ).

Hope this helps

Best regards

Bob

[Jason43]

The DMM is reading in ma. Confirmed this with a different analog meter as well. I used an eight pin bias probe plugged into the DMM to take the reading(this probe: SRS BiasTool).

After installing the "bias pot", have you checked the bias voltage to be still present on pin 5 ( control grid ) of each power tube? You should get a reading in the -40 to -50 VDC range between each power tube's pin 5 and GND, if this negative voltage is less ( negative ) than that or not present, this could cause your tube(s) to redplate and die, so should this be the case, re-examine the mod you performed as well.

I will give this a try in the morning and let you know.

[Robert M. Martinelli]

Jason,

Let us know about the above mentioned bias ( negative ) voltage measurement.

As to the bias current measurement, I strongly recommend you to throw away ( not literally! ) your bias tool. Be sure to have the amp disconnected from the mains voltage and the capacitors discharged beforehand, then disconnect each power tube socket's pin 8 from GND, get four 1 Ohm 1% 1/2 W resistors and connect one of these resistors between each power tube socket's pin 8 and GND.

Now, with the amp and the tubes back on ( having made yourself sure that the negative bias voltage on each power tube's pin 5 is OK ), set your DMM to read DC milliVolts and measure the voltage across each 1 Ohm resistor ( = between each cathode and GND ). Thanks to the Ohms law, 1 mAmp through a 1 Ohm resistor causes the voltage to drop 1 mV, so your mV reading will directly show the mAmps flowing through each resistor ( cathode current ).

If the +B voltage is OK, readings around 45 mAmps or even 50 can still be considered safe. ( Bear in mind that the cathode current is the sum of plate and screen grid current ).

Anything over 65/70 mAmps ( say, 60 mAmps of plate current and some 10 mAmps of screen grid current ) will cause the tubes to redplate, overheat and, eventually, die. The higher the PD, the earlier the tube will fail ( 6L6s are rated at 30 W, but I doubt any "modern construction" tube fits the bill ).

The four resistors can be left there for future measurements, as they don't cause any appreciable loss.

Hope this helps

Best regards

Bob

[Jason43]

Robert, checked the bias voltage. I got a reading of 0.0 with the DMM set to DCV. Stupid question, but I'm assuming this reading should be taken with no power to the amp, correct?

[Albert Kreuzer]

Uh, no, with the amp off there won't be much voltage in it (at least after the caps are drained).

But you can measure bias voltage without power tubes. Less risky for the tubes.
After you made sure all voltages are correct, you can switch off, insert the tubes, switch on and adjust the bias.

HTH
Albert

[52 Bill]

I've got a couple of questions here.

Pull out the power tubes and turn on the amp. Does the fuse blow?

Which version of the SRS Bias Tool do you have, the mA or the mV?

Other than that do as Bob and Albert suggest and pull out the power tubes, turn the amp on and carefully measure the available voltage to pin 5 of the output tube sockets. If you turn the bias pot that you added across it's full range, what range of voltages do you read at pin 5?

[Jason43]

Gotcha. Doing that, I got a reading of roughly -46.0 to -48.0 from all 4 tubes.

[Jason43]

I've got a couple of questions here.

Pull out the power tubes and turn on the amp. Does the fuse blow?

Which version of the SRS Bias Tool do you have, the mA or the mV?

Other than that do as Bob and Albert suggest and pull out the power tubes, turn the amp on and carefully measure the available voltage to pin 5 of the output tube sockets. If you turn the bias pot that you added across it's full range, what range of voltages do you read at pin 5?

With the power on and tubes out, the fuse does not blow.

I have the version of the tool that read in mA. Even checked with SRS to confirm that I have the correct tool. I will check the range at pin 5 while turning the bias pot and get back you in a minute.

[Jason43]

Checking the voltage on pin 5 while sweeping the bias pot, I got a reading ranging from -27.0 to -56.0.

[52 Bill]

Checking the voltage on pin 5 while sweeping the bias pot, I got a reading ranging from -27.0 to -56.0.

That sounds good, with that range you should be able to bias the amp with any output tube.

If you've had problems with 2 sets of output tubes, there may be something else wrong with the amp. When you installed the 2nd set of 6L6's did the fuse blow again, or was it just the low bias current problem?

[Jason43]

That sounds good, with that range you should be able to bias the amp with any output tube.

If you've had problems with 2 sets of output tubes, there may be something else wrong with the amp. When you installed the 2nd set of 6L6's did the fuse blow again, or was it just the low bias current problem?

The same thing happened both times. Put the new tubes in and switched it on, got the low bias reading. After about a minute or two, the 6l6 in the first position lit up like a light bulb and the fuse blew. Both times the bias reading was roughly the same(about .39ma). I could sweep the pot and get a range from about .20 to .60ma.

[52 Bill]

The same thing happened both times. Put the new tubes in and switched it on, got the low bias reading. After about a minute or two, the 6l6 in the first position lit up like a light bulb and the fuse blew. Both times the bias reading was roughly the same(about .39ma). I could sweep the pot and get a range from about .20 to .60ma.

Then apparently, even though you did the bias pot mod ok, something else was disturbed or disconnected or broken when you removed the board.

Go back and inspect all of the connectors and wires, etc. that were moved when you pulled the board out. See if anything looks wrong or loose. Check all of the wiring going to the power tube sockets.

With no power tubes installed carefully take voltage readings on the power tube sockets, specifically the plates (pin 3), the screen grids (pin 4). There should be high voltage there, so watch out.