Peavey Valveking VK212 100W Combo: No Tube Power

[keithb7]

Hi kind folks. A friend of mine owns a Peavey Valve King VK212. Its a 100W tube amp. He brought it to me to have a poke around in as it does not work.
I am a hobby tinkerer. I have some electronics training. I have built 3 tube amp kits. I have a pretty good understanding, but am no expert. I get the safety stuff, draining,
caps, lethal voltages ect. I consider myself still learning whenever I go into a tube amp. I have a lot to learn about reading schematics. I suck at that. LOL.
Your assistance to attempt to fix this amp is much appreciated.

Here are the amps symptoms:

When the power switch is engaged, the red LED indicator lights up, but none of the tubes light up. No sound of course.

Here is what I have done so far. I pulled the chassis and put it on my bench.

1. Verified all fuses, both internal and the one off the rear chassis are good.

2. Inspected the visible side of the board for compromised connection cables, burnt spots, etc. None spotted.

3. Checked mains and have 120V AC going into power transformer

4. Checked output of power transformer. Have the following:
14.85V AC at P211 and 15.54V AC P215
190.2V AC at P201 and 165.3V AC at P208

5. All 3 relays are clicking off and on when main power is shut off and on. Identified as SK101, SK102, SK103

6. 477V DC present on pin 5 of all main power tubes when powered up with tubes in and speaker load connected.

Pins 4&5 on both 12AX7 tubes, heater tubes I believe, have 5 milli-volts DC on them.
I measure 12.7V DC on what I think are the bridge rectifier diodes of the heater circuit. D206, D207, D208 and D209.
Plate voltages on 12AX7 pins 1&6 have 468V DC and 462V DC respectively.

It's early, but I'm guessing we have a heater problem. I am not sure where to go from here.
I did find schematic. It is located here: https://drive.google.com/file/d/0B6b...ew?usp=sharing
Any help, tips, appreciated. Thanks, Keith

[Jazz P Bass]

V1 & V2 are a 6Vdc supplied heater.

The rest of the tubes use a series setup.
Any one open heater will kill the string.
I use my meter to read the resistance actross each tube, working down the line.
This may help find an open, whether it is a heater element or a bad solder joint or trace.

Get on P301/P311 & make sure that you have the 6vdc & HTR1 supply.

valveking_100_212 _V1 & V2.pdf
peavey_valveking_100_212 _HTR.pdf
peavey_valveking_100_212 Series HTR.pdf

[keithb7]

Thanks Jazz. You are the man! I measured the 4 main power tubes. 1 heater circuit in one tube was wide open. No ohm reading.
The other 3 were giving ohm readings. Swapped it out with a good known tube. We now have glowing heaters. The amp is crackling and
noisy as heck. Especially when I tap the main power tubes. Bad snaps, blue hue, then red plating when I tap them. Unknown how old they are. I suspect they all need changing. Hopefully
that'll clear things up sonically. I plan to change them all tomorrow.

Thanks again Jazz. I did not recall that heater circuits were all connected in series. Good to know.

[Enzo]

Actually all of the tubes are in series V1 and V2 are connected to "6V" but that is simply the low voltage end of the string of the rest of the tubes. V1 and V2 heaters are in parallel with V3 heater. The 6L6s use 0.9A current, and with each 12AX7 wired for 6v needing 0.3A, three in parallel makes a fifth 0.9A. SO a 30vDC supply sends 0,9A through the string.

[Mick Bailey]

Check the power tube sockets before you put the new tubes in. If tapping a tube causes it to red-plate then I'd suspect the socket tension or a bad solder joint. Could even be a bad joint somewhere else causing loss of bias.

[Jazz P Bass]

Actually all of the tubes are in series V1 and V2 are connected to "6V" but that is simply the low voltage end of the string of the rest of the tubes. V1 and V2 heaters are in parallel with V3 heater. The 6L6s use 0.9A current, and with each 12AX7 wired for 6v needing 0.3A, three in parallel makes a fifth 0.9A. SO a 30vDC supply sends 0,9A through the string.

I don't know why I can never fully grasp Peavey's series heater circuits.
Thankyou, Enzo.

Maybe now it will stick.

[Enzo]

I find that little solution kinda clever really.

[keithb7]

Yes, I was educated when I forgot something I had learned.
I installed 4 new matched power tubes and the amp seems to
Be working as it should. Most symptoms are gone now except
High gain input jack 1, which is soldered directly to the board
has suffered an impact it seems. Jack tabs are loose on board.
I really don't like this design for this very reason. The connection
has been compromised. It appears all knobs and threaded nuts
On every pot and jack on the front of the amp have to be removed
to lift IC board to replace the jack. Missing the vintage Fender amp
style I like so much.

[km6xz]

What make of tubes were the bad ones? I see that symptom almost weekly with JJ power tubes but some techs in the US say they have pretty good luck with JJ. The ones shipped here are definitely unreliable and almost always the same problem, loose elements inside. If you got the loose element thing going on(band microphonics when tapped) be sure to check the screen resistors. They are the first to go with a shorting tube.
The VK is a cheaply made and designed unit and heater problems in the series output section is very common. Also, that power pot on the back is the most worthless "effect" going, and someone fiddling with it can cause the amp to sound broken. In the rehearsal studios here that use that model, I have removed that pot just to clueless players do not complain when someone turned it and made the amp sound horrible and broken.