Peavey Valve King - No Heaters

[earache]

I am looking at a Peavey Valve King VK100 head. It's original problem was that it's output would fade in and out when on. I chopsticked around the inside with it powered up and had a couple loud "pops" near one of the preamp tubes and the sound would fade then, so after draining the caps I touched all the connections near the preamp tubes with a hot soldering iron.

That didn't help. As a matter of fact, the amp won't make any sound at all any more. Now the tubes won't light up. I don't have any heaters at the preamp or power tubes. I measure 0 VDC at pin 2 and 35 VDC at pin 7 of the power amp tubes.

I am unsure if the original problem was the heaters.

I have the schematic, which I got from Peavey. I am having a hard time deciphering it as it has no voltages listed.

What should I be looking at here? Where on the schematic does the heater voltage originate from?

Thanks in advance for taking a look. The schematic is attached below.

peavey_valveking_100_212.pdf

[The Dude]

Heater voltage is marked at the top of C211. Check F203 (fuse) and make sure it is not open.

[Jazz P Bass]

Go back & check your work!

The amp worked, you touched up solder joints & now it doesn't work.

The chances are good that you have a solder bridge or a failed solder joint.

[earache]

Ok Jazz I will!
Dude - the fuses F201, F202 and F203 test oK.

[The Dude]

If the fuse is good, we should have voltage at the + side of C211 or R212. Is it there? Another possibility: There's a plug P301/P311 that carries, among other things, heater voltage to the amp board. If you unplugged it to do some soldering, possibly you missed a pin or bent one when you put it back together?

[earache]

If the fuse is good, we should have voltage at the + side of C211 or R212. Is it there? Another possibility: There's a plug P301/P311 that carries, among other things, heater voltage to the amp board. If you unplugged it to do some soldering, possibly you missed a pin or bent one when you put it back together?

Hey Dude

I checked all voltages across the ribbon, on both ends, so that I know the ribbon itself is OK. Here's what I got:
Pin 1: 0 VDC
Pin 2: 0 VDC
Pin 3: 0 VDC
Pin 4: 34.6 VDC
Pin 5: 0 VDC
Pin 6: -46.5VDC
Pin 7: 463 VDC
Pin 8: 463 VDC
Pin 9: 463 VDC

This amp has DC heaters and in series, so if one tube is dead, will they all be dead like a string of old Christmas lights?

As for the voltage at R212, there is 34.6 VDC at the ribbon cable side and 15.2 VDC at the other side.

What's broke on this thing, is it a bad capacitor somewhere along with a bad tube maybe?

[Jazz P Bass]

What is 'broke' is the heater circuit.

Pin 5 of the ribble cable must read 6 Vdc.

If it does not, then you have a broken series circuit.

Either one of the power tubes or V3 is not completing the string.

As was stated previously, the heaters worked & now they do not.

With power off & supply drained, you should be able to measure each tubes heater starting at HTR1 on the power supply.
Follow the schematic.
From HTR1 to 6L6 V4 you must have continuity to pin 7.
Then to pin 2, you should read the heater itself (about 2 ohms)
Keep going down the line.
Please do not fail to read the next pin 7, as that will prove out the connection.
By the time you reach 6L6 V5 you should read four x's 2 ohms of heater resistance.
Then on to V3.
Pin 4 & 5 are tied to ground.
Pin 9 is tied to 6L6 pin 2.
Measure pin 9 to ground to verify V3's heaters. (12AX7's have a higher resistance than 6L6)
Actually the reading will be V3 plus V1 & V2 which are in parallel with each other.
Removing V1 & then V2 should raise the resistance reading, thus proving there heaters out.

At this point if everything checks out, pin 9 'becomes' the 6V designater.
That 6V is what will be the power source for V3, V1 & V2. (marked as 6V1)

[52 Bill]

Here's what I got:
Pin 1: 0 VDC
Pin 2: 0 VDC
Pin 3: 0 VDC
Pin 4: 34.6 VDC
Pin 5: 0 VDC
Pin 6: -46.5VDC
Pin 7: 463 VDC
Pin 8: 463 VDC
Pin 9: 463 VDC

Pin 5 is missing 6 volts dc that supplies the heater voltage to V1 and V2.

This amp has DC heaters and in series, so if one tube is dead, will they all be dead like a string of old Christmas lights?

This is true of the driver tube and the 4 output tubes. The two preamps tubes are wired in parallel dc.

I was wrong here, the three 12AX7 are parallel wired with each other, but they are all supplied through the 4 power tube filaments.

[earache]

Hi... I tested the output tubes out of circuit by checking resistance across pins 2 and 7 of each tube and found that one output tube had a bad heater. I replaced that tube for now with another 6l6GB, but still have no output.

The amp does make a slight "pop" noise when I push either the "channel select"or the "gain", and also when I tap pin 3 of any output tube.

Additionally -is there any issue with mixing a 6L6GB with 6L6GC's for test purposes?

[Jazz P Bass]

As far as testing goes, the tubes can be mixed. (just don't overpower it)

Which one is the different one?

You still need to recheck the 6V voltage.

[Enzo]

Replace is one thing, but do all the tubes now have lit up heaters?

[earache]

Enzo - of course I left out the most important fact...yes, all of the tubes are now lit.

However there is still no output, save for the fact that the entire chassis is now microphonic. Anywhere I tap on it makes a sound like a cheap synthesizer drum. This happens whether something is plugged into the input or not.

It seems that there is a problem on the input side somewhere.

I have tried replacing each of the 12ax7's in turn with a known good tube and there is no change.

Touching pin 3 of any output tube yields a "pop".

Jazz P - I followed thru the schematic as you laid it out for me and found that, with the replaced 6L6GC tube, I have resistance across all points as you described. Thanks for the explanation of the schem, I understand it much better now.

[The Dude]

Now that you have the heaters working, you can narrow down your search by finding out if the problem is in the preamp or power amp section. To do this, plug a signal into the effects return and see if the amp produces sound. Also, run a cable from the effects send into another amp and see if you get anything.

[earache]

Now that you have the heaters working, you can narrow down your search by finding out if the problem is in the preamp or power amp section. To do this, plug a signal into the effects return and see if the amp produces sound. Also, run a cable from the effects send into another amp and see if you get anything.

With the heaters now lit, I am back to where the amp only works intermittently. When it works, both the preamp and power amp sections work but the entire amp is microphonic. As I said previously, tapping on the amp makes a sound like a cheezy electronic drum set. When I jiggle V1 in its socket it occasionally lets out a big static blast. Must be an intermittent solder point near V1 or a bad socket or really microphonic V1.

[earache]

Does anybody have any experience with an amp where a component makes the entire chassis microphonic? Could it be a filter cap or is it more likely a tube? Please share any experiences you have had.