Peavey Valveking 112 help needed please.

[BStringThumper]

Hello, Everyone.

I was hoping please for some advice from the many experts on this site.

Problem: I have a Peavey Valveking 112 that screams/howls like a banshee as soon as the standby switch is flipped. All tubes have been tested and new ones tried just to try them. All controls are set to zero, it makes no difference. Resistors have been tested, diodes tested, electrolytics have been replaced, this amp is the older first edition, preventative maintenance. The trouble prone relays have been replaced for good measure.

I took some measurement: No signal, all controls are at zero, clean channel, gong into 16 ohm dummy load. All measurements in DC with ground to chassis, even the heaters.

V1 V2 V3 V4 V5
1 166.4 193.3 191.7 122.2mv and goes up to 320mv 16mv up to 320mv NOTE; Pin 1 not connected/used for V4 or V5 6L6GC on schematic.
2 2.9mv .7mv 2.6mv 12.35 5.46
3 1.079 1.27 1.31 437 436
4 -1.2mv -4.8mv -.6mv 434 434
5 -1.1mv -4.8mv -.7mv -54.6 -54.6
6 180.6 257.7 289.9 n/c n/c
7 66.3mv 87.1mv 27.8 19.23 12.33
8 1.39 1.83 90.4 -2.6mv -2.4mv
9 5.42 5.45 5.45 n/a n/a

I've pulled the preamp tubes one at a time and it only quit screaming when I pulled V3.

Injecting a signal can be traced to V4 & V5 pin 5 but is very distorted. (still using a dummy load and signal tracer). The volume control at the zero position has much static, moving up you can hear the test tone, moving higher it breaks into more static.

Pin 8 of V4 & V5 have ground, tested not powered up, with 47 ohms of resistance. I'm not sure what that's all about, something to do with the texture control.

Here is the kicker. I wasn't worried about my bias with 437vdc and -54.6 of bias, but I tried my bias king tester. The tester showed no current going through pin 8, it didn't even light up the display. I tested the Bias King on another amp and it works perfectly.

I'm ready to solder pin 8 of both tubes directly to ground with a 1 ohm resistor and see what's going on.

If you've made it this far through this nightmare, THANK YOU! What am I missing here? Any suggestions/thoughts would be greatly appreciated.

Thank you!!!

[Enzo]

You cannot pull ANY tube on these amps and get reliable readings, the heaters are in series.

My initial reaction is that your output transformer primary wires are reversed. Try swapping places with the brown and blue wires from the OT.

Look at the power supply page of the schematic, depending on where you ground your meter, that 47 ohms is probably OK. See the grounds in the B+ supply?

[BStringThumper]

Hi Enzo,
Thank you for your response.
I tried swapping the connector around and no difference, unfortunately. It does seem like a ground problem. I even ran another ground from the front of the potentiometers to chassis ground. I will check/test the B+ and B++ grounds.

Question, please. How can I not have current flow through pin 8 when I have plate voltage, screen voltage, and bias, and ground?

Thank you, Enzo.

[g1]

It sounds like some kind of issue with the bias king.
I don't see how it can howl if the power tubes don't work. Does the bias king register anything when the amp howls?

[BStringThumper]

Hi g1,

I thought that, too, but I tested it on another amp and it works fine. Yes, it howls with nothing registering on the Bias King. So strange.

Thanks.

[Jazz P Bass]

Get on the decoupling capacitor for the preamp supply with your meter & try to ascertain how much Vac ripple is riding on it.
If the cap is bad, the signal itself will ride on it, possibly causing the howling in a positive feedback fashion.

[g1]

I just read the bias king instructions .
It runs off AC heaters only, will not work with DC heaters. That's why it didn't even light up.

[BStringThumper]

Hi Jazz,

I will recheck all the preamp supply caps. All the electrolytics are new. I even pulled them and tested them on my old EICO 950B. Tested good.
Thanks.

[BStringThumper]

Hi g1,

Cool! Thanks for finding that!!!! This is my first experience with DC heaters. Learning something new everyday. Yeah, my first tip off should have been when the heater pin was reading in DC. DUH.

Thank you!

[Enzo]

Eico 950B!!! Yay, what a great old thing. First real test equipment I got was an EICO VTVM, but the second was a 950B. Still sits here in my shop.

[BStringThumper]

[Enzo]

Yep, them's the stuff. Though I opted for the VTVM with the larger meter. Same exact innards, but the meter itself was larger, model 249, like this one:

Vintage Used Eico VTVM 249 Electronic Voltmeter Ohmmeter Tester Tested Working | eBay

[BStringThumper]

Nice, Enzo. I like the big meters. I've been using digital more lately because of the big simple numbers to read. Ha.

Thanks for pointing out the 47 ohm in the ground path.

I tried to fix my tube pin voltages on my first post. No matter how I space them in edit mode it slams them together like this. As anyone found a fix for this??

Thanks.

[BStringThumper]

Hi, Everyone.
I hope the voltage chart looks better than the first post.
Thanks again to everyone helping me out with this. I had to take it off the bench for a little bit, but I'm hoping to get back on it soon.
Any other thoughts on things to check?


PV VK112 Tube Votages.pdf

pv_valveking_112 schematics.pdf

[BStringThumper]

PEAVEY VK112 PROBLEM RESOLVED.

I resoldered everything in the texture/resonance board, replaced the new capacitor with an old Mallory. Powered it up and it would only scream when turning the texture knob far left or right. Big improvement. Then I tried Enzo's switching of the primary leads again.WOW! No screaming and total texture control! YAY! THANK YOU ENZO! The power board clearly says where the brown and blue wires should go, but it does not work in that position. It's never easy when someone else has worked on an amp before you. Which way was the connector from the factory when it worked?? The wrong way from written?? Why didn't it work the first time I swapped them?? This was a tough one.

Thanks again for everyone's help!