Peavey Valveking II 100w Bias Issue

[Nickyhalfstack]

Hey y’all! I have a Peavey Valveking II 100w from 2015 on the bench here with some bias issues.

The issue: TIC lights turn red after period of time playing (5-15 minutes) and cut off the output volume. I then adjust the internal bias pot, get TIC lights green again, volume comes back and then will go out after a short amount of time playing again. Other symptoms of the issue are - I have some flabby low end when I do have power, sounds like a bad cap but could be a bad resistor too.

I’ve found someone with pretty much the exact same issue on the peavey forums here:

viewtopic.php?t=45698


A tech told them this is a common issue with the 2015 batch of these that were produced and can be solved with a bias upgrade kit. I’ve contacted peavey and no such bias upgrade kit exists for this amp.

Wanted to see your guys opinions on what I should change the bias resistor values to. My bias seems to need to get a bit hot to get the TIC lights green and I definitely have a faulty resistor or cap in the bias circuit.

I have confirmed my issue is in the power section and tested my power tubes and phase inverter. Tubes are good.

[rogue777]

There was no upgrade kit.
Do you mean you need to get the bias colder (more negative voltage) for the green lights to come on?

I wish I could remember. There was a post about this. And the fix was to change the polyfuses to a different value. The polyfuses are components
F104, F105, F106 and F107. The guy had a picture of his work that was done in the forum somewhere. He didn't take the board out to do the work and soldered on top of the board. It was sloppy work, imo. But he got the job done. The polyfuses are .17A in the schematic, I believe they were changed to .2A or something. The component change value came from Peavey, but no formal upgrade kit.

The issue was that heat from the tubes would falsely trigger the poly fuses and the TSI leds would go red. I think it was also recommended to mount the polyfuses on the other side of the pc board to keep the fuses away from the tube heat. Good luck, maybe you can find more info about this. The poly fuses are located right in the output tubes cathode path to ground in series with a one-ohm resistors for each 6L6.

like this:

https://www.mouser.com/ProductDetail/L ... ng%3D%3D

https://en.wikipedia.org/wiki/Resettable_fuse

[Nickyhalfstack]

Is this the thread? viewtopic.php?t=52570

Few things I didn’t mention in the first post - When the TSI lights are green my tubes glow light blue and low E strumming sounds similar to bad caps - flabby. I can tell the power amp isn’t quite all there. I basically red plate when I turn the bias pot up for the TSI lights to go green so I’m thinking cold bias isn’t the issue. I’m wondering if I should be looking for a bad cap/resistor in the bias section or if one of those fuses is the culprit. All of the tubes are blue so it makes me suspect there may be a deeper issue in the bias section causing this. My tubes are definitely running hot so makes sense why TSI is turning green. I’ll report back with voltages. Thought I recorded them but forgot to write them down.

[j0nsnell]

Those resetable fuses are for the EL84 18Watt head. NOT the 100W amplifier. Changing the value of these Polyfuses allows the EL84s time to damage themselves as they will then be allowed to overheat and arc over inside. They are not triggered by temperature.
To reduce the bias current on the 100Watt amplifier, the -ve bias must be adjusted and if you are not sure of what you are doing, don't attempt it.
Replace R205 with 56k 1/2W.
There is no official mod as Peavey output valves are selected for lower idle current, as most suppliers can do. Normally badged Peavey or their own name, like Messa do.

[rogue777]

Below is the link were someone said the poly switch values were changed on the schematic. Its the 50W not 100W. Also the VK 2 100W schematic.
Jon, no doubt you right about R56 1/2W, and the tube heat, it was mentioned in the link. See picture.

https://forums.peavey.com/viewtopic.p ... e#p336046

>>>>>>>>>>>>>>>>>>>>>>

Quote
I’d like to give the update on how to fix my vk 50 eventually.
As noted before, the pcb borad marked wrong value for the F2 fuse, it is T1.0a on schematic and T0.5a marked on pcb. So I used to 1a time delay fuse, so it won’t blow up.

However, though the fuse is working, the TSI is not working, it still turns to red after short period of green light.
So I replaced the 2 resettable fuse connected to cathode of power tube.
Note: the schematic marks these 2 psr fuse should be replaced to 0.25a from 0.17a. And mine is 0.17a still. And the resistance I measure is way too high. So I suspected they are bad.
So I replaced them with 0.25a, now I put everything back. Boo, it is working like a charm!
Refer to picture, you can see the 2 psr resettable fuse that I replaced.
Thank you all!

>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Question:
Does ambient temperature effect a poly fuse?
Internet says:
Yes, ambient temperature can effect a Poly Fuse (also know as a Polymeric Positive Temperature Coefficient (PPTC) device). Here's how:
1. Trip Current: as ambient temperature increases, the trip current decreases. This means that at higher temperatures, the Poly Fuse will trip at a lower current than it would at room temperature.
2. Reset Characteristics: After the fuse has tripped, higher ambient temperatures can also affect the reset current. It may reset at a different current level or take longer to reset compared to lower temperatures.
In essence, a higher ambient temperature can make a Poly Fuse more sensitive to overcurrent conditions.
www.littelfuse.com www.electrio-tech-online.com

[Nickyhalfstack]

Replace R205 with 56k 1/2W.
do.

I think there may be some confusion here. R205 is one of the giant 25w resistors mounted to the chassis. Here’s the schematic.

[Nickyhalfstack]

For some reason I cannot attach the schematic file.
It’s posted in this thread, second schematic

viewtopic.php?t=52502

[j0nsnell]

Correct, different chassis.
Replace R67 with 5k6 and R2 with 2k2.
That will give you more -ve voltage and cool the output valves down a bit.
Adjust with VR14 for the optimum bias current of your choice.

(R205 in the larger 100W chassis is 39k not the larger resistors as you mentioned).
See first schematic in your link.