VK100 - Red Plating

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GTS
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VK100 - Red Plating

Post by GTS » Fri Nov 19, 2021 10:40 am

Hello

I received a Valveking witth a broken tube, it went so hot that a solder connection of a heater got unsoldered in the base of a tube.
I resoldered the pin but the Tube sonds realy terrible.

So i ordered 2 sets of matched tubes. Well yeah it sounds good but after a while two of the tubes V4 to V7 (cant remeber which tubes) start to glow red and get hotter and hotter. I connected my bias adapter and meassured the bias current which is constantly rising without an input to the amp.

I checked all parts around the tubes and they are OK except the C1 on the texture breakaway, i replaced it but without success.
I attached a schematic with some markings of parts i checked. Green means OK and red means damaged and replaced.

I hope you can help me to get this Amp back to life.
vk100.png
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Enzo
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Re: VK100 - Red Plating

Post by Enzo » Fri Nov 19, 2021 7:43 pm

Red plating means a loss of bias to the tube. Instead of just measuring a bunch of parts, check the operating voltages, specifically the pin 5 voltage of the power tubes.

GTS
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Re: VK100 - Red Plating

Post by GTS » Sun Nov 21, 2021 2:44 pm

Will do...

GTS
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Re: VK100 - Red Plating

Post by GTS » Mon Nov 22, 2021 5:41 am

I measured the Voltage of Pin 5 and it started at around +45mV DC and 17mV AC.
The Voltages have been very stable for the first few minutes of operation.

As i noted the Glowing Tubes the Voltage at Pin 5 was at-32V DC, didnt check AC Voltage, switched it off to prevent further Damage.
Any Idea?

Beach
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Re: VK100 - Red Plating

Post by Beach » Mon Nov 22, 2021 8:55 am

I looked up the rest of the schematic. It is unclear what voltages are utilised from there, but the plate voltage is probably some 500VDC. I do not own this specific model, but I do have a VT212 classic with 2x 6L6GC at 500VDC and in my amp the grid is some -60VDC. Your -32VDC might thus be just too high a voltage (remember is is negative). If that is the case then there is a leakage current somewhere in the grid bias voltage circuitry.

Switch it on for 60 secs. Measure the voltages at either side of R211 in the bias. Measure all the voltages at pins 5 of the four tubes.

It might be that due to overstress, the tube socket is leaky, Since you say you got it and it was overheated before, that might be the case. Pls measure the grid bias voltages and get back to us!

Oh.. and be careful out there!

GTS
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Re: VK100 - Red Plating

Post by GTS » Mon Nov 22, 2021 3:15 pm

I know, tubeamps are Dangerous...

Will measure all voltages with and without the Main Tubes installed.

Will Take the old tubes, its OK when they burn...

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Enzo
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Re: VK100 - Red Plating

Post by Enzo » Mon Nov 22, 2021 3:19 pm

SO pull the power tubes. The circuit voltages will still be there. -32v sounds way too low to me. You need to get it up to more like -50 at least. Scope that -32v, is there a lot of ripple on it?

45mv? 17mv? did you mean 45 volts and 17 volts? If you meant 45mv positive, then you have lost bias there for sure.

GTS
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Re: VK100 - Red Plating

Post by GTS » Tue Nov 23, 2021 9:45 am

With the tubes removed the Voltage at Pin 3, 4 and 5 doesnt change or i didnt wait long enough.

With the Tubes installed the Voltage at Pin 5 starts to drop to -48 Volt (dont know why i measured 45mv, maybe bad connection).
After a while the Voltage rises to -32 Volt and the Tubes start to glow.

When i switch to standby the Voltage Jumps back to -48 Volt and the glowing stops.
Also the Screen and the Plate Voltage drops down a bit when the glowing starts.

Beach
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Re: VK100 - Red Plating

Post by Beach » Tue Nov 23, 2021 10:18 am

Pls measure at every pin 5 of each tube. So four measurements. It should not drop so much in Volts.

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Enzo
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Re: VK100 - Red Plating

Post by Enzo » Tue Nov 23, 2021 11:06 am

Please. Take the power tubes out. There is no reason to burn them up while we hunt for your bias voltage.

SO you have -48v on the grids? (Pins 5) On the sockets that red plate the tubes, does this voltage now stay -48 or does it start sinking down to -32 or similar?


MArk the tubes you use, 1,2,3,4. If you install them, note which two red plate. It does matter which, because it identified which is which in the schematic. Now swap places. DO the same two tubes red plate in different sockets? Or do the same two sockets cause red plating, no matter which tube is installed? One shorted tube can pull down the bias on the tube next to it, thus red plating in pairs.

It is quite norma in anay tube amp for B+ to drop as tubes start ti red plate. Red plating occurs because excess current is flowing through tehm. That current drags down the B+.

GTS
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Re: VK100 - Red Plating

Post by GTS » Wed Nov 24, 2021 9:19 am

Yeah the Voltage of Pin 5 starts at -48V and slowly goes to -32V and maybe further but at -32V the glowing starts and i switched off.


I got 3 Sets of Tubes for the Amp.

The Old resoldered Tubes with the terrible Sound (Maybe the result of the Overheating).
Two new matched Sets of Tubes.

And the Amp didnt Work with all 3 Sets!

Today i swapped around the Tubes and found that Set 1 of the new Tubes had 1 bad Tube and Set 2 of the new tubes had 2 bad tubes.
There have been some scary moments while testing.

In one config, two tubes started to glow blue and made a hissing sound and in a different config the amp started to oscillate with a very high frequency.

after i sepperated the bad tubes from the good tubes i measured the bias current from all tubes in 1 place of the amp and selected 4 tubes which are close to each other.

Now i have a nearly matched set of tubes in the Amp and it works without red or blue glowing tubes.

I know thats not a perfect Setup but im waiting for the parts to build my own tube tester.
Ill write the company where i ordered the tubes and i hope to get at least 1 new set of matched tubes.

I tested the Amp for nearly 1 hour and i had no further trouble. But ill test it a few more times for a longer time.

Thanks for your help!!!

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