Valveking 112 (Alternative) Bias Mod

sizzlingbadger · Post by **sizzlingbadger** » Fri Jan 28, 2011 2:34 pm

Here is a photo of my bias mod to the VK 112. I couldn't get a 50K locking pot here in NZ so I did it slightly differently. I used a 50K trim-pot and mounted it inside the amp with access through the rear panel. There is a 15K resistor in series with it. I also added test points to the rear of the chassis using 1 ohm resistors in the power tube grounds on pin 8.

Tedzomplin · Post by **Tedzomplin** » Fri Jan 28, 2011 4:19 pm

Excellent ! Very NICE work.

CharlieP · Post by **CharlieP** » Fri Jan 28, 2011 4:39 pm

I like that! Great work!

CharlieP · Post by **CharlieP** » Fri Jan 28, 2011 4:42 pm

I don't see a resistor in series with the pot. Recommend install one! I took Enzo's advice to do that, protects the circuit from dead short!

Fourtythree · Post by **Fourtythree** » Fri Jan 28, 2011 9:05 pm

Nice looking work you did there. Where'd you find your bias jacks? I like yours better than the ones I got from Weber.

sizzlingbadger · Post by **sizzlingbadger** » Fri Jan 28, 2011 11:29 pm

Thanks for the kind comments.... I think it looks pretty tidy.

The resistor is in the wire between the pot and the PCB, you can just make it out in the photo under the white mains wire, it has clear heat-shrink on it. The jacks were just kicking around in my box of spares, I can't remember where they came from originally...

The tubes were biased at 24mA from the factory, I have set them to 35mA and the amp sounds much warmer now, definitely worth the effort. I have a couple of Tung Sol KT66's winging their way to me in the post

humbas · Post by **humbas** » Fri Mar 18, 2011 1:18 pm

What kind of resistor do you use?... 15 khom?... What Voltage (Watt)?

sizzlingbadger · Post by **sizzlingbadger** » Sat Mar 19, 2011 7:37 pm

A 15K resistor in series with a 50K pot. 1/4 watt is fine.

humbas · Post by **humbas** » Mon Mar 21, 2011 5:38 am

Thank you.

humbas · Post by **humbas** » Mon Mar 21, 2011 5:44 am

...sorry, but can i put 1/2 Watt?

CharlieP · Post by **CharlieP** » Mon Mar 21, 2011 7:06 am

humbas wrote:...sorry, but can i put 1/2 Watt?

Yes!

humbas · Post by **humbas** » Wed Jun 15, 2011 5:42 am

I made the mod and it works fine... i realy warm the tubes! when i test the stock tubes i read 12.5 mA, it`s this possible?... I put them at about 32 mA and the result was very good. The only thing that i didn`t like was a "humm" that appear after this mod. I didn`t ground the new pot.. can be this the cause of the problem?

humbas · Post by **humbas** » Wed Jun 15, 2011 5:45 am

PS: Brand new tubes "TAD 6l6WGC-STR"

humbas · Post by **humbas** » Wed Jun 22, 2011 5:55 am

Another problem is that with this setup, i can`t go above 32mA with the pot in maximum position... Do you know why? Thank you.

Fourtythree · Post by **Fourtythree** » Wed Jun 22, 2011 8:26 am

Try a different resistor value.

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Valveking 112 (Alternative) Bias Mod

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