Diezel Vh4 Bias Problem

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in_flames927

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Hello RT,

I recently bought a new matching quad set of JJ KT77's for my "2012" Vh4 to replace the ones that came stock with it, and also a bias probe from Eurotubes (ET Pro One). I installed the "new" quad of KT77's and hooked up the bias probe. My intent was to set the amp back to it's stock 35mA per tube. But when I try to go above 25mA, the amp suddenly starts to hum, and the bias probe stops reading correctly and shows 1. mA/ 1. vdc.

So I switched back to my (old... but still working) KT77's to see what kind of reading I could get. With these old tubes, I could adjust the trimmer up to 32mA, but after 32mA the same hum would appear, along with the bias probe suddenly reading 1.mA/1.vdc.

Any ideas why this is happening?

Thanks
 
Weird, I reckon your multimeter is set to the right setting?

I just recently biased a VH4 and didn't have such a problem

Maybe try the Diezel subforum?
 
You don't happen to have the 34/6L6 switch on the top of the chassis to 6L6 do you?
 
steve_k":2dtqnm73 said:
You don't happen to have the 34/6L6 switch on the top of the chassis to 6L6 do you?
Click to expand...

Do the more recent models still have that switch?
 
Are you adjusting the wrong trimmer? Did you make sure it was the bias trimmer and not the hum trimmer?
 
jonl":391htxkv said:
Are you adjusting the wrong trimmer? Did you make sure it was the bias trimmer and not the hum trimmer?
Click to expand...

Didn't think about that, makes sense with the described problem

The bias trim pot is next to the MIDI board, you can see a video on youtube (in spanish) describing the procedure
 
jonl":1g1l2zo3 said:
Are you adjusting the wrong trimmer? Did you make sure it was the bias trimmer and not the hum trimmer?
Click to expand...

I suspect this is the issue, too. Hopefully that's it, because it's an easy fix.
 
Thanks for the quick replies.

My Vh4 is definitely set in the El34 setting, not the 6l6.
My bias probe is an Eurotubes "Pro One". It doesn't have any switches or settings to mess with, you just hook it up to one of the tubes and turn the amp on.
In the first picture below, I have a white arrow pointing to the trimmer I was adjusting. I believe this is the bias trimmer.

I hope these details give a clearer look into what I was doing right.....or possibly doing wrong.
 

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The arrow in that picture is pointing to the bias trimmer. I'm not familiar with that Bias Probe thing. Does it show the milliamps? Or do you have to do some math to convert from what it's showing you to get to the milliamp value?

Have you seen this page? viewtopic.php?f=14&t=24410

It's for using a digital multimeter (which is what I use), but it at least talks about the general procedure. Nothing in there helps, you could post this in the Diezel sub-forum and someone (probably Peter) will contact you about what's going on.
 
Hello again,

I may have solved my issue. I don't know if this is a mistake on my part or not, (this being my first time biasing an amp), but the presence knob seemed to have an effect on the threshold at which I could set my mA to.

First, I turned the presence knob all the way down... and the probe would only let me bias the trimmer up to about 15mA.
So I did the opposite and turned the presence knob almost all the way up... Lo and behold the bias probe now lets me turn the the trimmer to 35mA and beyond!

I don't know if this was an amateur mistake on my part, or if it had something to do with the bias probe... but thanks for the quick responses and baring with me.
 
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