Bias-help please!

[Vigier Arpege]

I'm trying out a bias-probe, the multimeter is set to 200mV. When I turn the amp on the multimeter reads only 2.2 mV, I've turned the switch to EL34 since my new tubes are EL34BHTs.
I haven't touched the trimmer pot yet, just like to know if I've wired everything right, don't want to blow anything in my beauty.

So is this right? Thanks guys!

 

[Rødskjegg]

Somebody correct me if I'm wrong but you need to set your meter to mA.

 

[Vigier Arpege]

Is it not mV with a bias-probe?

 

[Rødskjegg]

I don't have any experience with bias-probes so I could be wrong but you could try and set it to mA.

 

[Vigier Arpege]

I'm pretty sure mV is the right one with a bias probe, but I'm surprised the multimeter only reads 2.2 mV when I turned the amp on. So I want to be sure I've done everything right before I'm turning the bias-trimmer...any experts here?

 

[duesentrieb]

mV is correct.

Please set DMM (to measure the internal resistor) to 200 Ohm and read the Ohms between PIN 1 and 8 (those left and right to the cavitiy). Report the value.

 

[duesentrieb]

Is this the bias probe from Tube Town?

 

[Vigier Arpege]

yes it's from Tube Town, is it really necessary with the Ohm-thing? Think I need some pictures/help with that...thanks!

 

[duesentrieb]

Then it is one Ohm.

2,2mV with the new tubes?
What do you read for the other tubes?

Don't worry, just set trimmer to read 33mV each tube.

 

[Vigier Arpege]

If you look at the picture it says 2.2 mV....if I hooked up everything correctly.

Can you see if it's right? (2.2 mV is very low isn't it?)

What can be the reason for this, the previous owner has never biased the amp can this cause it?

I try the other tubes too, bee right back...

edit: the first reads 2.2 mV, the second tube reads 2.0 mV, third reads 2.1 mV, the fourth reads 2.6 mV...so is it safe to turn them up to 33 mV, I don't blow anything?

The previous owner said the amp has been modded by Peter, not sure if that has anything to do with this though?

Really appreciate your help

 

[nbarts]

Turn the amp off, turn the bias trimmers all the way down & then try again. I had similar readings our of mine, turned out my bias trimmer were set way too high. Also check all the fuses inside the amp.

 

[nbarts]

Start with the trimmer all the way down & bias to where needed. Even in case your device shows the readings wrong (highly unlikely), when you go too high you'll see the tubes glowing really freaking bright. That would be the time to back off really fast!!!

 

[Vigier Arpege]

Ok, I'll try that.
The amp wouldn't work if the fuses inside the amp are broken, or...?

 

[duesentrieb]

Yes. Broken fuse = no signal.

Just turn the trimmer up while reading one tube to 33mA.

 

[Vigier Arpege]

Thanks for your patience and your help

 

[Hartmut]

Don't worry, just set trimmer to read 33mV each tube.

Just turn the trimmer up while reading one tube to 33mA.

Hmm now I´m confused... is it mV or mA??

 

[duesentrieb]

When using an adaptor like this, the (idle current) mA are "converted" to be read as mV (thats what you read on the DMM).

 

[Vigier Arpege]

I finally biased my VH4 to 33mV, will this change over time and is it a good idea to check this regularly?

The bias-trimpot was almost fully counter-clockwise so I had to turn it almost a 180 deg. clockwise, very strange...

After biasing:


33mV

 

[duesentrieb]

As long as the tubes don't get red plates (due to adaptor or user error), you're fine. Crank the master, hit a chord and see if they get red plates. If not: everything is ok.

Play it for an hour, then recheck (one tube is sufficient) and re-adjust. Then you are safe for a long time (say 6 months or so).

 

[Vigier Arpege]

OK, thanks!
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