Kinda URGENT: Some quick VH4 bias help please???

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philbag

philbag

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I'm in the middle of biasing my VH4. I have a 2002 model with 2007+circuitry. The output transformer has 5 taps instead of the more modern 3 taps; it only has 3 wires on the 5 taps though so I assume theyre basically the same. ANYHOW. I've hooked my black croco to the middle tap (7) and my red croco to the tap next to it (tap5 or 9, whichever side I wish to check with my DMM).

I have my DMM set at 200mA DC. I turn the amp on and I'm getting negative readings on my DMM (ie, -60mA etc). I wound the decreased the bias trim pot almost to nothing prior to turning the amp on just so I could bring it up steadily. I was thinking perhaps I'd go above 0 and into the 30-33mA territory Olaf/Diezel website describes as ideal for EL34 bias... but the pot wound out just under 0mA. I immediately wound the bias trimpot back down and turned the amp on standby.

What am I doing wrong? ANY info would help! This is my first time retubing/biasing an amp myself so I've got no experience, :lol: :LOL: .

Thanks in advance!
 
This is a shot of the OT in my 2003 VH4.

2003VH4.jpg
 
Thanks for the input. My red lead is indeed plugged into the mA socket on the DMM. I only have two options for red lead insertion on my DMM. The one I'm using is labelled 'V*ohm sign*mA*diode sign*', the other is listed '10A MAX'.

Picture181-1.jpg


Green circle indicates what setting my DMM is on during operation.
 
Mark2c, thanks for that pic! Yep, mine looks almost the same, except some different coloured wires around the tranny, but yes i have the black croco on the tap with the 'orangey' wire soldered to it, and currently have the red croco hooked to the tap with the 'yellowy' wire soldered to it.

p.s - thanks for the speedy pm reply, i will definitely send you an email in the next couple of days!
 
I am not so familiar with your meter. What is the reading of 200 in red, on the left used for ?
 
Ohms apparently. The other readings are Volts DC and Volts AC. I have it set in amps DC, selecting the 200mA option.
 
O.K., I found a photo of your meter. Try the 200m position under Voltage and see what reading you get.
It should appear as a -.032 etc.
 
thanks alot, i will try that out tomorrow. Would the .032v type reading in that mode be equivalent to 32mA?

Argh i wish i knew why the readings are negative. I'm not even sure if i can use the negative reading the same as a positive reading? Ergh, i just want my vh4 back online! It's been outta commission since march, im dying here.
 
Bias is a negative voltage. Once you get your meter sorted, the -.032V will be the negative volts being supplied to your tubes.
I just want to be sure you did not blow a fuse on the meter because we really need to see mA's.
 
Hi there,
So I've just tried reading the voltage DC (200m setting Mark2C described) and got a value of... 1?

I recorded this video to illustrate what happens when I'm measuring in 200m Volts DC. Doesn't change if I turn the bias trim pot.
https://smg.photobucket.com/albums/v90/philkore/?action=view&current=biasfun.mp4

This video is showing what happens when I'm measuring in 200m Amps DC.
https://smg.photobucket.com/albums/v90/philkore/?action=view&current=biasfunmA.mp4

I'm thinking I may need to go buy a good DMM. This one is my uncles and is probably busted for some reason.
 
The second video was correct in the 200mA setting. Set it to -70mA.
 
Swap the red and black on the OT--I had the same thing happen.
 
racerevlon":3t7ci4sr said:
Swap the red and black on the OT--I had the same thing happen.
Click to expand...

Phil has it right, so no need to swap the leads.
 
racerevlon":29lhmn7t said:
Swap the red and black on the OT--I had the same thing happen.
Click to expand...


So I'd be using the black clamp as the tester switching between the sides of the output transformer and using the red tester as the ground?

Might make it a little clearer to bias in that fashion.
 
Well I gave the VH4 a solid blasting last night for about an hour or so and it seems to be perfectly fine. Sounded HUGE. Bias complete! Though I got a small spark of my output transformer when changing the crocos over and it seems to have blown out my DMM :lol: :LOL: , can't recheck the bias just yet. Hopefully will get around to buying a new one tomorrow, double checking the bias and slipping it back in the chassis.

One thing, I turned my lights out and looked at the tubes while in operation, there seems to already be a very slight blue glow on parts of the tubes - hopefully they're not screwed up already. I have too much gas biting at me to have to buy more tubes.
 
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