Biasing question, faulty tube?

  • Thread starter Thread starter insomniac
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insomniac

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Hey!

I've followed the biasing tutorial/thread to bias my VH4 amp. When I test the yellow wire on the OT I get 70mA, but when I test the blue wire on the OT I get 35mA.

Does that mean I have a faulty power tube?
How can I spot it with the multimeter? From the outside, all the tubes look OK.

Ron.
 
You can also have a broken screen grid resistor.
 
Thank you Peter, that's a more serious problem that I thought.

Could you guide me and tell me where that broken resistor might be?

I'm writing from Israel. there is no reputable amp tech here that I could trust enough with this amp, and shipping it to you guys might cost around $1400 both ways (I might as well just get you a flight ticket :) ). I've built some guitar effects the last couple of years so I got basic skills with a soldering iron. Is that something I could do by myself?

Anyways - Here are some pictures of the power tube sockets and the mainboard just in case. How can I find out where the broken resistor is?

Ron.
 
It´s the one that looks burned. The red letters
are brown and seem to disappear.

You either can buy a 1 kilo ohm / 5 watt type or the
1 kilo ohm / 9 watt type we used in the amp.

Best,
Peter
 
Peter needs to give the folks at ENGL a mastercourse in customer support....NEVER receive a response back from any contact initiated through their support form.
 
Ok, that looks quite easy to replace.

Could that explain why there's a big gain reduction on channels 3 and/or 4?

Also, I've looked around the amp and there seem to be more broken resistors like this one, connected to the fault tube LEDs. Anything I should do with these?

Here's a picture.

Ron.
 
On newer models we use 100 kilo ohm / 2 watt resistors
and blue LED´s. This combination is more safe when it
comes to blown tubes.

Only hardship is to push out the old red LED´s. It can be done
with a rubber tipped hammer.

We have an extended service manual. Please send an
email to peterdiezel (at) diezelamplification (dot) com

Best,
Peter
 
I've been to Thomann two weeks ago (was touring with my band) and discussed the issue with Peter Stapfer, with his advice I got the TAD 7025WA for the V1 and TAD 12AX7A-C for V2-V7.

I replaced them all. No change.

I should mention that some time ago, a band that used my amp accidentally used a broken cable to connect it to the cabinet in another room. It was on for about a whole day before they noticed the cable was broken and no sound came out of the cabinet. When they got it back I found out that the 3rd and maybe 4th (if i remember correctly) fault tube fuses were blown up. I replaced them and the amp seemed to work again, but I think the gain reduction might be since that time. My guess is that's why the all these resistors are fried up as well.

I'm actually waiting for having a skype chat with Peter regarding this issue.

Ron.
 
Wow, I would murder them if they so non-chalantly did that to my amp... never lend your girlfriend or your Diezel away!
 
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