How to bias your VH4 w/DMM - attention lethal volts inside !

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duesentrieb

duesentrieb

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Biasing Tutorial

VH4

Here we go (keep in mind that there are 500 Volts on the OT, be careful, make sure the crocos are tight on the OT, use an insulated screwdriver for the trimmer . . . ):

1 ) take the chassis out of the headshell (and have a look if the bias selector-switch fits your tubes - it is labelled "EL34 and 6L6") and place the amp - pots facing you - onto two books, watch out for powertubes !

2 ) on the right you can see the output-transformer (OT). Its right side (red wires) is the secondary - you need to hook the crocos to the primary, so the left side


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3 ) Set DMM to 200mA DC current (many DMMs need another slot for the red for current for voltage !)


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4)Hook crocos to DMM leads and the black to the middle of the three primary OT wires, red to either left or right of it


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Switch amp on, standby run-position, hooked to load - be careful !

5 ) read current and adjust desired value with the bias-trimmer on the left side of the mainboard, close to the midi board . . .


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6 ) this way you are reading the current for one side of the quartet (so 2 tubes), which means that should read 60mA for EL34.

7 ) Switch amp off, you may want to repeat the whole procedure for the other side - in the case of this amp using the blue wire . . .

8 ) Put amp back into headshell after you've unhooked from mains

9 ) Enjoy your fresh biased VH4 !

Recommended current settings for pairs in a VH4:

5881: 50 mA
6L6: 60 to 70 mA
EL34 60 to 70 mA
6550, KT66, KT88: 80 mA

VH4S:
In the VH4S you have two output transformers, each hooked to 2 tubes. What you do is exactly the same as for the VH4, but twice. And the reading you get from one side of the output transformers (the current) is just for one tube (!).

So take those recommendations and use half of it . . .

5881: 25 mA
6L6: 30 to 35 mA
EL34 30 to 35 mA
6550, KT66, KT88: 40 mA

If you have further questions, let me know.


Olaf
 
That is the description I was talking about :) Thanks for posting!
 
It is on the official site (hidden though). Its always a tough decision, you know - for the average user, who never biased an amp before this is way too much. For experienced guys though . . .

So if you are unexperienced, please copy it and give it to your tech.

Olaf
 
I read in a different post about biasing each tube. I have a TAD Bias Master which can read levels of each tube at the same time. With only one bias trimpot to adjust aren't all of the tubes being adjust at the same time when adjusting that pot?
 
Yes. And your readings include the screen current (you are reading at the cathode), so you should set the bias pot to 33mA for each tube . . .
 
So, my man duesentrieb, howzabout an instructional on biasing a Herbert.
 
Hi,

I've just changed for the first my power tubes and adjusted the bias yesterday, following Diezel biasing instructions, and I have a few questions:

_is it normal to measure a negative current with the DMM?

_although my new tubes are a matched quartet, the currents measured between blue and red, and yellow and red are not the same: current between blue and red is nearly 5mA smaller than current between yellow and red....is it normal?

_what is other trim pot on the right of the head (when the head is upside down)?
 
1 - neg: when you flip the probes its positive, curent has no + or - , either flowing or not
2 - 5mA difference is ok
3 - that the hum pot which affects symmetry of the filtering to ground. Just tweak it a bit to the position with the least hum
 
ok thanx a lot for thoses informations...

but concerning the hum pot, should I tweak it with my guitar hooked on the input? on the clean channel or on Channel 4?
 
Hello - I'm about to attempt to bias my VH4 for the first time and this seems mighty helpful, but the pics aren't showing up - caould you please re-post it with the pics intact? Thanks!
 
Thanks for the pics and the very helpful video...I'm afraid I may have made an error though.

I did everything as indicated, and was getting DMM readings of about 52.3 and since I'm switching to 6L6s I was ready to adjust - I DIDN'T MAKE ANY CHANGES - but when I re-applied the DMM probes to the OT I now only get a reading of 0.0

Also, the the amp hum (I have it connected to a speaker) cut initially (when I had been able to get the reading) - now I can't get it to stop, which say to me that the DMM isn't interrupting the circuit, am I right?

FYI, I'm still able to read the voltage on the plate, so it's not a problem with my DMM, at least I don't think so. Other than changing the DMM's range knob from 1000 (DCV) for the plate and 200m (DCA) for the tubes there's no difference, is there?

Heaven help me, I hope I didn't ruin my amp - I'll die without it! Any suggestions? HELP! :-)
 
Can you check the fuse inside your Multimeter please?
 
Dear Olaf,

Thanks for the very clear tutorial! Will this information stay available at the top of the forum, as a "sticky"?

Once you have set the bias of a set of power tubes, is it necessary to check the bias at regular time intervals (due to aging of the tubes for example)? What is your advice on that?

Regards,

Jan
 
Hi Jan,

even if we don't make that a sticky, you'll find it via the search.

Under normal conditions (hobby-ist) I'd say twice a year is ok. Pros, playing 3 times a week, more often of course, monthly I'd say.
And: the colder you set the tubes, the less often you need to check the bias, thats why IMO 30mA (for EL34s) is cool: sounds good and the tubes will stay alive very very long . . .
 
Hey Olaf,

Quick question.

I went to re-tube my VH4 with 6550's, and when I went to measure the mA, following your instructions, I was getting a reading of -0.08 mA, and if I transferred the red lead to the other side (blue cable), it was measuring -0.14 mA. I could most certainly have it set up wrong, but just for reference :

IMG_4624.jpg


Do I have it right?
If I moved the red lead to the slot next to the one it is in (on the DMM), it reads nothing.
Could this be related to a faulty fuse? (read below)



Also, I transported my amp to a recording studio I'm currently in and out of, and noticed that when we went to start tracking guitars through the VH4, socket number one's (looking at the back, left to right) tube fault light was lit up.
I assume this means the fuse is toast?
This is back when the amp had EL34's.

I also noticed that the same tube that had failed, also seemed to not sit in the socket very well.
I tried the tube in the other sockets and it was fine, but it just seemed to be this one.

IMG_4623.jpg



Is this grounds for concern?


Thank you for your time!

-Taylor
 
Hi Taylor,

yes, please use the other socket (mA) and check the fuse.
yes, your tube fuse inside your may have been toasted as well
this happens: the tube is ok this way in the socket.

When you leave the black in common and put the red into the V (right) slot, and set your DMM to the "radar" symbol (almost 9 o'clock) then you have a beep when both probes are conencted. You can use this function to check fuses. If it beeps they are ok (hook the probes to the two metal sides of the fuse), if not they need to be changed/are blown.
 
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