VH4 Bias readings....

[Axegrinderturbo]

Hi guys,

I have recently changed all valves on my VH4 (matched quad of winged C EL34’s and a JJ set of pre amp’s from Watford Valves). I adjusted the bias when I put the new valves in set to 35ma a pair with no problems at all!

I was playing the amp a couple of days ago and I felt the sound wasn’t as great as it should be with nothing changed about my setup etc. so I though I’d check the bias over the weekend just for piece of mind. I checked the bias for the yellow wire on the OT which was showing as 37ma so I adjusted this to 34.5.

(The wire layout I have from the back of the amp to front is Blue, Red, Yellow – see the photo below)

I then repeated this checking the blue wire however the reading was 03.7! This may sound stupid but is this correct and merely another way of displaying 37ma or is there a problem?

Also with the bias running at 37ma when I initially checked the amp, will this cause any problems?

This is how I had my multimeter set up:

Any comments would be appreciated

 

[MARK2C]

37mA/mV is not a problem. Tubes drift a bit during break in and 75% MPD would be 39mA/mV, so again your fine.
It's most likely that the fuse blew in the meter to give you a reading of 3.7.

 

[King Crimson]

I adjusted the bias when I put the new valves in set to 35ma a pair with no problems at all!

If you are saying that you biased 2xEL34s in the VH4 as a pair to 35mA, then that is very cold. You should be more at 70mA per pair.

Typical Diezel power tube bias settings pair-wise:

EL34/6CA7: Aggressive, slender low end, higher total harmonic
distortion… Recommended bias setting: 60-70mA

5881/6L6: Great tube for clean sounds, good bluesy tones, also has
strong bass and excellent clarity… Recommended bias
setting: 50-70mA

6550: Very loud, very good dynamics, strong low end, amazingly
percussive… Recommended bias setting: 80-100mA

KT88/KT100: Similar to 6550, but very punchy with great clarity and
warmth… Expensive… Recommended bias setting: 70-100mA

 

[MARK2C]

I adjusted the bias when I put the new valves in set to 35ma a pair with no problems at all!

If you are saying that you biased 2xEL34s in the VH4 as a pair to 35mA, then that is very cold. You should be more at 70mA per pair.

Typical Diezel power tube bias settings pair-wise:

EL34/6CA7: Aggressive, slender low end, higher total harmonic
distortion… Recommended bias setting: 60-70mA

5881/6L6: Great tube for clean sounds, good bluesy tones, also has
strong bass and excellent clarity… Recommended bias
setting: 50-70mA

6550: Very loud, very good dynamics, strong low end, amazingly
percussive… Recommended bias setting: 80-100mA

KT88/KT100: Similar to 6550, but very punchy with great clarity and
warmth… Expensive… Recommended bias setting: 70-100mA

Very good point and thanks for clarifying.

 

[Axegrinderturbo]

Thanks for the responses guys,

Whenever I've biased the amp I've always followed the tutorial supplied on the Diezel website so I believe this is measuring one pair at a time (keeping the multimeter on the centre wire on the output transformer then checking the reading on the remaining two wires - in my case the yellow and blue).

The tutorial provided does not specify a way of reading each tube individually (can this be done?) so like you say the readings I have of 35ma are way too cold.

I'll try raising the bias on Wednesday when I'm next in our rehearsal room and keep you posted!

Thanks again!

 

[MARK2C]

The ground/black lead on the meter stays on the center lug/red wire. The red lead is moved to measure the reading at the yellow and blue wires.
It seems you have the procedure down, but if it read 35mA, each tube in the pair is biased at about 17mA.

This biasing method is very accurate, even more so depending on the quality of the meter. The only way you can check each
tube is with a Bias King or Bias Rite type meter. A good matched quad would make that type of purchase unnecessary.

 

[Axegrinderturbo]

Thanks again for the reply.

I did get a matched quad from a supplier I have used with no problems before so I trust that they were matched correctly. I'll try the adjustment tomorrow and let you know how it goes!!!

 

[Axegrinderturbo]

Hey,

I've been back into our rehearsal room yesterday in order to do a full re bias. J turned the amp on and noticed the tube failure lights on 1&2. I changed the fuses across all tubes and the lights went out. I then carried out the bias and increased it to 70ma however although on one side I was getting 70ma as recommended, the other side was showing 39ma.

The valves were purchased from a company that solely deal with providing valves an were a matched quartet. My question is this; are the different readings only down to a poorly matched quad or could there be another reason? Does the tube failure lights suggest anything?

I did try the amp with the bias readings I have mentioned and it sounded great. Any help would be appreciated.

 

[i.ak]

Does the same happen if you change the order of the tubes in the quartet? If the mismatched reading follows a particular tube the problem is then in the not-that-well-matched tube. If the mismached reading stays in the same tube position/socket even though you inserted different tube in the socket then there might be something else causing the problem.

I assume that your tubes were also properly warmed.

Did the tube fault leds come on again? If so are the tube fault leds always on at the same tube positions (1 & 2) even though you insert different tubes in those sockets?

 

[Axegrinderturbo]

Since I've changed the fuses the tube failure lights have remained off. I've checked the tubes in different orders but still get the same readings however I tried my original set off tubes (about 3 years old) and the bias readings were more or less equal for both front and back plate.

This must therefore mean the problem is simply the new set of tubes I got have not been matched correctly??? I must say that the new tubes even with the 70ma/40ma readings sounded absolutely awesome.

Final question is will playing with the tube set that are providing the 70ma and 40ma the other cause any damage to the amp?

Thanks again!

 

[i.ak]

To me it seems that the problem is in the new tubes and their matching, because the old quarted works fine.

You need to check this from someone more qualified than I am, but I think that if you use the tubes with pairwise "non balanced" current readings there will be no potential problems for the amp itself (despite possible audible tonal differences), but the tubes will very likely not live as long. Or then there is, thus check from someone who really knows.

By the way, if you missmatch the tubes I advice you to bias it according to the one which gives you the highest readings. From my experience, using mismatched tubes also adds the level of hum.