Marshall 2203 bias balance help sought

I was checking the bias on my 1988 2203 head tonight after fitting a set of new GT EL34Rs. I was using a biasright type tool. On the first tube I got a reading of 25 ma. Way low so I set it to 38ma. I then measured the third tube and got a reading of 45ma. A bit high from what I have read but I was more surprised about the difference between the 2. I went back and forth a few times and left it at 35ma and 42ma. I have biased 2 power tube amps before and never seen more than 1ma or so difference between the pair. This is my first 4 power tube amp.

What could be causing this? The tubes are a proper matched GT quartet so they should be ok. Could something in the amp have drifted? What effect would a difference in bias like this have on the sound? I did not try the other 2 tubes - didnt seem much point if there is a problem with the amp.

Can anyone help?
Thanks
Neil

 

Thanks for the input.

It didnt occur to me that a matched set of tubes would give different bias results. I went back and switched tube positions around to try and determine if the tubes were the cause or the amp was the cause.

The results. Between the 4 tubes in the 4 positions I noticed a maximum difference of 15ma between the highest and lowest bias current readings. The typical difference was 10ma. The highest reading was consistently achieved with one particular tube, the lowest with another particular tube. The other 2 tubes fell inbetween these readings. This confirms that the tubes are responsible for the problem.

I did notice that moving a tube to another location and then back to its original position did not necessarily give the same reading. It could be ~1+ma different. Not sure what causes this (amp could be drifting with time and or temperature, measurement error).

I thought matched tubes were matched for bias current draw along with transconductance etc etc. If so then these Groove Tubes are suspect. I understand that tubes are delicate and that a knock could change the bias but this seems extreme and such a susceptibility would render them useless for and amp that is to be moved around in a car or truck.

I dont know if this is range of differences in a tube is acceptable. I suspect it isnt and I think I just bought my last Groove Tube product.

 

Hi John,

The tubes are the Groove Tube EL34R (I think R stands for Russian - I dont know which Russian company though).

I did let the tubes settle before taking the reading and waited until the current measurement appeared to have settled before taking the reading. I probably could have got more reproduceable results by leaving it longer.

I havent spoken to GT yet thats a good idea.

Cheers
Neil

 

Here is the upshot.

I took the tubes back to the store and it was clear from looking at the boxes they came in that they were not a proper quartet. One box (of 2) was from a quartet and the other box was a duet. Somehow they had become taped together (as GT does for its Quartets) and sold to me. Impossible to say if this was a store problem or a GT problem. If I had been awake when I bought them I would have realised the problem and saved all this hassle.

The store replaced them with a (real!) quartet of GT E34LS's which are now fitted and sound amazing. I shouldn't really try and compare results from 2 (probably wrongly biased) duets with a quartet but the E34LS sounds much better than the EL34R.

Thanks for the help.
Neil