VH4 Repetitive Tube Fault - solved

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mivens

mivens

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First posting so here goes...

Bought a 1yr old VH4 recently. Gigged once with no probs but second gig I got tube fault LEDs on position 1 & 2 during sound check - a bit concerned but gig was probably best ever. Assuming I had lost 2 tubes (Winged-C EL34) & that they were over a year old anyway I ordered replacement matched quad.

Before fitting & biasing I measured bias current on one of the remaining 2 tubes which was 39mA - not too bad, but planned to set 35mA for new quad. After fitting new tubes & fuses I measured bias current using 1ohm type probe in position 4 - it levelled out at 35.5mA after about 10mins so I was happy no adjustment was needed. Then I put the probe in position 1 (for grins) measuring 33.8mA after switch-on & it gradually started to climb as it warmed up - put the kettle on. After about 10mins there was a pop & fuse fault LEDs 1 & 2 lit up again - Arrrgghh!

I can't believe 2 tubes from the new set are faulty too. I can only assume there's a problem in the amp. I'm about to check plate resistors in case one has broken down, but any other suggestions would be welcome.

Cheers
 
We have 30 % broken new tubes during our test managment ;)

Btw, welcome to the forum :thumbsup:
 
Many thanks for the super fast response & Wow that's a high % !

Got the new tubes from Watford Valves & never had a bad one from them over several years (maybe my time has come however) so before approaching them about this I plan to replace all fuses & switch over the 2 pairs of EL34s ie. 1&2 with 3&4. If the tubes really are bad then I would expect fuses 3 & 4 to blow next time.

I'll post update tomorrow.
 
I'm not even kidding go to the RUBY EL34BHT's.Mine was doing the same thing and when I switched over to those not one single problem. They sound great too. Also bias lower about 60ma to 64ma a pair no need to go higher, actually gives it a little more dynamic too, not so compressed.
 
First of all let the amp run for at least 5 minutes before your final adjustment.

Second, bias around 31-32ma, after a while in my experience it climbs up over 35ma anyway.

Third, check the bias in a few days again, I've seen readings going overhead within the first week.
 
Thanks for the tips. Here's the latest...

With the pairs switched over the amp seems to be stable, so I measured the bias current on each tube to check for anything suspect. Although these are supposed to be a matched quad I got 30.5, 35, 36.2 & 31.8 mA after 10mins. Also 3 of them leveled out OK after warming up but the tube measuring 36.2mA continued to drift between 35.5 & 36.2.

So I will take all the advice on board & return the winged C's as a bad deal plus potentially unreliable & get some Rubys instead (never tried these). The amp is being used on an album & it's nearly finished so only reason I went for the same tube type was to get a consistent sound across all tracks. Oh well a bit of variety is probably a good thing.

Cheers all
 
Asmall drift is ok and depends on Your wall power
 
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