Mark IV running on only 3 power amp tubes???

[boola]

Hi,

Just got my s/h Mark IV today and I think one of the tubes in blown.

It's one of the outside class A EL34's. It doesn't glow AT ALL. The other 3 glow - probably far too much!

I put in an EL34 from my Marshall and turned it onto standby, sure enough it started glowing so I'm sure the original tube is dead.

The strange thing is that the amp is working fine with the duff tube! It sounds ok, perhaps not as bright as it might but I think all the tubes need changing.

So is it ok to use it like this? Is the other EL34 going to blow quite quickly? I am doing a full re-tube but is it ok to play it in this condition while I wait for them to arrive?

Maybe it hasn't gone but the other is drawing too much current so it is not glowing? Does anyone have a clue what might be happening?

I wanted to use the Bias rite to check the current but looks like I'm going to have to take it out of the chasis to do that

Thanks!

 

[Shep]

it's most likely dead heater in the tube.. i have had that happen in amp's i have worked on and they still work fine.. just poor head room and awful tone.. put a new quad of power tubes in.. i wouldn't run it with that marshall in you have no idea what it's drawing..

 

[boola]

cool, yes that sounds a likely explanation.

I didn't run with the Marshall EL34, just kept the Mark IV on standby to see if the valve glowed and it did.

 

[HalfDeaf]

Make sure the tubes in the outer sockets are matched. If you want to run EL34s from a Marshall, its probably ok, just as long as you switch them both (and they aren't drawing more load than the Mesa tubes - this can be tough to check, unless you have a tube tester). The inner tubes should be matched as well, but they don't have to be a quad with the outers.

 

[phyrexia]

Hey Shep, if a heater died wouldn't the tube act like normal after you'd been playing it for ten or so minutes and the tube had heated up? Or do they require constant 6.3v in addition to signal to make them warm enough?

 

[Shep]

Hey Shep, if a heater died wouldn't the tube act like normal after you'd been playing it for ten or so minutes and the tube had heated up? Or do they require constant 6.3v in addition to signal to make them warm enough?

the heater is needed to excite electrons rich coating on the cathode to go to the anode..if the electrons can't get excited then your tube doesn't work..

And thats how babys are made.

 

[Shep]

you can run your mark iv in class A/B (two inner tubes only, pull the outers) and set to simul-class. mesa hollywood said this was ok to me once. not sure if this requires change to impedance speaker requirement however.

do this if you really have to play with that amp, otherwise, wait until full re-tube.

That sounds suss to me.. don't know why but..

if it was just the outer pair on A class then i can see that being ok..

but yeah if mesa said so then thats fine by me.

 

[boola]

The way I'm getting round it for now is to use 2 5881's from my JCM900 in the middle positions and the 2 Boogie 6L6's in the outer positions and running it in class A.

 

[boola]

ok cool, thanks. It doesn't say that you can in the manual so I just wanted to be safe.