Help! VHT 2/90/2 just blew a fuse on "B" channel

[Robert Burns]

its a 312 ... 1 amp... 250v glass fuse (front mounted).. I turned on
both switches (im in stereo) and my right channel...went "pow"!
So...my question is... can i safely play thru only my "A" channel...
and be ok? With the fuse out of "B'? Or do i need to wait
and have both A and B' functioning? If i remember right...i can play
thru A' alone...but should i at least put say a 3/4'amp fuse in, and see
if i have another problem? (as in fuse pops immediately again.)
Need some input...dont want to screw up (if it already hasnt been)
my favorite amp! Thanks... Should i take the (2) speaker cables
out of the back of the VHT? (going to my 2 212 crate cabs), or just
dont bother with any unplugging, since im only "hot" on "A" ?

[emperor_black]

so sorry to hear that Robert. Good luck.

[T@rgetDrone]

Personally, I'd just go buy a handful of the proper fuses (so you have a couple of spares). Replace the blown fuse and try again. Though I suspect it's gonna go poof (the fuse) again, either immediately or in a short time. If not, great and rock on! If it blows the same fuse, I'd suggest pulling it out of the rack and going directly to the best tech you can find. It is a mighty fine amp and I'd hate to see you risk it getting even more messed up trying to run it if there's an obvious problem.

That's my 25 cents worth. Replace the blown fuse and if it happens again, yank it to get it diagnosed and fixed. Go to whatever backup rig you may have. That amp is fantastic. Not worth risking more damage for a few minutes/hours/days running it under problematic conditions.

Let us know how it turns out.

[emperor_black]

what were the symptoms before the fuse-blow? It could just be as simple as changing the fuse or more work like changing the PT's and probably re-bias.

Here is a excerpt from the mesa boogie manual...

If a fuse blows, the problem is most likely a shorted power tube and shorts can either be mild or severe. In a mildly shorted tube the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try to identify one as glowing red hot. It is likely that two of a pair will be glowing since the “shorted” tube will pull down the bias for its adjacent mates, but one tube may be glowing hotter — and that one is the culprit. The other two are often fine — unless they’ve been glowing bright red for several minutes.
Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few moments
then back to ON will usually cure the problem...at least temporarily. Watch the tubes carefully now. Should the problem recur,
the intermittent tube will visibly start to over heat before the others and thus it can be identified. It should be replaced with one from
the same color batch, shown on its label. Call us and we will send one out to you.
The severe short is not nearly so benign. In the worst cases, a major arcing short occurs between the plate and the cathode with
visible lightning inside the glass and a major noise through the speaker. If this is seen to happen, IMMEDIATELY turn the amp to
STANDBY. By this time the fuse probably will have blown. Such a short is usually caused by a physical breakdown inside the tube
including contaminate coming loose or physical contact (or near contact) between the elements. Replace it and the fuse with the
proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.

[Robert Burns]

Thanks for that... i got an email from VHT today saying was most likely
a KT-88 Power tube... ive read some on allot of you guys changing
your own tubes, plus re-biasing...but im a total newb to this. I guess
i will have to pull it out of the rack case, and then open up the top...as
the VHT rep said that i would have to give him the "I.P." code on the
tube for them to send me the exact replacement? I was wondering if
i should just buy an entire set of tubes, just to be ready for any further
occurance? Should i just try to put a new fuse in, once? Or should i just
figure i have a major problem, and not put the system thru this again?
It is only a 1 amp 250 v fast blow fuse...so it's not like its the 6 amp
"Mains" fuse. Anyone? Err on the safe side and just wait? Or put in
a 3/4amp smaller fuse, and try again?

[emperor_black]

Look at it this way, a fuse costs much much less than a power tube does. so, I would try to put a new fuse in first and see if that blows too. yes, you would have to open it up. Try not to touch anything near the capacitors. THERE ARE LETHAL VOLTAGES IN THERE.

Usually power tubes come in multiples of two. If you replace one, you've got to replace the other too. Usually you are ok with not needing to bias if you are using the same brand, but you never know, it might need biasing.

[Robert Burns]

Look at it this way, a fuse costs much much less than a power tube does. so, I would try to put a new fuse in first and see if that blows too. yes, you would have to open it up. Try not to touch anything near the capacitors. THERE ARE LETHAL VOLTAGES IN THERE.

Usually power tubes come in multiples of two. If you replace one, you've got to replace the other too. Usually you are ok with not needing to bias if you are using the same brand, but you never know, it might need biasing.

thanks! Im feeling confident already...

[toejam]

Should i just try to put a new fuse in, once? Or should i just figure i have a major problem, and not put the system thru this again?
It is only a 1 amp 250 v fast blow fuse...so it's not like its the 6 amp
"Mains" fuse. Anyone? Err on the safe side and just wait? Or put in
a 3/4amp smaller fuse, and try again?

Whenever you replace a fuse, only use a fuse of the same exact rating.

[Robert Burns]

Whenever you replace a fuse, only use a fuse of the same exact rating.

Thanks...im an electrician... but just not an electronic tech! :-)
I plan on trying a fuse 1/4 amp "Less" than the original..expecting it
to "Blow". (update as of today...put the 3/4amp new fuse in...and without even
going off of "Standbye"... my B' channel starts making a bunch of racket, and
hissing... and low volume "Pops". So without wanting to do any further damage..
its retired until i either feel confident enough to try the tube change myself (given
that i can even identify the shorted Tube!) or get it to an authorized service
center...or worst case scenario, i have to ship it back to Burbank! Arrrrgg!