main fuse blows

[slowboy]

...all locating pins are there on main tubes and look good...

[guitician]

Series heater arragment in the VK100... Did you use the same tubes?http://peavey.com/forum/download/fil...58307ae1d9730a
If so then, you really aren't testing anything until you have known good tubes. It may just be that the heater circuit is shorted by some piece of metal too.

[slowboy]

original preamp tubes...same main tubes which I had bought new 4 months ago and put in amp, but I figured if a tube didn't blow fuse, it was still good...I put in a tube and turned amp on...didn't blow fuse [didn't light up either]...replaced 2 more checking each time to see if fuse blew and it didn't...4th tube, [all tubes lit up] and fuse blew...
don't know what the above link is for, but when I click on it, it says I'm not authorized to view, download or link to/from this site

[R.G.]

Just goes to show you, I should have repeated my own advice from many other posts - the first thing to do in any tube amp repair is to replace the tubes with a known good set of tubes.

The fact that the secondary fuses didn't blow fooled me. Good catch.

[Jazz P Bass]

Peavey uses a series heater circuit on the power tubes.
No tube will power until the four heater circuits 'see' the heater resistance.

So, unless you have a 'Known Good Set' of tubes, you are chasing your tail.
It would appear that you have a bad power tube, though.

Aside: I have a quad of 6L6 & EL84 tubes where all of the pins are removed 'except' for the heaters, just to test tubes in this type of circuit.

[Enzo]

I think removing the tubes and fuses not blowing pretty much screams power tube failure. When the amp dumped, it jarred the tubes and one croaked.

Preamp tubes really have no way to blow fuses. Power tubes do, so even if a preamp tube has failed, I doubt it is this issues cause. A new quad of 6L6 is likely required.


And I agree with the rest, that all the tubes must be in for them to light.

[g1]

The odd thing is that originally he said it only blew coming out of standby mode.
But the most recent time it blew while in standby, just turning power on (post #14).
So this seems to indicate a shift of the problem, or a new problem.

[Enzo]

Not necessarily, remember this is a Peavey, so B+ to the plates is present even in standby. The standby removes B+ from screens and the small tubes. SO a shorted tube will likely blow something. SOmetimes it can be a race to see whether the B+ fuse or the mains fuse blows first in an amp.

That it won't blow fuses without the tubes tells me the flyback diodes are OK and the OT is not shorted to ground. So bad power tube gets my vote still.

[Chuck H]

If the old tubes that were replaced months ago are still around, and were working but replaced for a non failure reason, try plugging those in. Otherwise you may need to buy new tubes just to see if they're the problem. There may still be a physical glitch in the heater circuit that will manifest when ANY tube is plugged in whether that tube is good or not.

[slowboy]

I know it's a long shot, but maybe if I take an old tube and try in each spot?...only problem is not knowing which old tube is good.

[Chuck H]

You may need a lot of fuses to get that worked out. So you have the old tubes then, but didn't throw out the bad one? And you still may have a heater issue. More likely a plate to heater short on one of the tubes that's in the amp now. Was it the same failure last time? Fuse kept blowing?

[slowboy]

actually last time it wasn't blowing any fuse...it just kept popping and crackling and sound was cutting in and out

[Chuck H]

That doesn't sound like a typical power tube failure to me, but I've hardly seen everything. My first thought would be intermittent pin contact due to fatigue on the sockets, dirt or compromised pads on the board. This could cause pops and crackles, cutting in and out and WRT your new failure it could have caused a failed bias condition resulting in a shorted power tube that's causing the fuse to blow.

Not sure how bending the input jack would exacerbate things, but the board would have flexed. If there was any (unreported) actual shock to the amp that would seem a more likely cause.

Plugging in power tubes at this point then may be more like using them as test fuses. An expensive way to go for sure.

[Tom Phillips]

Any one or more of your power tubes could have an intermittent short that causes the fuse to blow. The fuse could blow at different times and under different conditions such as immediately at power up or when coming out of standby or after playing a while. That's the characteristic of an intermittent fault.

When the tube doesn't stay dead shorted then it can be very difficult to isolate the problem. One approach is to replace the whole set of tubes, set the old ones aside and then determine if the fuse blowing problem returns. If the problem does not return then you can be pretty sure that there is a bad tube in the old lot. Then you need to decide if it's worth your time to try to isolate the problem to a specific tube or scrap the old set and move on. It's also prudent to inspect the tube sockets and verify that they are clean, the individual sockets are tight, the wiring is good and there is no evidence of arcing (Tell tail carbon tracks).

[Enzo]

Actually, in my experience, popping and crackling is often the sign of a failing power tube.


Due to the wiring in this amp, it is difficult to use a one at a time tube approach. A new set of power tubes is recommended. You can then use some other amp, like an old Fender, to test individual power tubes.