main fuse blows

[slowboy]

my son's Peavey VK100 has started blowing main fuse beside power chord...I don't know if this has anything to do with it, but my son said he about turned head and speaker cabinet over and caught it...when he caught it, it bent the plug on the guitar chord where it was plugged into amp head, and broke the little plastic nut that holds connector to front of amp...my son says it didn't start blowing fuses until that happened...I have taken amp out of it's cabinet...I did this just to look for any blown fuse/s...the 3 fuses on circuit board look fine...I tested fuses to see if there was any continuity and they all seemed ok...
we can turn power and standby switches on, but as soon as it is taken off of standby the main fuse blows...is there any ideas on this and maybe something I can do to fix amp before spending money to have technician fix it?
we replaced main tubes a few months ago, but haven't replaced preamp fuses...don't know, but wondering if amp has bad preamp fuses...thanks for any and all help and advice
slowboy

[R.G.]

It is most likely that the physical trauma caused some wire or component to be shoved into a position where it is overloading the input fuse. This might be in the AC mains power wiring, might be in the high voltage secondary stuff.

Unless you have some skills at working with AC mains voltages and higher AC and DC voltages, you are at severe risk of being electrocuted or further damaging the amp by messing around inside it while the power is live. And probing inside the live chassis is the next step in debugging.

If you don't already know how to debug inside a high voltage/AC mains circuit safely, and know full well that you DO already know, the internet is not where to learn it.

Without that knowledge and skill, you're stuck at replacing anything that's in a socket and can be unplugged and replaced - cables, fuses, and tubes.

[slowboy]

yes, not able to, or have/had any intentions of probing around any of the electrical...haven't lived for 58 years being stupid...was just wondering if it could be bad preamp fuses or something that I could replace...ie - "cables, fuses, and tubes"...

[R.G.]

I dug up the schematics and those are not preamp fuses. They are fuses in the secondary of the power transformer, to protect the transformer. Since the primary side fuse blows and the secondary side ones don't, the problem is likely in the AC mains wiring part of the circuit. the primary fuse is the only part you're likely to be able to service there.

I did notice that Peavey's schematic specifies a fast blow fuse on the primary side. That's counter to normal practice, but I haven't done any analysis to be able to second guess their schematic.

Probably best to take it to a tech. It may well be simple, possibly just a broken wire, but maybe as bad as broken PCBs inside.

[slowboy]

I dug up the schematics and those are not preamp fuses. They are fuses in the secondary of the power transformer, to protect the transformer. Since the primary side fuse blows and the secondary side ones don't, the problem is likely in the AC mains wiring part of the circuit. the primary fuse is the only part you're likely to be able to service there.

I did notice that Peavey's schematic specifies a fast blow fuse on the primary side. That's counter to normal practice, but I haven't done any analysis to be able to second guess their schematic.

Probably best to take it to a tech. It may well be simple, possibly just a broken wire, but maybe as bad as broken PCBs inside.

oh, I meant didn't replace preamp tubes when replaced main tubes, not preamp fuses

[g1]

One thing to try is removing the main power tubes.

[guitician]

Look closely for loose objects like an internal star washer, or any conductive foreign object, that may have fallen into the circuitry when the jack broke.

And you may want to take a close look at the inside conductors of that jack that broke and make sure none of the contacts are shorting anything out.

[slowboy]

One thing to try is removing the main power tubes.

what happens if I remove main power tubes...would amp power up?
what if I remove the preamp tubes...would amp power up?
if I did either one of the above, how would it affect main fuse?...

[guitician]

Removing a potentially shorted tube will remove the short and the fuse won't blow.

[slowboy]

is it the same for preamp tubes as well and do I remove one at a time, or remove all at once

[guitician]

Less likely that pre-amp tubes are shorted because they are smaller and lighter and usually have covers. Start with the power tubes, or circuit inspection, which would be my 1st step.

[slowboy]

I removed all tubes, the preamp and main...turned amp on and it didn't blow the fuse...now do I just replace [1] tube at a time until a tube makes fuse blow?...

[g1]

Put all the preamp tubes back, it should still not blow the fuse.
Then, if you want, you can try one power tube at a time to locate the defective one.
Usually, you will want to replace the power tubes as a pair. You can save your good one as a spare.

[slowboy]

Put all the preamp tubes back, it should still not blow the fuse.
Then, if you want, you can try one power tube at a time to locate the defective one.
Usually, you will want to replace the power tubes as a pair. You can save your good one as a spare.

put preamp tubes back didn't blow fuse, but preamp tubes didn't light up either...then put one main tube in at a time, and main tubes didn't light up either until...after I put the last main tube in [all tubes in place-preamp and main, all tubes lit up and fuse blew...this time I didn't have to take amp off of standby, just turned power switch on]...

[g1]

Looking at the bottom of the 6L6 power tubes, in the very center there should be a round plastic locating pin. Is it broken and missing from either 6L6 ?