Blowing fuses

[Lindz]

I have an 18 watt (1974x tmb type circuit) that I built and have been modding for a while and recently converted to fixed bias. It was working great but I bought a Hall fixed bias VVR that I installed today in an effort to keep my neighbors happy

thought I was careful, but the amp blew a fuse when I switched from off to standby after the install (2amp slo blo on the mains)

very carefully went over everything and tried with a new fuse and popped it again going to standby (did not make it past standby to fully on either time).

Pulled out the vvr assuming I had wired something incorrectly, and put it back as was, and popped another fuse.

Pulled the tubes and still popped another fuse.

Stopped at that point, did some reading and thought I would post for some help

So my questions are:

I would guess that it has to be the PT, indicator lamp or heater wiring that are bad if it pops fuses with no tubes? correct? or are there other possibilities I should check. The lamp lights for about a second then goes out as the fuse pops.

How would one best troubleshoot this? I am still a bit of a noob in spite of having built a few amps. Generally my builds have worked without much issue so I have not had a problem to troubleshoot like this before.

One extra wrinkle is that I have a pretty big first cap for an 18 watt - a 50/50 can (was playing around trying to tighten up the bass a bit) so I am wondering if I perhaps I get more inrush than I otherwise would with that big cap. (or is this even an issue since it is in standby)

Maybe the 2 amp fuse is a little light? It did work for a couple weeks with the 50/50 before today but perhaps that is contributing. That said, it seems more likely that I have a bigger problem since it stopped working after my vvr install and fuse popping today

[tubeswell]

If it pops a fuse, then there's a short somewhere from one of the PT windings to ground that's pulling too much current. If if happened after you did the VVR mod, but wasn't happening before you did the mod, then yes, my money's on you did in the mod.

[sliberty]

Are you using a tube rectifier? If so, I'd start by replacing it.
Next I would replace the power tubes.

Tube replacement is always the easiest thing to do when troubleshooting an amp. Andd when it comes to blowing fuses, the rectifier and the power tubes are often the culprets.

[Lindz]

I kinda feel stupid, but since it was popping fuses with no tubes, I assumed it was the PT and bought a replacement even though I could find no shorts on any of the leads from it, or anywhere else. I just figured there must be a problem with the windings where it must be drawing too much current for some reason

Anyway, tranny arrives with the mailman this morning so I hook it up t, flip the switch and "pop" I blow another fuse $%$#& Grrrrrrrrr....

Hmmmmmmm? Nothing except the switch and lamp are hooked up so I just stare at it for a minute wondering what else it could be ... the only things hooked up at that point were the switch and lamp - then when I am tracing the lead looking for a short to the chassis I look at the rectifier diodes??

Even though I figured it was highly unlikely that would be the issue, I replaced them. I thought if it was a tube rectifier that would be a no brainer to try replacing it. but diodes ??

Sure enough somehow they were shorting. Could have been a very easy repair last weekend, but instead I overlook something simple and now have an extra PT.

The one good thing that came out of it is that I bought a lower voltage PT and now I do not need to use a dropping string to lower the b+ like with the previous one which was way to high with solid state rectifier, fixed bias etc

One question for knowledgeable folks - I am kind of surprised that I zapped some 1n4007 diodes - maybe I should re-evaluate how I am rectifying the b+. I am using very simple 2 diode setup- 1 from each 280v line joined at the cathodes then running to the power switch -

I was under the impression that diodes are pretty robust..? I've seen many amps that rectified in a similar fashion, but is there a better way to set it up for an 18 watter? Or, did I just have some bad luck

[martin manning]

Bad luck, unless the previous PT was up around 350-0-350, but you could improve the reliability with another pair of diodes. In theory it should work fine with just two 1N4007's since they are rated 1000V PIV, and they will only see 2x 280VAC or 791V peak. Often two (or more) are used in series, sometimes with 0.01uF caps across each one to insure that they share inverse voltage equally.

Troubleshooting is both an art and a science. Take some solace in that you are now better off without the heat and wasted power from the dropping resistors.