Orange AD30 Blowing HT Fuse

[AtomicMassUnit]

I was thinking tubes, bought a new set, it worked fine for an entire 3 hour practice, then blew both fuses (.500a ht and 4a main). It appears the GZ34 rectifier tube is bad, so I put in a good 5u4 I had, and the amp worked. I bought another GZ34, played it, all was fine, turned on again and the HT fuse blows. Replaced the HT fuse, then turned on, and saw the brand new rectifer arc and both fuses blew.

This just killed my new rectifier tube. Bad PT? What can I check next?

[Jazz P Bass]

I would start by putting the 5U4 back in & see how she goes.
Some of the new GZ34's suck eggs

[R.G.]

There's some problem lurking there, all right. One thing that may help keep your PT and/or filter caps alive longer until you positively diagnose this is to put high voltage solid state diodes in series with the PT wires to the rectifier tube. These will be inaudible if the rectifier tube works, but if it shorts, they will keep this from putting high voltage AC on the filter caps and/or effectively shorting the PT.

Shorting the PT HV winding is not a disaster as long as the primary fuse blows. But that's not what the primary AC fuse is there for. It's put there to keep the amp from literally starting a fire if something really bad happens. One fuse in each side of a center tapped HV secondary can be tailored to the current in that half of the winding only, and offer some real protection from soft shorts on the HV winding.

[Prairie Dawg]

There's some problem lurking there, all right. One thing that may help keep your PT and/or filter caps alive longer until you positively diagnose this is to put high voltage solid state diodes in series with the PT wires to the rectifier tube. These will be inaudible if the rectifier tube works, but if it shorts, they will keep this from putting high voltage AC on the filter caps and/or effectively shorting the PT.

Shorting the PT HV winding is not a disaster as long as the primary fuse blows. But that's not what the primary AC fuse is there for. It's put there to keep the amp from literally starting a fire if something really bad happens. One fuse in each side of a center tapped HV secondary can be tailored to the current in that half of the winding only, and offer some real protection from soft shorts on the HV winding.

Good information as always RG. The diode mod is a good one for any tube amp which uses a rectifier tube.

There have been similar problems with the AC30 custom classic series and the fix there was to just forget about the standby switch and leave it on all the time. With a slow warm up tube like a GZ34 you don't need a standby.

I've had indifferent results with the current run of GZ34s and the only good thing you can say is that they're inexpensive. Shuguang, JJ, Sovtek, they're all dog waste.

If it was me I would try and get my hands on a good old military 5R4 or a Mullard GZ34 for rectifier service.

[AtomicMassUnit]

Ok, I put the 5U4 back in, it immediately arced and blew the mains fuse. With no tubes at all, the fuses hold and I measure 6.5vac across pins 2-8, and 658vac across pins 4-6. This is the 3rd rectifier tube I have tried, so I am hesitant to try another.

The tube socket is pc mount, so I'd really like to avoid putting the diodes there.

***edit*** when I pulled my 5U4, I noticed a crack in it I hadn't seen before... retesting with another.

[AtomicMassUnit]

With a new (sovtek) rectifier the amp is working... reading 150 watts on my killawatt. I guess I got two bad JJ5ar4's in a row. And my bench 5u4 got cracked some time between when I tested it the first and second time with that tube. Burning in now. Just to be on the safe side for gigs, I am going to forgo using the standby switch.

[R.G.]

Even though it's PC mount, do at least consider putting SS diodes somewhere in series with the tube anodes. They don't have to be right at the tube socket. You can find the traces from the transformer wires going to the rectifier tube anodes and insert the diodes anywhere along those traces.

The slick way to do this is to find a section of trace long enough to hold the diode's length and wide enough to solder the diode lead to, then drill through the PCB into the trace two places (for the diode leads), scrape off enough solder mask around the holes to let you solder the diode leads, then solder it in and cut the trace between the two diode leads. If there's a convenient place, it can be made to look like the maker intended that to be the way it was.

[AtomicMassUnit]

Very slick RG! Thanks!!!