Cataclysmic failure!

[axpro]

Just finished a 5e3x2 build from Ted weber. It seemed to work fine at the shop, just a touch noisy and hard to control the volume (normal for a 5e3) I brought it home, and decided to make a couple of minor changes. added some more filtering (extra 40 uf cap after the rectifier (which is tube)) and a .22 mallory in the first stage (As on Steve A's blues express plus 8Ha)

I also added a resistor network from the volume controls to the tube stage in an attempt to make the volumes a little bit less interactive.


IT seemed to work fine... the controls were still interactive but a little less so, then i heard a squeal start, turned around and saw the Rectifier pop (flash brightly and go out) and then all 4 power tubes in quick succession... i hit the power and have left it to cool down. Any ideas of what it could be? I have a few suspicions....

-Parasitic oscilation got stronger until i could hear it and it could do dammage.
- too big a filter cap after the rectifier (the weber SS rect was defective so i was using Tube)


Any other ideas???

[stokes]

What kind of rectifier tube? It sounds like the rectifier shorted out.

[Don Moose]

As I recall, tube rectos tend to fail short. R.G.Keen recommends putting a pair of silicon rectos in series with each leg for just such an event.

... too late to help.

[axpro]

It was a chinese 5ar4....


I think I have more Rectos around... but now i am out of 6v6's... ah well, it'll wait till next week!

[axpro]

Just a quick add on, all the tubes that came with the kit are cheap-*ss chinese tubes... could that be an issue as well? I replace the recto and it fires up, but seems noisier... I suspect output tubes.

[Paul P]

As I recall, tube rectos tend to fail short. R.G.Keen recommends putting a pair of silicon rectos in series with each leg for just such an event.

Dave Funk recommends this as well but I don't understand the idea. Won't
the diodes rectify they AC coming out of the power transformer ? If so then
what does the tube rectifier do ?

Paul P

[Enzo]

The diodes pass current only one direction, so yes they rectifiy, but since they do that, if a rectifier tube shorts plate to cathode, the high voltage AC from the transformer will not make it to the filter caps - the diodes will block it. It will be pulsing DC just like the tube makes.

The diodes stop the reverse voltage half of the AC cycle from getting to the tube, but the remaining pulsing DC is what the tube would have passed anyway.

One reason many people like tube rectifiers is that they have a dynamic to them - they sag under load. That means the voltage across them changes with changes in curent. That means the voltage will drop on the B+ when a loud sound is played. This affects the sound of the amp.

A solid state rectifier - a diode - doesn't do that. But putting one in series with a tube doesn't stop the tube from still doing it.

[Don Moose]

To clarify - the silicon diodes go between the HT secondary and the tube recto, not between the recto and the 5V heater winding. ... unless you used the CT on the 5VAC winding as the filter input instead of usual one recto cathode.

[stokes]

Back in the '80's there were some Chinese 5AR4's that were just 5Y3's labeled as 5AR4's.I doubt Ted would deliberately pass one of these off,though.Where did you get the 5AR4?If it were one of those,the 5E3X2 would definately fry it.

[Bruce / Mission Amps]

How To Do It

Dave Funk recommends this as well but I don't understand the idea. Won't
the diodes rectify they AC coming out of the power transformer ? If so then
what does the tube rectifier do ?

Paul P

What I've done (and still do sometimes for working, pro players who can't afford a blown amp due to a failed rectifier) is this:
Get two 1N5408 diodes and wire one from lugs 5 to 4 and one from 7 to 6 with the cathodes pointing at 4 and 6 respectively... then you solder your high voltage leads from the PT secondary to lugs 5 and 7 respectively.
Now all the Hi-V AC has to go through the diodes first.
Blow a rectifier and (except that the B+ will be higher without the drop the rectifier tube and the SS diodes are more efficient)... it doesn't seem to matter because the HI-V is already rectified.
Done.