Problem with a rectifier tube I think!

[coreysan]

I just built a kit of the Fender Super Reverb, and 2 months ago it worked great. I stored it away, and got it back out
today to test with new speakers, but when I powered up, there was no sound, no hum, no B+, no nothing.

I noticed that all the tubes were nice and warm except for the GZ34 - it was cold as ice.

I checked the heater wires for the GZ34, and they are providing 5.2vac, and the power transformer is putting out
360vac per side, just as the specs said it would.

Can I safely assume the GZ34 is the problem, and not something else?

(I'm new to amp building, so I thought I'd double-check!)

[eleventeen]

Sure sounds like it. That tube (or any tube) is either lit up or it's not. = heat. Can you see the heater glowing? Other tubes are lit up?


I don't have dozens of examples for you, but there have been a few stories around here of strange GZ34 behavior. By that I mean failure modes. I find it kind of strange, back in the day, nobody paid much attention to rectifier tubes, they just worked. But there were not off-brands of GZ34 (aka 5AR4) tubes. They were either RCA, GE, or Mullard, or Amperex. Today they are all almost certainly made in China or Russia now, and while those countries have gotten their tube mfr act far more together than they once were, I get the feeling that these tubes, even though they are simpler than little triodes, cause problems.

[sluckey]

The filament is probably open. Use your ohm meter to check resistance between pins 2 and 8 on the tube, not the socket.

[tubeswell]

Either the GZ34 filament is dead, or the socket pins for the rectifier tube filament aren't properly wired/connecting.

[coreysan]

Thanks you all. Your help is so appreciated.
I measured the ohms across pins 2 & 8 and got <= 1 ohm, so the
filament is not open.

I sure hope it's not something worse. It worked absolutely perfectly last month.
Should I test for a possible short somewhere?

[sluckey]

You have voltage on the socket pins 2 and 8. The tube filament is not open. The only thing left is the socket. Try wiggling the tube around in the socket with power on. Can you get it to light up?

[coreysan]

Everyone, I can't thank you all enough for all your advice. I'm really grateful for your interest in helping
others, especially me.

I found a backup rectifier tube and fired up the amp it works just fabulous! So I'm so relieved that it was just
the tube. I can't explain why it stopped working- no cracks that I can see, and the filament appears closed, but
there's definitely no glow at all coming from that tube, and when I connected the back up tube, it fired up right away.
I re-seated it twice, and it fired up both times, so I'm going to get a new rectifier tube!

I'm just really relieved!

So now that I was able to test it, my conclusion is that it's got a wonderful sound, and I'm more than satisfied with it.

It does have the typical amount of hum, it's a low level.

My question is this: most engineers say the first thing is to put shielded cable in for the inputs and volumes.

My friend's 1968 Vibrolux Reverb was modified with shielded cable but it has the same hum mine does.

So I guess my question now is, what to check?

[eleventeen]

Chasing down hum is perhaps the most challenging thing there is in amp building. It can be caused by layout, lead dress, weak or bad parts, a heater to cathode short, relative transformer placement, lack of shielding, ground loops, and bad solder connections. I've undoubtedly left out a few possible causes. We try to duplicate existing designs more to avoid the possible hum problems than to be able to claim that what we have built is a perfect clone of something else.


There is no overarching answer. However much hum you hear is more than likely the sum of half a dozen things. You could fix one of them to utter perfection and still have annoyance going on. You just have to do the best you can in all those areas, worry about whether you have dimmer noise getting into your amp through the power line or via EMF and try to incorporate as many best practices as you can.