Lost a rectifier :(

2nd harmonics for the masses

Lost a rectifier :(

Postby Ty_Bower » Sat Sep 12, 2009 2:48 pm

I lost the rectifier on my GetSetGo. I never had that happen before. Maybe a week ago, I thought I saw a flash of light from the 6AX5GT. I quickly turned the amp off, and started checking the output tubes. I had been swapping in different Sovtek 6B4G which I recently picked up from eBay, and suspected one was somehow bad. Couldn't spot anything wrong on the tube tester, so I powered the amp back up and everything was fine.

No problems for a week or so, then last night the rectifier started arcing again. I watched it *SNAP* a few times before I quickly hit the power switch. Scratching my head, not knowing exactly what to do, I replaced the RCA 6AX5GT with a GE. I thought maybe I should check a power supply cap to make sure it wasn't shorted or something, but was too lazy to actually do it. Everything seems fine again with the new rectifier in place. I've marked the old one as BAD.

While thinking about things, I observed the RCA seems to have only a single cathode in the center of the tube, with the two plates on either side. The GE appears to have two complete cathode/heater structures, and the plates are some kind of cladded copper material. Any comments?

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Postby TomMcNally » Sat Sep 12, 2009 3:59 pm

Rectifier tubes are (and always have been) the weak link when
it comes to failure. Turning the amp on and off many times to
test tubes may have contributed, or you put a shorted one in.

I bought dozens of 6AX5's (they are cheap) and have been
giving everyone that buys and amp from me a spare, just
in case. No feedback so far.

I have been using an Amperex in my g*s*g and it has the
copper colored plates that look like they are glowing red when
they are off.

Thankfully JJ doesn't make 6AX5's or we'd have heard of
lots more that popped !

You can see the copper in this pic ...
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Postby TerrySmith » Sat Sep 12, 2009 6:10 pm

Here are two 6AX5's and a 6BY5. From what I've seen, RCA, Sylvania and some GE's look like the one on the left. GE made the one in the middle and it looks more robust. I've had one in my GSG since day two.

The one on the right is a 6BY5GA, looks like a 5V4 but with 6.3v isolated heaters. The heater draws 1.6a which might be a little much if using a PA-774 power trans. You can jumper pins 3&4, 1&8 and it will drop in. There's also a 6BY5G, wouldn't that be cool! ;)

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floppy filaments ?

Postby EWBrown » Sun Sep 13, 2009 4:13 pm

I prefer the GE 6AX5s, with the two separate complete rectifier "structures" (like the center one depicted in the previous photo), the "lesser" 6AX5 with the single filament / cathode structure and two plates, is essentially a slightly upgraded 6X5 / 6X4, and it falls short of the G*S*G's plate current requirements.

Now, for the meat of the issue:

Before you plug in that vintage Russian 6B4G (6C4C), especially the early version with the twin plates, look down inside the elements, and make sure that the filament isn't flopping around and getting too close to the grid, which could then short out and make for some unwanted "fireworks".
The filament should be neatly centered inside teh grid, and not "leaning" to one side.

I have seen a couple of vintage Russki 6C4Cs with just this problem, and they will usually test as "short" in even the simplest emissions tester.

In a similar vein with droopy or floppy filaments,

The earlier Russian made 811As had a similar problem with droopy filaments, and they could not be safely used in the Collins 30L1 tabletop RF linear amplifier, as the filament would sag and contact the grid.

The tubes in this particular unit are horizontally mounted, so the problem was aggravated. The simple "cure" was to rotate the four tube sockets by 90 degrees, so that the long axis of teh plate (and grid) was vertical, and the saggy short problem could be avoided.

Later 811As had more "restrained" filaments, so the retrofit wasn't required.

HTH

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