Help with 12ax7 identification and GZ34 repair

[schmee]

And here's that Mullard (maybe GE-branded) GZ34, with the broken guide-pin. I've glued one back on before when I had it, but I don't have the broken pin for this one. I knew that going into the sale, but I figured I'd either repair it or just be careful to insert it with the correct orientation. I'd much prefer to repair it. Any experience with this?
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And here's that Mullard (maybe GE-branded) GZ34, with the broken guide-pin. I've glued one back on before when I had it, but I don't have the broken pin for this one. I knew that going into the sale, but I figured I'd either repair it or just be careful to insert it with the correct orientation. I'd much prefer to repair it. Any experience with this?
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I've not successfully glued a broken pin back on. They always seem to come off again. What I do is take red finger nail polish and locate the guide on the side of the tube base with a red dot. I then put a red dot on the chassis where the guide slot is. It's really not hard to put the tube in correct if you watch closely.
heck I'll buy any broken stem Mullard I can find out there cheap!

 

[theprofessor]

I've not successfully glued a broken pin back on. They always seem to come off again. What I do is take red finger nail polish and locate the guide on the side of the tube base with a red dot. I then put a red dot on the chassis where the guide slot is. It's really not hard to put the tube in correct if you watch closely.
heck I'll buy any broken stem Mullard I can find out there cheap!

Thanks, schmee! I've done the same, but with a red paint-pen. And you don't want to know the price I paid for this Mullard GZ34. We'll just say it was well worth it.

 

[Wally]

I once bought a ‘59 Bassman for a little bit of money because 1)the amp blew fuses, and 2)the fellow who had bought it for $5 at a church bazaar would not spend the money to have the amp fixed. After about 4 years of not showing me the amp, asking questions as to what would cause fuses to blow, and not accepting that it could be any number of things...he wanted to sell the amp...never having heard it work! He brought it in...first time for me to see it.
It was covered in green and gold paisley vinyl with about 15 blown fuses in the bottom of the cab.Since he had a couple of new fuses in the box down in the botttom of the amp, I said I had to see what it did...I put a fuse in, flipped it on...and the fuse blew immediately. I asked him what he wanted..,he didn’t know. I made him an offer. He asked for $100 more, and I paid him. He left. A few fellows had watched the transaction. I said something like ...”I wonder what I have bought?” I pulled the power tubes....both locator pins were gone. I reoriented them correctly, put in a fuse....and the amp made loud sounds!
I got lucky. The original trannies were good. I spent a lot of money resurrecting it, but the fellow who owns it today has a killer 5F6A.
Can you imagine buying a 5F6A for $5, owning it for 4-5 years, and never having a tech get it going???? I must have offered to fix it for him at least twice a year. Lol.....

 

[Les Paul lover]

I recently got a small lot of 12ax7's, 12ay's, and a GZ34 for a good price, and I'm trying to sort a few things out. First, what each of the 12ax7's is. Secondly, how to repair the broken guide pin on the Mullard GZ34.

Here's a photo of the 12ax7/ecc83 I'm unsure of. It's a short smooth plate with two star-shaped micas. Is it an EI (Elektronska Industrija Nis)?

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I can't be sure as the pics are quite dark, but that looks like a RFT ECC83 to me?

Are the plates wing shaped?
With 3 perforated holes in the wing bend?

The star shaped micas and angled getter are typical RFT too.

Sonically, it should break harder, faster, and sound tighter than other ECC83. Some people say they lack treble, but I find that's only when compared to other NOS. Compared to modern ECC83, they still enhance the top end.

They don't always sound great in all amps. They can really alter the tone/temperament in some circuits.

I love them in my Genz Benz Black Pearl 30 and my Orange R30. I really disliked it in my RV50 ( some might have preferred it there though... ymmv.).

 

[Les Paul lover]

These look to be four Chinese 12ax7's

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The 3 on the right are definitely Chinese. I don't recognise the plate structure for the leftt one.

 

[Les Paul lover]

I'm a dumbo. That tube in post #1 is an RFT, as a member over at The Gear Page pointed out to me. I've had one or two of those before and even have pictures. The micas sure looked like RFT to me, but I did not recall that plate structure. Here are some pics of an RFT I used to have.

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Ah, i see you figured the RFT out already.

 

[Wally]

Something tells me I have seen the silver colored plates like the left tube...but I can’t identify it.

 

[Les Paul lover]

Something tells me I have seen the silver colored plates like the left tube...but I can’t identify it.

I'm wondering if it's a Sovtek?

Rounded square mica are typical from saratov, and the top and bottom plate rib.... Looks Russian to me.

 

[uriah1]

I seem to recall the Mullard rectifiers had a pinhole in bottom
plastic.

 

[Silverface]

I've glued one back on before when I had it, but I don't have the broken pin for this one. I knew that going into the sale, but I figured I'd either repair it or just be careful to insert it with the correct orientation. I'd much prefer to repair it. Any experience with this?

See below

They make these dudes to fix the guide pin:

https://www.tubedepot.com/products/octal-tube-keyway-replacement

Just slide it over the existing tube, drop of silicone to hold it together, and you're good.

^^^^^^THESE!!!!

There is NOTHING better or safer.

for the broken key, i just mark on the base of the tube with a red marker. no need for actual repair of the key.

nice collection, prof!!

play music!

The problem remains that you can get distracted and accidentally turn on pin direction or the other and POOF! And the problem is with high-voltage tubes, where "POOF!"m can be damned expensive.

A handful of those slip-on guides are chap and permanent. What a GREAT idea!

 

[theprofessor]

A handful of those slip-on guides are chap and permanent. What a GREAT idea!

Just got some of these in the mail. I'll report back when I get a photo of one on and then one on the broken tube base.

 

[theprofessor]

Just had a look at this GZ34 once I got in the key-way replacements. Easy. Here's what happens.


Guide pin is broken.

Guide looks to be between pin 8 and pin 1. Pin 8 is just to the left of the guide, and pin 1 is just to the right of the guide.

Get a key-way replacement. The guiding part of the pin is a bit smaller on these than on the original tubes, but it still gets you oriented in the right direction.

Check an octal tube socket I had lying around, just to be sure that the ridge on the guide-pin is between pins 1 and 8. Yes. But the 1 is on the left of the ridge here and the 8 is on the right, because this is the socket -- a mirror-image of the bottom of the tube.

 

[theprofessor]

Slip it over the pins, noting carefully that pin 8 is to the left of the guide and pin 1 is to the right of the guide. I did it wrong the first time.

Doesn't seat perfectly. Will need to file some.

There's the culprit...

 

[theprofessor]

Done with the filing. Now put it on again, carefully noting the orientation.

Here it is, all finished. Add a dab of silicone adhesive underneath, if you like. The base is now slightly taller, which means the pin insertion into the tube socket will be slightly shorter than normal, but still plenty of contact to work properly.

 

[Silverface]

The base is now slightly taller, which means the pin insertion into the tube socket will be slightly shorter than normal, but still plenty of contact to work properly.

I usually have to file down raised edges of the broken keyway AND open the pin holes up just a bit to get the adapter to fit as flush as possible. Then - after gluing with silicone or superglue gel - the pins are "shortened" only by the thickness of the adapter plus about 1/128".

The've been made for years. No idea why I hadn't mentioned them before either!

 

[Opaltone]

This one was fun. The seams on the top of the envelope and the internal construction, as well as the print on the side, made me think first of a Mullard I63-type or the like. But it's an NEC Hitachi gold pin branded "Realistic Lifetime." These are sometimes referred to as Japanese Mullards. Thus the "Made in Japan" on the side.

Does anyone have any experience with these who can compare them to any other tubes I might know?

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Howdy Professor!

FYI - that's a Matsushita, not NEC or Hitachi. Glad you got the RFT sorted out. Nice job fixing that GZ34!
- Thom

 

[theprofessor]

Thanks much, Thom! Great to see you here again. And I'm glad to know about the Matsushita. Much appreciated!

 

[theprofessor]

The GZ34 story has a good ending. I called Brent Jessee, and he agreed to test it for me, since I have no amps that use a GZ34. He doesn't normally do testing like that, but he did this time. I just got the tube back. It's a 1965 Mullard and tests at 265 mA out of 360 mA, where 180 mA is minimum good. Mr. Jessee said it tested "pretty much like an unused tube." And I got the base and guide-pin affixed to the tube with some silicon adhesive. So there you go. Now I just need an amp to run it in!

 

[Mr Ridesglide]

Fun stuff Professor!

 

[jimytheassassin]

the silver plate tube shows sovtek very faintly on the side in your photo.

I'm wondering if it's a Sovtek?

Rounded square mica are typical from saratov, and the top and bottom plate rib.... Looks Russian to me.