Rectifier tubes?

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Rectifier tubes?

Postby Blair » Sat Apr 20, 2013 7:04 pm

What is one if the better new production GZ34 tubes?

I recently got a ST-70 on trade and unfortunately, it's in too good of condition to scrap up and part out. Came with cage and bottom. The guy said there is a bad transformer, but it all ohms out just fine cold. It definitely has been butchered by someone in the PS, but very fixable.

Anyone interested in this amp?

Thanks,

Blair
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Re: Rectifier tubes?

Postby Geek » Sat Apr 20, 2013 11:02 pm

You're in luck! The tougest new tube I 've experienced for ST-70 duty is also the cheapest - Sovtek :))
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Re: Rectifier tubes?

Postby Blair » Sun Apr 21, 2013 2:24 pm

Thanks Gregg,

I may as well give this a try since it's all ohming out.
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Re: Rectifier tubes?

Postby Blair » Tue Apr 23, 2013 10:31 am

Has anyone used a JJ multicap in place of the metal can? I know these run hot! Will the JJ can survive?

I can't bring myself to cut this fellow up even though it had a power transformer that broke down and the rectifier and HV taps were fused. I had a spare PA-060 and it had a bell dent, so I robbed the bell off the old one and replaced it. The original board looks pretty decent also. Is there anything I really need to watch for on these amps?

Image
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Re: Rectifier tubes?

Postby kheper » Tue Apr 23, 2013 1:53 pm

Blair wrote:Has anyone used a JJ multicap in place of the metal can? I know these run hot! Will the JJ can survive?

I can't bring myself to cut this fellow up even though it had a power transformer that broke down and the rectifier and HV taps were fused. I had a spare PA-060 and it had a bell dent, so I robbed the bell off the old one and replaced it. The original board looks pretty decent also. Is there anything I really need to watch for on these amps?


The JJ caps are rated at 500V. The turn on surge could over-volt them. You could series two 385V or two 500V JJ caps, if there is enough space. F&T makes smaller 550V axial electro-caps.

I would not use the old driver board. The caps probably need to be replaced. Coupling caps can catch fire. The bias supply (electro-caps and old selenium rectifier) needs to be rebuilt. Bite the bullet. Strip it, get a new board, re-cap it, get a new bias diode and new sockets. You'll be glad you did.
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Re: Rectifier tubes?

Postby Blair » Tue Apr 23, 2013 2:49 pm

Hi,

I kind of "bit the bullet". I ordered a 525V replacement can from TubeDepot along with replacement coupling caps. This is going to be an eBay amp, so I don't want to put too much into it. If it were mine, I'd strip, powder coat, and ground up the whole thing with a bigger power transformer.

Thanks,

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Re: Rectifier tubes?

Postby Blair » Tue Apr 23, 2013 8:27 pm

Is there anything really wrong with the original 7199 if the traces look good and I replace the vitals?

The selenium was already changed for a SS diode.
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Re: Rectifier tubes?

Postby DeathRex » Tue Apr 23, 2013 9:20 pm

Mine has the original circuit in it, but a new PCB like yours. I wired in a 6GH8A I think and to me it sounds good.
I also put in Russian capacitors, and rebuilt the main electrolytic.
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Re: Rectifier tubes?

Postby Blair » Tue Apr 23, 2013 9:59 pm

Thanks,

I'm interested in a flip on this one to fund other toys. I don't want to short change anyone, but I also don't want to sink a few hundred in an amp that will only bring around 400 bucks.
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Re: Rectifier tubes?

Postby kheper » Wed Apr 24, 2013 3:13 pm

The driver board says "dynaco" on it, but it is green. Is it a new board made from the original design?

If the 7199s test good, the amp may fetch a higher price than if it has substitute tubes or even an after market board. Just mention that the tubes test good (if true).
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Re: Rectifier tubes?

Postby Shannon Parks » Wed Apr 24, 2013 7:11 pm

Geek wrote:You're in luck! The toughest new tube I 've experienced for ST-70 duty is also the cheapest - Sovtek :))


I second this.

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Re: Rectifier tubes?

Postby Blair » Thu Apr 25, 2013 3:25 pm

Image

New couplers, PS cap and rectifier. The KT77s are a bit loose in the sockets, but with my phone as the input signal, it's dead quiet.

Thanks guys!
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Re: Rectifier tubes?

Postby pmarcin » Sun Apr 28, 2013 4:32 am

I third the motion on the rectifier; the Sovtek is the most robust. I've had JJ's that arced over from the onset and blew the fuse, using a standard can. Replaced with Sovtek and worked perfectly. :(
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Re: Rectifier tubes?

Postby TerrySmith » Sun Apr 28, 2013 8:24 am

What you have there is a later model '70. The driver board is factory as they did go to fiberglass later on and the date codes on the output transformers (74-16) means they were built on the 16th week of 1974.

If you still have the original can cap you could list it on ebay, yes people pay good money for used caps! (lol)
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Re: Rectifier tubes?

Postby Blair » Sun Apr 28, 2013 6:23 pm

Hi Terry,

Yes, I could also tell it was newer because if the leads being vinyl, or whatever it is. I could never sell a used cap from the 70s in good conscience.

Time to list the amp on fleabay though:)
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