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Tube matching jig recommendations?

Jfaustus

Jfaustus

AK Subscriber
Subscriber
Hi all-

I am (slowly) restoring a pair of Julius Futterman H3aa monoblocks (in fact, the first pair he ever made). Thread here. Latest round of bench testing points to the failure of one or more power tubes. This really sucks because the unit uses 6LF6 tubes which are few and far between. And they have to be closely matched. At 6 per channel I will eventually need two matched sextets. 12 came with the amps and I have another 8 or 10 singletons. I'm hoping that out of those I will at least be able to cobble together one channel. Hey, I could do some mono listening!

To get there though I will need testing capabilities I don't currently have. So.....

Can anyone recommend a DIY jig for matching current at operating voltages?

Fingers crossed...
 
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Mr Boochie said:
That's a tough tube to measure the actual operating parameters. Do you know anyone with an Amplitrex Tube Tester? I think Jim McShane used to have one. But, I don't know if he still tests tubes. You can try emailing him. He may have more suggestions.

This was his last email address that I used: jimmcshane@mcshanedesign.net
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Thanks! I'll hold that as a backup. I may just use the amp itself if I can get the voltages to behave. But please don't let that stop anyone from chiming in with more recommendations!
 
Jfaustus said:
Thanks! I'll hold that as a backup. I may just use the amp itself if I can get the voltages to behave. But please don't let that stop anyone from chiming in with more recommendations!
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You know, I read that this isn't a great amp for measuring the idle bias current (the usual cathode-tied, precise 10-ohm resistor technique). I am not at all familiar with output-transformer-less amps, so I really am out of my neck of the woods here. It is not a simple push-pull, Class AB1-type thing.

What did Julius Futterman recommend?

Sorry! And good luck with your search!

:thumbsup:
 
Mr Boochie said:
You know, I read that this isn't a great amp for measuring the idle bias current (the usual cathode-tied, precise 10-ohm resistor technique). I am not at all familiar with output-transformer-less amps, so I really am out of my neck of the woods here. It is not a simple push-pull, Class AB1-type thing.

What did Julius Futterman recommend?

Sorry! And good luck with your search!

:thumbsup:
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Thanks! I'll get there. Futterman put precisely two pots in the H3aa monoblocks. One adjusts the bias of the input stage (6EJ7 andf 6FS5) and the other, I believe, evens out the screen voltage between the two banks. He does give detailed instructions for what to do in the event you suspect the power tubes are off. I quote in full: "If a zero reading can not be obtained, test the six power tubes." Keep in mind that in 1974 (when mine were made) or 1978 (when those instructions were written) you could probably by a dozen perfectly matched 6LF6s at your corner store for the price of a pack of gum.

Can you say more or recall what you read about it being hard to measure idle current? I actually put in cathode-to-ground sense resistors for one of the banks and read from them. But since the voltages aren't cooperating the readings don't mean much. But they were were stable -- which doesn't necessarily mean anything.
 
Jfaustus said:
Can you say more or recall what you read about it being hard to measure idle current?
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If you look through the posts on the Net, you get a general feeling that Google AI sums up.

Futterman OTL amp - adjusting bias.jpg

As I stated, I know little about OTL amps. You should seek a tech who is competent in this design.

My gut feeling is not to experiment with the amp unless you really know what's going on. TAKE CARE with this thing. That's my final word on this.

Good luck!
 
Can knock together a test rig using regulated power supplies, but thats only single point / idle bias matching. If you really want the feline's butt for matching you want a curve tracer, optimally one that can run the tubes at the plate and screen voltages the amp actually uses.

Something that will do idle bias with known plate and screen voltages plus a transconductance test will generally be fine though. If the idle and transconductance line up pretty well, generally the curves should be acceptably close as well. Better if you can set the voltages close to what the amp runs them at of course but there are only so many options.

problem with many of these things though is they tend to test a specific list of tubes, not just anything, and also usually not things that aren't common audio tubes. You're basically doing TV tubes here. A Utracer or whatever can test anything you want but the Amplitrex does what it does.
 
gadget73 said:
problem with many of these things though is they tend to test a specific list of tubes, not just anything, and also usually not things that aren't common audio tubes. You're basically doing TV tubes here. A Utracer or whatever can test anything you want but the Amplitrex does what it does.
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Yeah. I think he needs access to an Amplitrex tube tester. I have never heard of anyone DIYing one of those. The tubes need to be accurately matched for his amp.

He should reach out to Jim McShane. If Jim doesn't do it anymore, maybe he knows of someone who does.
 
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gadget73 said:
will an Amplitrex test a Compactron sweep tube though?
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You will need a socket adapter. He needs to contact Jim McShane. Amplitrex services are listed on eBay. Maybe contact one of them

I am not at all familiar with this amp, that tube, or that tester. It's better that he gets information from someone who does know.

:idea: :)
 
I'm not either, I just know 12 pin socket, and most modern tube testers don't test those. Also sweep tubes generally tend to have high plate but low screen voltage ratings, so ideally both would need to be adjustable to test it somewhere within datasheet spec, or even more ideally somewhere near where the amplifier runs it.
 
gadget73 said:
I'm not either, I just know 12 pin socket, and most modern tube testers don't test those. Also sweep tubes generally tend to have high plate but low screen voltage ratings, so ideally both would need to be adjustable to test it somewhere within datasheet spec, or even more ideally somewhere near where the amplifier runs it.
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That's why he needs to talk to someone who knows those tubes and the Amplitrex or alternatives.

I did come across some posts on what Ham Radio guys and TV repairmen used to test the 6LF6. Those were super old articles (2010 and before — most of the posted links didn't work) — prolly many of those techs are no longer available. :( Might as well try to get a hold of McShane while he's still in business. That's what I would try to start with.

:thumbsup:
 
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I was about to post, the most reasonable and sensible thing to do, is send them out to get tested properly and matched. The amount of cost and your time, to figure out and construct your own set up would greatly outweigh the cost of sending them out for a test. What QE_Eng is offering, is why AK is so awesome.
 
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