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Old 18th March 2012, 01:54 AM   #1
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Default Tube recommendations anyone?

Hi guys.
I am about to pull an old preamp out of mothballs and know that it needs work. It is a circa 1980 Theta tube preamp. I plan to take my time and restore it but don't have deep pockets so I am searching for reasonable costing tube choices.
The preamp requires 6 6DJ8s and I have been to looking. In the cost effective (cheap) category are the JJ E88CC/6922, the Sino 6922/6N11, and the ElectroHarmonix 6922EH. Any of these seem viable? Is it critical to search out matched sets?
Also, I'm a noob at this so any personal notes or advice regarding this preamp or related procedures are welcome.
Thanks in advance for any help!
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Old 18th March 2012, 02:52 AM   #2
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Location: Monroe Township, NJ
Definitely use the Russian 6Н23П-EB (6n23-ev) in the phono section, as it is low in both noise and microphonics. The EH 6922 is, in fact, 6Н23П-EB. Sonics are good, if unspectacular. The variant is also tough as nails. So, use 'em everywhere, should the Theta have a tube eating reputation, like stock AI preamps.

Recommending JJ E88CCs used to be safe. Unfortunately, JJ is having quality control problems with much of their production. If you can afford the expense, "reissue" GEC 6922s elsewhere for "tone" makes some sense.

As for matching, it depends on the circuitry employed. Can you post a schematic of the unit?
Eli D.
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Old 18th March 2012, 04:20 AM   #3
tomchr is offline tomchr  Canada
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Are the current tubes shot or missing? What prompts the tube change?
Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits.
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Old 18th March 2012, 08:21 PM   #4
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Thanks for the info Eli Duttman, I appreciate the insight. I will look at the cost of outfitting with your suggestions. The tube eating comment is not unexpected but I have little intelligent to say there. I bought this preamp new in the early 1980s at the recommendation from a manufacturer when I was in audio retailing. I used it for some time with a pair of bridged hafler 200s and for some reason both amps went up in flames so the whole system went into boxes and has not been handled with care since. My most recent look at it was a few years ago and I noted a few tubes broken and rattling around within so i removed them all. The big reason for the mass replacement, tomchr, is really just trying to chase away gremlins. With no more knowledge of what went wrong, I thought it better to spend a few bucks and remove the condition of the remaining tubes from the equation. Am I being paranoid?
ps I WISH I had a schematic for this unit... now that I can read one. I will try to generate one directly from what I see. It is a minimal layout. Thanks for the input!
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Old 18th March 2012, 09:50 PM   #5
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Originally Posted by pcb121055 View Post
I used it for some time with a pair of bridged hafler 200s and for some reason both amps went up in flames
That's a shame, I guess they didn't like being bridged.

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Old 19th March 2012, 01:20 AM   #6
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well vinylkid, I didn't see anything that would have taken them down. used to build the hafler kits and they each had the bridging adapter. They were driving a pair of dahlquist dq-10s. Ran beautifully for a good while (months) before flaming out. Thing was, they both went at exactly the same time so I blamed the preamp. gonna find out now. this is kinda like an old murder mystery. i hope this preamp gets exonerated
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Old 2nd April 2012, 02:09 AM   #7
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hi guys... an update! finally got off my rear and dragged the preamp out of the attic and when i opened it, i realized the missing tube story was for my old heathkit preamp. the theta had all six tubes in place. they turned out to be Tungsram E88CC/6922, "made in hungary" is printed on their sides as well. i plugged it in by itself and they all lit up. after 30 years in a box! so i left it this way for several (10 maybe?) hours. the only thing i could think to do was to check for dc on the output and that was with a cheap radio shack meter. nothing found. i threw caution to the wind and put it into my system and it worked just fine! no noise of any kind. there is a gremlin though... if you tweak the volume control just right, it pops loudly thru the speakers. i listened to it at a moderate volume for about two hours and it did pop once about mid-session without my having touched it. so i have it back out on my counter for full cleanup. i guess i need to check it over for bad solder joints? great to have it available. any tips on cleanup are welcome. i have read the sticky about careful power up but i guess i'm past that. sorry for the miscue on which preamp i was rescuing. thanks for the help/responses about the tubes. i'll post pics if i can figure out how.
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Old 2nd April 2012, 03:09 AM   #8
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Location: Adelaide South Oz
Get a can of spray contact cleaner and do the tube sockets - check the tube pins to see if they need a clean up as well.
That may well fix the problem - even if it doesn't it is a good start.
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Old 2nd April 2012, 12:38 PM   #9
DF96 is offline DF96  England
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Try getting some switch cleaner into the volume pot too.
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Old 2nd April 2012, 12:56 PM   #10
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Thanks for the recommendation, guys. Chemical cleaning is high on the list. I bought the Caig Deoxit and propose to be very thorough. It would be nice if that's all it needed. I am anxious to have it back in the system for good.
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