: Substitute audio tubes  ( 731 )

electra225

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Substitute audio tubes
« : January 25, 2020, 02:42:53 PM »
I have an old Dynaco SCA-35 that needs a good going over.  The circuit boards have been hardwired more than once.  Looking online for replacements, I find one can use their credit card and end up with all the parts necessary to build a new SCA-35.  I also see there are tube adapters to replace the spendy and semi-unobtanium 7199's with 6GH8-A's. 


6GH8's are essentially dumpster tubes to everybody but guys who fix old color TV's.  Sylvania made a contemporary for the 6GH8-A, the 6KT8, used mainly by Zenith in color TV sets.  The 6GH8 is a triode-pentode, while the 6KT8 is reversed, a pentode-triode.  I assume the 6GH8 is triode-connected for audio use.  If this tube is good enough for the venerable old Dynaco amps, wonder could they, or the 6KT8 be used to replace the 6EU7 in Magnavox tuners and amps.  I don't have my tube manual here, and realize that basings may differ.  What do you guys think?
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TC Chris

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Re: Substitute audio tubes
« #1 : January 25, 2020, 10:50:01 PM »
I think that my tube manual is out in the back room, that's what I think.  And the first question is what the design characteristics are, which the manual would reveal.  I'll bet yours is in a warmer place than mine. It would be easy enough to either rewire the socket or make an adapter but only if the principal characteristics match closely enough.

Chris Campbell

firedome

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Re: Substitute audio tubes
« #2 : January 27, 2020, 10:27:50 AM »
6eu7s were used quite successfully in lots of Heathkit stuff without any performance or longevity issues, I've had at least 4-5 of those Heath amps, including an AA-32 for 40 years, that I liked a great deal. Not sure what the issues were in Maggies, perhaps the circuit design. Still have a bunch of NOS 6eu7s as well as 7199s and 6gh8s (both used by Scott) but keep them as Heath amps are fairly common and no doubt one will fall into my lap again at some point.

Had an SCA-35 once, well worth restoring, that had really excellent output transformers.
The real experts are Joe Curcio, also Kevin Deal out in Chicago... www.triodeel.com iirc.
He has boards and even repro transformers, and there's lots of mods... I'd leave it alone.
Happy Motoring! from Roger in NY

TC Chris

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Re: Substitute audio tubes
« #3 : January 27, 2020, 07:00:02 PM »
Is the SCA-35 the integrated amp with P-P EL-84s?  My brother has one that's retired and I'd love to pry from his hands.

Chris Campbell

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Re: Substitute audio tubes
« #4 : January 27, 2020, 07:48:31 PM »
Well IMHO 7199's are neither hard to find or too pricey.  Yes they don't sell for $3.00 a tube but you can still find NOS tubes for under $10 if you keep your eyes open and take your time to look.  Right after Christmas I bought a sleeve of 5 NOS Sylvania branded 7199's for $25 at the local Vintage Swap meet attended by literally thousands of people.  They were in a box of mostly TV tubes that were all jumbled up by the time I found them and I would venture to guess that many people had probably glanced at the stuff in the top of the box and not bothered to look any further.  The same box also had 3 NOS 6L6g tubes in RCA boxes that I bought for $12!


Buy some new stock style PCBs and rebuild that amp the way it was designed. 
-- Aloha, Ken

No Console Left Behind!

TC Chris

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Re: Substitute audio tubes
« #5 : January 27, 2020, 08:18:10 PM »
  Right after Christmas I bought a sleeve of 5 NOS Sylvania branded 7199's for $25 at the local Vintage Swap meet attended by literally thousands of people. 

Just to keep things straight here....  I'm pretty sure "swap meet" is California-speak for what passes as a "flea market" in Michigan.  Neither quite makes sense since you're neither swapping stuff nor buying fleas.  My old friend from L.A. who died this past year helped me understand the dialect and we taught him Midwestern when he visited. 

Chris Campbell

firedome

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Re: Substitute audio tubes
« #6 : January 29, 2020, 08:09:33 AM »
Chris: yes the SCA-35 is a EL-84 PP integrated at 17W/ch and introduced around 1962. 2 x 12ax7s for phono and line stages, 2 x 7199 for driver/phase inverters, rectification is solid state, and Z-595 output transformers that are considered some of the best ever used by Dynaco. Those trannies are frequently sought after and scavenged for custom amp designs also unfortunately. Dyna also made a basic SC-35 stereo amp that is similar although using 7247 as a driver/phase inverter. These are quite rare and also exceptionally good in their power range and typically used with the famous PAS-3x preamp that has to be one of the most modified components ever. Kevin Deal in Chicago is the go-to guy for all things Dynaco; info, mods and parts. He's even had Dynaco's trannies repro'd to the exact original design, so anything can be resurrected.  Chris - I'd get hold of that baby if at all possible!  IMO it'd put about any console amp to shame with very few exceptions, those output trannies make for an awesome amp!

Ken: that's a heck of a tube find, seeing 7199s typically run about $25 apiece or more, not to mention NOS 6L6Gs...sometimes we get lucky!  Used to be that Hamfests were a good source for bargains like that but no longer, thanks to you know where... :-(
Happy Motoring! from Roger in NY

electra225

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Re: Substitute audio tubes
« #7 : January 29, 2020, 06:01:00 PM »
Why, then, are the SCA-35 gurus going to the 6GH8-A's if the 7199's are common and cheap?  The 6EU7 in the Magnavox equipment I own have proven problematic.  I don't know if it is design or environment, but I suspect a 6EU7 first if I have a performance problem of any kind.  Cathode/heater shorts and having one half dead is what I see.  Granted, they are 60+ years old.  I realize tube basings are different, but my question is why could not a 6GH8 or a 6KT8 be used in place of the 6EU7......
I don't need Google.  My wife says she knows everything.