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Old 30th June 2010, 04:40 AM   #1
bst is offline bst  United States
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Default Modify Dynaco MKIII to use 6AR6s ?

I'm bringing a pair of neglected Dynaco MkIIIs back to life, and am considering changing to 6AR6WA/6098 output tubes. Is this a viable option?

I'm retaining all of the original iron, and installing Triode Electronics input boards. I'll also upgrade the PSU caps, and may consider switching to a stiffer choke.

My incentive for changing the output tubes is primarily cost, with quality an additional factor. I have several matched quads of NOS Tung-Sol JAN 6098s. I don't think I can find a current production 6550 to equal the original Mullard/GEC pentodes, and the price of NOS 6550s is ridiculous.

Is this a good avenue to travel? If not, are there Russian milsurp tubes which would be a better choice?

Thanks in advance for your opinions and/or experiences.
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Old 30th June 2010, 05:19 AM   #2
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What about 6CA7?
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Old 30th June 2010, 05:27 AM   #3
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I thought about 6CA7s, but unfortunately, I only have three. Bird in the hand, and all that...

BTW, here's a link to the entry on another forum which got me thinking about the 6098 option: Tubes Asylum - 6AR6/Triode Input Board/MIT-RTX/max build - Chris Spafford, February 11, 2003 at 17:28:35

I don't know if the poster's obvious enthusiasm stayed so high over the long run, but he sounded pretty pleased with the initial results.
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Old 30th June 2010, 06:40 AM   #4
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I don't see why not if you rewire the sockets and keep dissipation below say 18-19W /tube. From past experience the published 21W max dissipation really is max. George (Tubelab) would not be impressed at all.
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Old 30th June 2010, 12:59 PM   #5
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screen voltage max is 300Vs with 3.2Ws dissipation and with a plate dissipation of 19Ws, it's certainly a different beast than the 6550/KT88.

However, I have a friend who ran 6AR6s in his Mark IIIs - coincidentally, he used the Triode Electronics input board too. If I remember correctly, he ran them with the stock B+ voltage and dialed back the bias. The 6AR6s _were_ being heavily abused and would last six months before he would have to replace 'em. Since he had about 100 of the little buggers, he really didn't care!

In a trade, I ended up with these Dynaco Mark IIIs and rebuilt them from the ground up. I ended up running SED 6550s and also played with the top cap EL38 (required a bias supply change).
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Old 30th June 2010, 02:53 PM   #6
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George (Tubelab) would not be impressed at all.
I have a box full of used Tung Sol 6AR6's. I "tested" a few and was not too impressed. They bases are rewired to solve the problem inherent in 6L6GC, EL34, and KT88 types where the plate (pin 3) is adjacent to the heater (pin 2). I have seen several guitar amps arc over at the socket pins when driven to clipping with an incorrect load. This means that you can crank up the plate voltage with 6AR6's, but the dissipation limits remain.

The Bendix 6384 version is a different animal that can be cranked hard. It is unfortunately expensive and hard to find.
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Old 30th June 2010, 03:31 PM   #7
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I don't think I can find a current production 6550 to equal the original Mullard/GEC pentodes, and the price of NOS 6550s is ridiculous.
While far from inexpensive, "reissue" Gold Lion KT88s made by New Sensor at Saratov, Russia are good tubes. FWIW, feedback from the field finds the "reissue" GLs as being sonicly EXCELLENT.

IMO, the best NOS option is employing TT21s and adapters.
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Old 30th June 2010, 10:05 PM   #8
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Eli, after reading more about the Gold Lion KT88 reissues, I'm going to follow your recommendation. Jim McShane has matched quads for $220, which is reasonable considering the tubes' reviews and Jim's reputation for standing behind his wares.
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Old 30th June 2010, 11:55 PM   #9
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No doubt about it, McShane is the man.

You don't need a matched quad. Each monoblock needs a well matched pair. Pairs are easier to cull out than quads, which saves some cash. The GNFB loop brings the amps into alignment.
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Old 1st July 2010, 02:14 PM   #10
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How about 6BG6GA's or the Russian 6P3S's? I have used the 6BG6GA's with adapters in Dynaco Stereo 70's. I have used the Russian tubes also and they sound great.
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