The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Power tubes for an original ST70

    Share

    sailor

    Posts : 269
    Join date : 2011-04-04

    Power tubes for an original ST70

    Post by sailor on Tue Dec 20, 2011 8:46 pm

    I found a very clean ST70 which had the can cap replaced otherwise original. It was equipped with EH brand of big bottle 6ca7's I guess when the last owner lost the main cap. it damaged the left rear tube. Anyway, to make a long story short I am not sure if I will want to resell or keep the amp but in any case the power tubes will need to be replaced.
    I am looking for ideas on replacement tubes. I could just add 2 new EH 6ca7, or buy a set of JJ EL 34s, what about 6L6's or KT77. My main problem is I don't think the EL34's have enough Bass. I like the sound of KT88's much better, {The reason I may sell the amp.} The tube needs to be reasonable in price and something the next buyer may want. Any ideas?

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Power tubes for an original ST70

    Post by Bob Latino on Tue Dec 20, 2011 9:36 pm

    Hi Sailor,

    If you have an ORIGINAL ST-70 PA-060 power transformer on your amp, I would not recommend the use of KT88 tubes as output tubes in that amp. (An original PA-060 power transformer has a stack lamination of about 1.5 inches to 1.625 inches.) KT88's draw more current than EL34's and will stress an already undersized and hot running PA-060 power transformer even more. In your situation I would just replace the TWO EH 6CA7's (EL34's) on the left channel and play the amp. I would just toss that left rear tube and use the left front tube as a spare. On an original ST-70 or any ST-70 with the original driver board (or a reproduction of the original driver board), the output tubes should always be replaced as matched pairs.

    Bob

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Power tubes for an original ST70

    Post by tubes4hifi on Tue Dec 20, 2011 11:49 pm

    EH 6CA7's are excellent tubes and sound better than EL34s

    bigmm79

    Posts : 23
    Join date : 2010-09-15

    Re: Power tubes for an original ST70

    Post by bigmm79 on Wed Dec 21, 2011 12:12 am

    I really like the EH 6CA7.
    I also like JJ KT77s, I think they are better than EL34 types I have tried.
    On the more expensive side, I bet the Gold Lion KT77s are really good,
    I know the KT66s are.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Power tubes for an original ST70

    Post by sailor on Wed Dec 21, 2011 4:22 pm

    Thanks for the comments.

    Sled108

    Posts : 40
    Join date : 2010-02-03

    Re: Power tubes for an original ST70

    Post by Sled108 on Tue Dec 27, 2011 4:41 pm

    I'll second the JJ KT77's...You'll want to keep the Amp after hearing Them...And, the same for the Gold Lions at 2X the $$...

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Power tubes for an original ST70

    Post by sailor on Wed Dec 28, 2011 1:05 pm

    I finally concluded that it was between the JJ-KT77 and the 6ca7EH. Thanks to a post by J4570 about EAS having a sale on tubes I was able to save about $16 on a quad set. I figure if I put it up for sale a new quad set would add to the value. Plus from the present tubes, I have at least 2 good tubes, maybe 3 that could be used in one channel when or if in the future, one of the new tubes fail.
    I now have an SCA 35 and a Dynaco 70. They are about as different as night and day. I will probably comment on the differences once I have the new tubes.
    By the way, On DEC. 24, on EBAY someone paid $455 for a "it works but as is", no internal pictures or closeups SCA35.
    In the future, I think the SCA35 will become very scarce because so many are being bought up for there transformers and tubes used to make ST35 clones. More when I compare the ST70 and the SCA35.
    Thank again for the comments.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: Power tubes for an original ST70

    Post by burnedfingers on Thu Dec 29, 2011 9:31 am

    I believe the 6550 and KT88's draw 1.6A each. 6CA7's draw 1.5A, Since the Dynaco Stereo 70 was designed to run an external preamp that piggybacked onto the octal sockets on the front of the 70 there is additional heater current that is available. The front preamp provisions/power take offs were rated to 6.3v @ 1A each and 350VDC @ 10mA.

    Other tubes.... this will probably require a modification to the bias circuit. My 70 has been modified to use 6550's,KT88's, 6BG6GA's with adapters, 6L6GC's, and russian 6N3cE's.

    Sponsored content

    Re: Power tubes for an original ST70

    Post by Sponsored content Today at 4:40 am


      Current date/time is Sat Oct 01, 2016 4:40 am