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

    7199 to 6GH8A conversion

    Share

    wharf-creek

    Posts: 22
    Join date: 2008-12-18
    Age: 62
    Location: Baltimore, Maryland

    7199 to 6GH8A conversion

    Post by wharf-creek on Thu Apr 16, 2009 11:49 am

    Hello All,

    Just spent the morning here playing catch-up on some of this stuff. Read thru a BUNCH of posts and saw some GREAT pics!! It's very humbling to see the level of skill in which some of these amps have been built...and re-built. I salute you!! Now, on to the subject of my post.

    I built a 'clone' ST-70 a year ago, and listen to it on and off with other amps I rotate thru my system. This 'clone' used an old OE Power Transformer, some Output Transformers that were obtained off ebay and designated as being specifically for EL34 tubes, and I also used one of Sheldon Stoke's SDS Power Supply boards. The Driver Board is a STOCK replacement board from Dynakitparts.com. However, this was just about the time that 7199s were worth their weight in gold....so instead of using them, I also purchased a set of the 6GH8a Adapters from Dynakitparts.com...and I've been using this amp with those tubes instead of the 7199s. I later found that there was a recommendation to also change some parts on the board to better accommodate these tubes, but I did NOT do that as yet. Thus my question; Is there any current recommendations on what to do to 'convert' the original board from 7199 to 6GH8a beyond just the pin-out changes? I know there was something....but I can't find my records on those changes....and, I'm thinking there might also be an Generation III change in order...with even more info. I have a spare 'stock' board that I've gone thru and actually cut the traces and re-wired for the 6GH8 tube...and I plan to install that in this 'clone' amp at some point. But I wanted to just complete any other up-dates before I do so. I would also mention that with 7199s in the amp, it sounds EXACTLY like another ST-70 I have that is also stock except for the SDS board. So, the OTs are not really subject to criticism in this amp, and in fact, may actually be a bit superior. I wish I knew more about them...all I can say is they are a bit larger than the OE units, and they only have 4 and 8 ohm outputs. So, I had to adjust the NFB circuit to account for that. But, this is common now on most all the newer kits available, so the changes in this circuit are pretty well engineered as far as I know. I can't remember the last time I saw a 16 ohm speaker!! Anyway, the bottom line is that the two amps sound the same when played with 7199s....and nearly identical when the one is played with the 6GH8a and the other with the 7199. I think my best evaluation is to call it a 'draw'!! Both tubes sound good...and the differences are so slight that I could well be saying the same thing about two different 7199 tubes. So, while the cost of 7199s today is a bit less than last year's $100/pair prices.... I still think the conversion to the 6GH8A is worth trying...and it's a fun project that will not harm your amp, and gives you something to do on a rainy day...of which I've just had 3 of in a row!! I'm going nutz!! By the way, I also tried the adapters in one of my SCA-35 with similar results....no real distinguishable difference!!

    OK.....thanks to all for reading this....good luck...and "Happy Soldering"!!

    TSD

      Current date/time is Thu Apr 17, 2014 2:19 am