Results: Dyna ST-70 Mods

Jon Yaeger

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Nov 2, 2002, 4:03:53 PM11/2/02

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For some reason, I've taken to the Dynaco ST-70 amp. IMHO, it offers one
of the best bangs for the buck. When "properly" modified, it sounds even
better!

1. My first Dyna 70 mod used a new driver PCB from Sound Valves in Ohio,
which featured 6GH8s in lieu of the temperamental & rare 7199's. I didn't
notice a big sonic change, but I felt that the newer components were more
reliable. The mod sounded competent, but not "knock your socks off." I
definately recommend their $35 hardware upgrade kit which has ceramic
sockets and new shiny hardware).

2. My 2nd project was a Dyna-70 with the Welborne Labs mod designed by
Alan Kimmel. This is not a project for the beginner or the
faint-hearted. It requires a bit of drilling to the chassis and heavy
modification.

The circuit is a mu-follower using a 6U8 and a 5751 per side. Installing
the circuit board is a bit like building a ship inside a bottle - heaven
help you if a cap goes bad once the board is installed. It will take a
lot of desoldering and maneuvering to expose it again . . . . .

The results are well worth it. Everyone that heard it thought it sounded
terrific. I had someone over to buy another amp and they insisted that I
sell them this one, too. (I've got more than a little seller's remorse,
but I needed the funds).

3. My 3rd mod was a Triod Labs PCB, which used a 12BH7 (or 12AU7) and
EF86s for drivers. I really went all out on this one - repainted the
iron, chromed the chassis, installed all new hardware, etc. It looks
awesome for a Dyna. I installed mullard EF86's and silver foil output
stage coupling caps I bought from a nice fellow in England.

I'd have to say, (subjectively, of course!) that the sound is about 85% of
the Welbourne. It's fine but doesn't seem to have the dynamic range or
clarity that Kimmel's has.

4. I made some encapsulated front end filters (ala Van Alstine) and
installed them on the Triode Labs unit. I don't know if this made much
difference . . .

I'm not trying to advertise for Welbourne, but their's was the clear
winner, and maybe my experience will help another R.A.T. . . . .

- Jon

Tube747

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Nov 3, 2002, 3:47:49 AM11/3/02

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What is your comment about the Alan Kimmel's mod Dynaco MKIII? How you compare
this to the Alan Kimmel's ST-70 you built?

Jon Yaeger

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Nov 3, 2002, 3:58:53 PM11/3/02

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I don't know. I don't have specific experience with the Mark III, stock
or modified. Sorry.

- Jon

In article <20021103034749...@mb-fu.aol.com>, tub...@aol.com

Dean Hiebert

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Nov 4, 2002, 2:05:08 PM11/4/02

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Try triode wiring the EF86 and reducing overall NFB in project 2. I like
mine that way.

Dean

onogos

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Dec 3, 2002, 2:11:54 AM12/3/02

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Hi Jon,

I agree with your findings. I have the SV board and the Welbourne
Labs board and have listened to at least 6 different variations of the
ST70 including stock, and the ST70 with the Welbourne Labs mod is, to
my ears, the best sounding, especially with upgraded parts and NOS
driver tubes.

Francis

jo...@mindspring.com (Jon Yaeger) wrote in message news:<jono1-02110...@172.16.1.10>...