Refurbish a Dynaco St70

Depending on how in-depth you'd like to go; Joe Curcio may be the best person to contact. He has decades of experience, in rebuilding/modding the ST-70. These amps have some excellent transformers, which are(arguably) the most important components, of a tube amp. The only major problem, with owning a completely stock ST-70, is finding a source of NOS or high grade 7199 tubes(my favorite was the Sylvania). They're almost extinct(Unobtainium) and there is no direct/plug-in replacement. It's not too hard to rewire the tube socket, to receive the pin configuration of the 6GH8A tube and adapters are also available. NOTE: When I say, "completely stock," I'm not saying one with the original caps and resistors, which absolutely should be replaced, to obtain the best sound and any kind of reliability. The only part that can be hard to find(for a stock rebuild), is the quad-cap. There's a seller on eBay(http://www.ebay.com/itm/NEW-30-20-20-20-550V-ELECTROLYTIC-CAPACITOR-DYNACO-DYNAKIT-MKIII-MKII-ST-70-/331196180628) and Curcio Audio also offers a kit/upgrade for it: (http://curcioaudio.com/dynadr_3.htm) I used to buy mine from Antique Electronics Supply, in Tempe, AR. Replacing the inputs, outputs and power cord, without chopping up the chassis, is pretty much a cakewalk. Have fun!

Personally- if I were going to rebuild an ST-70 now days and retaining a MOSTLY stock(tube rectifier/greater filter cap capacity) power supply(like I used to), I'd use this board: (http://www.curcioaudio.com/st7upg_3.htm), stuffed with higher quality components. I just happen to love the Siemens CCa and this board would make it's use possible.

Hello Ralph- "the original design is quite competent except for the power rectifier system, which is handled by a single 5AR4." Just curious as to whether you've ever added a second 5AR4(and heater transformer) to an ST-70.

"but that is only if you consistently push the amp hard." I had the same Mullard GZ34(Blackburn), for about 15 years, but always had the system bi-amped, below 200Hz(ergo: wasn't pushed). "I really don't like modifying the chassis of older units- it usually damages their resale." That's what stopped me from making a permanent, on chassis mod. I was thinking about punching out the quad-cap hole, to accept an octal socket, then installing four filter caps and a heater transformer, internally. Two GZ34s, each used for half the full wave(plates tied together, per the Radiotron Designer's Handbook) did sound nice though. Then I got power-hungry, gave the ST-70 to my son and moved on.

"Pry- I meant with modifying Dynacos, not where he lived. :)" Actually: Mr Johnson began a company known as Electronic Industries, in Minneapolis, Mn, in 1951. He was building some amps back then. Another company assumed ownership, but- he stayed on as Research Director. He didn't start modding ST-70's until the late 60's. Started ARC, in 1970.

YEP! ARC's first FM tuner was based on the Dynaco FM-3. The M-60 MkIII C3 amp was a redesign of the Dynaco MkIII. ARC offered kits, to remake the ST-70, using only the chassis and transformers. Imagine what we'd have had, back then, if Hafler and Johnson had formed a company. But, HEY- over 350,000 STOCK ST-70s were sold, making it the most popular tube amp ever. Hafler's design wasn't too shabby, on it's own merit(what an understatement)!

A good decision, no doubt.