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.

Definitely waste your time on refurbishing. As point of comparison look at the Van Alstine update.

Thanks much! The responses are exactly in the form I was hoping for.

Atmosphere, am I correct in thinking that you (your company) rebuilds them for others that supply the amp?

Yes, we have done that many times. A typical rebuild (such that the chassis is reliable) is about $200.00. This is mostly just repairing the power supplies and making sure that the unit meets spec with good tubes, but does not include any tubes that might need replacement.

IMO its best to keep the amp as stock as possible; perhaps replace the coupling caps as an upgrade; the reason why is that the original design is quite competent except for the power rectifier system, which is handled by a single 5AR4. It should have been a pair of rectifiers. Some people replace the tube with solid state but you run into problems with too much voltage which can lead to other problems down the road- short filter cap life and possible damage to output transformers.

This issue is not addressed properly with most of the update paths I have seen- for example adding more tubes to the driver stage puts more load on the already stressed rectifier... I find that as long as the amp is not pushed hard it is very musical, so selection of the loudspeaker is important for best results. The harder you push the amp, the shorter the rectifier life will be, but it can last decades if the amp is loafing. So an efficient speaker is really helpful.