Refurbish a Dynaco St70

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.

Thanks much Atmosphere! I will contact you regarding having your people do this. I believe it would be fun to play with and should I not find a place for it in a system and am sure I can find a AG member to enjoy it without much $$ loss.

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.

No- not done that. There's not a lot of room for another rectifier, and unlike some I really don't like modifying the chassis of older units- it usually damages their resale. For something like that I figure if a person has a better idea, best to build it up from scratch.

But if you did have dual rectifiers then the amp would be more reliable, as the 5AR4 is the most likely tube to fail, but that is only if you consistently push the amp hard.

"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.