Audio Research ST 70 Mod

[Robert Cham]

Is anyone out there familiar with it. I have one that is misbehaving. i haven't used it for awhile so can't really remember tha problem, probably low gain in one channel that doesn't follow tubes when they're swapped.

Anyway, I'm interested in opinions of the sonic quality versus the DIY Tube driver board mod. Maybe it makes sense to just put a new driver board and cap board in and get it ready for use in a biamp setup.

[Pyre]

Hello, I assume you are referring to the ST-70-C3 mod. Since the mod is more of a complete rebuild using the ST-70’s base and transformers, swapping in a new cap and driver board would be a lot of work. You would have to take out everything and build a ST-70 from scratch really. A lot of the power supply circuit has been moved to the driver board on the C3 and tube rectification has been completely removed. In my opinion it would be a shame to disassemble a C3.

As to what may be wrong with your amp it is tough to say. I would assume that one of the components has drifted out of spec causing power to be diverted away from the driver stage or output tubes. The fact that you still have sound is a good thing. Is it just lower volume or distorted with any hum? If you do not have a manual for the C3 I would be happy to send you a copy.

As for sonic quality versus the DIY driver board that is really a matter of opinion. The C3 has 6L6GC’s and the DIY board on an otherwise unmodified ST-70 would use EL-34’s. Not really comparing apples to oranges but definitely different, both have had a lot of testing and engineering go into the circuits. The C3 in my opinion is overly complex. I have a C3 and a couple of modded ST-70’s. I prefer the sound of the C3 hooked to Electrostats and a set of Kef 104 a/b’s. I prefer the modded ST-70’s when hooked to A-25’s. I have always been impressed by the bass capabilities of the C3.

If you are looking to build a simple amp, trade the C3 to someone who can restore it and get yourself a good ST-70 to put a cap board in or driver board. It’s always best to start with something that’s working right.

Shane

[Robert Cham]

I never had any intention of disassenbling the AR moded ST 70. I was just interested in peoples opinions of the two different mods. I use Tympani IIIs, so your electrostatic opinion might apply.

I plugged it in again after several years and was greeted by lots of hum and 120 Hz buzz. Looks like it's time for new electrolytics at least. I noticed thata spare SDS Labs ST 70 Cap board could be easily modified to work. I don't see anything wrong with that, and the price is right.

Bob

[Pyre]

That SDS board is a pretty good idea. You could replace one of the 50uf sections with a 70uf, parallel it to the 160uf to get you the 230uf needed and keep the last two 50uf’s as is. Are you planning on trying to fit it underneath or on top?

[Robert Cham]

There is one section that is 190 ufd. I planned to use that to replace the 230ufd. The other sections should be sufficient for the rest. I planned to use the diodes on board as well and the two caps for the center tap of the OPT. All of this frees up plenty of room underneath.

Comments?

Bob

[Pyre]

That should work just fine. I would stick with the diodes on the C3 driver board, maybe replace them if they need it.

[Robert Cham]

Any parrticular reason to stick with those diodes?

Bob

[EdStiles]

I have an original ARC ST-70C3 kit that is still unbuilt (an "in progress" project). After I purchased it on eBay, I wrote Audio Research asking guidance on what mods I should make to update it. I received a detailed response from William Z Johnson about some changes he would recommend, if you PM me with your email address, I'll forward them to you.

On mine, I have replaced the original power transformer with a Triode PA-060 which allows more power reserve and I can run EL-34s.

Hope to finish it soon, need the time to rework the driver board with new resistors and caps, a 2008 project.

Ed