1964 Dynaco ST-70 Stereo Tube Amp Rebuild

scott spencer

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910
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Rochester, NY
Hi ya gang - There was zero woodworking on this project, but nonetheless am happy enough about doing it that I wanted to post it here.

This is a Dynaco ST-70 stereo tube amp built from a kit (Dynakit) by my uncle in 1964. He used it for many years, and gave it to me sometime in the late 1980's. Shortly after, I ran into a 1961 factory wired version of the same amp, and ran both amps in mono mode in my stereo setup for around 25 years. Wonderful sounding amps. A couple of years ago the amps started acting up, as any 50+ year old electronics do, so I pulled them from service to give them an overhaul. My uncle's 1964 amp is now finished. I stripped it down quite a bit, gave it a good cleaning, bought and soldered up a new and upgraded circuit board for the driver stage from VTA electronics, replaced all 8 vacuum tubes, installed a new quad capacitor, gave it a new power cord, new inputs, new outputs, added some toggle switches that allow me to switch between pentode and triode mode, and added an "on" light indicator to help my wife remember to turn it off! :giggle: I got it wired up, biased the output tubes, and watched in glow!

The amp uses a GZ34 rectifier tube, (3) 12AU7 driver tubes, and (4) EL34 output tubes. It puts out 35 watts per channel in pentode mode, or about 20 watts per channel in triode mode. I've got new 12AX7 tubes ordered for my 1990s Lazarus Cascade Basic preamp, and will continue to use a nice pair of custom speakers I finished in the mid 90s. For now, I'm going to stick with one stereo amp, and use it in triode mode that will put out a beautiful sounding 20 watts per channel. Hope to get another 25 years of service from the old Dyna!

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scott spencer

Member
Messages
910
Location
Rochester, NY
Just a long overdue follow up with the 2nd amp mentioned in the original post. I did the same mods and used the same tubes on the 2nd amp as the first, so they're now identical. Both are hooked up in what's called a vertical bi-amp configuration that features one 2-channel amp to power each speaker. On each amp, one channel drives the woofer, and the other channel drives the midbass and tweeter. I get hair tingling clarity and separation from this setup. Can't believe how the mods and this bi-amp configuration have breathed new life into what was already a pretty slick setup.

The vertical bi-amp configuration was suggested by Bob Latino who designed the circuit updates. The logic is that two separate amplifiers have zero crosstalk between the left and right channels, so you get greater separation. There's also significant benefit from dedicating one channel of each amp to just the woofer, since the woofer is much harder to drive and control than the mid and tweeter....that gives the 2nd channel of each amp the easy task of driving the mid and tweeter, which results in greater clarity than if the woofer were being driven by the same channel. This setup allows me to choose output tubes that excel at their respective tasks too...kinda of like dedicated saw blades vs general purpose....the set of tubes for the woofers has tighter deep bass, and the other set has clearer midrange and top end (yes even the same tube type can sound very different from one another). It does require an extra set of wires, and that the speakers have separate inputs for each.

It's not exactly your basic setup, but ooh la la! I've been doing this stuff for a while, and my system has never sounded better, and I've never enjoyed it so much. It was a great quarantine project during early spring this year.

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Vaughn McMillan

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ABQ NM
That's awesome, Scott. :thumb: Too bad there's not a way to capture the sound of that rig and post it here. I'll bet it's sweet.

Back in my touring days, we used to run a quad-amped mono setup for our PA, with separate amps for bass, mid-bass, mids, and highs. It was all solid state (a combination of Phase Linear and Crown amps), but that way we could dedicate the watts where they were needed the most (bass and mid-bass).
 
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