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Dual ST-70s dilemma

H

hairyandy

New Member
Hey all, looking for some ideas from the braintrust here.

I’ve recently acquired two, original, factory-wired Dynaco ST-70s. I have an Acoustat preamp with dual stereo outs that I originally thought I would just run to each ST-70. Now I’m thinking that I might need something in between that will be able to attenuate the signal to one or both of the amps to even out the probable volume discrepancy. One ST-70 will be powering a pair of 1966 Altec Carmels (16-ohm) and one will be powering a pair of JBL 4311Bs (8-ohm) and I know there will be a volume difference. Ideally, in my head, what I’m thinking is a box that can take the stereo line-level signal from the preamp and split it to multiple outputs (at least 3 separate pairs of outs) with an attenuation/volume control on each output. Two outputs will be for the ST-70s and the third output will be sent to a Sonos box so that I can have my turntable on the Sonos network and play it in other rooms/back deck if I want. One of my big concerns is that the ST-70s run full out so any switching done after the preamp could potentially send a loud DC spike to the speakers which would suck. Also, and obviously, I would want this to be as transparent as possible.

Does someone already make something that can do this? Am I going to have to make my own?

Thanks for any and all help/insight/ideas on this.

Andy
 
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Search Amazon for rca volume attenuator. There are plenty of them (most designed for car use, but will work fine in the home). You should be able to split the output once more using Y cables, then put a volume control between each output and each amp.

Otherwise, what you're talking about is an audio distribution amp. Most don't have volume controls; here's one that does.
 
swechsler said:
Search Amazon for rca volume attenuator. There are plenty of them (most designed for car use, but will work fine in the home). You should be able to split the output once more using Y cables, then put a volume control between each output and each amp.

Otherwise, what you're talking about is an audio distribution amp. Most don't have volume controls; here's one that does.
Click to expand...


Thanks Swechsler! Funny, I did some Amazon searching first and completely missed that Rolls piece. ‍♂️

Andy
 
Have the ST-70s been recapped? I highly suggest taking a look at the restoration article I wrote for pointers. With regards to DC spikes, that shouldn't be a problem with the ST-70s (or most other tube amplifiers) due to the use of output transformers, which don't pass DC.
-Adam
 
Maybe you'd consider replacing the stereo/mono switch with a potentiometer for input attenuation on each amp. tubes4hifi has kits for that.
 
hairyandy said:
The Selenium rectifiers and bias caps have been replaced.

Andy
Click to expand...
That's a good start, but I highly recommend also replacing the molded-paper capacitors (usually branded 'Black Cat' or 'Pyramid') on the driver boards. I used mylar capacitors I happened to have lying around, and it sounds fine to my ears. I also recommend replacing the filter capacitor can, either with a replacement can or with discrete capacitors placed under the chassis. Here's a photo of my ST-70 after recapping:
st70_3.jpg
 
AdamAnt316 said:
That's a good start, but I highly recommend also replacing the molded-paper capacitors (usually branded 'Black Cat' or 'Pyramid') on the driver boards. I used mylar capacitors I happened to have lying around, and it sounds fine to my ears. I also recommend replacing the filter capacitor can, either with a replacement can or with discrete capacitors placed under the chassis. Here's a photo of my ST-70 after recapping:
st70_3.jpg
Click to expand...
I plan on checking the cans. I will check the Black Cats as well, but these are both early 60’s, factory wired, completely stock and in REALLY amazing shape. I’m not replacing anything that doesn’t need replaced. I’m a touring guitar tech and I make guitar amps and pedals so I understand how these things work. Obviously if something is bad I’ll replace it, but I want to keep them as original as possible.

Andy
 
hairyandy said:
I plan on checking the cans. I will check the Black Cats as well, but these are both early 60’s, factory wired, completely stock and in REALLY amazing shape. I’m not replacing anything that doesn’t need replaced. I’m a touring guitar tech and I make guitar amps and pedals so I understand how these things work. Obviously if something is bad I’ll replace it, but I want to keep them as original as possible.
Click to expand...
There wasn't much difference between factory-wired and kit-wired units in that regard; the driver board was supplied pre-assembled even if you bought the kit version. Besides, changing the molded-paper capacitors is a good way to prevent red-plating of the output tubes. When they were made, molded-paper capacitors were supposed to last longer than the wax-paper capacitors which had preceded them, but this turned out to not be so. Replacing the paper dielectric with mylar and other plastics vastly increased their reliability; supposedly, some ST-70s were equipped with mylar capacitors which looked the same as the old Black Cats, but I'm not 100% on this, and I think it's a good idea to replace them as a matter of course.
hairyandy said:
Your ST-70 looks awesome Adam!!!
Click to expand...
Yeah, it's in nice shape. :) It's an example of a later production unit, with parts bearing 1974 date codes. The chassis is chrome-plated rather than nickel-plated as with older units, and the driver board is green fiberglass rather than the typical red phenolic. In addition, they equipped it with a black tube cage bearing the Tyco-era Dynaco logo, rather than the usual brown cage with script 'Dynakit' plate. Here are some more photos:
st70_1.jpg

st70_2.jpg
 
Nice set of Dynas! Man, do those pics ever bring back the memories.
A pair of these babies with 2 DQ's and a good preamp will send anyone off on a listening binge.

We need a word for that kind of listening: the one we all do when we make a change that seems to work; re-listening to about half or more of our source collections.
I guess I'd call it a "Reference Binge." That pays homage to the audiophile craft and the flow of alcohol that both play such important roles during those late hours when we hear our craft transformed, as if to art.

Need someone to come up with a more provoking idea.
 
I put the volume control in place of the mono/stereo switch in my ST-70 when I built it.
It is much easier to add it when building it. Putting it in after the fact will involve more work. Possibly removing the board, or at leased removing the screws and a few wires to make some room to install it. If you don't feel like going to all that trouble, there are plenty of 100k volume controls in a box with RCA connectors for input and output.
20200727_134252.jpg
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