Sound Valves VTA ST70i

[Vaejoviscarolinanus]

I had a Dynaco st70 back in my early years and it was a fun little amp . I ended up modding it and I do not know what happened to it but I just decided to get a similar one. I found a kit on eBay and I am going to try to assemble it for a weekend project. Okay I have not done anything like this in years so I am just trying to get any ideas from the community about suggestions to help make it go easier. Advise

 

[Doodski]

I had a Dynaco st70 back in my early years and it was a fun little amp . I ended up modding it and I do not know what happened to it but I just decided to get a similar one. I found a kit on eBay and I am going to try to assemble it for a weekend project. Okay I have not done anything like this in years so I am just trying to get any ideas from the community about suggestions to help make it go easier. Advise

Can you link us to the eBay kit so we can see some details and pics?

 

[Vaejoviscarolinanus]

 

[mhardy6647]

I believe those date from the early/mid 1990s and may have used Panor-era Dynaco parts.
I reckon most of the assembly is physical/mechanical -- the PC boards may be (?) pre-stuffed?
No real advice if you know how to read instructions, cut and strip wire, solder, and are very careful with installation of polarized parts (e.g., electrolytic capacitors). Take your time and double-check (at least) all work.
If you're comfortable using a DMM or VTVM with high (lethal) voltages, and if the instructions include voltage charts for first power-up tests, it's a good idea to perform them. If you don't know what you're doing with HV and/or don't feel confident, comfortable and aware enough to be careful without being afraid -- don't do it!

FYI: There are 'current production' Dynaco-style kits (and, reputedly, pretty good ones, too) available -- or, at least there were (you know, pre-pandemic/supply chain meltdown). That's probably a better route (depending on how your eBAY find works out).

Dynaco replacement parts & vacuum tube audio amplifier kits from Dynakit

Find the right Dynaco™ parts to restore, repair or upgrade your original Dynaco™ vacuum tube amplifier. We also offer an array of new "select" Dynakit™ classic vacuum tube amplifier kits based on the original legendary designs. Orders typically ship within 24 hours.

www.dynakitparts.com

 

[Vaejoviscarolinanus]

I believe those date from the early/mid 1990s and may have used Panor-era Dynaco parts.
I reckon most of the assembly is physical/mechanical -- the PC boards may be (?) pre-stuffed?
No real advice if you know how to read instructions, cut and strip wire, solder, and are very careful with installation of polarized parts (e.g., electrolytic capacitors). Take your time and double-check (at least) all work.
If you're comfortable using a DMM or VTVM with high (lethal) voltages, and if the instructions include voltage charts for first power-up tests, it's a good idea to perform them. If you don't know what you're doing with HV and/or don't feel confident, comfortable and aware enough to be careful without being afraid -- don't do it!

FYI: There are 'current production' Dynaco-style kits (and, reputedly, pretty good ones, too) available -- or, at least there were (you know, pre-pandemic/supply chain meltdown). That's probably a better route (depending on how your eBAY find works out).

Dynaco replacement parts & vacuum tube audio amplifier kits from Dynakit

Find the right Dynaco™ parts to restore, repair or upgrade your original Dynaco™ vacuum tube amplifier. We also offer an array of new "select" Dynakit™ classic vacuum tube amplifier kits based on the original legendary designs. Orders typically ship within 24 hours.

www.dynakitparts.com

Thanks for the Dynaco parts site , I somehow missed it during my search. I have had a kit years ago and it was my introduction into the tube world but after that I always had already built units. These are really not much of a challenge because the boards are already made and if you follow the “ build sheet” and check it off then you will probably be able to make it work . I have a little bit of nervous thinking but I’m going to follow the instructions and double check it . I am sure that it is just been a long time since I have done this type of thing. I will post the results of this project in a few days or so.

 

[mhardy6647]

Should go smoothly, and net you a respectable 35-ish watt per channel amplifier when you're finished.
The nice thing about vacuum tube hardware is that it's fairly forgiving -- wire the sockets (if there are any point-to-point sockets, which may not be the case) & transformers right and don't reverse polarity of the electrolytics and you should be good. Most errors can be 'fixed in post-production'.
Check the physical integrety of connections before soldering and solder "decisively" -- i.e., don't under- or over- heat. Use top-qualiy rosin-core solder. Heat the connection, not the solder, and let the heat of the connection melt & flow the solder.
Heathkit & EICO both have good, simple soldering instructions in their kit manuals.

Heathkit:


EICO:


Doc Bottlehead (Dan Schmalle, of Bottlehead kits in Washington State, US) recently did a YT 'how to solder' tutorial that seemed pretty good.

 

[anmpr1]

One thing to consider: the 7199 driver tube is out of production for many years. So unless you can get those somewhere, you'll want to get a kit that uses a substitution, such as the 6GH8A. Or one of the drop-in board mods using other tubes, which you can find on line.

Practically, it's pretty easy, and almost anyone can build a Dyna clone--all you need is a wire stripper/cutter. screw drivers, a soldering iron, solder, and an inexpensive battery powered multi-meter to set the bias.

Unfortunately, now is probably not the time to be looking to build any tube amp...

 

[mhardy6647]

Looking at the image in the OP -- I think that the driver section of this particular ST-70 variant (which is fairly modern) has been updated to use something else (maybe 6U8 or 6GH8, or maybe totally redesigned). I have some literature on this amp... someplace... as I seriously considered buying one when I was inching back towards vacuum tube hifi in the mid-1990s.

I also presume that the kit includes tubes.

 

[JeffS7444]

I couldn't keep up with Stereo Cost Cutters / Sound Values / Sound Valves back in the day, but that looks like a pretty decent redesign of the old amplifier and does stuff I'd want to do including: Eliminate 5AR4 rectifier tube, any selenium rectifiers and multi-section can capacitors, and replace 7199 input/driver to something more readily available. So build per the instructions and enjoy! SV-supplied parts are likely more than good enough.

Multi-section can capacitors can work perfectly well in other products like McIntosh, but in the ST70, they're hit with too much heat, too much ripple current, and they're kind of costly to replace.

 

[airborne]

on my own forum ( where i am so far the only poster ) i have explicitly banned discussion of vacuum tubes

people can't handle freedom - they waste all resources that are available to them

which is why the solution is to make sure nothing is available

you probably noticed our so-called "shortages" ...

i wish there was a better way but there isn't ...

freedom was a mistake ...

it's getting fixed.

 

[BDWoody]

on my own forum ( where i am so far the only poster ) i have explicitly banned discussion of vacuum tubes

Sort of like a note to self?

 

[airborne]

Sort of like a note to self?

when i have the time i'm going to create ( not here LOL ) a few humorous sock puppet accounts like "Noob" and "Audiophool" and have them post foolish things, and then i'm going to troll them ...

might have to temporarily allow tubes discussion otherwise i would have to ban myself from my own forum !

on ASR people are careful not to make claims that tubes / alnico / whatever sound "better" and simply say that they enjoy those things or want to build them for fun etc ... not sure how i feel about that TBH.

i think i would rather have them happily discussing tubes elsewhere than torture them by forcing them to admit that tubes aren't serious audio.

but the forum has to start with something - what better than trolling Audiophools !

 

[Vaejoviscarolinanus]

Well , I understand that some folks are not going to be Vaccum tube fans. I enjoy the warm coloration and the midrange and I am happy to listen to Solid state gear as well. Sort of like some days I drive a 1968 Fastback mustang with points in the distributor instead of the 2022 vehicle I use as a daily driver. It is just because I can have a variety I enjoy it.

 

[egellings]

I have a tendency to prefer my tube amps, largely because they are home-brewed and that biases me to favor them. Of course, they are no slouches when it comes to output impedance & distortion. No SE 300B for me. Too mushy.