• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

New to the Dynaco ST70 Community

peterh said:
Nice to see, enjoy the music !

As for the rectifier tube ; they use to keep alive very long. Two things that kills them ( independent of brand) is overcurrent and hotswitching. Both these kills by creating arc that shorts the elements and physically damages them.

The "yellow sheet" diode mod does some help here as the arc won't be continous as the diodes block during the negative half period. This is the only thing i would do if this was my amp.
Click to expand...
Thanks I was considering that mod myself. Good to hear other recommendations for it. BTW what do you mean by hotswitching in this case? The ST70 doesn’t have a standby switch to cut plate power.
 
Last edited:
If your lucky, hot saitching will just blow the fuse, but it can and usually does impact the rectifier tube. How switching is turning the amp off and back on quickly instead of waiting a few minutes.
 
Power blips can do the same thing, and those you can't really control. There are some devices that will sit between the wall and the plug that will kick out in the event of a power loss to prevent that. Some GFCI will do it, and there are ones designed for drill presses and other tools to prevent an auto-restart if the power goes off and comes back
 
Listening today Kenny Burrell’s guitar never sounded this good. It sounds like he is right here in the room with me! I thought I had a good system before (I am still using the Marantz 2220B receiver record output and the same Pioneer CS77 loudspeakers). I think I am going to really like this amp.
 
As much as been said about how awful the ST70 is, they haven't sold a metric crapload of the things over the last 70 years for no reason. They were introduced in 1959 and you can still buy a brand new kit today with the stock circuit if you want.
 
gadget73 said:
Power blips can do the same thing, and those you can't really control. There are some devices that will sit between the wall and the plug that will kick out in the event of a power loss to prevent that. Some GFCI will do it, and there are ones designed for drill presses and other tools to prevent an auto-restart if the power goes off and comes back
Click to expand...
I actually made a device that controls this using an arduino controlling an SSR relay.
The artduino is always powerd on. When it received a + signal ( a typically remote control from the pre) it starts a timer
and after 40 seconds it powers on the ssr. If mains power is lost it will always restart that delay.If the +signal is lost it will also force a new delay.
In addition it has a shorter delay that powers on the pre, and makes sure that when + signal is lost it will power down SSR first and the pre ( with another relay) last.
( the +signal is not in my case emanates from the pre it comes from a different source, that's how i can control the pre and power individually)
And yes the device was inspired from the burned rectifiers and fuses. Since then no more rectifiers are lost.

Building a much simpler device that only forced a delay to the poweramp is of course easier.
 
gadget73 said:
As much as been said about how awful the ST70 is, they haven't sold a metric crapload of the things over the last 70 years for no reason. They were introduced in 1959 and you can still buy a brand new kit today with the stock circuit if you want.
Click to expand...

The ST70 was sorta like the VW Bug of audio in the 50s and 60s. It was economical, it was an everyman appeal, it was easy to repair and maintain, it lasted a long time. It worked well and very practical. And, to top it off, they are still a good sounding budget amp today, although a lot more expensive than a Class D starter amp.
 
yeah but I can't build or fix a class D amp, so for me its of very little interest. Not that I own a Dynaco either, my ST70 was built more to be a smartass than anything else. Dynaco chassis, Eico ST70 transformers, and mostly the Eico circuit but I modified it somewhat.
 
gadget73 said:
yeah but I can't build or fix a class D amp, so for me its of very little interest. Not that I own a Dynaco either, my ST70 was built more to be a smartass than anything else. Dynaco chassis, Eico ST70 transformers, and mostly the Eico circuit but I modified it somewhat.
Click to expand...
Yeah, like a sand buggy! Cool! Nobody I know is going under the hood of a Class D amp. Certainly not me. But if a ST70 landed at my door, I could do just about anything to it.
Rock on!Pic_0563.jpg
 
I remeber seeing that goofy solid state driver board for the ST-70. Some people actually bought and used them! And why not, all the cool kids were buying solid state amps. ;)
 

Attachments

  • 62098635.jpg
    62098635.jpg
    115 KB · Views: 13
gadget73 said:
that looks like an awful lot of passive parts for the number of active parts I see.
Click to expand...
Right? There is another one with less passive parts I've seen before.
Maybe from Curico years ago? Curico is selling a four tube driver board at the moment.
 
The more I listen the better it sounds! I’ve been too busy listening to do the other mods and recap the input board like I planned.
 
pac1085 said:
Don't waste your time on those kits, especially from the hifiaudio guy. Junk..

This is what I would buy if I wanted to do a stock rebuild and didn't want to source the individual components

PC-3B ST-70 MF Resistors - Dynakit Parts (this one uses the 6gh8a tubes, but you can get their other version that uses the 7199's if you have good ones)
ST-70 BIAS CIRCUIT KIT - Dynakit Parts (Bias components kit)
MULTI-SECTION CAPACITOR - Dynakit Parts (I like the upgraded 80-40-30-20 can capacitor but they sell an original value one as well if you want to stick to lower capacitance, but it appears to be out of stock)
C-354 CHOKE - Dynakit Parts % (optional but I like to replace them if the wax is all melted out of the originals and stuck to the bottom cover - cheap enough)

There are a ton of other directions you could go with it too, of course
Click to expand...
I’ve rebuilt mine with these, and the individual bias kit. The yellow sheet mod on the rectifier was done as well. The amp is now likely better than new and will outlast me. Love my ST-70.
 
I purchased a JJ GZ34 to use as a test rectifier after one of my ST70 builds. Amazon overnight shipping was the key reason...it didn't last 2 hours.

But the last ST70 build I did for a friend. I didn't want to come off any of my Mullards as they just keep rising in price. I ended up buying a couple Mesa Boogie 5ar4/GZ34 from sweetwater. My friend uses his ST70 almost daily & he's still using that Mesa rectifier I supplied him. It's been just about one year now & no issues.

I added the yellow sheet mod & in comparison to the mullard I had in the amp the Mesa gave a livelier & more open perception. My experience anyway..maybe just much stronger.
I haven't tried enough of these rectifiers to say are all reliable..but the 2 I purchased for Dynaco use have held up.
 
Mybig8ohm said:
I purchased a JJ GZ34 to use as a test rectifier after one of my ST70 builds. Amazon overnight shipping was the key reason...it didn't last 2 hours.

But the last ST70 build I did for a friend. I didn't want to come off any of my Mullards as they just keep rising in price. I ended up buying a couple Mesa Boogie 5ar4/GZ34 from sweetwater. My friend uses his ST70 almost daily & he's still using that Mesa rectifier I supplied him. It's been just about one year now & no issues.

I added the yellow sheet mod & in comparison to the mullard I had in the amp the Mesa gave a livelier & more open perception. My experience anyway..maybe just much stronger.
I haven't tried enough of these rectifiers to say are all reliable..but the 2 I purchased for Dynaco use have held up.
Click to expand...
Another story to confirm JJ 5AR4's are crap. They aren't even good for testing.
I have a cheap PSvane 5AR4 for testing thats held up great. It was literally the cheapest 5AR4 I found at the time. The TAD brown base worked great in my
ST-70 until I finely replaced it with a SS WZ-34 coppertop rectifier.
 
It's great to know that the WZ-34 Coppertop is working great for you. Definitely gives me the added confidence to finally try it.

And yeah, I had read beforehand that the JJ wasn't very reliable but I figured it would at least last a few months or give me the ability to test voltage points on new builds or changes every once in awhile.

Between what I've read & my experience I would rate it as Worthless Tube.
 
really sort of amazed that after all these years JJ hasn't re-designed their 5AR4. Surely someone there has heard feedback about them.

Their other tubes generally seem OK, so its not like they can't figure things out.
 
You must log in or register to reply here.
Back
Top Bottom