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Dynaco ST 70

vitorzoom

vitorzoom

Well-Known Member
Hello everybody. After many years of solid state, I ended up purchasing an original Dynaco ST 70 Amplifier, without any modifications.
The only change was a recap on the capacitors, following the original design.
I really liked the sound, compared favorably to the Adcom GFA 545 ll and other amplifiers I have. The valves are my question. Some have their specs erased and I would like to know which ones would be good options for me to buy. I would like guidance for all valves. It doesn't have to be anything esoteric, very expensive. But just make sure it's within the original amp's specs.
Any guidance is welcome.
At the time my speakers are the Yamaha NS 1000.
 
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vitorzoom said:
Hello everybody. After many years of solid state, I ended up purchasing an original Dynaco ST 70 Amplifier, without any modifications.
The only change was a recap on the capacitors, following the original design.
I really liked the sound, compared favorably to the Adcom GFA 545 ll and other amplifiers I have. The valves are my question. Some have their specs erased and I would like to know which ones would be good options for me to buy. I would like guidance for all valves. It doesn't have to be anything esoteric, very expensive. But just make sure it's within the original amp's specs.
Any guidance is welcome.
At the time my speakers are the Yamaha NS 1000.
Click to expand...
EL34 , JJ is a good brand available from eurotubes.com ( no affiliation). Make sure you get a matched quad. By buying
a well-matched quad you don't have to keep memory of tube positions.
Tuen the bias pots fully anti-clockwize before tube change, then after mounting new tubes turn on power
and after a minute connect a voltmeter to the "biaset" and start adjusting bias until 1.5V on the biaset connector

7199 is probably fine, if they need replacement ecf80 and a pin adepter is a perfect replacement.
( you could also replace the circuit board that is already made for ecf80/6U8 or some of the alternatives,
this might be at the same cost as 2 tube adapters. Boards may be obtained from dynakitparts.com and others.
 
The following are all new issue tubes. I've tried the vintage NOS thing. I personally don't feel it's worth the cost or time to search for good ones, but that's up to your ears.

I like KT77s in this amp. Both the JJ and Genalex Gold Lions sound great. A bit more bass, harder hitting, than the EL34 variants I've tried.

EL34s were great too. I've tried JJ, Tung-Sol and Electro Harmonix. I favored the Tung-Sols. I listen to rock primarily and the KT77s held up better IMHO.

Another option is the 7581A reissue from Tung-Sol. Closer to a 6L6GC than a EL34 or KT77, but you can use them and they really sound great. I am running them right now.

Genalex and Tung-Sol for rectifiers. Using them in several amps, no problems.

No help on the 7199s, I have VTA drivers in all.
 
vitorzoom said:
Hello everybody. After many years of solid state, I ended up purchasing an original Dynaco ST 70 Amplifier, without any modifications.
The only change was a recap on the capacitors, following the original design.
I really liked the sound, compared favorably to the Adcom GFA 545 ll and other amplifiers I have. The valves are my question. Some have their specs erased and I would like to know which ones would be good options for me to buy. I would like guidance for all valves. It doesn't have to be anything esoteric, very expensive. But just make sure it's within the original amp's specs.
Any guidance is welcome.
At the time my speakers are the Yamaha NS 1000.
Click to expand...

@vitorzoom that had to be quite the eye opener going from Adcom sand power to dyna stereo 70. I still remember the first time I went to tube power many many moons ago. Was quite the holy cow moment sonically. If your 70 is completely original you may want to make sure the power supply caps are not leaking. And to save your rectifier tube, may want to add a diode on one leg of the high voltage AC. You will still get the sound of the tube rectifier, but the rectifier will not see the full wave coming off the power transformer.
 
Lavane said:
EL-34's, 6CA7's, KT-66's and KT-77's are all drop in replacements. For the power tubes.
Just stay away from JJ brand GZ34/5AR4 rectifier tubes.
Click to expand...
Matched pair output tubes in each channel are advised.
Replacement for the 7199 may be 6CG8 on suitable adapters.
 
Blackdog61 said:
I had those many years ago and liked them. Still using the Chinese Nesstones and they are doing well, but I should do something else.
I will say that an updated ST-70 using the stock circuit is a very good amp. Although there are surely better amps available, I am content.
Click to expand...
Couldn't agree more. I have a few different amps (power, integrated, tube, ss) in the stable and I always come back to the ST-70. I could listen to it all day long.
 
There are a few good reasons why the ST-70 has really never went out of production by one company or another.
Bang for your buck cost.
They just plain sound good.
Easy to assemble or work on.
Other than the iron, uses easy to source parts.
Probably more documentation and support than any other tube amplifier made to date.
Lots of modifications, if your into that.
And plenty of easy to find choices for tubes.
Even though mine is in a second system now, it still gets used and isn't going anywhere soon. Plus it's nice to have a decent backup for my main system.
 
I'm using JJ 6CA7's in mine. They were the only reasonably priced 6CA7's I could find when I was looking due to shortages from the Russian-Ukranian war. I've had them 5 or so months, and they sound very good. I really prefer them to the EL34's I had previously. I also use 6GH8's in adapters instead of 7199's. Mine is has the original board but has been recapped, had the 'yellow sheet' mod performed, and has diodes vs the selenium rectifier.
It's always impressed me with how powerful and musical it is. It was definitely my best Goodwill find to date!
Shown as purchased:
Bare.jpeg
 
David_NC said:
I'm using JJ 6CA7's in mine. They were the only reasonably priced 6CA7's I could find when I was looking due to shortages from the Russian-Ukranian war. I've had them 5 or so months, and they sound very good. I really prefer them to the EL34's I had previously. I also use 6GH8's in adapters instead of 7199's. Mine is has the original board but has been recapped, had the 'yellow sheet' mod performed, and has diodes vs the selenium rectifier.
It's always impressed me with how powerful and musical it is. It was definitely my best Goodwill find to date!
Shown as purchased:
View attachment 2675486
Click to expand...
I've tried various EL-34's, KT-77's and even a set of KT-88's. I finely bought a set of the EH 6CA7's and really like them also.
 
Lavane said:
There are a few good reasons why the ST-70 has really never went out of production by one company or another.
Bang for your buck cost.
They just plain sound good.
Easy to assemble or work on.
Other than the iron, uses easy to source parts.
Probably more documentation and support than any other tube amplifier made to date.
Lots of modifications, if your into that.
And plenty of easy to find choices for tubes.
Even though mine is in a second system now, it still gets used and isn't going anywhere soon. Plus it's nice to have a decent backup for my main system.
Click to expand...

+1 for @Lavane comments. In the audio world the Dyna stereo 70 is akin to the small block Chevy V-8 in car enthusiast circles; tons and tons of mods and aftermarket parts are available for them.
 
mcgjohn said:
yes, have tried em, but not in a Dyna 70, but they are really nice sounding tubes.
Click to expand...
Thank you, glad to hear that. They remain relatively reasonable in price.

I have found that the drive section tube has every bit, if not the most impact on the final sound from my ST70. Certain brands of tube in that spot sound less dynamic and engaging in my system. It is actually pretty surprising to me just how much affect it has. While waiting on the Psvanes to arive, I have been using a matched pair of Tung Sol EL34B in one channel, and a matched pair of Winged C EL34 in the other and hear no perceivable difference. In fact I will keep them as a quad for emergencies.
 
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