Review and Measurements of Dynaco ST 70

[anmpr1]

SIY, is there a driver board you'd recommend that ditches the hard to find 7199? I have a pair of clean MKIV's collecting dust for the last 30 years.

I may be the only guy on ASR that owns both amps at opposite extremes of the SINAD graph.

I've dealt with this outfit, and found them to be knowledgeable and reputable. This board replaces the 7199.

https://www.dynakitparts.com/shop/pc-4b-mkiv-mf-resistors/

 

[anmpr1]

Stereophile measured the Dynaco Stereo 70 II, not the same, but...

https://www.stereophile.com/content/dynaco-stereo-70-ii-power-amplifier-measurements

Here's a link to a 1959 review:

http://classictubeaudio.com/wp-content//hfst70.gif

 

[DKT88]

I used these driver board replacements in similar Dynaco amps in the 90s. They use 6DJ8/6922tubes which are currently produced new. http://www.curcioaudio.com/st7upg_3.htm I was happy with the sound
.

 

[Jaimo]

Wow. That exchange between Stereophile and Bob Rapoport is just just horrible. Never seen anything like it.

it was reviews like this that prompted me to cancel my Stereophile subscription.

 

[Jaimo]

@SIY who do you recommend for power and output transformers. I need a ST70 power tranny as well as a pair of ST35 output trannys.

My ST70 power tranny came out of some scrapped military gear and hums like crazy. I have a pair of Hammond output trannys on my ST35 and while this sounds great, I think there's room for improvement.

 

[anmpr1]

https://dynaco.com/

A new outfit owns the Dynaco name, and has an ST-70 replacement. 1500 dollars. The original kit was $99.00, which works out to about 860 of our inflato dollars. No kit is offered--they say due to fear of being sued if someone electrocutes themselves. How times have changed.

 

[SIY]

All of my Dyna sources are memories from 15-20 years ago. Last time I bought any Dyna transformers, I got them at Antique Electronic Supply. The link that @anmpr1 posted above looks pretty solid.

I think there's better stuff around these days, but the Dyna equipment does have a nostalgic buzz.

 

[anmpr1]

it was reviews like this that prompted me to cancel my Stereophile subscription.

This is pretty mild stuff compared to 'noted' (/sarc) analog turntable reviewer, Michael Fremer. Some of his grotesque outbursts against those who call him out over his nonsense are exceptionally cringe-worthy.

 

[DKT88]

I used to use http://triodeelectronics.com/dytr.html for tubes and parts 20 years ago. They still have dynaco replacement transformers.

 

[DonH56]

A lot of mods, and some poor builds, left the Dynaco's susceptible to motor-boating -- LF oscillating due to power supply coupling (and/or lack of decoupling) and feedback to the input. Could also be HF oscillation, again IME these things were often modified by folk with no real understanding of what they were doing and lead dress and other things led to stability issues. Note tube amps get very unhappy when the outputs are unloading; I sent a few off into space by removing the load resistors during bench testing -- didn't want to power the thing down and up again as many tube amps take a while (minutes) for the output to stabilize. My old ARC preamp would put out massive LF sine waves for a minute or three after turn-on until everything stabilized. Replacing the tube rectifiers with SS for instant turn-on would hugely exacerbate this problem (and the in-rush current did things no good either). It was an elegantly simple circuit and far too many folk modified them without realizing the implications and how things relate to one another across the different circuit blocks.

Some things never change, but at least the Internet means some "bad" mods get blasted and revised or removed.

I, @restorer-john, et. al. could probably post a lengthy and sadly hilarious list of bad mods and ones with unintended consequences....

 

[DonH56]

https://dynaco.com/

A new outfit owns the Dynaco name, and has an ST-70 replacement. 1500 dollars. The original kit was $99.00, which works out to about 860 of our inflato dollars. No kit is offered--they say due to fear of being sued if someone electrocutes themselves. How times have changed.

See previous links to Frank van Alstine's site -- he still offers bare boards, complete kits, and complete rebuilds for Dynaco tube and SS amps.

 

[Burning Sounds]

Here's a couple of pics of my Curcio ST70.

The first shows it in it's original rebuild guise - it was exceptionally reliable for more than 20 years and would probably have gone on for longer had the homemade circuit boards not started to show heat stress. It drove my Maggie MG1s for years.

The second is the rebuild I did a couple of years ago with a larger case and independent bias supplies. The replacement power transformer (about twice the size of the original) is a Sowter - not inexpensive, but very well made and still available - http://www.sowter.co.uk/classic-amplifier-transformers.php

The power supply caps are Black Gates of mythical audiophile fame .

 

[HammerSandwich]

Is that much DC offset normal?

 

[SIY]

Is that much DC offset normal?

That's not DC offset, it's a measure of the voltage across the output stage cathode resistors to determine idle current. That's a standard thing on tube amps.

 

[HammerSandwich]

Got it, thanks.

 

[sergeauckland]

Valve amplifiers shouldn't have any DC offset, as their output is via a transformer that doesn't pass DC. However, most valve amplifiers have feedback going to the cathode of the first stage, so there will be a small DC path back to the loudspeaker. With a loudspeaker having a DC path to ground, this makes the offset very small. If the loudspeaker has a series capacitor, hence no DC path to ground (my 801s in passive form had a 1000uF capacitor in series with the bass unit, so no DC path) then the DC offset may be higher, but then it doesn't matter as the loudspeakers block DC.)

It's pretty much academic with valve amplifiers as large DC offsets are blocked and small ( a few mV) DC offsets don't really matter.

S.

 

[rebbiputzmaker]

That's a fabulous FR (the good channel) for a 60+ year old tube amp into 4 ohms IMO. Must be some really good OPT TXFs.

The output transformers were actually an excellent design with a sophisticated winding pattern not matched in modern designs.

 

[SIY]

The output transformers were actually an excellent design with a sophisticated winding pattern not matched in modern designs.

Yes, they were good transformers. No, modern units from Lundahl, James, Tamura, Sowter, and the like will outperform them.

 

[rebbiputzmaker]

A bit of work on the filtering, regulation and tightening up the tolerances to bring the channel balance back into line and it'd be a fun project.

A stiffer power supply helps. Back in the day photo flash caps were popular in mods.

 

[SIY]

The 7199 is 1/2 12ax7 1/2 12au7 in one envelope

No, it isn't. It's a combination pentode-triode. The pentode first stage is where you get all the gain for the input and driver because the cathodyne phase splitter has slightly less than unity gain, so a triode like a 12AX7 won't work as well here.

edit: Basically, you need to drive the output stage to about 30-35V for full power out, so with a gain of 100 in the first stage (which isn't going to happen without a CCS plate load, 70 is more realistic), you could only get 9dB or so feedback- and not even that in the top octave or two.