ST70 rebuild, hard-wired

Hi everyone,

I am new to this forum and want to share my efforts in rebuilding a ST70 amplifier.
I bought a stock ST70 (1961) on EBay that looked like this:



I completely stripped the amp and cleaned the chassis:



I did not change the original topology of the amp except for the power supply.
The power transformer was replaced by a Triode Electronic transformer.
I needed a 230V version anyway, so I decided to buy a beefier transformer instead.

Modified the multicap for the 80-40-30-20 version of Dynakit and replaced
the selenium cell for an ultrafast rectifier diode type BYV29.

All other components are new except for the chassis, the A470’s,
the choke,  the speaker connectors, the bias pots and the cathode resistors.

I used Belton tube sockets (except for the sockets up front that are still the
original Amphenol sockets), resistors are Beyschlag (red brown ones),
Takman and Kiwame (green). All carbon types. For coupling I used Auricap capacitors.
Tubes used: a matched quad JJ EL34L, Silvania and/or RCA 7199, Philips nos GZ43.

The mono-stereo switch was left unused.

Most rebuilds are done using PCB’s for the front end of the amp.
I disliked the PCB on top, so I decided to follow another approach.

You hardly ever see a hard wired  ST70, so I fully hard wired the frontend using
stand offs, terminal strips and a sub chassis that is covered with a laser-cut
chrome steel plate on top. First attempt sub chassis:



This one worked fine but I was not completely satisfied.
I used this one for another ST70 that I rebuild for my brother.
I thought it over and designed a less chaotic topology.
Layout of the final version of the front-end:



Dimensions (in millimeters) of the sub chassis:



Dimensions (in millimeters) of the cover:



Top view of the completed ST70:



This is how it looks underneath:



Some detail shots:







Comments and critics are welcome.

Those of you who might be interested in (re)building the ST70 this way,
may benefit from this layout. Mail me and I will send you the drawings
in PDF format, or if so desired, in Autocad DWG format.
All drawings are in a 1 to 1 scale.

See also my triode - ultra lineair mod., Post n°35:

https://dynacotubeaudio.forumotion.com/t41p25-run-your-dynaco-st-70-mark-ii-mark-iii-or-mark-iv-in-triode-mode-photo

Here are the two finished ST70’s and my ‘Classic One’ preamp:



The Classic One is the incredible sweet sounding design of
Triode Dick consisting of a SRPP stage (6N30) and
a cathode follower stage (6N6) and here it is:

http://www.triodedick.com/pdf_files_td/Classic%20One.pdf

If you want to know more about this beauty, mail me.

All ST70 photos (click on the photo displayed in the
center for a larger view) can be found here:

http://www.mijnalbum.nl/Album=J4WJS6CU

At this time I (temporarily) replaced my ST70 for the Allen Wright's PP-2C power amps (monoblocks)

Text on this forum is all in Dutch, but there are a lot of photos and schematics, if you’re interested. Use Google Translate.

Man you must enjoy soldering !!!!!!! I have a rebuilt ST-70 here under willieher and as much as I enjoyed my rebuild I would never attempt the rebuild you did. I think you did an excelent job.
I dont post very often but when I see extencive work on a driver board like this I had to
say something. You must enjoy the sound of carbon resistors over metal film. I have added
2 Riken carbon resistors in the signal path to add a little noise, I have also changed the 4 output caps To Mundorf Silver in Oil. My amp sounds a lot better now.
Willie...

Yeah, I don’t mind a little soldering.
Saw your amp too and it is clear that you don’t mind to experiment. Nice job, Willie!
I wanted to rebuild my ST70 to stock, but as always with the kind of breed audiophiles are,
I am planning to build another ST70 with a completely other setup.
Just to see and experience the difference in sound. (and because I like to solder!)
I may even want to experiment with metal film resistors and a solid state bias adjustment system.
Maybe I will try a SRPP frontend as in my Classic One preamp, don’t know yet.
Of course this amp is also completely hard wired:

http://www.mijnalbum.nl/Album=WTZPO3OZ

I think that point to point wiring is the way to go in glass sound, but I am probably a bit nostalgic here.
I am quite pleased with the combination of these two amps, but there is always room for improvement.

Thanks for your reaction,
Roland

Hi Roland, speaking of GLASS I have been rooling some power tubes
and I have come across and am curently using a quad of NOS Russian 6n3ce
there not expensive, and these things sound sweet.
Good luck on your next project.

Willie...

Thanks Willie,

Before I start with this new project, I first have to restore two Dynaco FM3 tuners.
Still looking out for a replacement power supply transformer (230V), by the way, so if anyone has a tip for me, I would be much obliged.
(maybe I will hard-wire one of them too!)
Then I have to build a SET amplifier based on the Decware model SE84C of which I have all the components in store,
I still have to modify my AVO CT160 tube tester and on top of that,
I am planning to build a short wave receiver (ARRL 1963 Simplex Super), so I have a lot to do.

I have tried the 6N3CE myself, but I found these inferior to the EL34.
But that is purely my personal opinion of course.

Oh, I forgot to mention, I have got my ST70 hooked up on a pair of Tannoy Cheviot speakers.
I find these speakers very suitable in combination with tube amps.
Is there someone else here with experience of the famous Tannoy sound?

Roland

Interested in your Classic one preamp. Do you have complete schematics that you can share.

Schematics Classic One:

http://www.triodedick.com/classic_1/classic_one_schema_versterker.GIF
http://www.triodedick.com/classic_1/classic_one_schema_voeding.GIF

I also made elaborate drawings of this amplifier. If you want I can mail them to you.
This amp uses double powersupply!! Transformers are made in the Netherlands by AE schagen:

http://transformatorspecialist.shoplin.nl/pd-27033-7-53881/product/classic_one.html

Thanks for responding. Would like to see the drawings, but can't send external links yet as i am new to forum. Would like to hear about your dual power supply also.

You’re welcome.
This preamplifier can be seen as a pair of monoblocks, which means that both channels are completely separated including the power supply section(s).
It will all become clear when you study the photos of this project:
http://www.mijnalbum.nl/Album=WTZPO3OZ
Because you are new here, I am not able to mail you because your address is not visible for me.
In a week or so I will mail you my drawings.

Wow, those are very nice. I, like you, am modifying a ST70, but wanted something a bit more than a modded Pas 3. I will wait the week so you can send me the drawings

Wow? This is just how it looks. Wait until you HEAR this baby....

Measurements of my ST70 in triode mode:

Square wave respons 1kHz:



Square wave respons 10kHz:



Square wave respons 20kHz:



Frequency response:



THD+N vs Frequency, 1W:



THD+N vs Frequency, 10W:



THD+N vs Frequency, 15W:



THD+N vs power:

Measurements of my ST70 in UL mode:

Square wave respons 1kHz:



Square wave respons 10kHz:

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Square wave respons 20kHz:



Frequency response:



THD+N vs Frequency, 1W:



THD+N vs Frequency, 10W:



THD+N vs Frequency, 30W:



THD+N vs power: