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by **EWBrown** » Sun Jul 10, 2011 2:12 pm

I suspect that "unconditionally guaranteed" somehow doesn't mean half a century later

I always liked the funky look (but not the funky tactile feel) of those beeswax-dipped paper capacitors. I have a variety of these in the parts stash, but I've been somewhat nervous about putting them in a working circuit, they may emulate the other form of "Black Cat" which I previously mentioned. =:o

/ed B

by **Dynaco_WJW** » Sun Jul 10, 2011 2:45 pm

OK, this is clear to me. I will buy some new capacitors. Now I have to find out which capacitors are fine for a PC-3 board. I will buy a new board at Dynakitparts.

I found a nice website regarding a capacitor test:
http://www.humblehomemadehifi.com/Cap.html

by **EWBrown** » Mon Jul 11, 2011 1:18 pm

The ST70 gets pretty hot on top of the chassis (we used to call these amps "dorm room heaters", and those wax dipped caps could melt, and make a bit of a gooey mess, because beeswax hasd a low melting point.

I wouldn't throw them wawayey could still be perfectly good, just try them in a linestage or a simple low power SE amp, instead.

/ed B

by **Dynaco_WJW** » Wed Jul 13, 2011 2:01 pm

Thanks for the replies!

I finally ordered the components I need.

Can capacitor and Vitrohm carbon resistors @
www.http://askjanfirst.de/eindex.htm

Jupiter Vintage Tone condensers and metal film resistors @
http://www.tonefactory.nl/

Bare circuit board and bias resistors @
http://dynakitparts.com/store/

It was hard to get wire with the right diameter. Now I have black, massive 0,5mm2.

If I will have all the components I can start reassembling again.

by **Dynaco_WJW** » Thu Jul 21, 2011 11:11 am

Yesterday I finished the ST-70. After checking the wiring again (found two wrong connections) I could set the bias. Though late in the evening I wanted to test it. It was obvious that is has a lot of power! Now I'm curious about the sound. I am comparing it to a SCA-35 with EFB (Dave Gillespie / George Audioregenesis).
Is it true that you have to 'play in' new components? New is the circuit board with all the components, the can capacitor and new resistors inside the amplifier.

Here are the photo's

PAS-3X is the pre-amp.

by **TomMcNally** » Thu Jul 21, 2011 11:59 am

Beautiful job ! Thanks for the pictures, they might inspire some of us to restore our Dynaco gear to its original condition.

by **DeathRex** » Thu Jul 21, 2011 5:43 pm

I'd bias those EL34s kinda soft to help out the transformer, like about 1.2-1.4 instead of 1.56.

by **Geek** » Thu Jul 21, 2011 8:38 pm

DeathRex wrote:I'd bias those EL34s kinda soft to help out the transformer, like about 1.2-1.4 instead of 1.56.

I did a lot of listening tests of an ST-70 at various bias levels and agree. My "sweet spot" was 1.216 (39mA/tube).

Just my twonie

Cheers!

by **dcgillespie** » Thu Jul 21, 2011 9:17 pm

More superb work! And what a sizable Dynaco collection you are getting! I would be interested to know how hot the power transformers get using the 220 volt version. Here in the states, with the original 117 volt transformers running on typically 121 volts or so, they can get pretty hot -- they won't burn you, but you can't hold your hand on them very long, either. Do yours run that warm?

Dave

by **soundmasterg** » Fri Jul 22, 2011 2:53 am

dcgillespie wrote:More superb work! And what a sizable Dynaco collection you are getting! I would be interested to know how hot the power transformers get using the 220 volt version. Here in the states, with the original 117 volt transformers running on typically 121 volts or so, they can get pretty hot -- they won't burn you, but you can't hold your hand on them very long, either. Do yours run that warm?

Dave

I wonder if mine will run that hot when I get around to restoring it? Might be a good idea to get an upgraded transformer from Triode Electronics and use my Stereo 70 PT for something else?

Greg

by **Dynaco_WJW** » Fri Jul 22, 2011 3:57 am

Tank you very much for the compliments.

I will measure the temperature of the power transformer soon.

About the bias current, I don't understand it well. If you are changing the bias current, you will also change the working point of the tube or not? This is also the setpoint for the class of the amplifier? Maybe it would be a good idea to show the working point in a graphical diagram for this amplifier?

Until now I have the idea that my SCA-35 gives a more spacial effect to the music. I don't know if this is because I have to play with the new components for several hours, the PAS-3X or the ST-70. But the ST-70 gives a lot of power!

@soundmaster, tell us more about your ST-70 project.

Here is a picture of the working amplifier. (better sharper photo placed july 22th)

by **Geek** » Fri Jul 22, 2011 4:22 am

TUBEGLOW!!!! (love)

That pic would make a heckuva nice wallpaper.

Cheers!

by **Dynaco_WJW** » Fri Jul 22, 2011 2:33 pm

@ Dave -- I ran the amplifier with cover and a lot of dB's for some time and I can keep my hand on the power amplifier. I don't know the exact temperature but is can keep my hand on the transformer for hours (if I want) The OPT's are not warm at all. I'm not satisfied about the sound yet. I don't have any idea how the tubes are. Maybe it is a wise idea to buy new EL34's.
What is your opinion about the brands of EL34? Is it true that components like resistors and capacitors (and tubes) have to 'play in' for several hours? Or is that just a fairy-tale?

by **rmyauck** » Fri Jul 22, 2011 5:13 pm

Great work! If you bias your output tubes a little lower like Gregg mentioned they will last possibly much longer, and sound better. There was a fellow named Joe Rosen on another form audioasylum.com whom liked the 7199 driver board from back in the day like you have. He thought the sweet spot was in the 30's also. If you google him with audioasylum , you may find his posts from years past. He mentioned other small mods to improve the bass etc. I believe upping capacitance in PS is one and It's mentioned in the dynaco tube audio form, by dynakitaudioparts.

The SS diode mod (added with 5AR4) is worthwhile too to make old and new rectifier tubes last. (put UF4007 in series from the output of power trans former to pins 4 and 6 of the 5AR4). You can use the adjoining pins of the tube socket as they are empty to mount the diodes, say 5 & 7. The transformer wires are then moved to these spots. The diodes have to be oriented correctly, striped end to pin 4 & 6.

by **soundmasterg** » Sat Jul 23, 2011 6:19 am

Thanks for the interest. My Stereo 70 project hasn't started yet. I got it for $10 back in 2004. It came with four Mullard EL34's, an Amperex GZ34, and two RCA 7199's, that were all still good. The GZ34 went into my Vox AC30, I still have the Mullards, and I sold the 7199's on ebay for decent money. I picked up one of the SDS boards to use in the amp. I got the board where he uses two EF86's and a 12AU7. I've gathered all the parts but haven't been able to get to working on it yet as my guitar amp projects and repairs, and lately school have kept me too busy. I hope to be able to get to it soon however!

I'm curious how it will sound with the different board.

Greg

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Restoring a ST-70

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