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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 2nd Sep 2017, 4:47 pm   #21
Herald1360
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Default Re: Dynaco Stereo 70



Given the lack of shutters on an octal, such a gadget might actually have some merit, unlike the 13A abominations.
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Old 2nd Sep 2017, 6:43 pm   #22
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Default Re: Dynaco Stereo 70

I was joking about just that, but actual period dummy octal plugs existed as decorative covers to hide sockets on pnels.... like on Wearite-decked Ferrographs where one looked like a control knob.

The ST70 has quite a following in the states and all sorts of items are made for them.

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Old 19th Sep 2017, 10:05 pm   #23
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Default Re: Dynaco Stereo 70

Coming a bit late to this thread (September's been busy), I can add my voice to all the positive comments about this amp. I bought one from the US about ten years ago, because of recommendations from there, and because it is one of best-known and well-supported kit amps. The 110V power transformer is inconvenient in the UK and a bit under-powered anyway, so it is worth getting a 240V drop in replacement. US firms like Triode electronics make them, and may be a cheaper source than getting a bespoke one made here. I was lucky to find a big 240V transformer from an organ which fitted perfectly to the chassis. I had to devise a bias voltage supply from a spare winding and drop the HT a bit with a 5R4G rectifier and a bucking transformer, but it works well. The small footprint makes it particularly useful.
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Old 20th Sep 2017, 3:26 pm   #24
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Default Re: Dynaco Stereo 70

I don't have the schematic or parts list for this amplifier.

My main concern is that the electrolytic capacitors in the power supply will need to be 'woken up' gently before you apply full voltage. They first need to be reformed. A 1N4007 diode and a 50k resistor will do that.

The mustard coloured capacitors are made with a polyester dielectric and are normally OK.
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Old 23rd Sep 2017, 2:25 pm   #25
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Default Re: Dynaco Stereo 70

Hi,
It had been my intent to use a series lamp connected to a 110V supply to remind the components that there is a need for them to come back to life. Unless of course others may suggest otherwise?
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Old 23rd Sep 2017, 5:57 pm   #26
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Default Re: Dynaco Stereo 70

I feel that series lamp limiters and variacs don't have the necessary current limiting ability to protect old electrolytics.

Dedicated reformer circuits usually limit the current to a few milliamps.
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Old 24th Sep 2017, 3:18 am   #27
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Default Re: Dynaco Stereo 70

In an amplifier of this standard, why would you bother reforming caps If it's REALLY important (which it isn't) restuff the old cans with modern capacitors. If done well it's nigh on impossible to see the difference, BUT you will certainly HEAR the difference. Humming the song is for PEOPLE to do, NOT the amplifier.

The Dynaco is capable of superb results, the output transformers being at the peak of ultra linear development (using what was available at the time) and STILL, as I said before, can run with the best today.

Joe
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Old 17th Nov 2017, 6:41 pm   #28
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Default Re: Dynaco Stereo 70

A quick thanks to everyone for their supportive comments. Stripped and cleaned and finally undertook a 6 hour soak using a reduced Voltage primary and then energised. I used a quad for the test plus an old pair of Mission 700s just in case of any through fault damage. Just needed a bit more tightening on the txfr laminations but all sounded rather nice.
I now need to decide if I keep it or not.
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Old 17th Nov 2017, 6:47 pm   #29
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Default Re: Dynaco Stereo 70

I wish I had a house oops amplifier like that!
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