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Old 31st Aug 2017, 7:42 pm   #1
Bookman
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Default Dynaco Stereo 70

Judging by the historical feedback on this item it would seem that there are several appreciative owners of this item. I came by one today with booklet serial number 6402151; the attached pics refer.
It will be a long term project but I need to plan accordingly with the spare parts.
The main item on the agenda are the two octal pin preamp input plugs and dare I ask; a connection diagram for the plugs relative to preamp O/P and ST70 I/P. Pages 6 & 7 of the book has the overall diagram but the latter connectivity is unclear.
The previous owner assured me that the valves are all okay but I will get them checked out. I would also use a 110V source to undertake initial switch on tests.
Would appreciate any other aspects of risk analysis and suggested solutions and of course where to source the plugs.
In terms of preamp I have a Pioneer SA8100 which I intend to use?
Thanking you all in advance; I remain yours sincerely
Joe
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Old 31st Aug 2017, 10:00 pm   #2
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Default Re: Dynaco Stereo 70

Octal plugs are available on eBay but you don't need them as you will be using a self-powered pre-amp which will be connected via the phono-sockets on the Dynaco's front.
There is a small industry in the USA devoted to the rebuilding of this amplifier (which was available as a kit) particularly circuit boards designed to replace the rare 7199 triode-pentode valve, usually with a double-triode..
Alternatively you can buy (from Hong Kong) plug-in sockets which allow the use of 6U8/ECF82 triode-pentodes, which are much more easily findable.

If you search around on the internet you'll find loads of rebuilding articles for this amplifier, apparently the highest selling of all time.
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Old 31st Aug 2017, 10:01 pm   #3
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Default Re: Dynaco Stereo 70

Park electrical USA have replacement boards for these. Parks web site also has a lot of feedback from users with mods. I have used there pcbs for small EL84 amps.

Bob.
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Old 1st Sep 2017, 10:43 am   #4
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Default Re: Dynaco Stereo 70

I offered a couple of octal plugs recently. See post #20 in this thread:-

http://www.vintage-radio.net/forum/s...d.php?t=131297
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Old 1st Sep 2017, 12:34 pm   #5
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Default Re: Dynaco Stereo 70

The chassis and above-chassis ovecomponents look aged. Do you know how this was stored?
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Old 1st Sep 2017, 3:58 pm   #6
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Default Re: Dynaco Stereo 70

This amplifier was first issued in 1959 so the components on the input circuit board could be over 50 years old. They do look like 1960s resistors and capacitors.
The octal sockets on the front were designed to be used with two Dynaco PAM mono preamplifiers, which took their power from the ST70.
The initiator of this thread is not going to be using these devices so octal plugs do not need to be bought.
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Old 1st Sep 2017, 5:36 pm   #7
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Default Re: Dynaco Stereo 70

The coupling caps on the pre-amp/input board are Mullard mustards and Domino silver mica's. They should be ok but worth checking before you switch it on for leakage etc. As are the resistors to make sure they're not SC/OC are way off spec.

It's also worth using a lamp limiter or other form of current limiting before powering it up.

Very nice amplifier's which use good quality "iron" ; shorthand for mains and output transformers.

Andy.
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Old 1st Sep 2017, 5:40 pm   #8
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Default Re: Dynaco Stereo 70

Here's a bit of clarification from the manual accompanying the kit version of the amplifier. This document can be found in loads of places on the net and would be most useful for anyone rebuilding the amp.
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Old 1st Sep 2017, 7:29 pm   #9
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Default Re: Dynaco Stereo 70

Quote:
Originally Posted by Station X View Post
I offered a couple of octal plugs recently. See post #20 in this thread:-

http://www.vintage-radio.net/forum/s...d.php?t=131297
Given that I am forever tinkering it would be nice to have them in my keep; please pm to continue and thanks.
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Old 1st Sep 2017, 7:35 pm   #10
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Default Re: Dynaco Stereo 70

Quote:
Originally Posted by Edward Huggins View Post
The chassis and above-chassis ovecomponents look aged. Do you know how this was stored?
Yes Ed, it does look aged but when I collected the item it was rigged up in a seriously eclectic array of sound equipment. There was a Tascam R2R the size of a barn door that was part of the set up as well as myriads of quad equipment and an ADC Sound Shaper Three which I secured at the same time. Will monitor the situation with the other stuff but the valves on ST70 are pristine and generally it is only the chassis that has been left uncared for.
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Old 1st Sep 2017, 7:38 pm   #11
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Default Re: Dynaco Stereo 70

Quote:
Originally Posted by Diabolical Artificer View Post
The coupling caps on the pre-amp/input board are Mullard mustards and Domino silver mica's. They should be ok but worth checking before you switch it on for leakage etc. As are the resistors to make sure they're not SC/OC are way off spec.
Thanks Andy, I am only a hop, skip and a jump from Watchet Radio Museum (fantastic establishment) where Neil has been most helpful in the past in testing my valves.
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Old 1st Sep 2017, 7:55 pm   #12
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Default Re: Dynaco Stereo 70

If I remember rightly, these were designed by David Hafler no less, and were also sold in 'Dynakit; form.

David
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Old 1st Sep 2017, 9:00 pm   #13
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Default Re: Dynaco Stereo 70

I really do not understand the obsession with the octal plugs. Unless you are going to construct a pair of mono valve preamplifiers which work with 350V at 10mA for their HT and 6.3v at 1A for the heaters, they are of absolutely no use to you. Yes, there would be a connection between the power amp and them on a fully-wired octal plug to octal socket lead but for your preamp you would have to construct a phono plug to octal plug lead, taking care not to connect the 350V, when in fact all you have to do to connect the Pioneer to the Dynaco is use a pair of phono leads between the appropriate phono sockets on each piece of equipment. Simples.
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Old 1st Sep 2017, 11:56 pm   #14
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Default Re: Dynaco Stereo 70

Whatever anybody else says, the amps are well worth "doing it properly".
I do agree however, on forgetting the octal plugs.
Output transformers like those pictured are second to none.
That amplifier will still stand with the best today, sixty years old or not.

Best wishes with your restoration.
Joe
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Old 2nd Sep 2017, 6:41 am   #15
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Default Re: Dynaco Stereo 70

If an owner of a Dynaco Stereo 70 does not install the octal plugs on the front panel, some way will have to be found to provide a test point for measuring idle current during the bias adjustment process. In the stock amplifier one point in the octal plug, labeled BIASET if I remember correctly, served that purpose.
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Old 2nd Sep 2017, 7:47 am   #16
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Default Re: Dynaco Stereo 70

I don't see how an octal plug would help that process at all. In fact an octal plug would obscure the test point on the octal socket marked BIASET. All you need to do is insert the positive probe of a multimeter into the BIASET socket and connect the negative probe to the chassis, which is ground. Exactly the same procedure as on a Radford STA25
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Old 2nd Sep 2017, 8:36 am   #17
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Default Re: Dynaco Stereo 70

Sorry...misunderstood and mistook octal PLUG for octal SOCKET! I agree, octal PLUGS are not necessary unless one is using the Dynaco PAM-1 preamp with the Dynaco Stereo 70.
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Old 2nd Sep 2017, 8:46 am   #18
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Default Re: Dynaco Stereo 70

Quote:
Originally Posted by barretter View Post
I really do not understand the obsession with the octal plugs.
I do!

You can't have too many odds and ends which "might come in useful"

Anyway, they'll fill up the empty sockets neatly. Plugged like that the electrons won't be able to leak out so easily
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Old 2nd Sep 2017, 2:53 pm   #19
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Default Re: Dynaco Stereo 70

Thanks for sharing my sentiments Chris those darn electrons do get everywhere especially more so when the current is greater such as with these old amps; skin effect and all?
Actually at very high voltages (<400kV) steel particles will levitate within an enclosure without the presence of a magnetic field?
My feet have trod this planet for many a year and many a mile over several continents and I wish I had a fiver for every time I have said "if only". Here we have such a situation and I am a stickler for completion of optional interconnectivity product selectivity?
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Old 2nd Sep 2017, 3:07 pm   #20
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Default Re: Dynaco Stereo 70

We'd better keep quiet about this one or some firm will be bringing out a mock octal plug with plastic pins and shortened body 'to protect the kiddies'.

James Thurber's aunt spotted the problem with electricity and wall sockets many decades ago and just look where that led...

David
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