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Dynaco Stereo 70 amplifier
Dynaco Stereo 70 amplifier
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Old 6th October 2004, 11:59 AM   #1
Burnedfingers is offline Burnedfingers  United States
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Default Dynaco Stereo 70 amplifier

Just thought I would throw this one out for comment. I have noticed a furry of buying and selling of this particular model that was very popular as a kit in the 60's. Has anyone had experience with this unit that they would like to share? It seems like a very cheap way for some to start with tube equipment.

I had heard a repaired amplifier at a friends house and frankly was quite impressed with the sound that I was hearing. I don't know if it was the custom speakers I was liking or the sound of this animal.

I was wondering how it would sound up next to my Quicksilvers so I purchased one and will soon find out when it makes its way to me.

Anyone owned one? Hot rodded one? Care to comment on their experiences with one?

Thanks
Joe
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Old 6th October 2004, 12:22 PM   #2
SY is offline SY  United States
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I've built/owned/modded/rodded at least a hundred of them over the last 35 years. It's probably the most popular tube amp of all time and rightfully so. Very good performance at a rock-bottom price. The iron was quite fine, the rest of the amp... less so. But pull out all the old parts, drop in a good driver stage, rebuild the power supply, and you can have a pretty sweet amp.

The ST-70 unit I'm using at the moment has solid state rectification, a good sized cap bank, regulation of all supplies (used the Curcio regulator), a SYclotron input stage (my own design), and EL34 outputs configured as triodes. It's not a monster amp, but it does a great job of driving my ESL panels above 200 Hz. I've also had the Curcio driver stage in this particular chassis, and that was quite excellent.
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Old 7th October 2004, 11:05 AM   #3
Burnedfingers is offline Burnedfingers  United States
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SY

Thanks for the reply. So, how much money would I have to pump into one of these in order to be able to equal my "Quicksilver Mono Block Amps"?

I have received several emails from people letting me know that these amps are more or less run of the mill pieces capable only of questionable performance.

If I may judge from the response of this thread that either no one except you and I have even encountered this brand or no one considers it worth commenting on.

I understand it originally sold in the early 60's for $99.00 plus shipping and if I remember the average take home pay for that period it was roughly the take home pay for an average blue collar worker.

Basically what is worth keeping on this amp? Naturally I would assume the chassis provided it was in good shape. Are the output transformers capable of providing good sound? How about the power transformer? Big enough? I understand the multisection capacitor needs changing to a higher voltage one as a age consideration there. The selenium rectifier in the bias circuit needs tossed if favor of a 1N4007 diode. People scrap the driver board.

What is the best front end to go with? I've seen a 6922 input stage and others that use different tubes than the 7199.

Joe
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Old 7th October 2004, 12:25 PM   #4
SY is offline SY  United States
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Quote:
I have received several emails from people letting me know that these amps are more or less run of the mill pieces capable only of questionable performance.
People gladly say all sorts of questionable things.

Dave Hafler, the original designer, had a philosophy of using the fewest and the cheapest parts. So dump any notion of trying to use his power supply or driver stage. Keep the iron and the chassis, toss the rest.

Put in decent solid state rectification, a regulator (you can get fancy, but a 317/HV bipolar standoff will work very well), and a good triode driver stage (constant-current diff amps, for example). 6922s are a fine choice for the driver, though by no means the only choice.

For maybe a $200 investment, you can end up with an amplifier which will exceed the Quicksilvers in performance and reliability, though at somewhat lower power.
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Old 7th October 2004, 02:31 PM   #5
analog_sa is offline analog_sa  Europe
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Quote:
decent solid state rectification
This sounds very similar to 'military intelligence'

I also have an ST70 sitting in a cupboard somewhere and have to admit it sounds surprisingly reasonable considering the cheap-skate design and appaling quality of components.

Never got round to modding it as it seems impossible to do any worthwhile improvements while retaining the original chasis. The PS is just way too weak. Improving it may be an option if you have sensitive speakers and a power o/p of around 10W/ch is enough.
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Old 7th October 2004, 02:35 PM   #6
SY is offline SY  United States
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Quote:
The PS is just way too weak.
In the original chassis, I've fitted about 1000 uF of HV capacitance and a very tight regulator. That's not exactly a "weak" PS. Be creative!
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Old 7th October 2004, 02:43 PM   #7
analog_sa is offline analog_sa  Europe
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A 1000uf? I doubt i have this much in all my amps combined. The problem as i see it is the mains transformer and choke, not the capacitance
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Old 7th October 2004, 03:02 PM   #8
SY is offline SY  United States
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Well, after regulation, I get 405V. It does not sag under full load. What else could I want out of that power transformer?
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Old 8th October 2004, 02:42 AM   #9
Bandersnatch is offline Bandersnatch  United States
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Default hot-rod St.70

Well, weak inthe PS is weak in the PS. monoblock it with a PA060 for each channel and do a choke input supply. You'll be down to about 25 watts instead of 35, but it will work much better. Lengthen the primary with the spare 6.3 volt heater winding...

As for front ends, check out the audioXpress when it comes out with an article by Pete Millet on the E-Linear circuit. he has experimented with SE but the Emotive Audio stuff which gave him the name( and arrived at a comparable circuit *ABOUT* concurrently ), is PP and really rocks with EL34's.
regards,
Douglas
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Old 8th October 2004, 11:04 AM   #10
Burnedfingers is offline Burnedfingers  United States
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Douglas


Do you happen to have a link to that article? I can't seem to find it.

SY

Would you care to share your driver schematic?

Joe
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