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Old 6th October 2004, 11:59 AM   #1
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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
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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
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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
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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
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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
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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
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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
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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
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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,
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Old 8th October 2004, 11:04 AM   #10
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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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Old 8th October 2004, 11:25 AM   #11
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Joe, I posted my input stage here.

At the moment, I'm not using the input transformer option, though if I had some spare cash and could buy another pair, I would.
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Old 8th October 2004, 11:27 AM   #12
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It has not been published yet. No skill with electronic schematic software either. I have a prelim draft, but don't think I am allowed to share it.

Two stage amp, common cathode diff amp, one grid grounded or feed them both a balanced signal. B+ is sourced from the OPT's screen taps. Just watch the phase, or you'll have positive feedback...
regards,
Douglas
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Old 9th October 2004, 06:08 AM   #13
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Thanks SY.

I was kinda thinking of a 6SL7 6sn7 arrangement for my front end. Actually the 6sl7 for the input stage and the 5692 for the phase splitter with a hand made PC board. Will this work?


Joe
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Old 9th October 2004, 10:39 AM   #14
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I think these are excellent choices. What I would do, though, is use the 6SL7/5691 as an input/phase splitter (either CC diff amp or SYclotron) and the 6SN7/5692 as a differential driver.
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Old 21st October 2004, 11:11 AM   #15
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Default Auditioned the Dynaco

I received my Dynaco this week and naturally had to fix it. Frankly
outside of the cheap original cost of this "KIT" amplifier I cannot imagine why someone would buy one.

I purchased a new driver board from Vacuum Tube Audio from (Ray) off ebay. It has been setup to use (3) 12AT7's but I am told I can sub other tubes in their place. I will soon have the parts to put it together.

Listening observations:

1). No bass 2). Lack of midrange detail 3) Strained high frequency response.

I have severe doubts that this amp is going to turn into a worthwhile project. It may have to be sold for scrap just to recoop
my investment.

I hope I am wrong...

Joe
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Old 21st October 2004, 12:25 PM   #16
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I'll be delighted to take that horrible piece of scrap junk off your hands!
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Old 21st October 2004, 02:08 PM   #17
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HEy-Hey!!!,
I will dispose of the Iron for you. No need for that cramped chassis. E-mail me when you're properly fed up with it.
regards,
Douglas
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Old 21st October 2004, 02:26 PM   #18
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Quote:
Listening observations:

1). No bass 2). Lack of midrange detail 3) Strained high frequency response.
I don't know anything about that driver stage (and will assume it competent), but in all seriousness, if this is the way your amp sounds, you need to do some troubleshooting- you clearly have a wiring error, bad solder joint, bad component, bad tubes...

Is there a schematic of your driver stage somewhere on the 'net?
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Old 21st October 2004, 09:38 PM   #19
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Ok

Removed the SS rectifier and installed stock 5AR4(new). Changed coupling caps, installed bias diode, new 10K pots, and caps. Resoldered all connections!

Checked voltages with schematic-OK

Changed 7199's to new RCA's, changed EL34's to new. Bias now sets and stays.

Sounds somewhat better but in stock form not worth writing home about.

I will stuff the new board and report my observations.

Quote:

I will dispose of the Iron for you.

Quote:

I'll be delighted to take that horrible piece of scrap junk off your hands!

You guys really crack me up.
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Old 21st October 2004, 09:40 PM   #20
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Do you suppose the power supply has enough guts to run KT90's in place of the EL34's?

I believe the heater current on a EL34 is 1.5A and on a KT90 1.6A

Anyone have an idea here?

thanks
Joe
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Old 21st October 2004, 10:31 PM   #21
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Quote:
Originally posted by SY
Joe, I posted my input stage here.
SY, I wish I had seen this a few days ago, I already ordered Curcio's premium driver board.

I'm a noob with a couple of kits under my belt and studying Rosenblit's tube design book. Can you provide me any more clues on improving the power supply? You mentioned the 317 which I assume is the LM317 regulator?

After the cap and premium driver board purchases, I'm feeling the desire to curtail further purchases for a while with other mouths (projects) to feed.

Thanks.
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Old 21st October 2004, 10:34 PM   #22
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The Curcio board is excellent, a built-in 317-based regulator and a constant-current cascode diff amp. You did fine.

For more reading, do get Morgan Jones's two excellent titles, "Valve Amplifiers" and "Building Valve Amplifiers".
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Old 22nd October 2004, 10:58 AM   #23
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Quote:

I'll be delighted to take that horrible piece of scrap junk off your hands!


Make me an offer.
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Old 22nd October 2004, 11:42 AM   #24
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If the transformers check out OK (I have no interest in the aftermarket driver board, my cupboard is full of ST-70 drivers of every imaginable topology!), I'll recoup what you spent for the ST-70 itself. Plus shipping.
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Old 22nd October 2004, 04:27 PM   #25
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FYI

Free advice column for ST-70s

Assuming not a huge mod to be done, like changing the driver board:

- replace all resistors with MF/MO types
- replace all caps with film caps - the ceramics with Silver Mica
- put ~100ufd AFTER the choke in the PS.
- remove the cathode resistors - or reduce them by 10x in value
(they do serve as fuses) this will dramatically improve the bass
- remove the mono switch/replace RCA jacks
- add cap to the bias supply - replace the old caps there
- strap the tubes to triode (tie the screens to the plate with a current limiting resistor)

This will make the unit sound pretty decent.

The power tranny came in two sizes: small and larger
The larger one is more better.
There were several after market drop in replacements, higher current bigger core, which were suitable for dumping on the PS filtering.

What the above mods will do is to improve the amp to the point where it is listenable, and actually good, and not cost you very much money or time...

No doubt the worst part of the amp is the driver board & icky compensation circuit that resides there.

The second worst part is a slightly anemic power supply.

I have some definite ideas on a nifty driver board, but have yet to get through the 101 other projects that have priority over that. But if it ever got done, it would be a definitive approach.

Sy - neat cross coupled stage! Did you see the one that was around back 20 years ago? Andy something out of Jersey did that one...

_-_-bear
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