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Old 9th Feb 2012, 8:19 pm   #1
Cre8anet
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Default Any Dynaco experts please??

Are there any ST-amp owners out there? (were the transistors ones SCO- series, from memory?)

...There may be some dynaco m/f sheets going as part of a clearout and maybe a few original manuals if you can show a good home.

(maybe drop me a PM with model you own if you want me to find out for you)

All the best,

Dave
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Old 9th Feb 2012, 10:40 pm   #2
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Default Re: Any Dynaco experts please??

I have a ST70 living happily at my address.
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Old 9th Feb 2012, 11:46 pm   #3
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Default Re: Any Dynaco experts please??

I have 2 dynaco st70 amps in use, powering a pair of klipsch horns running in vertical Bi-amp mode, being driven by a Mcintosh c220 valve pre, being run by a ampex 600-2 reel to reel (valve)

-Chris
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Old 10th Feb 2012, 9:33 am   #4
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Default Re: Any Dynaco experts please??

I've got an ST70 too .

Cheers,

GJ
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Old 10th Feb 2012, 10:41 am   #5
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Default Re: Any Dynaco experts please??

I have been very happy with the Vacuum Tube Audio (aka tubes4hifi) board in place of the stock ST70 board which uses now scared 7199 valves.

Someday I will replace the power transformer, and then all that will be left from the original will be the chassis, cover and output transformers!
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Old 10th Feb 2012, 11:13 am   #6
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Default Re: Any Dynaco experts please??

Quote:
Originally Posted by qualityten View Post
... the stock ST70 board which uses now scared 7199 valves ...
I'd be scared too if I was as rare as a 7199 (did you mean sacred ?) ! In a conventionally wired amp there is a mod which allows the 7199 to be substituted by a 6U8A which, I believe, is more or less an ECF82 http://site.tubedepot.com/pdf/7199_6u8.pdf. This would be more difficult and potentially destructive to implement on a pcb though.

Cheers,

GJ
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Old 10th Feb 2012, 1:34 pm   #7
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Default Re: Any Dynaco experts please??

I like the sound of a sacred valve, though I'd be more inclined to elevate KT88s or some such to that rarefied level. Maybe scarce is the mot juste here.

Since the application is audio, I should think a fabricated inline B9A plug to B9A socket adaptor would work fine for a PCB application, as long as clearance height is OK.
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Old 10th Feb 2012, 5:32 pm   #8
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Default Re: Any Dynaco experts please??

Yes, scarce. Sometimes my fingers just have a will (word?) of their own. But Freud would have something to say about the variants.

The VTA boards are beautifully manufactured and drop in replacements. (No connection with them, except as a satisfied customer.)
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Old 10th Feb 2012, 5:39 pm   #9
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Default Re: Any Dynaco experts please??

Quote:
Originally Posted by Herald1360 View Post
I like the sound of a sacred valve ...
I imagine I would too. But I've never actually heard one .

Cheers,

GJ
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Old 10th Feb 2012, 8:01 pm   #10
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Default Re: Any Dynaco experts please??

Don't you use any of yours

ISTR there's a veritable (or is that venerable) concentration in South Oxon somewhere...
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Old 10th Feb 2012, 8:03 pm   #11
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Default Re: Any Dynaco experts please??

I was lucky enough when i bought both of my amps from a guy at a hamvention, to have received spare valves for each amp
I would never mod mine out, the original sound better then the mod kit imho.

-Chris
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Old 10th Feb 2012, 9:57 pm   #12
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Default Re: Any Dynaco experts please??

Hi,

I have a modified ST70 with the Curcio long-tailed cascode pair driver as featured in Glass Audio. This is basically the Hedge circuit from the June 1956 Wireless World.

I didn’t bother with the power supply mod and kept this standard.

I laid up the PCB myself and got it made by a local PCB company.

Picture attached.

Terry.
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Old 16th Apr 2015, 12:13 pm   #13
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Default Re: Any Dynaco experts please??

My ST70 has been recapped except for the quad electrolytic, and the biasing selenium rectifier changed to a BY251 diode.

The problem is that although when set up all appears to be ok, after a period of use the two EL34's on the right hand channel overheat (they are JJ brand and the red logo goes transparent) and one or other fails. The EL34's on the left hand channel remain ok.

Static checks and visual examination reveal nothing amiss, but can anyone suggest where to start looking for the cause of the problem.

Thanks

Alex
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Old 16th Apr 2015, 4:21 pm   #14
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Default Re: Any Dynaco experts please??

Quote:
Originally Posted by awc View Post
The problem is that although when set up all appears to be ok, after a period of use the two EL34's on the right hand channel overheat (they are JJ brand and the red logo goes transparent) and one or other fails. The EL34's on the left hand channel remain ok.
That sounds suspiciously like a biasing problem. Check the current through both output valves which are overheating. My money would be on those two drawing too much current. I don't have a circuit diagram of this amp, so I'm "shooting from the hip" here. If you are afraid of killing valves by turning the amp on and allowing it to warm up, you could check what the manufacturer suggests the control-grid bias voltage should be and without the output valves in place, set it a few volts more negative so that when present, the valves are drawing less current than they are supposed to. You can then monitor the grid-voltage and/or anode current and adjust the bias so that the manufacturers optimum current is obtained. You can also hear how the amplifier sounds, too, and set it how you like it to be.

The other thing is, are the EL34s all JJ, or are the ones on the LH channel a different brand? Some manufacturers make valves which will cope with higher anode currents/voltages better than others. Electroharmonix (EH) are usually a good brand, but don't be fooled, they will all be manufactured in the East somewhere and then "badge engineered" with a different brand.

It would probably be advantageous to use matched output valve pairs, too, although opinions vary.

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Old 16th Apr 2015, 4:55 pm   #15
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Default Re: Any Dynaco experts please??

I agree with Colin that since it's just one channel it sounds a lot like bias failure. Each channel derives the bias for both of its valves from one pot. I'd suspect the right channel pot or something in the wiring around it. All it takes is for a wire or a solder joint or the pot wiper or track to go open circuit and the bias will disappear, at which point the valves will run suddenly very hot and sooner or later very dead.

It would be worth connecting a meter across the bias monitor resistor and watching its voltage as time goes on. A bias problem will cause it to rise suddenly and a long way.

VB
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Old 16th Apr 2015, 5:51 pm   #16
awc
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Default Re: Any Dynaco experts please??

Thanks to you both for your replies. I am attaching the circuit diagrams and perhaps you could let me know if there is anything you want to add before I start your recommended checks.

Thanks

Alex
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File Type: pdf Dynaco 2.pdf (512.3 KB, 85 views)
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Old 16th Apr 2015, 6:10 pm   #17
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Default Re: Any Dynaco experts please??

Wasn't the ST70 designed by David Hafler?

David
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Old 16th Apr 2015, 8:29 pm   #18
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Default Re: Any Dynaco experts please??

If not already present, add a high value resistor from the bias pot slider to the negative end. This means that the valve still gets bias (probably too negative, so safe!) even when the slider lifts off the track.

Yes, I believe this was a Hafler design.
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Old 17th Apr 2015, 6:51 am   #19
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Default Re: Any Dynaco experts please??

Looks a pretty straightforward circuit. My guess would be either an intermittent bias preset or even a hairline crack in the PCB tracking that is expanding / breaking when the amp is warm.

SimonT.
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Old 17th Apr 2015, 12:30 pm   #20
awc
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Default Re: Any Dynaco experts please??

Thanks for the further comments. Putting an analogue ohmmeter on the biasing pot on the right hand channel showed a jumping about of the pointer indicating poor contact.

Treating this with Deoxit has sorted this and the pointer now moves smoothly from one end of the track to the other. Although the left hand channel pot seemed ok, I decided to give this a dose of Deoxit as well as a precaution.

As this should have sorted the problem I am inclined to run it up and monitor things for a while unless anyone feels further precautions ought to be taken first.

Once again many thanks for your knowledgeable help.

Alex
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