|
Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
|
Thread Tools |
9th Feb 2012, 8:19 pm | #1 |
Pentode
Join Date: Nov 2005
Location: Banbury, Oxfordshire, UK.
Posts: 204
|
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
__________________
~~~~~~May your Waves be Long~~~~~~ |
9th Feb 2012, 10:40 pm | #2 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,308
|
Re: Any Dynaco experts please??
I have a ST70 living happily at my address.
|
9th Feb 2012, 11:46 pm | #3 |
Retired Dormant Member
Join Date: Sep 2010
Location: Ohio, USA.
Posts: 757
|
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 |
10th Feb 2012, 9:33 am | #4 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: Any Dynaco experts please??
I've got an ST70 too .
Cheers, GJ |
10th Feb 2012, 10:41 am | #5 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,308
|
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! |
10th Feb 2012, 11:13 am | #6 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: Any Dynaco experts please??
Quote:
Cheers, GJ |
|
10th Feb 2012, 1:34 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
|
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.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
10th Feb 2012, 5:32 pm | #8 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,308
|
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.) |
10th Feb 2012, 5:39 pm | #9 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: Any Dynaco experts please??
|
10th Feb 2012, 8:01 pm | #10 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
|
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...
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
10th Feb 2012, 8:03 pm | #11 |
Retired Dormant Member
Join Date: Sep 2010
Location: Ohio, USA.
Posts: 757
|
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 |
10th Feb 2012, 9:57 pm | #12 |
Octode
Join Date: Aug 2008
Location: Rayleigh near Southend-On-Sea, Essex, UK.
Posts: 1,852
|
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. |
16th Apr 2015, 12:13 pm | #13 |
Retired Dormant Member
Join Date: Jul 2005
Location: Somerset, UK.
Posts: 334
|
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 |
16th Apr 2015, 4:21 pm | #14 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,464
|
Re: Any Dynaco experts please??
Quote:
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. Last edited by ColinTheAmpMan1; 16th Apr 2015 at 4:23 pm. Reason: A bit more practical stuff |
|
16th Apr 2015, 4:55 pm | #15 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
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
__________________
http://www.ampregen.com |
16th Apr 2015, 5:51 pm | #16 |
Retired Dormant Member
Join Date: Jul 2005
Location: Somerset, UK.
Posts: 334
|
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 |
16th Apr 2015, 6:10 pm | #17 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
|
Re: Any Dynaco experts please??
Wasn't the ST70 designed by David Hafler?
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
16th Apr 2015, 8:29 pm | #18 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
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. |
17th Apr 2015, 6:51 am | #19 |
Heptode
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
|
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.
__________________
The honesty of imperfection.......... |
17th Apr 2015, 12:30 pm | #20 |
Retired Dormant Member
Join Date: Jul 2005
Location: Somerset, UK.
Posts: 334
|
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 |