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In Reply to: RE: Dynaco mod posted by Tube747 on May 08, 2017 at 10:48:18
The problem you can run into is stability and you no doubt will when using stock circuitry.
The Dynaco ST-70 has two inherent problems with the power supply right off the bat.The first is the lack of adequate filtering to be able to keep up with the transient demands of today's material.The other is that choke they use which is an absolute joke for this amp.It is rated at 200ma and the output tubes alone are running at 205ma.
Before reducing feedback,I would change out the filter cap and put in a higher capacitance module and a better choke in the 240ma to 300ma range.I would even go further and put a 35uf film cap off the GZ34 and put in a dual 100uf JJ or equivalent after the choke and those two things alone will surprise you. I have done this on two of my ST70s that are otherwise stock circuitry.
See how this does before you start changing values in the GFB loop.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 05/09/17Follow Ups:
"a better choke in the 240ma to 300ma range"
2 Triad models, the C-17X and the C-24X, are cost effective candidates. Available space may be the deciding factor.
C-17X Data Sheet ==> http://www.mouser.com/ds/2/410/media-1068142.pdf
C-24X Data Sheet ==> http://www.mouser.com/ds/2/410/media-1068217.pdf
BTW, don't be fooled by the Dyna C-354's ratings. It saturates in use and the claimed 1.5 H. goes down the tubes. :> (( As Mikey said, the C-354 is a POS .
Eli D.
"BTW, don't be fooled by the Dyna C-354's ratings. It saturates in use and the claimed 1.5 H. goes down the tubes."
I'm wondering how you know this, Eli. Has someone measured the choke with DC flowing? Don't get me wrong, I'm not suggesting there aren't better chokes to use. It's just a point of interest for me whether this has been done.
--------------------------
Buy Chinese. Bury freedom.
Jack
I have measured the current going into the choke and out of the choke at idle as well as with full power driven by a 1khz signal into the amp.The choke saturates and becomes somewhat magnetic but not only that,its effective filtering is almost non-existent vs a resistor in its place.In the Dyna Mk3s,they do better as far as filtering but where that choke would work well is an ElCheapo where the total circuit draws 100ma or less.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 05/09/17
"The choke saturates and becomes somewhat magnetic but not only that,its effective filtering is almost non-existent vs a resistor in its place."
So you compared the amplitude of 120 Hz ripple at the input and output of the choke? You did this both at idle and at full power? Just want to understand how the test was performed.
--------------------------
Buy Chinese. Bury freedom.
Chokes with DC flowing through them do get magnetic...it is afterall a single coil, forming an electromagnet. It is decidedly unclear how the saturation status was measured...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
A Triad C-24X ($9.81) costs less than a "Dynaclone" C-354 ($23.95). The C-24X is the currently recommended part, for "El Cheapo" builds.
Eli D.
Edits: 05/09/17
The St.70 is very cramped for space. Best to use it as a monoblock chassis. Remove an OPT, put an end-belled Hammond in its place. Put a pair of damper diodes in place of the EL34's on that side( 1.2A 6AX4's will do *JUST* fine ). No more pushed over the edge 5AR4's need be acquired...:) and the OE PA060 will support a single channel quite well. Use the C354 to decouple the filter stage used by the split-load PI...or what ever the third stage filter is feeding.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
You can't use a Triad C-17X choke in a Dynaco ST-70 with a stock type chassis. Inside the Dyncao ST-70 chassis, you only have about 1.75 inches of height. The C-17X is about 2.25 inches in height. The C-17X is also to large fit the mount hole spacing for the stock C354 Dynaco choke. The Triad C-24X IS a good substitute for the original Dynaco choke.
Many Dynaco ST-70 users who have never taken off the bottom cover of their amp might be surprised to find a tan colored waxy substance on the inside of the bottom cover directly under the choke. Over the years this "filler material" can and will melt out of those original chokes. When people "restore" an original Dynaco ST-70 they usually replace the quad cap, the selenium rectifier and possibly the driver board. They should also REPLACE THE CHOKE. That choke was cheaply made. The Triad C-24X is a much better made choke. I have used many Triad C-24X chokes in the past 11 years. To my knowledge, not one has failed ...
Bob Latino
It would be interesting to know what the C-24X choke measures at lower currents. It's rated 1.0H at 240mA, but as we know (I hope), inductance varies with current, inductors being some of the less "ideal" components out there.
The C-354 at Dynakit Parts is rated 1.75H @ 200mA and 62 Ohms. Triode Electronics claims 1.5H @ 200mA and 55 Ohms for theirs.
I checked the inductance of an old stock C354 choke in a Dynaco MkIII on the bench at the moment. It measures 1.38H with a Q of 7 with no DC current applied. The measurement was taken with an impedance bridge, and it does have provision for measuring inductors with up to 100mA bias current. It is, however, somewhat tricky to set up the test.
Thanks for the specifics, Bob. I was wondering about the available space.
The C-354 is, most definitely, a POS, which should be routinely replaced, like selenium rectifiers, given almost certain long term failure.
FWIW, I have a C-17X on hand, for a project. Triad has kept standards up. The "U" bracket part has a bottom plate. There are no exposed laminations. :> )
Eli D.
The picture of the choke on the data sheets looks worse than my old Dynaco choke. Did they pull this out of a 1960s amp?
Could a larger value be put in its place? Say 6h?
ZAC
For a first stage cap I/P filter, 6hy would be humongous in the current you would need and you couldn't fit it in an ST70 chassis.You could split up a couple chokes but you would have to design it properly but I'm not sure what you would gain in this circuit.
BTW,the choke ELI posted is much heftier than the C354 choke.I'll take some side by sides pics when I get home later.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
These modifications I agree with.
Can't hurt by beefing up the power supply within reason.
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