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Dynaco mod

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Posted on May 8, 2017 at 10:48:18
Tube747
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Posts: 419
Joined: May 11, 2004
 I read an article about the mod of stock Dynaco ST-70, and the writer suggest that reducing the feedback from -20db to -12db by changing resistor from 1k to 2.7k. Also, remove the 82pf and 390pf capacitors. And what do you think?

 

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RE: Dynaco mod, posted on May 8, 2017 at 11:03:51
tubular.well
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 Any data that supports the changes ? Or is it just a random walk in the park ?

 

David Hafler, posted on May 8, 2017 at 11:30:22
gusser
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Location: So. California
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 And his engineering team are highly respected even to this day by fellow industry professionals. I think they knew what they were doing when they specified the stock feedback values.

If in doubt, run the formulas and see if the values are correct for the circuit.

But I would not blindly change component values, especially in a a feedback loop, based on some audiophile magazine. Improper feedback can cause instability and oscillations that can destroy tubes.

 

RE: Dynaco mod, posted on May 8, 2017 at 12:22:13
Triode_Kingdom
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Location: Central Texas
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 Then as now, designers sometimes employed large amounts of NFB in order to produce competitive specifications. The question is whether performance such as distortion and frequency response deteriorate so significantly when NFB is reduced as to cause audible degradation. Considering that the ST-70 includes NFB through the UL screen taps, 12dB in a global loop should be more than sufficient for good technical performance. Whether the NFB reduction is sonically beneficial is another story.


 

RE: Dynaco mod, posted on May 8, 2017 at 13:13:56
SteveBrown
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Posts: 2454
Location: Portland, OR
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 If you want to read a library of comments about the ST70 from someone who seems knowledgeable, search in the vintage forum for posts by Joe Rosen.

 

RE: Dynaco mod, posted on May 8, 2017 at 17:36:58
Michael Samra
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Location: saginaw michigan
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 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

 

RE: Dynaco mod, posted on May 8, 2017 at 19:27:20
gusser
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Posts: 3649
Location: So. California
Joined: September 6, 2006
 These modifications I agree with.

Can't hurt by beefing up the power supply within reason.

 

RE: Dynaco mod, posted on May 8, 2017 at 19:51:38
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "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.

 

RE: Dynaco mod, posted on May 9, 2017 at 03:42:11
zacster
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Posts: 2177
Location: NYC
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 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?

 

RE: Dynaco mod, posted on May 9, 2017 at 04:24:18
Michael Samra
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 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

 

You can't use a C-17X choke in a Dynaco ST-70, posted on May 9, 2017 at 05:51:00
Bob Latino
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 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

 

Choke Saturation, posted on May 9, 2017 at 06:46:39
Triode_Kingdom
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Posts: 10037
Location: Central Texas
Joined: September 24, 2006
 "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.







 

RE: Dynaco mod, posted on May 9, 2017 at 07:20:21
LinuxGuru
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Posts: 582
Location: European Union
Joined: November 11, 2008
 It could make sense.

Mid-range tube amp usually have decent output transformers with relatively low resonant frequency and high leakage inductance (leading to considerable phase shift as signal frequency increases).
Thus, if you measure THD/IMD across entire audio spectrum, you may notice that with 20 dB GNFB & high output power THD rise from 0.5%/1 KHz to as much as 5%/20 KHz (IMD raises proportionally).
In order to reduce side effect, decreasing GNFB may help.
Capacitor you mention one should select manually with minimum overshoot at square wave around 2 KHz. Use variable capacitor from old radio in series with 0.01 - 0.05uF film cap and oscilloscope to do so.

 

RE: You can't use a C-17X choke in a Dynaco ST-70, posted on May 9, 2017 at 09:09:29
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Dynaco mod, posted on May 9, 2017 at 12:15:11
Tube747
Audiophile

Posts: 419
Joined: May 11, 2004
 The article also mentioned the change of cathode resistor of el34 from 15.6 ohm to 300 ohm for pure class A operation rather than class AB plus 50uf 50v cathode capacitor?

The output power would drop from 35W to 28w.

 

RE: Dynaco mod, posted on May 9, 2017 at 12:52:00
LinuxGuru
Audiophile

Posts: 582
Location: European Union
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 I use fixed bias on output stage, so can't comment on this change, prefer to stay with it without any cathode resistors and capacitors. IMHO pure class A for push-pull doesn't make sense.

PS. My previous post backed up by real experiments and measurements with spectrum analyzer.

 

RE: Choke Saturation, posted on May 9, 2017 at 13:05:55
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
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 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

 

RE: Choke Saturation, posted on May 9, 2017 at 13:33:18
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Dynaco mod, posted on May 9, 2017 at 18:19:03
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
I did something similar to your inquiry about 20 years ago when I last played with ST70s. I used a 1kohm pot (+ the 1kohm stock resistor) for each channel and dialed in the feedback for best sonics. If I remember correctly, It sounded best with the total feedback resistance 1.2-1.4kohm range. You may like different values. Too little feedback makes the sound bloated.

Leave the caps in place, they correct a HF phase inversion that produces positive feedforward at high frequencies.

 

RE: Choke Saturation, posted on May 9, 2017 at 21:13:02
Triode_Kingdom
Audiophile

Posts: 10037
Location: Central Texas
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 "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.


 

RE: Dynaco mod, posted on May 10, 2017 at 08:44:16
Triode_Kingdom
Audiophile

Posts: 10037
Location: Central Texas
Joined: September 24, 2006
 "pure class A for push-pull doesn't make sense."

Class A push-pull offers the significant advantage of reducing signal current through the power supply filter caps to near zero. That takes those components out of the sonic equation.


 

RE: Choke Saturation, posted on May 12, 2017 at 04:20:40
PakProtector
Audiophile

Posts: 12360
Joined: May 14, 2002
 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.

 

RE: Choke Saturation, posted on May 12, 2017 at 04:45:41
PakProtector
Audiophile

Posts: 12360
Joined: May 14, 2002
 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.

 

RE: Dynaco mod, posted on May 12, 2017 at 09:41:17
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 Having owned one and tinkered with it in various ways >20 years, I feel confident in stating that the real performance bottleneck of the ST-70 is in the power supply, particularly in the use of a single GZ34 and a marginally adequate power transformer, along with the underrated choke.

Once you've dealt with these issues, the stock circuit works reasonably well. I think it can be upgraded, but the power supply, IMO, should be the first target for performance upgrades. The PT can be upgraded with the unit from Triode Electronics.

If you've ever heard a pair of MkIVs vs a ST-70, you will see what I mean.

 

RE: You can't use a C-17X choke in a Dynaco ST-70, posted on May 12, 2017 at 15:40:09
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 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.

 

RE: You can't use a C-17X choke in a Dynaco ST-70, posted on May 13, 2017 at 14:15:46
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 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.

 

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