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Direct input to ST-70

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Posted on April 3, 2020 at 15:53:12
hvlee
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Posts: 129
Location: Northeast Georgia
Joined: February 12, 2007
I have an Adcom GFP-565 preamp coming. A couple of the outputs are direct, bypassing any output capacitors.

I wonder if I should be concerned using one of the direct outputs with my stock ST-70. If so, there's a normal output using caps I can use but my OCD says direct is best.

I realize there's a Van Alstine bandpass mod for the ST-70, but my days of neat soldering are far behind.

Thanks, Harry

 

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RE: Direct input to ST-70, posted on April 9, 2020 at 18:54:36
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
I wouldn't use a preamp lacking output coupling capacitors with an original Stereo 70.

 

+1, additionally the Adcom is an older preamp and DC offset may have increased...., posted on May 8, 2020 at 16:53:37
....due to aging of the components. I would not go there, but if you do, make sure to measure the DC offset before doing anything else. Even if it worked, the Dyna is not comfortable with infrasonic signals, hence the VA mod. Your OCD is whispering troubled thoughts in your ear.

 

RE: Direct input to ST-70, posted on May 8, 2020 at 17:10:12
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
The Van Alstine filter is not really needed, IMO. There's a simpler way. Here's the fairly recent, landmark lab study by Dave Gillespie of the original Stereo 70 circuit... the entire document is a "must read," but regarding the DC offset thing, have a look at the top third of page 10, where Dave provides a simple procedure for adding DC blocking capacitors to the Stereo 70 inputs:

 

RE: Direct input to ST-70, posted on May 8, 2020 at 17:31:19
hvlee
Audiophile

Posts: 129
Location: Northeast Georgia
Joined: February 12, 2007
Thank you. I've read that in the deep dark past but without an eye to any mods.

I'm running the ST-70 from the "Normal" output of the GFP-565 so I'm ok. Sounds great.

Thanks,

Harry

 

RE: Direct input to ST-70, posted on May 9, 2020 at 06:52:30
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
Interesting that you mention "mods." While this procedure for installing a DC blocking capacitor at the inputs is, by definition, a modification, the original driver and power supply etc., remain. Also remaining should be the original sonic character - or so I would think.

I'm not crazy about the idea of swapping out the original circuit design for a different one. I find that the original Dynaco Stereo 70 circuit design, properly functioning, is a fine-sounding amplifier even by today's standards. It's objective, measured performance in the lab is as unimpeachable as always.

 

RE: Direct input to ST-70, posted on May 9, 2020 at 09:49:58
hvlee
Audiophile

Posts: 129
Location: Northeast Georgia
Joined: February 12, 2007
I can't claim to be a virgin, mod-wise, but in general, I like to leave things alone, assuming the designers were more informed than I. I've made mods to some of my amateur radio equipment which were peer-reviewed and popular with the community but actually improved function of my old, tube type gear.

But this is a special case. My ST-70 is a Bill Thomas and I think it would be sacrilegious of me to mess with it. He even aligned the screw slots.

And, I gave up and used the "Normal" output from the preamp so it's not an issue. I can't imagine that my aged hearing could tell the difference anyway. As I said, it was just my OCD telling me that Direct was pure.

Regards,

Harry

 

RE: Direct input to ST-70, posted on May 9, 2020 at 13:09:58
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
Bill Thomas... that does make it special. What a forum this was at one time. The archives are invaluable.

 

RE: Direct input to ST-70, posted on May 9, 2020 at 13:54:12
hvlee
Audiophile

Posts: 129
Location: Northeast Georgia
Joined: February 12, 2007
It's a responsibility.

Bill held my hand through five rebuilds of an ST-120. He was always available to help.

Regards,

Harry

 

"I'm not crazy about the idea of swapping out the original circuit design for a different one", posted on June 27, 2020 at 08:00:28
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
I would agree with you, if only I had a small stockpile of 7199s.

I have tried the popular three 12AU7 board sold elsewhere. Despite trying different resistors and capacitors, adjusting level of feedback, etc, a few of us find that circuit too sterile, too "solid state-y" for our tastes.

And so, the search continues.

 

RE: "I'm not crazy about the idea of swapping out the original circuit design for a different one", posted on June 27, 2020 at 11:49:31
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
Thank you for your feedback. I have read others reporting the same about the popular triple 12AU7 board. My assumption is that the transistors used on that board may be the cause of its somewhat closer to solid-state sound.

 

Transistors?, posted on June 27, 2020 at 16:50:47
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
Ahh, I'm guessing you mean the LM334, which is used as a CCS (constant cathode source) on the newest version of that input board.

If so, I have the earlier, pre-CCS version, but the guy who I compare notes with has the CCS version.

I always assumed my assessment of the sound was due to the use of the non-linear 12AU7s. I had one guy look at the schematic and tell me the lack of musicality I note is due to the large amount of negative feedback in the circuit.

I tried a variety of output tubes, including the original 1960s Mullard EL-34, and re-issue Gold Lion KT-77, neither of which did it for me. I was able to coax more enjoyment out of the amp by using the Russian
6P3S-E, a 6L6 variant which Mike Samra recommended.

 

RE: Transistors?, posted on June 27, 2020 at 17:08:03
petercapo
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Posts: 665
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Thanks. I should have said "solid-state" devices.

 

RE: Transistors?, posted on June 27, 2020 at 21:33:28
1973shovel
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Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
I didn't mean to sound like I was trying to correct you, only that since mine doesn't have them, I was lost for a second.

As always, it's good to see your posts here.

Thanks.

 

RE: Transistors?, posted on June 28, 2020 at 03:46:23
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
Not at all. I desire correction. If not, I would continue to sound like even more of a moron than I already am! :-)

 

RE: Transistors?, posted on June 30, 2020 at 06:35:26
1973shovel
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Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
I should have added that I was speculating about the CCS vs non-CCS versions of the same circuit. My version has no sand (other than the bias diodes) and my impression has always been that it lacks a certain musicality.

It's always good to read your posts!

 

Reimagined Dynacos, posted on June 30, 2020 at 06:44:09
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
Very interesting. Wonder if it has anything to do with the long tail pair topology. In 2018, Stereophile magazine reviewed a Cary amplifier where the reviewer said he prefers its cathodyne phase inverter topology to other designs - I find his description interesting to read and reminiscent of what I hear from my Dynacos. As we know, the original Dynaco used the cathodyne phase inverter.

Always good to read your posts, as well!

 

That's a good question, posted on July 2, 2020 at 13:34:43
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
I don't know enough about circuits to determine why one sounds more musical than the next.

Next fall/winter my plan is to try and tweak the three tube 12AU7 input circuit a bit by trying different coupling caps and adjusting feedback. If that doesn't make it to my liking, my plan is to take the board out and try a new input stage.

I've been thinking I'll try the 6GH8A circuit, which I already have a board for, except that I'll try the 7643 tubes in place of the 6GH8A. I write to someone who modified his Sherwood S5000 to take 6GH8As in place of the original 7199s. He said he much prefers the 7643 tubes over the 6GH8A, so that gives me some hope that it'll improve my ST-70.

 

RE: That's a good question, posted on July 2, 2020 at 14:17:35
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
It's good to have options. Please let us know how you do!

 

I will post my results, posted on July 3, 2020 at 06:57:00
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007



But it's not going to be anytime soon. I have a few other projects ahead of the ST-70, most notably, a ST-35 clone on a ST-70 sized chassis.

I'm going to build it using a 12AX7 and a 12AU7, instead of the single envelope 7247/12DW7. The new production 7247 doesn't sound nearly as good as old stock, according to my source. And NOS are scarce and expensive, so it doesn't make sense to build a new amp around it.

And until winter, when motorcycle season slows down, all the Hi-Fi projects are on hold.

Happy Independence Day!



 

Looks Great! (nt), posted on July 3, 2020 at 13:18:23
petercapo
Audiophile

Posts: 665
Joined: December 29, 2012
nt

 

I hope mine turns out as well, posted on July 4, 2020 at 17:32:13
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
I should have added that I lifted the picture, just to show the general set-up I'll be using. Mine won't have the wood sides, although I suppose I could add them if I got ambitious.

One thing I know won't look as good as that picture is my power transformer, which is from an SCA-35. Every original one I've ever seen, including the two I own, look like they were plucked from the depths of the La Brea tar pits.











 

Jed Clampett's property?, posted on July 4, 2020 at 18:32:52
petercapo
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Posts: 665
Joined: December 29, 2012
Isn't that the spot on the ground where Uncle Jed was shootin' for some food, and up through the ground come a bubbling crude?

 

Uncle Jed's Black Gold is the only oil I will use in my capacitors, posted on July 6, 2020 at 07:36:10
1973shovel
Audiophile

Posts: 10109
Location: Greenville SC
Joined: February 25, 2007
But the transformer tar comes straight from La Brea. Apparently David Hafler felt it was the best sounding tar, after extensive testing.

 

RE: "I'm not crazy about the idea of swapping out the original circuit design for a different one", posted on March 5, 2021 at 18:38:11
Mushroom Soup
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Posts: 310
Location: Western New York State
Joined: November 1, 2003
I seem to remember long long ago that the 12AU7 wasn't considered a "hi-fi" tube.

 

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