Dynaco-Doctor Forum

Welcome! Need support, you got it. Or share your ideas and experiences.

Dynaco-Doctor Forum

RE: Direct input to ST-70

73.123.207.47

In Reply to: RE: Direct input to ST-70 posted by hvlee on May 08, 2020 at 17:31:19

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.

Edits: 05/09/20 05/09/20

Follow Ups:

"I'm not crazy about the idea of swapping out the original circuit design for a different one" - 1973shovel 08:00:28 06/27/20 (15)
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" - petercapo 11:49:31 06/27/20 (14)
  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.
  
  Edits: 06/27/20
  - RE: "I'm not crazy about the idea of swapping out the original circuit design for a different one" - Mushroom Soup 18:38:11 03/05/21 (0)
    I seem to remember long long ago that the 12AU7 wasn't considered a "hi-fi" tube.
  - Transistors? - 1973shovel 16:50:47 06/27/20 (12)
    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? - petercapo 17:08:03 06/27/20 (11)
      Thanks. I should have said "solid-state" devices.
      - RE: Transistors? - 1973shovel 21:33:28 06/27/20 (10)
        
        In Reply to: RE: Transistors? posted by petercapo on June 27, 2020 at 17:08:03
        
        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? - petercapo 03:46:23 06/28/20 (9)
        
        In Reply to: RE: Transistors? posted by 1973shovel on June 27, 2020 at 21:33:28
        
        Not at all. I desire correction. If not, I would continue to sound like even more of a moron than I already am! :-)
        
        Edits: 06/28/20 06/28/20 06/28/20 06/28/20
        
        RE: Transistors? - 1973shovel 06:35:26 06/30/20 (8)
        
        In Reply to: RE: Transistors? posted by petercapo on June 28, 2020 at 03:46:23
        
        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 - petercapo 06:44:09 06/30/20 (7)
        
        In Reply to: RE: Transistors? posted by 1973shovel on June 30, 2020 at 06:35:26
        
        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!
        Stereophile review of Cary Audio SLI-100 (Open in New Window)
        
        Edits: 06/30/20 06/30/20 06/30/20 06/30/20
        
        That's a good question - 1973shovel 13:34:43 07/02/20 (6)
        
        In Reply to: RE: Reimagined Dynacos posted by petercapo on June 30, 2020 at 06:44:09
        
        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 - petercapo 14:17:35 07/02/20 (5)
        
        In Reply to: RE: That's a good question posted by 1973shovel on July 02, 2020 at 13:34:43
        
        It's good to have options. Please let us know how you do!
        
        I will post my results - 1973shovel 06:57:00 07/03/20 (4)
        
        In Reply to: RE: That's a good question posted by petercapo on July 02, 2020 at 14:17:35
        
        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) - petercapo 13:18:23 07/03/20 (3)
        
        In Reply to: RE: I will post my results posted by 1973shovel on July 03, 2020 at 06:57:00
        
        nt
        
        I hope mine turns out as well - 1973shovel 17:32:13 07/04/20 (2)
        
        In Reply to: RE: Looks Great! (nt) posted by petercapo on July 03, 2020 at 13:18:23
        
        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? - petercapo 18:32:52 07/04/20 (1)
        
        In Reply to: RE: I hope mine turns out as well posted by 1973shovel on July 04, 2020 at 17:32:13
        
        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 - 1973shovel 07:36:10 07/06/20 (0)
        
        In Reply to: RE: Jed Clampett's property? posted by petercapo on July 04, 2020 at 18:32:52
        
        But the transformer tar comes straight from La Brea. Apparently David Hafler felt it was the best sounding tar, after extensive testing.
RE: Direct input to ST-70 - hvlee 09:49:58 05/09/20 (2)
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 - petercapo 13:09:58 05/09/20 (1)
  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 - hvlee 13:54:12 05/09/20 (0)
    It's a responsibility.
    
    Bill held my hand through five rebuilds of an ST-120. He was always available to help.
    
    Regards,
    
    Harry

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio