Angela Instruments Dyna St-70 6B4G

Discussion in 'Tube Audio' started by ronton3, Jan 12, 2019.

  1. ronton3

    ronton3 ronton3

    Messages:
    171
    Location:
    Macon Mo
    "To fill a military order Philips internally triode connected some 6AV5GA tubes, which when triode connected are pretty close to a 6B4G. These have been available for a number of years. It would seem Philips either couldn't or chose not to make a run of "real" 6B4G. IIRC these date from the 80s, and I doubt a real 6B4G could have been made at that late date."

    I have read that Angela's paper catalogue from the late 90's gives easy instructions for simple changes to make these usable in a stock Dyna St-70. I am reading Dave's wonderful thread with all the meaningful responses about baseline testing for the St-70, I want to keep mine stock, but also try the matched quad of these 6B4G's I have. I have searched the Angela Site but the paper catalogue is no longer available. Could someone please post the instructions. Thank You
     

     

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  2. trainbuftony

    trainbuftony Electron Herder Subscriber

    A ST 70 is an ultralinear design amp. It uses pentode tubes because part of what makes it a ST70 is that it runs EL34 pentodes in ultralinear mode, meaning that the transformer has a winding for feedback to the screens. The last set of tubes I bought for a ST70 were brand new matched mullard EL34 and we're under $100 shipped. In this hobby that's not a lot of money, and the combo is bound to result in a great sounding amp. Installing a modified TV deflection pentode, may work, however the screen is going to be way over 200v in ultralinear mode. Forcing triode mode, will result in less power and musher sound. You may need to mod the bias arrangement and feedback in order to get it working right. I personally would order some new EL34s from someone you trust, bias the amp, and enjoy it for what it is. I won't discourage you from playing with deflection tubes, I play with them myself. I don't think that putting them in the st70 will result in better sound or lower distortion.
     
    petercapo and peterh like this.
  3. ronton3

    ronton3 ronton3

    Messages:
    171
    Location:
    Macon Mo
    Thanks for responding: I have 2 St-70's both recapped and sounding great, one I am listening to right now, is in triode mode with EL-34s, which I can easily reverse. I began with tubes about 1995, and went on a collecting binge at hamfests, flea markets and used hi-fi shops(In particular Buzze's Hi-Fi, in Gladstone north of KC), things were much cheaper then, I bought stuff often not knowing what it really was, if it had tubes I bought it, often regardless of condition. I have several sets of Mullard El-34s some original Dyna's. I just want to try the Jan 6B4G's, I do not think I should just stick them in the triode mode amp, that is why I am asking for the Angela Catalogue instructions. I will then probably change it back to stock form.

    I have recently ran across this info. Can anyone tell me if this is the same as the Angela Catalogue. I will not be drilling any holes, there is enough room do do this without that, also I often leave the bottom plate off and place the amp on blocks to help with heating.

    1. Acquire a pair of 510 ohm RH-25 (if 5U4 rectifier), or 750 ohm
    RH-25 (if 5AR4 rectifier). Drill holes matching the pattern of the
    resistors into the ST-70. Secure tightly with #4-40 screws and locnuts. For each resistor, the the 6.3V center tap pin to the resistor. The other resistor pin, wire to signal ground.

    2. Disconnect the negative bias supply from the grid leak resistors.
    Connect the grid leak resistors to ground. Make sure you dont short out the negative bias supply by accidently connecting it to ground.

    3. Remove the EL34 Triode mode if in place... I.e. pull out the 100
    ohm screen resistor between pins 3 and 4. If this is not in place, then you can safely let the ultralinear tap sit on pin 4 as there will be no connection to the tube.

    4. Clip the NFB line from the OPT secondary back to the driver board.
     
    Last edited: Jan 13, 2019
  4. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

    Messages:
    847
    Location:
    Gainesville, FL
    I can't tell you if it's the same. Why don't you just give Angela a call and ask?
     
  5. ronton3

    ronton3 ronton3

    Messages:
    171
    Location:
    Macon Mo
    I sent an email: and promptly received this response: Ron,
    Some years back we, along with twenty other individuals and organizations associated with tube sales and distribution, were dragged into a frivolous lawsuit claiming that tubes we sold contained mercury that had injured our of our hi-fi repair shop customers. There was
    no physical evidence showing that we had ever sold this individual tubes containing mercury and, as predicted, the suit was eventually thrown out of court because it had absolutely no merit. This experience cost us lots of money and upset our over 35 year good relationship with our business insurance company. We had to make a number of changes to our business to reflect the new realities. Our lawyer advised us to take down the entire "HOW TO..." section of our website because it could result in more frivolous litigation.
    Steve Melkisethian/Angela Instruments.

    I sent another email asking if there is a copy of the catlogue I can buy, I haven't heard back yet. I set things up the way it is described, and will try it with low voltage on the variac(closely watching), and will report back. Again the 6B4Gs I have are GE JAN not the real old ones. Thanks for responding.
     
  6. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Sad that useless lawsuits have such an effect. I always expected any how-to article comes with the standard "try at your own risk" statement implied if it wasn't directly stated.
     

     

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  7. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

    Messages:
    847
    Location:
    Gainesville, FL
    Do a search for 6B4G ST-70 over at Audio Asylum. There are lots of posts on their Tube DIY board and some of their other forums too.
     
  8. ronton3

    ronton3 ronton3

    Messages:
    171
    Location:
    Macon Mo
    Yesterday I removed the connections from the .02 ceramic to the 6.3 taps(leaving it in place for restoration later), I then put in place 2, 800 ohm 20 watt cements, configured the same as the .02, held in place with an added tie point in the corner of the chassis. The bias caps had been replaced 20 years ago with 100/100 Sprague Atoms.

    I then removed the wires to the center taps of the bias pots and tied them to ground.

    Next I moved the wires from points 11 and 14 on the board that went to the 4 pins on the front two EL34 sockets, placing them on the front sockets which are not wired for a preamp. I left the OPT on the pins winding in place. The board had been recapped 20 years ago with 4 .1 Solens and 2 .047, 716P Orange Drops(I may change the Drops to russian oils), the Solens will stay. The Can Cap is stock and was slowly reformed last month, when I rescued them from the shelf, where I now think they were for far to long.

    I believe this completes the instructions found in the first post, this is also how I had the other ST-70 wired that I was asking in a post last month trying to id the mod I had made when they were recapped. I now realize it was for the Sylvania JAN 6B4Gs, those were returned to stock triode EL-34 form.

    I waited until dark so I could monitor the lights in the tubes, and slowly, with my hand near the switch, ran it up to about 60 volts over a half hour, I had detected small glimmers of life then suddenly, very dimly, the burn in tones started from the Sheffield test CD, via an M4A file on the Iphone connected directly to the inputs. I ran it on up to about 100V without any bad signs. I took it to the main system and hooked it to a pair of Marantz Imperial 6's which are sensitive, and plugged it into an RCA isolation transformer at the 106V level. The Iphone was connected to an Audioengine1 blue tooth streamer, connected to the St-70 furnished with 4 Sylvania JAN 6B4G's Triodes(these are not like the earlier 6B4G's), a Mullard 5AR4 and 2 7199's. I selected the Statler Brother's Flower's on the Wall, there is something about the opening banjo twangs that tell me something. I was not disappointed, rather a bit shocked it sounded as good as it did, about 75 or 80 percent of the Iphone volume is a comfortable listening level, when there is a pause between songs there is no hum. This morning I plugged it into a 115V outlet and played Sitting Still Within, a meditation file with a gurgling stream and bell strikes in the background I have heard 1000's of times, I do not think I have heard it better, but who knows, we can be so influenced by different factors.
    I am now doubly happy I was referred to Dave's,18 page base-line-test post, because I was seriously thinking about changing the driver board not any longer. The 7199s are doing well, the ST-70 sounds great. Right now I am listening to Sirius XM 50's on 5 it has been on for about an hour, no red plates, I can hold my fingers on the side(not just the top), of the PWR TRF, the can is just warm. I have not been able to find any voltage info on the 6B4Gs, and I have not checked any, I believe things are going quiet well. I should mention that the new 6's and the caps only have a few hours of playing time, it seems it already sounds better this afternoon, but again we all know about subjectivity don't we?

    http://www.audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/page-1 Thanks Dave.

    I also thank a guy named Jim on DIY, for posting the directions I used to do this, I must have reviewed a hundred posts on several sites before I came across something simple enough for an amateur to implement.
     

    Attached Files:

    Last edited: Jan 15, 2019
  9. joeriz

    joeriz Active Member

    Messages:
    113
  10. ronton3

    ronton3 ronton3

    Messages:
    171
    Location:
    Macon Mo
    Joeriz: Thank you so very much for solving the mystery that I first posted about in the thread below. I now remember this, that picture did it, 20 years ago I did this mod and recently restored the one I did it on to regular Triode mode. I had a problem back then and put it on the shelf, I recently discovered that the problem was a bad 7199 that leaked and did not even move the needle on one of the plates. The one I have since converted to 6B4G still does not have the 1.5 meg resistor changed, but I did it on the first one, I will swap those around. Also on the first one I used 500 ohm resistors but the one I just did I used 800 ohm I may change that back, In this latest version I did not remove the tap from pin 4, I did the first time I am now wondering if I should do it again. Even as is it still sounds good and is more powerful than I anticipated. Thank you, Thank you, mystery solved.

    http://audiokarma.org/forums/index....-70-mod-id-please.848869/page-2#post-12356442
     

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