The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


+9
peterh
cheehpogi
Bigron865
Roy Mottram
oatstao
SFORZANDO0
Bugs
jrethorst
Bob Latino
13 posters

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by peterh Wed Mar 11, 2015 2:39 am

    If it is a "dynaco" the bias should be 1.5V
    avatar
    cheehpogi


    Posts : 4
    Join date : 2015-03-05

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by cheehpogi Wed Mar 11, 2015 9:59 am

    peterh wrote:If it is a "dynaco" the bias should be 1.5V  
    Reading on Bob's previous post, he stated that stock dynaco tubes should run at around 1.25 since we are not using the original mallard tube.
    avatar
    rovano


    Posts : 10
    Join date : 2011-09-25

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by rovano Sat Mar 02, 2019 8:51 pm

    Bob Latino wrote: You can also install DPDT (dual pole dual throw) triode/pentode switches and make the move from triode to pentode easily switchable. The only problem is where to put the switches. Bob

    When you still use the bias pots and you don’t want to compromise the chassis, you may want to consider this possibility.
    I created a sub frame that I mounted on extended screws of the tube sockets so that the switch handle sticks through the ventilation slot. This is how I did it:

      Top view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode16

      Bottom view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode15

     Sub frame:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Triode14

      Left channel:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1040910

     Right channel:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1040911

     Top view:
    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 P1100710

    See my complete point to point wired rebuilt:

    https://dynacotubeaudio.forumotion.com/t973-st70-rebuild-hard-wired

    Rovano
    avatar
    Dogstar


    Posts : 361
    Join date : 2014-06-23

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Dogstar Tue Sep 03, 2019 3:37 am

    Last night I had my amp cranked listening to ZZ Top and I noticed what to me sounded like harshness when listening to higher pitched notes. So....I switched to Triode mode and the music was much smoother. I’m using Gold Lion KT88’s with maybe 500 hours on them, a Webber rectifier and a 6SN7 tube in the center socket on the board. Speakers are Klipsche with Bob Crites crossovers and custom cabinets.

    I thought the tubes might be going bad but the system really did sound good in Triode mode.

    avatar
    tubonos


    Posts : 2
    Join date : 2020-03-03

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by tubonos Tue Mar 03, 2020 3:40 pm

    I think that there's differences between this post, and the possibly newer versions of the instructions and the diagrams that come with the ST70. I'm comparing against these documents, and the diagram for the output transformers, and it seems that in the recent diagrams, the amp is shown with pins 3 and 4 having one wire each to the output transformers with no resistor.
    What mode would this be: pentode? Triode?
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Bob Latino Tue Mar 03, 2020 3:59 pm

    tubonos wrote:I think that there's differences between this post, and the possibly newer versions of the instructions and the diagrams that come with the ST70.  I'm comparing against these documents, and the diagram for the output transformers, and it seems that in the recent diagrams, the amp is shown with pins 3 and 4 having one wire each to the output transformers with no resistor.
    What mode would this be: pentode?  Triode?  

    In order to switch between pentode ultralinear mode and triode mode you need a pair of switches > one for each channel. If the amp has a single wire connected to pins 3 and 4 of each output tube, then the amp is hard wired into pentode/ultralinear mode. See below ..

    Bob

    Below > VTA ST-70 wired to switch between pentode ultralinear mode and triode mode.

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 ST-70-photograph-new-K42-Y

    Below > Original Dynaco ST-70 pictorial hard wired into pentode ultralinear mode.

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 ST-70-pictorial-with-stock-driver-board
    avatar
    tubonos


    Posts : 2
    Join date : 2020-03-03

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by tubonos Tue Mar 03, 2020 4:24 pm

    Thanks bob, you're awesome as always. This shows as a new account but you've been helping me now for 5+ years (not that I often need it) Smile
    Pillo69
    Pillo69


    Posts : 176
    Join date : 2012-04-11
    Location : Granada (España)

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Pillo69 Fri Mar 06, 2020 4:41 pm

    A few years ago, when the forum was not exclusive to VTA, there were schemes and ideas for some modifications on the original ST70.
    There was the option of placing the Bias potentiometers on the front, using the location of the chassis for Pentode / triode switches.
    (I hope the attached photograph is not illegal).

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Bias-frontal
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Bob Latino Fri Mar 06, 2020 5:14 pm

    Pillo69 wrote:A few years ago, when the forum was not exclusive to VTA, there were schemes and ideas for some modifications on the original ST70.
    There was the option of placing the Bias potentiometers on the front, using the location of the chassis for Pentode / triode switches.
    (I hope the attached photograph is not illegal).

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Bias-frontal

    The forum is not exclusive to VTA amps. The forum is for all types of Dynaco TYPE tube gear. Your photograph shows an option for bias from Dynakitparts that you may use on original Dynaco type amps or the amps from Dynakitparts.

    Bob
    avatar
    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by jrethorst Sun Jul 04, 2021 12:23 am

    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Bob Latino Sun Jul 04, 2021 8:50 am

    jrethorst wrote:
    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John

    People like to run EL34's in a Mark III because EL34's are a relatively inexpensive output tube. An EL34 is not make to look at 500 volts DC on the plates of the tubes. An EL34 is supposed to run about 435 - 450 volts on the plates. If you run the amp in triode mode, that does not change the amount of voltage on the plate of the tube (pin # 3). Running the amp in triode mode just removes the ultralinear connection and allows the output tube to run in a simpler, less efficient mode (with less power).

    Bob

    jrethorst likes this post

    avatar
    jrethorst


    Posts : 39
    Join date : 2009-02-01

    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by jrethorst Mon Jul 05, 2021 11:59 am

    Bob Latino wrote:
    jrethorst wrote:
    Bob Latino wrote:If you use EL34's in a Mark III they will probably have a short life. KT88 or 6550 tubes are best in a Mark III.

    I've seen advice elsewhere to bias a Mark III down to .9 if running EL34s. Would that still be good advice after the mod to triode mode?

    Thank you
    John

    People like to run EL34's in a Mark III because EL34's are a relatively inexpensive output tube. An EL34 is not make to look at 500 volts DC on the plates of the tubes. An EL34 is supposed to run about 435 - 450 volts on the plates. If you run the amp in triode mode, that does not change the amount of voltage on the plate of the tube (pin # 3). Running the amp in triode mode just removes the ultralinear connection and allows the output tube to run in a simpler, less efficient mode (with less power).

    Bob

    Point well taken, but I like the sound of the EL34 as compared to KT88 or 6550. Would setting a lower bias reduce the voltage presented to the tube?

    Sponsored content


    Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo - Page 2 Empty Re: Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo

    Post by Sponsored content


      Current date/time is Fri Mar 29, 2024 4:30 am