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

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

    Post by Bob Latino on Sat Jan 17, 2009 9:17 am

    Any Dynaco ST-70, Mark II, Mark III or Mark IV may be run in TRIODE mode as well as the normal pentode ultralinear mode of operation. You can hard wire these amps in the triode mode by two simple modifications.

    1. Disconnect the GREEN or GREEN/WHITE wire from pin 4 of each output tube and insulate the bare end so it doesn't short out against the chassis.
    2. Solder in a 100 ohm 1 watt resistor between pins 3 and 4 of each output tube.

    You are now in triode mode. The amp will now have only about 60% of the power it had before but you may like the subtle change in sound. The amp will have a little more mid range projection. Singers will move slightly forward in the sound stage. The extreme top and bottom end of the frequency spectrum will be rolled off very slightly from what it was in pentode mode but you probably won't notice any change in frequency response. The amp will sound a little "smoother".

    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. If you replace your stock driver board with the VTA driver board the choice is easy. You don't need the stock bias potentiometer with the VTA so just pull it out and you now have a nice 3/8 inch hole which will be suitable for installing your DPDT switch. If you have a stock driver board then you may be able to find a place for the switch(es) inside the chassis. You could also drill a couple of extra holes in the chassis somewhere. Get a high quality switch - one that can deal with 400 + volts DC. The diagram below shows a switch installed for one channel of an ST-70 which would also be applicable for the mono Mark II, III and IV.

    Bob


    jrethorst

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

    Post by jrethorst on Sun Feb 01, 2009 6:17 pm

    For the Mk III in triode, what differences are there between the KT88 and 6550 and EL34 output tubes (the EL34 requires another small circuit modification)? I'm using the Triode Electronics upgrade board, if that matters.

    BTW nice forum! Just found it.

    Thanks, John

    Bob Latino
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    Re: Triode/pentode on a Mark III

    Post by Bob Latino on Sun Feb 01, 2009 7:21 pm

    John,

    When you go from pentode to triode the sonic changes are pretty much the same in all Dynaco amps whether you are using a KT88, 6550 or EL34 tubes. Your power drops to about 60% of what it was but the amp seems to sound a little "smoother". Which mode you like best depends somewhat on your musical tastes and your associated equipment. Triode seems to sound better with small group acoustic jazz combos and vocalists. If you are a rock fan you may favor the pentode mode ? You will get a little more bottom end smack with pentode.

    As for the three tubes that you mention the "tubiest" sounding is the EL34 with a little more mid bass output than the KT88. An EL34 has a little less extreme bottom and extreme top end response than a KT88. The 6550 is somewhere in between the EL34 and the KT88. In a Mark III I really wouldn't use an EL34 because it wasn't designed for 475 - 500 volts DC on pins 3 and 4. 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. A relatively inexpensive and good sounding KT88 is the Sovtek KT88. About $100 for a matched quad. These have proven to be durable and have that interesting "coke bottle" shape. The Sovtek's also have a tri-alloy plate structure which is said to give a higher minimum plate current for better transconductance.

    Yep - we are all friendly here. No one has any axe to grind ...

    Bob

    jrethorst

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

    Post by jrethorst on Mon Feb 02, 2009 2:10 am

    Bob Latino wrote:Triode seems to sound better with small group acoustic jazz combos
    Just what I'm looking for, along with classical, which would also probably benefit from the increased smoothness, and not miss the enhanced high and low ends that rock listeners might prefer.

    I have not installed the Triode Electronics boards yet, but that's my next project, after which I'll look for tubes. Any thoughts on KT88 vs. 6550? Thanks!

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

    Post by Bob Latino on Mon Feb 02, 2009 6:49 am

    jrethorst wrote:
    Bob Latino wrote:Triode seems to sound better with small group acoustic jazz combos
    Just what I'm looking for, along with classical, which would also probably benefit from the increased smoothness, and not miss the enhanced high and low ends that rock listeners might prefer.

    I have not installed the Triode Electronics boards yet, but that's my next project, after which I'll look for tubes. Any thoughts on KT88 vs. 6550? Thanks!

    John,

    KT88's can take higher plate voltage and dissipate more wattage, so they have the potential with the right power supply to give a few more watts than a 6550. Some say a KT88 is more of a "hi-fi" tube and a 6550 is better in guitar amps but I don't find that to be necessarily true. KT88's tend to be a little more expensive than 6550 tubes. The best production KT88 out there now is the Genalex Gold Lion KT88 reissue but it is expensive > $45 - $50 a tube. A great 6550 tube is the Svetlana Flying =C=. I bought a pair of Mark III's from someone and they came along with the two amps. These are very nice sounding tubes also.

    Bob

    Bugs

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

    Post by Bugs on Fri Apr 17, 2009 2:13 pm

    Bob, how does this work with the VTA board if you want to switch between triode and
    pentode? I believe the VTA is in triode to start with.

    Thanks,
    Bugs

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

    Post by Bob Latino on Fri Apr 17, 2009 3:16 pm

    Hi Bugs,

    The DRIVER CIRCUIT on the VTA driver board is all triode circuit but that doesn't make the amp run in triode. To run the ST-70, Mark III etc. in triode you must disconnect the ultralinear screen tap at pin #4 ON THE OUTPUT TUBES and then connect a 100 ohm 1 watt resistor between pins 3 and 4. You can do this easily with a switch on any ST-70, Mark III etc. with a VTA driver board because the VTA driver board has its own bias system doesn't use the original bias system. You can then just pull the original bias potentiometer and now you have a nice 3/8 inch hole where you can install a DPDT switch. On a mono amp like the Mark II, Mark III or Mark IV you need just one DPDT switch. On an ST-70 you need two DPDT switches. The diagram for the switches is above for the left channel of an ST-70. You would just do the same thing for the right channel on an ST-70.

    Bob


    Last edited by Bob Latino on Thu May 28, 2009 3:59 pm; edited 2 times in total

    Bugs

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

    Post by Bugs on Fri Apr 17, 2009 3:45 pm

    Thanks Bob.

    I'm probably showing how new I am to all of this, but what is the difference between a triode circuit and running in triode mode?

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

    Post by Bob Latino on Fri Apr 17, 2009 4:44 pm

    Bugs,

    An all triode driver circuit is a circuit that uses only triode tubes. Triode tubes have THREE parts > an anode, a cathode and one screen. Triode driver circuits are simpler and if properly designed are usually superior to driver circuits that use pentode tubes. Pentode tubes have five parts - an anode, a cathode and three screens, are more complex and are difficult to use in a driver circuit because they are not as linear as triode tubes at all drive levels. Dynaco used pentode/triode driver circuits because it let them get by with only ONE driver tube (a 6AN8 or a 7199 tube) in each channel. This allowed Dynaco to save ONE tube when you really need "a tube and a half" per channel if you use an all triode design. In most upgraded Dynaco ST-70 driver boards like the VTA driver board you have THREE dual triode tubes. The center tube is the initial voltage amplifier for BOTH channels while the two side tubes do the phase splitting/phase inverting for each channel.

    To run the amp in "triode mode" you take a tube which is a pentode (5 section tube) like an EL34 or KT88 and you tie one of the screens to anode with a resistor making it a "quasi triode" - in other words you make the pentode output tube act like a triode output tube. The only downside is that power output is now only about 60% of what it was as a pentode BUT the amp will sound a little different. In triode mode a pentode tube will sound a little "smoother" and the midrange will be projected forward a bit. Depending on the type of music you listen to and your system components you may or may not like the sound. I like the triode mode for acoustic jazz and vocals. If you are into classical music or are a rock fan you will probably like the pentode mode since it has more power and a little bit more output at the extreme top and extreme bottom of the frequency range.

    Bob

    Bugs

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

    Post by Bugs on Sat Apr 18, 2009 12:15 am

    Thanks Bob.

    SFORZANDO0

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

    Post by SFORZANDO0 on Sun Jul 05, 2009 7:07 am

    Hello Bob,

    I have two sleeping dynaco st70 II (1992 version)different of the original one.

    Do you thing it is possible to modify this version in triode mode without many problems?

    Nice site!

    Regards from Belgium,

    Marc

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

    Post by Bob Latino on Sun Jul 05, 2009 7:16 am

    Hi Marc,

    Although the driver circuit on the '92 version of the ST-70 is different from the older original ST-70, the output tube pin outs are the same. Pin 3 is the plate lead and pin 4 is the ultralinear screen tap. Alteration of this ST-70 to be set up for the amp to work in triode (with or without a switch) will work exactly the same way.

    Bob


    Last edited by Bob Latino on Tue Jul 07, 2009 5:10 am; edited 1 time in total

    SFORZANDO0

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

    Post by SFORZANDO0 on Sun Jul 05, 2009 7:24 am

    Hi Bob,

    Good to know, thank's for this very quick response!
    I 'll go this way and let you know,

    Regards,

    Marc

    PerryC
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    Triode Convert

    Post by PerryC on Sun Aug 15, 2010 5:07 am

    I love the way it sounds. That's what you call SWEET!!!

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

    Post by Guest on Wed Sep 15, 2010 7:08 pm

    Would there be a problem if you use a 110 ohm resistor, rather than the 100? I have some 1 watt 110 ohm on hand that are 1% tolerance. I am not sure if you need a 1% in this application. Thanks!

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

    Post by Bob Latino on Wed Sep 15, 2010 7:18 pm

    Michael Noroña wrote:Would there be a problem if you use a 110 ohm resistor, rather than the 100? I have some 1 watt 110 ohm on hand that are 1% tolerance. I am not sure if you need a 1% in this application. Thanks!

    Hi,

    No - I am sure that a 110 ohm resistor would work also. I have used a 50 ohm resistor in there and the amp sounds the same. The 110 ohm resistor will work just fine ..

    Bob

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

    Post by Guest on Wed Sep 15, 2010 7:38 pm

    Bob: Thanks for the prompt reply with the info. I am thankful that you have this explanation posted here. The following are directions I received from a major supplier of Dynaco parts, on how to covert to triode. They just didn't make sense so I checked the web and found your instructions.

    Find the green (not green/white) wire (one from the output transformer) hooked to pin 4 on an EL34, there is also a wire leading from that pin to pad 4 on the circuit board.
    Disconnect the green wire from pin 4 and run directly to pad 4 of the circuit board.
    Remove the wire that previously ran from pad 4 to the EL34/6550 sockets.
    Disconnect the screen tap lead (green/white) from pin 4 of the other output tube, then insulate the end of the wire so that the bare leads are not exposed.
    Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of each of the output tube sockets.
    Reinstall the output tubes, and recheck bias (BIASET on front panel). This will result in a 10 to 20 watt per channel.


    Guest
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    Should you increase the size of the bias supply caps?

    Post by Guest on Wed Sep 15, 2010 7:48 pm

    Is it a good idea to increase the size of the 50uF bias supply caps ?

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

    Post by Bob Latino on Wed Sep 15, 2010 7:54 pm

    Hi Michael,

    Yes - those instructions that he gave you for converting your amp ARE confusing.

    One other thing that I didn't mention at the beginning explanation is that the bias will drop slightly when you convert to triode mode. If you were at the standard 1.56 VDC when you convert to triode the bias will drop very slightly (and insignificantly) to maybe 1.53 VDC. If it bothers you then just turn the bias back up again.

    A 1.56 VDC bias on a stock ST-70 gives a bias current of 50 milliamps per each output tube. This is a little high by todays standards. Dynaco really pushed those EL34's hard but those original Mullard EL34's were a lot tougher than today's EL34 tubes. Conventional wisdom is to run a stock ST-70 at about 1.25 VDC which will give about 40 milliamps per each output tube. The amp will sound the same and your output tubes will last longer.

    Yes - you can increase the size of the 50 uF bias supply caps to 100 uF with no problem. All you need is a 100 uF @ 100 volt cap because those caps only have to deal with about -70 VDC. There is no need to increase those two caps beyond 100 uF and no real need to go above a 100 volt cap.

    Bob

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

    Post by Guest on Wed Sep 15, 2010 11:17 pm

    Bob: It turns out that the 100 Ohm resistors I have are only 1/2 watt. Will they work, or should I get some 1 watt?

    Finally I was reading H. L. Eisenson's Tu-be, or not tu-be, and he proposes some pretty serious mods for the ST-70, which I assume could be applied to the MK IV's. Do you know of any one who has done the mods he proposes, and whether or not they felt the changes were good? Thanks again for your help!

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

    Post by Bob Latino on Thu Sep 16, 2010 7:27 am

    Michael Noroña wrote:Bob: It turns out that the 100 Ohm resistors I have are only 1/2 watt. Will they work, or should I get some 1 watt?

    Finally I was reading H. L. Eisenson's Tu-be, or not tu-be, and he proposes some pretty serious mods for the ST-70, which I assume could be applied to the MK IV's. Do you know of any one who has done the mods he proposes, and whether or not they felt the changes were good? Thanks again for your help!

    Michael,

    A 1/2 watt resistor should work fine in there. You probably don't really need a 1 watt resistor.

    If you want to upgrade your Mark IV's then get in touch with Roy Mottram on the tubes4hifi web site > www.tubes4hifi.com He offers an upgrade board for the Mark IV that would give you a significant increase in sound quality from your Mark IV's.

    Bob

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

    Post by Guest on Thu Sep 16, 2010 11:37 am

    Bob: I apologize because I feel as if I am beating a dead horse at this point. I once again e-mailed the other guy that had given me the instructions on how to set up the MK IV's for triode, and this is what I received. I have included the most recent, and the original, in that order.


    For Runningyour MkIV in Triode Mode:

    1. Remove the Green Wire from the OT that goes to Pin 4 on one of the powertubes. (Do not disconnect the Green/White wire going to pin 4 on the otherpower tube.)

    2. Remove the wire that went from pin 4 on that socket to pad #7 on driverboard.

    3. Run the disconnected Green wire directly to pad #7 on the driver board.

    4. Install a 100 ohm 1/2W or better resistor between pins 3 & 4 on eachpower tube socket.

    You may try both ours and Bob Latinos method to see which yields the best soundto your ears.



    Find the green (not green/white) wire (one from the outputtransformer) hooked to pin 4 on an EL34, there is also a wire leading from thatpin to pad 4 on the circuit board.
    Disconnect the green wire from pin 4 and run directly to pad 4 of the circuitboard.
    Remove the wire that previously ran from pad 4 to the EL34/6550 sockets.
    Disconnect the screen tap lead (green/white) from pin 4 of the other outputtube, then insulate the end of the wire so that the bare leads are not exposed.
    Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of eachof the output tube sockets.
    Reinstall the output tubes, and recheck bias (BIASET on front panel). This willresult in a 10 to 20 watt per channel.
    I can't believe that they are both different, and that there is no mention, or correction of the first mistake on his part. The lastest version makes more sense in that there is a wire that goes from pin 4 to board # 7.

    Michael

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

    Post by Luddite on Thu Sep 16, 2010 3:30 pm

    Michael Noroña wrote:Bob: I apologize because I feel as if I am beating a dead horse at this point. I once again e-mailed the other guy that had given me the instructions on how to set up the MK IV's for triode, and this is what I received. I have included the most recent, and the original, in that order.


    For Runningyour MkIV in Triode Mode:

    1. Remove the Green Wire from the OT that goes to Pin 4 on one of the powertubes. (Do not disconnect the Green/White wire going to pin 4 on the otherpower tube.)

    2. Remove the wire that went from pin 4 on that socket to pad #7 on driverboard.

    3. Run the disconnected Green wire directly to pad #7 on the driver board.

    4. Install a 100 ohm 1/2W or better resistor between pins 3 & 4 on eachpower tube socket.

    You may try both ours and Bob Latinos method to see which yields the best soundto your ears.



    Find the green (not green/white) wire (one from the outputtransformer) hooked to pin 4 on an EL34, there is also a wire leading from thatpin to pad 4 on the circuit board.
    Disconnect the green wire from pin 4 and run directly to pad 4 of the circuitboard.
    Remove the wire that previously ran from pad 4 to the EL34/6550 sockets.
    Disconnect the screen tap lead (green/white) from pin 4 of the other outputtube, then insulate the end of the wire so that the bare leads are not exposed.
    Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of eachof the output tube sockets.
    Reinstall the output tubes, and recheck bias (BIASET on front panel). This willresult in a 10 to 20 watt per channel.
    I can't believe that they are both different, and that there is no mention, or correction of the first mistake on his part. The lastest version makes more sense in that there is a wire that goes from pin 4 to board # 7.

    Michael

    Michael,

    I will not comment on the validity of either method proposed, but as far as comparing the two sets of instructions, it appears that you may have confused apples with oranges. IMHO (after referring to both schematics) it seems apparent that the gentleman you consulted first gave you instructions relating to a MKIII, wheras the second set of instructions relates to a MKIV. That should account for the differences.

    Best Regards,
    Charlie

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

    Post by Guest on Thu Sep 16, 2010 11:34 pm

    Charlie: Thanks for the clarification!

    Michael

    Luddite

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

    Post by Luddite on Fri Sep 17, 2010 12:30 am

    Michael Noroña wrote:Charlie: Thanks for the clarification!

    Michael

    You're most welcome. What prompted me to look further was that he had mentioned circuit board pad #4 in the first instruction and then circuit board pad #7 in the second. I checked the MKIII, MKIV, and ST70 schematics and determined that he must have been referring to the MKIII and MKIV in that order.

    Best Regards,
    Charlie

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