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

Post n°1

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

by Bob Latino on Sat Jan
17, 2009 5:17 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

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Post n°2

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

by jrethorst
on Sun Feb 01, 2009 2:17 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

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Post n°3

Re:
Triode/pentode on a Mark III

by Bob Latino on Sun Feb
01, 2009 3:21 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

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Post n°4

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

by jrethorst
on Sun Feb 01, 2009 10:10 pm

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
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Post n°5

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

by Bob Latino on Mon Feb
02, 2009 2:49 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

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Post n°6

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

by Bugs
on Fri Apr 17, 2009 11:13 am

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

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Post n°7

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

by Bob Latino on Fri Apr
17, 2009 12:16 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
12:59 pm; edited 2 times in total

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Post n°8

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

by Bugs
on Fri Apr 17, 2009 12:45 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
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Post n°9

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

by Bob Latino on Fri Apr
17, 2009 1:44 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

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Post n°10

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

by Bugs
on Fri Apr 17, 2009 9:15 pm

Thanks Bob.

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Post n°11

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

by SFORZANDO0
on Sun Jul 05, 2009 4:07 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

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Post n°12

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

by Bob Latino on Sun Jul
05, 2009 4:16 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
2:10 am; edited 1 time in total

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Post n°13

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

by SFORZANDO0
on Sun Jul 05, 2009 4:24 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

PerryC

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Post n°14

Triode
Convert

by PerryC on Sun Aug 15, 2010
2:07 am

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

Guest

Guest

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Post n°15

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

by Guest on Wed Sep 15, 2010
4:08 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!

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