If it is a "dynaco" the bias should be 1.5V
+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- Posts : 1823
Join date : 2012-12-25
Location : gothenburg, sweden
cheehpogi- Posts : 4
Join date : 2015-03-05
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.peterh wrote:If it is a "dynaco" the bias should be 1.5V
rovano- Posts : 10
Join date : 2011-09-25
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:
Bottom view:
Sub frame:
Left channel:
Right channel:
Top view:
See my complete point to point wired rebuilt:
https://dynacotubeaudio.forumotion.com/t973-st70-rebuild-hard-wired
Rovano
Dogstar- Posts : 361
Join date : 2014-06-23
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.
I thought the tubes might be going bad but the system really did sound good in Triode mode.
tubonos- Posts : 2
Join date : 2020-03-03
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?
What mode would this be: pentode? Triode?
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
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.
Below > Original Dynaco ST-70 pictorial hard wired into pentode ultralinear mode.
tubonos- Posts : 2
Join date : 2020-03-03
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)
Pillo69- Posts : 176
Join date : 2012-04-11
Location : Granada (España)
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).
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).
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
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).
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
jrethorst- Posts : 39
Join date : 2009-02-01
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- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
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
jrethorst- Posts : 39
Join date : 2009-02-01
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?
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