Ask far as the sound of anything who can say what's right. A friend of mine bought a bunch of Mark Levinson gear (ML-2s ML-7, plus a pair of Quad 63s) from a guy named Peter McGrath. Michael said that McGrath always said that with any gear, it was a case of choose the distortion you like best. It's the old: Ford or Chevy, Martin or Gibson, etc. I honestly feel that the pursuit of audio perfection can be a very long road with no end. I like to sit and listen every once in a while, so I don't what to let tweaking become an end in itself. I can't remember exactly who, but I think it was Picasso that said that, paintings are never finished, they are merely abandoned.
+9
peterh
cheehpogi
Bigron865
tubes4hifi
oatstao
SFORZANDO0
Bugs
jrethorst
Bob Latino
13 posters
Run your Dynaco ST-70, Mark II, Mark III or Mark IV in TRIODE MODE - photo
Guest- Guest
Ideally, if you had an audio version of heavy duty jumpers, similar to non-fused HVAC service disconnects, you could do it that way. It still would not be advisable to change modes it with the amps on, but you would not have to worry about voltage passing through the thing.
Ask far as the sound of anything who can say what's right. A friend of mine bought a bunch of Mark Levinson gear (ML-2s ML-7, plus a pair of Quad 63s) from a guy named Peter McGrath. Michael said that McGrath always said that with any gear, it was a case of choose the distortion you like best. It's the old: Ford or Chevy, Martin or Gibson, etc. I honestly feel that the pursuit of audio perfection can be a very long road with no end. I like to sit and listen every once in a while, so I don't what to let tweaking become an end in itself. I can't remember exactly who, but I think it was Picasso that said that, paintings are never finished, they are merely abandoned.
Ask far as the sound of anything who can say what's right. A friend of mine bought a bunch of Mark Levinson gear (ML-2s ML-7, plus a pair of Quad 63s) from a guy named Peter McGrath. Michael said that McGrath always said that with any gear, it was a case of choose the distortion you like best. It's the old: Ford or Chevy, Martin or Gibson, etc. I honestly feel that the pursuit of audio perfection can be a very long road with no end. I like to sit and listen every once in a while, so I don't what to let tweaking become an end in itself. I can't remember exactly who, but I think it was Picasso that said that, paintings are never finished, they are merely abandoned.
Bigron865- Posts : 45
Join date : 2013-03-15
Age : 55
Location : Birmingham Alabama
Bob I know this post is kind of old but its new to me. Does all of this still pertain to the newest low gain or classic vta mods that I just did? I'd love a sweeter sound. I have the PIO caps and an upgraded quad cap on order. Should I leave it in pentode till I break those in and then try the triode mode?
Ron
Ron
Bob Latino- Admin
- Posts : 3226
Join date : 2008-11-26
Location : Massachusetts
Hi Ron,
Yes - The triode mode also applys to the lower gain CCS VTA driver board. In triode mode you only have about 60% of the power of ultralinear but the amp but the amp sounds a little "smoother" with more mid range projection. This is nice for vocals and acoustic jazz.
I don't think it makes any difference which mode of operation you use to break the amp in ...
Bob
Yes - The triode mode also applys to the lower gain CCS VTA driver board. In triode mode you only have about 60% of the power of ultralinear but the amp but the amp sounds a little "smoother" with more mid range projection. This is nice for vocals and acoustic jazz.
I don't think it makes any difference which mode of operation you use to break the amp in ...
Bob
Last edited by Bob Latino on Sat Mar 02, 2019 9:16 pm; edited 1 time in total
Bigron865- Posts : 45
Join date : 2013-03-15
Age : 55
Location : Birmingham Alabama
Bob I bought 2 switches today from a local electronics supplier. They look like the blue ones in your pictures but these are red. The rating was 10a 250v ac. Will these work for triode/pentode switches?
cheehpogi- Posts : 4
Join date : 2015-03-05
Hello from KY, my first post here and I would need some help. I did the resistor mod on a newly acquired st 70,well before the mod for me I think the amp sounded great but after the mod and adjusting the bias it seem flat so I revert it back to the original pentode mode and bias was still at 1.30 but to me it still sounded flat, is there anything that I did wrong? maybe I biased it wrong. What I did was the black probe on ground and the red probe I stuck it to where it would read 1.5++ and then adjusted the bias screw until the numbers went down to 1.30. Did I miss or messed up something? Newb here need some guidance. Thanks.
peterh- Posts : 1797
Join date : 2012-12-25
Location : gothenburg, sweden
If it is a "dynaco" the bias should be 1.5V
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 : 3226
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 : 3226
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 : 3226
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?
Sponsored content
|
|