Replacing ST-70 driver board - a RETORATION or REBUILD ???

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Replacing ST-70 driver board - a RETORATION or REBUILD ???

Postby cheap-Jack » Mon Oct 31, 2011 10:50 am

Hi.

There've been so many ST-70 driver broads of different designs for sale in the marketplace since day one. Most most if not all, of those driver boards are of totally 'alien' design from its orginators: Halfer & Laurent in 1959.

Put aside the sonic aspects of the so called upgraded driver boards so readily available todate, what get into my nerve is: are those new boards designed to RESTORE the original ST-70 sound - to make its orginal sonic signature sound better or to REBUILD it to deliver some alien sound qualities?????? (???)



I say so because I own a pair of stock PAS-2 & ST-70 which I heavily UPgraded them. When I say upgrade, I mean RETORING them without rebuilding it with a driver board of alien design in order to reserve ST-70 sonic signature as much as possible.

In fact, there are tons of improvement can be made on ST-70 WITHOUT need of replacing its stock driver borad at all.

I've done it by still keeping its old driver board. It sounded so slow, muddy & noisy which was totolly unacceptable to my picky ears when I first acquired it (original 1959 stock version) - a donation from an audiophile friend of mine about 5 years back. I could have returned them back to the donor if I knew nothig electronics as I could not stand the aweful sound at all.

But now this ST-70 sings like a nightingale - fast, see-thru transparent, ultra detailed, musical & pretty quiet - still retaining its 50-year-young stock driver board. ;)

Very briefly, like many DIYers did, I rebuilt the entire 450VDC HV PS by replacing the old stock composite 'lytic cap with discrete AC caps (10uF 400VAC oil capx1+40uF250AC PP-film motor-run can capsx3 ), SS voltage regulator for the stock driver board, rewiring the entire old-school multi-point signal reference grounding system to one-point star format with RFI earthing device into a 3-wire power cord format. Replacing old coupling caps with PP film caps & 1% metal film resistors on the stock driver board.

The most crucial sonic upgrade is to add a special triode-strapped topology to the EL-34 O/P power tubes so that the power O/P stage can be operable in either UL or triode mode by flipping a toggle switch. I have found its triode-strapped mode sound so so so much better than its stock U-L format though the amp delivers much less power (from 35W to say 15W).
So I've the amp working on triode mode since day one of its upgrade years back as I love triode sound over all.

I reduced its stock 20dB global NFB to 10dB to help boosting a little bit on the lower power O/P.

Mind you, this is not a conventional 100R resistor P-g2 strapping as seen using in all triode strapping powerstage designs so far. It is a special 32V g2 level-shifting device keeping Vg2 potential 32V lower than its plate while operating triode mode. Probably the first time ever used in ST-70!

So do we need to replace a new driver board of some alien designs to get better sound out of a ST-70, my answer is NO if one knows enough the business. [:) (wine)

c-J
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Postby rmyauck » Mon Oct 31, 2011 1:07 pm

Hi Jack,

Maybe the 7199 is probably another big reason for all the new boards! What's a good sub for 7199?
I also read low bias settings ( I think 32-33 mA) helps too! Google Joe Rosen in Audio Asylum from years ago for his mods to the stock board.

Randy
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Postby cheap-Jack » Mon Oct 31, 2011 1:42 pm

Hi. Randy.
rmyauck wrote:Hi Jack,
Maybe the 7199 is probably another big reason for all the new boards! What's a good sub for 7199?I also read low bias settings ( I think 32-33 mA) helps too! Google Joe Rosen in Audio Asylum from years ago for his mods to the stock board.Randy

OK. Please tell me technically why 7199 need to be replaced?

I just can't stop wondering why so many vendors out there still offer ST-70 driver board upgrades with alien designs? Is it truly a technical necessity to make ST-70 sound better or simply take advantage of its 50-year uncontested reputation to do business? Be a smart consumer, my dear readers.

Please also outline what Joe did in his ST-70 driver board upgrade to save my time of searching.

Thanks.

c-J

PS: To be fair to the orginators of Dynaco electronics, I think whoever selling ST-70 upgrades should pay a loyalty fee to its orginal designers.
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Postby rmyauck » Mon Oct 31, 2011 2:20 pm

Jack,

Try and find them, and if you do for reasonable prices. Also finding good ones is a problem now. Sovtek did make them for a while, but they were not the same as the originals and so sounded poor.

Randy
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Postby burnedfingers » Mon Oct 31, 2011 3:41 pm

Well Jack I just cannot resist throwing cold water over your post. You are the first I have ever heard that actually thinks that a stock Dynaco Stereo 70 sounds good. Having had a number of them I have listened to about every design from the stock to modified stock with 1% parts and good coupling caps and a redesigned input to just about all the aftermarket boards. Totally stock the Dynaco driver board sounds awful. The bass is very sloppy and muddy and the design is a joke. When the power supply is beefed up with better and larger caps it starts to sound better. When 1% parts are used along with several small modifications one can almost listen to it without it being to tiring. When the driver board is replaced with say a board from "Geek" of a Mapletree board or several others on the market the amplifier starts to sing. Jack I buy them for the transformers and then modify the hell out of them and they sound good.

quote:

OK. Please tell me technically why 7199 need to be replaced?

I just can't stop wondering why so many vendors out there still offer ST-70 driver board upgrades with alien designs? Is it truly a technical necessity to make ST-70 sound better or simply take advantage of its 50-year uncontested reputation to do business? Be a smart consumer, my dear readers.

Please also outline what Joe did in his ST-70 driver board upgrade to save my time of searching.

The 7199 is hard to find and expensive.

The reason you find Alien designs is the 70's driver board is antique and marginal at best.

Quote:

To be fair to the orginators of Dynaco electronics, I think whoever selling ST-70 upgrades should pay a loyalty fee to its orginal designers


Pay a loyalty fee? Jack you can make whatever modifications you want and nothing is due the the original designer.

You ever done distortion tests between the stock dynaco driver board equipped amplifier and a dynaco with an aftermarket board? I didn't think so. If you had you would see the distortion figures are lower than with the stock board.

Oh, triode is fine if you like it. Some do and some don't to each his own. I've had several dynaco's with a switch so I could change on the fly. I'm not really impressed with triode.
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Postby joeriz » Mon Nov 07, 2011 12:31 pm

burnedfingers wrote:You are the first I have ever heard that actually thinks that a stock Dynaco Stereo 70 sounds good.


Actually, there are quite a few folks around that think the stock ST-70 sounds good...some of them right here on this forum, including our esteemed host if I'm not mistaken. Just sayin'...

Joe
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Postby 20to20 » Mon Nov 07, 2011 7:10 pm

joeriz wrote:
burnedfingers wrote:You are the first I have ever heard that actually thinks that a stock Dynaco Stereo 70 sounds good.


Actually, there are quite a few folks around that think the stock ST-70 sounds good...some of them right here on this forum, including our esteemed host if I'm not mistaken. Just sayin'...

Joe



Ditto... (Raises hand)


Jack I buy them for the transformers and then modify the hell out of them and they sound good.


In other words, a different amp, with Dynaco trannies.

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Postby burnedfingers » Tue Nov 08, 2011 5:49 am

quote;
In other words, a different amp, with Dynaco trannies.


Correct

I did have one Dynaco stereo 70 that sounded very good. It had a modified circuit and contained 1% hand picked parts with premium caps.
The problem for me still lies with the ability to purchase reasonably priced 7199's.

I'm up for moving on moving to a newer circuit design that will provide better performance and lower distortion. There are a number of driver boards on the market now. Geek as I mentioned before has probably one of the very best boards available in my opinion. A close second again my opinion is the Mapletree driver board of which I bought a board from Lloyd and also made a clone of the basic design with slightly different operating voltages.


The Dynaco trannies are very good for the money and can usually be purchased for $150 a pair or less. The chassis are usually rusted to high heaven and generally end up being sand blasted and painted or replaced with a new SS chassis or other custom chassis.

Cannot agree with those that might be considered to be a "Triode Junkie" amps do sound different with aftermarket driver boards. Speakers also make a difference as do the other components in a system. To some triode mode sounds good and to others little or no difference can be heard. I have read pro and con on triode operation and do believe that the mode of operation is up to the user. Listen to both modes and see which suits your fancy but please do not come out with a statement that triode is the only way and or the best mode.

I currently have a driver board of my own design that uses 2) 6SL7's per channel one as input and the other in a long tail config. This amp seems to sound the same in triode as it does stock.

We each have our own opinion based on what we feel sounds good to us. I will apologize if I have offended anyone by my comments. If you don't like my comments then contribute your comments and ideas and it could very possibly be a learning experience.
Last edited by burnedfingers on Tue Nov 08, 2011 6:00 am, edited 1 time in total.
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Postby burnedfingers » Tue Nov 08, 2011 5:49 am

Cheap-Jack

Please accept my apology if my comments offended you. Didn't mean to chase you or anyone away. I would look forward to hearing from you via email if you so desire. I would especially like to hear more of your comments and ideas here on this forum.
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Postby 20to20 » Tue Nov 08, 2011 10:03 am

burnedfingers wrote:quote;
In other words, a different amp, with Dynaco trannies.

Correct

I did have one Dynaco stereo 70 that sounded very good. It had a modified circuit and contained 1% hand picked parts with premium caps.

The problem for me still lies with the ability to purchase reasonably priced 7199's.

The Dynaco trannies are very good for the money and can usually be purchased for $150 a pair or less.

If you don't like my comments then contribute your comments and ideas and it could very possibly be a learning experience.


Fingers,

Here are some ideas and we'll use your starting points

1.) So the 7199's are fine if they didn't cost so much, ...

2.) You didn't mention the 6CA7's so we'll assume you don't have a problem with those.

3.) You like the trannies, too...

OK... Let's analyze the amp one step at a time with a perfect signal from the matching input Z and we don't overdrive the amp into clipping the bass. Is that a good starting point for you?

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Postby EWBrown » Tue Nov 08, 2011 10:53 am

PS: To be fair to the orginators of Dynaco electronics, I think whoever selling ST-70 upgrades should pay a loyalty fee to its orginal designers.


I'm pretty sure that the original Dynaco designers (Hafler, Keroes, etc) are no longer on this earthly plane O:) O:) O:) If they are still among us, they'd have to be REALLY old.

I'd be pretty sure that they never imagined, in their wildest dreams, that Dynaco and Dynakit tube amps would still be popular in the 21st century.
Even they scrapped the tube desitns for those awful sounding solid state beasts they designed and produced in the 1970s and beyond.

Dynaco (anad before that, Acrosound) published a wide variety of schematics and designs in the back pages of their transformer catalogs, including most of the Dynaco amp and preamp designs.

Once teh 7199 or the 7247 driver boards are replaced, they really are no longer a true "Dynaco" amp, but are upgrades, or even totally new designs, with newer, better circuitry and passive components. .

/ed B
Last edited by EWBrown on Tue Nov 08, 2011 7:02 pm, edited 1 time in total.
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Postby burnedfingers » Tue Nov 08, 2011 6:17 pm

quote:
Once teh 7199 or the 7247 driver boards aer replaced, they really are no longer a true "Dunaco" amp, but are upgrades, or ven totally new designs, with newer, better circuitry and passive components. .

Yes, they are no longer "True Dynaco" amplifiers because the driver board has been changed. Also, can you consider ANY Dynaco that isn't a 100% per original design a true dynaco? I have listened to and owned a few with a modified power supply. Now these aren't True Dynaco's either but rather an upgrade from the original. One has to admit that one of the weak points of the original Dynaco is the power supply. Not only have I come to that conclusion but have also read it on the internet.

Yes, I like the 6CA7's as well as other output tubes in the Dynaco. I am rather fond of the 6BG6GA's. The more I listen to the 6BG6GA's the more I like them. The price on them is cheap even when you purchase the parts to make the adapter. You have to admit that the price of some of the better 6CA7's has gone thru the roof.

Part of this hobby or obsession as I will call it is to come up with something that can bring enjoyment into our lives. This however differs with different people. This is why I have a supercharged Camaro for a toy while others may like the economy of a Fiat or VW.
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Postby TomMcNally » Tue Nov 08, 2011 6:39 pm

EWBrown wrote:I'm pretty sure that the original Dynaco designers (Hafler, Keroes, etc) are no longer on this earthly plane O:) O:) O:) If they are still among us, they'd have to be REALLY old.
/ed B


David Hafler would be about 93 if he were alive, he passed away in
2003. He was part owner of the radio station our company owns
in Trenton, NJ. Needless to say, they had a lot of Hafler amps.
I never had the pleasure of meeting him though. I worked there
before his time, and also after.
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Postby battradio » Tue Nov 08, 2011 7:07 pm

TomMcNally wrote:
EWBrown wrote:I'm pretty sure that the original Dynaco designers (Hafler, Keroes, etc) are no longer on this earthly plane O:) O:) O:) If they are still among us, they'd have to be REALLY old.
/ed B


David Hafler would be about 93 if he were alive, he passed away in
2003. He was part owner of the radio station our company owns
in Trenton, NJ. Needless to say, they had a lot of Hafler amps.
I never had the pleasure of meeting him though. I worked there
before his time, and also after.


I did get to met David Hafler and Saul Marantz at the Vegas CES in 79 , the Hafler 200 had just came out , Two very nice gentlemen .Nelson Pass was just down the hall never heard of him at that time wearing his jeans and flannel shirt .

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Postby EWBrown » Tue Nov 08, 2011 7:12 pm

The ST70 can work very well with 6BG6As, 6L6GCs, 807s, and even 1625s, if the filament windings are connected in series for 12.6VAC fore them, and the driver tubes are also re-eired for 12V operation. It can even be used, in a low-power version, with 6V6GTs, if a more modest voltage power trannie, and different OPTs are used.

It's a pretty "universal" design, and is perfect for hot-rodding, upgrades, mods, etc.

My strangest "ST70" build is a 300B SET, and the only true "ST70" parts are the chassis and the choke, I used a same-sized power trannie, which I pulled off an old Magnavox console, with slightly lower (700VAC CT) HV secondary. The OPTs are the Transcendar 3K, 15W SE variety, and they have the same mounting footprint as the ST70 OPTs, so no chassis mangling was required. The Chassis was from Soundtastic, sold as "blemished" for a discount, but the only blemishing was some slightly discolored areas under the chassis, and those aren't visible during normal operation.

/ed B
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