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Complications of using 2c22/7193's on ST-70 Driver Board?

RibbonAddict

RibbonAddict

New Member
Hey all, it's finally time to replace the 7199's on my ST-70, so I've been taking a look at just upgrading the entire driver board entirely. The 12AU7 and 6SN7 boards seem to be the common swaps, but I've been curious as to whether it would be more effective to simply create a board with two 2C22/7193's

http://www.tubebooks.org/tubedata/hb-3/miscellaneous_types/2C22.PDF

I've done repairs on the Dynaco, but I've not attempted any modifications. So my knowledge is very limited on this subject. But to me the 7193 looks very comparable, and is single triode tube just like the 7199's. So I'm just assuming this could be something of an easy job, but I could use the opinion of somebody more experienced in this type of work.

Apparently the 7193 is a very worthwhile tube commonly held in higher regard sound-wise than the 6SN7. So it seems like it would be more worthwhile to create a board capable of accepting these tubes than it would be to replicate or purchase one of the commonly used 12Au7/6SN7 boards. Anything major preventing this from being done?
 
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I'm planning to dip my feet into the ST-70 mod community while polishing some things up along the way. I've looked into adapting it to the 6U8/6GH8S for a cheap and easy 7199 alternative, but I'm looking towards improvement as much as I am getting it back up and running. I'm replacing/have replaced every tube and several components already.. Which is pointing me towards ordering the VTA70 board and throwing in my 12AU7's of choice and just ending up doing the full overhaul.

By no stretch do I believe that the amp is seriously lacking with the original equipment, but I very much so enjoy putting in the effort to improve or recondition something. Nothing like a good before and after!
 
Seems to be basically a miniature 6J5, which is half a 6SN7. For any sort of reasonable driver arrangement you'd need at least 4 tubes total and I don't know that you've got the room.

The 7199 is not a single triode, its a pentode and a triode in one bottle. The pentode is a voltage amp, the triode is used as the phase inverter.
 
Yeah whoops, that would certainly be a tight fit to say the least. Doing some reading on the driver board upgrades it seems the VTA Classic is the one people firmly stand behind. Which certainly looks like an easy enough install for a tube newbie like myself to handle.
 
My usual complaint about the alternate driver boards is they don't provide performance data for the amp. The stock stuff is well documented. Can't offer any sonic impressions of either, frankly I've never heard an ST-70 with any driver board in there, it just makes me very leery when there isn't any measured test data to go with something that is being sold as an upgrade.
 
I may be alone here but i would go for the Tubes for HiFi 6SN7 board , normally i would suggest a diytube ST70 board but the have been discontinued.I bought three of the 6SN7 boards and building them for new builds .

I just had a ST70 in here totally stock , and rebuilt it totally stock , brand new can cap , new bias caps , the owner did let me put better coupling caps in it . All the tubes are labelled Dynaco and tested Good on my Hickok 539C .

It sounded good imaged ok , BUT it was hard to listen to , the listening fatigue factor is terrible , i had trouble listening to it for more that 20 minutes at a time , a modified st70 and the two Magnavox 9304 i have here can be listened to here for hours .


Also at resale time it will be easier to sell with the Tubes for hi Boards

http://www.diytube.com/st70/diytube_st70_B.pdf

Mark
 
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RibbonAddict said:
Hey all, it's finally time to replace the 7199's on my ST-70, so I've been taking a look at just upgrading the entire driver board entirely. The 12AU7 and 6SN7 boards seem to be the common swaps, but I've been curious as to whether it would be more effective to simply create a board with two 2C22/7193's

http://www.tubebooks.org/tubedata/hb-3/miscellaneous_types/2C22.PDF

I've done repairs on the Dynaco, but I've not attempted any modifications. So my knowledge is very limited on this subject. But to me the 7193 looks very comparable, and is single triode tube just like the 7199's. So I'm just assuming this could be something of an easy job, but I could use the opinion of somebody more experienced in this type of work.

Apparently the 7193 is a very worthwhile tube commonly held in higher regard sound-wise than the 6SN7. So it seems like it would be more worthwhile to create a board capable of accepting these tubes than it would be to replicate or purchase one of the commonly used 12Au7/6SN7 boards. Anything major preventing this from being done?
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The 7199 is a triode / pentode tube in 1 envelope. the 7193 is just a single triode so you would need 2. You will also need to source the pentode part of the equation of which you would need 2 as well. You will also need to supply enough filament voltage for the 4 tubes which will be at least twice as much as the 2 7199 tubes. The Dynaco power trans is already somewhat undersized and adding 2 more octal tubes to the equation might be too much to ask for the pt. You really have your work cut out if you want to do this.
 
[QUOTE="primosounds, post: 10865184, member: 176210"well The Dynaco power trans is already somewhat undersized and adding 2 more octal tubes to the equation might be too much to ask for the pt. You really have your work cut out if you want to do this.[/QUOTE]

Actually the ST70 was designed to power two PAM 1 preamps besides its' self so it isn't being overloaded , it has the power to provide four more 12AX7's filaments and B+ .
 
battradio said:
I may be alone here but i would go for the Tubes for HiFi 6SN7 board , normally i would suggest a diytube ST70 board but the have been discontinued.I bought three of the 6SN7 boards and building them for new builds .

I just had a ST70 in here totally stock , and rebuilt it totally stock , brand new can cap , new bias caps , the owner did let me put better coupling caps in it . All the tubes are labelled Dynaco and tested Good on my Hickok 539C .

It sounded good imaged ok , BUT it was hard to listen to , the listening fatigue factor is terrible , i had trouble listening to it for more that 20 minutes at a time , a modified st70 and the two Magnavox 9304 i have here can be listened to here for hours .


Also at resale time it will be easier to sell with the Tubes for hi Boards

http://www.diytube.com/st70/diytube_st70_B.pdf

Mark
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I've looked at the 6SN7 modifications but it takes a significant amount of current compared to the 7199 and 12AU7 boards which really pushes the already undersized transformer to the limit. So really that would be the new board, new tubes, and a new trans to do it right.

I definitely don't have the same experience with the SQ stock. In my case I was running all Mullard EL34's with Philips 7199's through a Super PAS and it was the easiest listening amp I've ever heard.


primosounds said:
The 7199 is a triode / pentode tube in 1 envelope. the 7193 is just a single triode so you would need 2. You will also need to source the pentode part of the equation of which you would need 2 as well. You will also need to supply enough filament voltage for the 4 tubes which will be at least twice as much as the 2 7199 tubes. The Dynaco power trans is already somewhat undersized and adding 2 more octal tubes to the equation might be too much to ask for the pt. You really have your work cut out if you want to do this.
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This is my main concern, I've been looking at this for a couple days and I'm just going to run with the VTA70 12AU7 board. A transformer upgrade at some time would be worthwhile, but that time is not now as this is one of several projects I'm going to be throwing money at in these upcoming months. It might be manageable to run four of those on that driver board, but it would certainly be a tight fit if so.
 
battradio said:
Actually the ST70 was designed to power two PAM 1 preamps besides its' self so it isn't being overloaded , it has the power to provide four more 12AX7's filaments and B+ .
Click to expand...

If this is the case I'd be much more interested to try the 6SN7's, but it seems like everyone I've ever talked to seems to agree the transformer isn't really built for it stock so I'm quite wary.
 
I suspect the reason some people don't like the stock driver board is they don't like the sound of small signal pentodes.

Honestly if you like the sound of the stock setup, I'd vote for keeping what you have.
 
Dynaco would never put the two octal sockets on the front of the amp if it couldn't safely power the PAM preamp , they would have had so many warranty claims it would have bankrupted them
 
I'd keep what I have if it weren't simply for NOS 7199's costing as much as the 12AU7 mod in itself. At that rate I'd rather do the mod and have the ability to pop in a tube I actually have laying around in plentiful bunches. Figure if I'm already completely satisfied with the current stock sound there's a possibility I could be overjoyed with anything beyond it.
 
I believe they do make boards that are circuit-wise the same as the original but with the socket wired for a 6U8 or 6GH8. 100% agree about the 7199 availability problems. If I owned one I'd probably convert away from it for exactly the same reasons. The obvious question becomes what to go with. I'd personally be likely to keep it close to the stock design but thats just me. I could be persuaded otherwise with objective data, or having the chance to hear both variants back to back in person so I could make my own decision.

The problem (IMO) is that what might sound good to me may not sound good to you so its always hard to say what the best choice is for someone else. I mean I could tell you what to eat for lunch and what kind of car to drive but I suspect you may not agree with my selections there either.
 
I completely agree with what you're saying, the Dynaco was built for that 7199 tube. Though, on that same point it also means that the measurements people get with the 6U8 swaps aren't quite optimal either from what I've been seeing. So really the only way you're going to get the 7199 sound is.. with the 7199. Which is a problem with the declining numbers of that tube.

Which really brings me back to the primary logic I see in the mod, is finding a reliable way to go beyond substituting a compatible alternative and revamping it to something rethought entirely to sound far better than the original. The chances of satisfactory results are more likely I believe. Essentially if either way I'm going to end up eventually needing alternative tubes, I feel I should give it the best chance it's got with a mod that was made to beat the original rather than attempt to get somewhat close to it. Because really, I could dislike either one comparatively.
 
well, you can fiddle with the screen resistor to get the voltages right. Also given how variable those pentode tubes are in general its no guarantee that a given 7199 will have it all exactly right. The problem is the phase inverter side. Its direct coupled to the pentode, so any variables in the pentode can throw the phase inverter out of balance. Get that too far off and its going to sound funny.
 
There is a four tube board available, but I can't find much on it othere than the designer's claims. I have the VTA 12AU7 board with the larger power trany and like it. With the larger trany you have more options for signal tubes. The RCA clear top and Sylvania 12AU7's sounded darn good on their own. And you can buy an adaptor to run a 6SN7 in the center if you wish. I thought about the octal board, but decided to go with the 12AU7 board due to the ease trying different tubes without adaptors. As for the current draw, would three 6SN7's draw more than a couple 7199's AND the two PAM's? Most of what I've read says running anything more than three or four 12AU7 or 12AX7 types over heats the trany. Looking at temps posted by ST-70 owners the original power trany tends to run pretty warm in stock form to begin with.
 
Single 12au7 heater is 300ma in 6.3v mode
single 7199 heater is 450ma
single 6SN7 heater is 600ma
single 12AX7 in 12v mode is 150ma. The PAM1 used a pair wired this way, so 300ma of draw per PAM

basically if you go by that data, each channel can have 750ma of current draw for the heater in stock configuration and be within design spec

that allows for one and a quarter 6SN7 or two and a half 12A*7 per channel without exceeding original heater circuit loads.

Mind you thats just the load to light the tubes up, the plate load is an additional factor to consider.
 
6DZ7 said:
This is pure imagination. What does, "sound far better" mean? Take the numbers out of your mind's eye and post them here for us so we can understand just what you want to achieve.
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Ehm, I didn't say I'm looking for something with objectively superior output and measurements, nor did I imply as much. To say " something rethought entirely to sound far better than the original " is exactly what it sounds like, a design that the creator had invented with the purpose of creating something that sounds better. If I myself claimed it to sound better that's one thing, but in this context there was no such implication. I only observed that the action of using a modification created with the mindset of a better sounding driver board would be perhaps worth a shot before a substitution for the original.
 
Lavane said:
There is a four tube board available, but I can't find much on it othere than the designer's claims. I have the VTA 12AU7 board with the larger power trany and like it. With the larger trany you have more options for signal tubes. The RCA clear top and Sylvania 12AU7's sounded darn good on their own. And you can buy an adaptor to run a 6SN7 in the center if you wish. I thought about the octal board, but decided to go with the 12AU7 board due to the ease trying different tubes without adaptors. As for the current draw, would three 6SN7's draw more than a couple 7199's AND the two PAM's? Most of what I've read says running anything more than three or four 12AU7 or 12AX7 types over heats the trany. Looking at temps posted by ST-70 owners the original power trany tends to run pretty warm in stock form to begin with.
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That's the board it's looking I'm going to go with. I've only been hearing great things about it. Very excited to see how it changes the sound.
 
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