Dynakit ST-70 pcb

[rembrandtste]

My newly acquired St-70 is still sporting the original 7199 pcb and I've found a chinese replacement with all the components less the 7199 tubes. 6GH8's are the way to go I'm reading. Anyone have any experience with this? rembrandtsteve

 

[peterh]

My newly acquired St-70 is still sporting the original 7199 pcb and I've found a chinese replacement with all the components less the 7199 tubes. 6GH8's are the way to go I'm reading. Anyone have any experience with this? rembrandtsteve

If your ST-70 works fine then keep the board.
When one of the 7199 times out ( usually increasing hum) then think of action. You
then have a choice of :
- 6GH8
- 6U8
- 6BL8
- ECF82 all with an adapter that makes the original board suitable for these tube
OR
you can cut 3 traces and solder some wires on the original board ( skipping the adapters )
OR
get a new board that is already ready for 6U8
OR
get another replacement board that does not mimic the original board, tubes4hifi is one source

But for the moment , if the amp sounds fine, buy music for the money !

 

[rembrandtste]

Afraid I've already ordered the chinese board. zhangsheng is doing a lot of business and I think I'll take a chance with him, it was only $42.00. The amp had/has a crackle in the left channel, popular opinion was a resistor on that PCB gone bad. That's why I went with the new board, if the one resistor is failing, how far behind can the rest be?
Thanks for your opinion! I'm going to change that c-354 choke, too...It really looks sad.

 

[primosounds]

Thanks for your opinion! I'm going to change that c-354 choke, too...It really looks sad.

My experience with replacement circuit boards for the Dynacos is that the USA made ones have no problems and are usually made on milspec materials.
Seriously, the choke on the st70 is probably the last thing on the amp that will have a problem. Since it under the chassis appearance doesn't really matter. The only problem with chokes if they test good is they sometimes can develop a buzz, but that is rare in the Dynacos.

 

[Dandy]

You should be fine with your replacement board. There is a diagram here showing how to rewire a standard board to use the 6GH8. http://www.tronola.com/6gh8a_tubes.pdf

 

[AdamAnt316]

I believe 6EA8s will work anywhere where 6U8s will. If the board you ordered is meant to use 7199s, you'll need an adapter to use those or any of the other tubes peterh mentioned. DynakitParts sells adapters which should do the trick. As others have mentioned, though, crackle can be caused by several things. Has the filter capacitor can in your ST-70 been replaced? My restoration guide should give you some pointers on the basics of what's needed to bring these ST-70s to full operating health. Good luck!
-Adam

 

[airtime]

Rewire the 7199 socket for the 6U8. It's easy and the 6U8 tubes are just as good as the 7199's. Just cut the tracings to the socket were needed and rewire as indicated below.

Step 1. Remove wire from pin 2 and relocate it to pin 6
Step 2. Remove wire from pin 6 and relocate it to pin
Step 3. Remove wire from pin 7 and relocate it to pin 2

Don't bother with the adaptors. It only adds more connections and pin socket problems.

https://d1sjrnpi226dnf.cloudfront.net/spree/attached_files/7199_6u8.pdf?1382390378

 

[rembrandtste]

Thank you to all of you who have weighed in to help me out! Lotta good tips here and I am pursuing them.

 

[Lavane]

It might or might not be a resistor. There might be some other cause of the crackling. Contact issues in the tube sockets can cause noises.

The choke is probably fine. The only way to tell is to desolder one lead and take a reading - it should be about 62 ohms DC resistance between the two leads. It looks "sad" because it was dipped in wax when it was made. Its appearance can be improved with some VM&P Naphtha applied to the metal frame of the choke with Q-Tips. I would not use any of the Naphtha on the choke core/winding. Just clean the metal frame.

I had a crackle in one channel of my more modern VTA ST-70. It turned out to be a failing driver tube on the board. One half of the tube shorted out internally.

 

[Tom Bavis]

If you're buying new boards, why not get boards that take a 6GH8?

 

[Shadowdog]

https://www.dynakitparts.com/shop/pc-3b-st-70-mf-resistors/

 

[spantou01]

Sorry you bought the 6GH8 board already as it retains a compromised circuit, albeit a very clever and successful one . If I were you I would have gone for a modernized replacement with CCS on the phase splitter and three 6SN7 octal tubes like this at only $39 for the bare board - http://www.tubes4hifi.com/ST70.htm
(one caveat is a caution about power transformer capacity - I have not checked if the stock Dyna ST70 can support this configuration)

I have build the noval tubes4hifi board with nearly identical circuit using (plentiful) 12AU7s and it is a significant improvement over the original ST70 as well as another “modern replacement board” using 6922 tubes. However, I am sorry I did not go for the Octal board.

 

[Lavane]

Sorry you bought the 6GH8 board already as it retains a compromised circuit, albeit a very clever and successful one . If I were you I would have gone for a modernized replacement with CCS on the phase splitter and three 6SN7 octal tubes like this at only $39 for the bare board - http://www.tubes4hifi.com/ST70.htm
(one caveat is a caution about power transformer capacity - I have not checked if the stock Dyna ST70 can support this configuration)

I have build the noval tubes4hifi board with nearly identical circuit using (plentiful) 12AU7s and it is a significant improvement over the original ST70 as well as another “modern replacement board” using 6922 tubes. However, I am sorry I did not go for the Octal board.

Running the octal board with an original power transformer is pushing it. The 6.3v winding isn't rated for the extra current needed for the 6sn7 heaters.

 

[peterh]

If you're buying new boards, why not get boards that take a 6GH8?

He did buy a board that supports 6GH8 ( and ecf82, 6U8 etc). I suggest you read all posts !

 

[spantou01]

How, specifically, is the original Dynaco driver circuit "compromised" to the point that it generates sympathy for someone using it?

As far as the notion of "a modernized replacement," for those interested, the following thread is, IMO, eye-opening: https://audiokarma.org/forums/index...s-the-outdated-tube-amps-of-years-ago.862848/

You seem to disagree without explaining why. I will respond to your question.

Replacing the driver PCB with merely a different tube does not realize the improvement potential with great Dyna transformers. There are (to my ears and to thousands of happy upgraders of Dyna amps) significant improvements possible when changing the concertina phase splitter for a better balancing device. The article you referred to imply there is nothing new in tubes since 1950s. That may be true, but solid state devices make it more affordable and practical to use CCSs for example, i.e. a more modern implementation of an long-known, but previously expensive improvement.

 

[peterh]

You seem to disagree without explaining why. I will respond to your question.

Replacing the driver PCB with merely a different tube does not realize the improvement potential with great Dyna transformers. There are (to my ears and to thousands of happy upgraders of Dyna amps) significant improvements possible when changing the concertina phase splitter for a better balancing device. The article you referred to imply there is nothing new in tubes since 1950s. That may be true, but solid state devices make it more affordable and practical to use CCSs for example, i.e. a more modern implementation of an long-known, but previously expensive improvement.

A concortina is perfectly balanced. An LTP may be close to balanced, more so with a CCS.
And i have to say that a large portion of "upgrades" offered to dynaco is more or less expensive snakeoil.

 

[spantou01]

... But, let us not indulge the notion that the original Dyanco circuits cannot possibly be as satisfactory to others.

I did not wish to offend anyone, I merely offered my opinion to the question regarding the replacement of the St70 boards. Or is this the AudioKarma initiation ceremony as this was my first post .

After replacing Dyna boards three times, with different replacement boards, I have observed significant improvements in each case. While some improvements may have been due to the simple substitution of newer for older parts I believe much of the improvements were due to different circuits.

I have great admiration for Dynaco and Hafler’s achievements with his designs, but those Dyna designs can be and have been improved upon.

 

[spantou01]

What all did you change on your ST-70?

I did two ST70s and a pair of MkIIIs and used a different PCB every time. But basically, the changes made were:

1. The front-end circuit and the bias supply. (None of these retained the original pentode VA and triode split-load phase inverter).
2. Replaced the multicap with a fresh one, and in two cases added capacitance with snap-in caps.
3. New power cable, on some new speaker terminals, tube sockets and RCA connectors, etc.

I also refreshed a SCA35 using Dave Gillespie’s EFB power supply/bias board and amplifier boards which retain the original circuit with the few changes needed to swap the 7199 for a 6GH8A, or 6U8A.

 

[spantou01]

....you do seem determined to diminish the perception of the qualities of the original Dynacos - ....

..... BTW, while the AK software does not seem to have a specific provision, it would be appreciated to include a reason for changes to your post if the changes come after someone else has posted (posts # 21 and 22).

If I seem determined (which I’m not) to diminish the perception of the qualities of the original Dynacos, others seem determined to place them on an unimprovable pedestal.

Post #21 occurred while I was editing #20, thus I did not know someone was responding to it. The reason why I edited it was to avoid offending someone, which seem difficult to achieve here, and I did not want to sidetrack the thread.

 

[spantou01]

I am motivated by what appears to be short-sighted and/or under-informed denigration of the originals, or denigration of the originals possibly (even likely) motivated by business interests. This has been going on for a while now and seems likely to have the effect of denying people the opportunity to enjoy the originals, not to mention disrespect for the legacy of one of the greatest names in audio history. David Hafler and Ed Laurent are not here to defend their work.

I am not singling you out. But, you have also not answered the question of whether all the other parts refreshing you did to your Dynaco came before or after you changed-out the original PC board with a different driver design?

I have the greatest admiration for David Hafler and Ed Laurent. I thought I made that clear in my very first post on AK. I called the ST70 driver a very clever and successful design. I maintain it could and have been improved upon.

In everyone of my Dyna projects I changed everything at the same time.