Help with modded Dynaco ST-70?

[Raccoon1400]

I'm working on this Dynaco ST-70.
It has been modded, does anyone recognize the mods?
I replaced the choke a couple years ago and put in a new set of power tubes. It came back for redplating power tubes after the client put maybe 1000 hours on it.

It had 6H23 preamp tubes in it. They seemed to be a bit microphonic so I ordered a new set of JJ E88CC tubes because I thought they were equivalent numbers. However they don't work properly plate voltage drops down to about 30V (200V with the 6H23 tubes). The sound is then distorted.
There is 30k of plate resistors (2x15k). I tried dropping it to 10k figuring the new tubes were drawing more current and got 60V on the plates. Voltage rose to about 300V on powerup then went down to 60V.

What differences between these tubes did I miss? Can I compensate for them or should I bite the bullet and order a new set of 6H23? Is this a weird design that brought out tube differences that normally wouldn't matter?

I replaced coupling caps and hope between that and another set of power tubes, and preamp tubes the redplating will be fixed.

 

[gadget73]

6N23P is supposed to be pretty equivalent to an ECC88/6922/6DJ8 so I wouldn't expect significant voltage differences between them. I would also wonder if it was actually designed around those tubes in the first place.

post 14 looks like the same board, just with different trimmers. Per that its supposed to run 6922 / ECC88 / 6DJ8.

Mystery ST-70 Driver Board

Does anyone recognize this ST-70 driver board? It has 4 wima .47 uf caps and I believe it takes four 6DG8 tubes. It has no markings on it what so ever. I am sure I could figure out how to hook it up but a schematic would sure make it easier. Thanks in advance Shane

www.diyaudio.com

not much on the board that would typically cause the output tubes to red plate unless its a bum coupling cap. Might want to check out the bias supply if multiple tubes were glowing.

 

[Raccoon1400]

Seems like this is the schematic, or close to it.

The redplating I should be able to figure out myself if I can get the preamp tubes working properly. If changing the coupling caps didn't fix that already.

 

[ivandezande]

With the heat damage and the lack of info, I would rip it out and start fresh with a mod/main board of your choosing.

 

[peterh]

The best way to obtain a good working with proper documentation is to rip out the mess and install a proper preamp board, possibly a 7199 but more likely a replecement one for ECF82/6U8 that follows the Dynaco design.
As for the redplating of power tubes, this is one way old tubes die. Install new matched pair EL34 will solve this.

A new board is available at dynakitparts

 

[ivandezande]

Another option:

Dyna 70 Upgrade Bare Board Only

The AVA enhanced pure vacuum tube amplifier for your original Dyna ST-70 chassis. Over the years, we have received may requests asking for service and upgrade help with the long-out-of-production Dynaco ST-70 vacuum tube power amplifier. The Ultimate 70 is a AVA circuit set for the Dyna ST-70...

avahifi.com

 

[gadget73]

As usual I'm going to advocate the less sensible but certainly more challenging path of figuring out what is currently there. Buying things that just work is boring.

 

[Raccoon1400]

As usual I'm going to advocate the less sensible but certainly more challenging path of figuring out what is currently there. Buying things that just work is boring.

I posted what seems to be at least close to the correct schematic above from that thread. Does it tell us anything?

 

[peterh]

I posted what seems to be at least close to the correct schematic above from that thread. Does it tell us anything?

Yes. If it was a Dynaco board or at least a documented clone, all needed documentation would be supplied, no guesswork and no uncertenties were needed.
There is a huge difference servicing a known good design and servicing some undocumented stuff. Not including the availability of correct spareparts.

 

[gadget73]

I posted what seems to be at least close to the correct schematic above from that thread. Does it tell us anything?

Maybe, it doesn't specifically call out the tube type but the other posts in that thread say 6922. 6922 = 6DJ8 = ECC88 so I'd expect any of those to have sane voltages. We also have voltages specified so maybe worth comparing what exactly you have vs this for voltage coming into the board as well as what the plates show. Looks like it should be 400-420 volts coming in and the plates should be at 225-250 and the cathode of the "upper" section of the tube at 112-125.

Same drawing, just not cropped wierdly



per this it also should have a regulated supply, not sure what that looks like but I do see some extra stuff underneath that looks to be more than just the usual handful of caps and resistors that would sub for the old can cap.

 

[Lavane]

I see at leased 11 diodes, wow! Personally I would strip it out, either get a stock style board kit from Dynakit, or a boars kit from 4ubes4hifi that us3sbeither three 12AU7 tubes and also has CCS. shecematics and assembly instructions for both style amps are available.

 

[gadget73]

Y'all have no sense of adventure.

 

[ivandezande]

As usual I'm going to advocate the less sensible but certainly more challenging path of figuring out what is currently there. Buying things that just work is boring.

I would agree except what’s there is not well done, good looking, or documented. I would encourage the OP to start over and do it right with a well made board. That would also be an adventure, an easier one with a better outcome.

 

[gadget73]

Thats what makes it both less sensible and more challenging. Anyone can buy new things and follow instructions. Reverse engineering someone else's mess and making it work correctly is a fun challenge.

 

[peterh]

The issue with this ampis that it is delivered to a customer, returned for not working properly and might eventually be sent back to same customer. One has a chioce to send a fully complient documented and most likely wirking amp OR a homemade partially documented with unknown performance.
I woud have sent the original Dynaco amp in original form.

 

[gadget73]

This is why I generally don't do work for other people. It lets me do what I find entertaining. If its not fun, whats the point? When I do work on things for people it tends to be very boring like for like stuff, and frankly I find it boring to the point where I rarely will do it unless I have some underlying reason. Last time was because it let me run a Marantz 8b / 7c combo in my rig for a little while. Next one will be an ARC SP9 / D75 combo.

Not going to say you're wrong, clearly I recognize my penchant for this sort of insanity defies logic.

 

[loudnoises]

This is why I generally don't do work for other people. It lets me do what I find entertaining. If its not fun, whats the point? When I do work on things for people it tends to be very boring like for like stuff, and frankly I find it boring to the point where I rarely will do it unless I have some underlying reason. Last time was because it let me run a Marantz 8b / 7c combo in my rig for a little while. Next one will be an ARC SP9 / D75 combo.

Not going to say you're wrong, clearly I recognize my penchant for this sort of insanity defies logic.

And to the benefit of many of the rest of us...

I support this flavor of insanity.

 

[ivandezande]

Thats what makes it both less sensible and more challenging. Anyone can buy new things and follow instructions. Reverse engineering someone else's mess and making it work correctly is a fun challenge.

Totally fair. I personally am not very good at that haha, rip and replace is much safer for me and my customers.

 

[Lavane]

I get liking a challenge and all, but I wouldn't be comfortable sending something back to a customer or friend that has sketchy documentation to begin with. I would want it to be something thats been well documented, and easier to work on in the future if needed. If this amp was something I was going to keep I might try and see if it could be made to work properly as is first before ripping its guts out and starting over. For me I would defenetly give the owner the choise of making a few changes to the existing amp to make itwork right as is and use it with these mods, or go with something thats a stable proven design.

 

[StepheK]

I'm a fan of those "three 12AU7 boards" as service replacements. Easy to source tubes, each output tube has it's own bias pot and I personally think they sound and look nice. For a non-techy end user, those digital bias displays that replace the octal sockets make a lot of sense too. On this one I added a volume control to eliminate the need for a pre-amp as well.