Dynaco Mark III?

[Lavane]

All you need to do on the Mk3 octal board, depending on the parts list, is increase R39 between the two bias filter caps. It would help to have a schematic for the octal board. I bought mine from an eastern European seller that makes/markets the auto bias boards. It was a driver board/bias board combo. I think his original R39 was less than 20 ohms. Changing to 470 ohms reduced the ripple on the bias voltage quite a bit.

Here's ripple with the stock resistor value at 0.5V vertical scale.
View attachment 3849611

And after increasing R39 to 470 ohms. Same vertical scale.
View attachment 3849612

Are the boards you bought copies of the VTA octal board?
Is there even a difference parts values wise, like a few resistor value differences between the 12AU7 and 6SN7 VTA boards? Or are the sockets and tubes used the only difference between them? There isn't much difference in gain , and some have used 6SN7's on the 12AU7 boards with socket adaptors. I tried a 6SN7 in the voltage gain socket of my 12AU7 ST-70 board. I didn't leave it there though.
I just didn't care for how it looked having to use the octal socket adaptor on a 9 pin socket. I'm surprised someone hasn't traced out the VTA octal boards and posted a drawing anywhere, like here, or at DIY, or the Dynaco Forum.
The schematic for the VTA ST-70 using the 12AU7 tubes comes with the amp kits, but no schematic for the amp using 6SN7's even if you order the kit with the 6SN7 board instead of the 12AU7 board.

 

[ceylon]

Does anyone know why Dan is neglecting the business?

there are online “where’s dan?” discussions going back 3 years. at some time he suffered a personal crisis. dunno whats going on the rest
of the time, but its somewhat common to take over a working program and tank it, as we are seeing happen elsewhere atm.

 

[buglegirl]

but its somewhat common to take over a working program and tank it, as we are seeing happen elsewhere atm.

Well I prefer to say take over a dwindling business. Usually what should happen is a consolidation and eventual trimming of these businesses into something viable.

That is a lot of work. And what seems like a good idea quickly becomes a huge PITA. There are ways to do this. But going the quick buck route like a lot of these "Failures" go is not viable also. You have to really love doing this and put up with the drudgery. And that sorta "Spirit" what is in short supply these days.

Frannie

 

[wdecho]

I would not say a dwindling business when there is a 3 month waiting period for shipping at Dynakit. I see a market for tube4hifi myself. Hope someone buys, takes it over.

 

[buglegirl]

I would not say a dwindling business when there is a 3 month waiting period for shipping at Dynakit. I see a market for tube4hifi myself. Hope someone buys, takes it over.

I own most of the older PAS upgrade stuff from Tubes4Hifi. I kinda have some insight into this.

3 months is excessive though. (But honestly its easy to sit here and throw rocks) I hate even asking people to wait a week or two. But supply chains are a MESS now for one. That is another subject.

Frannie

 

[ceylon]

Well I prefer to say take over a dwindling business. Usually what should happen is a consolidation and eventual trimming of these businesses into something viable.

VTA/tubes4hifi wasnt a dwindling business. dan took it over during a tube feeding frenzy that stil hasnt ended.

 

[Blackdog61]

One should change the title as this thread has drifted away from Dynaco . Will "VTA MkIII " be more accurate ?

As someone who has enjoyed a mostly stock ST70 for about a decade and endured all of the VTA hype, I don’t mind. Yeah, we agree that any Dynaco unit with an aftermarket circuit is no longer a Dynaco. If I do end up building the amps with these boards I may change the title, but since it’s my thread……………

 

[nerdorama]

AND, in future when someone is searching for info regarding Mk3's they will likely search for Dynaco Mk3 regardless of the PC board they are intending to use.

 

[wdecho]

As someone who has enjoyed a mostly stock ST70 for about a decade and endured all of the VTA hype, I don’t mind. Yeah, we agree that any Dynaco unit with an aftermarket circuit is no longer a Dynaco. If I do end up building the amps with these boards I may change the title, but since it’s my thread……………

All long threads have a way of getting off track.

 

[Lavane]

Even though they can get off track sometimes, there can be some added info that can still be useful.

 

[Blackdog61]

My 70 came to me in similar fashion; iron and little else. It didn’t even have the bias resistors so I used the individual bias system from Dynakit Parts, but retained the 7199 circuit. I soon moved onto the 6GH8a board after realizing that matched 7199s are tough to get.
So? I’ve got a 70 built with new parts but otherwise similar to the original design. Been there, done that and I wasn’t convinced that stock IIIs would be an improvement. Time for something new.

 

[gadget73]

One of these days I'll hear a set of Mark III with the stock circuit and get to compare them to my big Bogens. Basically the same front end, 6AN8 running a pair of 6550/kt88/etc tubes. Main difference is pentode vs UL mode, and obviously different output transformers. Would be an interesting comparison.

 

[albusark]

My 70 came to me in similar fashion; iron and little else. It didn’t even have the bias resistors so I used the individual bias system from Dynakit Parts, but retained the 7199 circuit. I soon moved onto the 6GH8a board after realizing that matched 7199s are tough to get.
So? I’ve got a 70 built with new parts but otherwise similar to the original design. Been there, done that and I wasn’t convinced that stock IIIs would be an improvement. Time for something new.

The dynakitparts 6GH8A is the only board I'd put in an ST-70, it sounds and looks fantastic. Otherwise I restore original pcb's, which isn't difficult, and keep it original. You can easily fix traces on the original boards. The only thing you need to watch for is darker areas on some older pcb's as heat has burned out the epoxy and what you've got there is carbon, semi conductive. You can test old pcb conductivity with a cheap wood moisture meter you'd use to test splits for bbq, the one with pins (you're looking for a close to 0% result otherwise toss it and get a new pcb).