Modified Dynaco ST70

Dear all,

I will get my hands on this modified st70 soon.   As Im a newbie in hi-hi audio and Dynaco as well, I will be glad if you guys can help me with some tips concerning upgrading the amp.  Also would like your opinion concerning current changes already made on the aplifier.

Well, looking at VTA upgrades, I would like to understand what kind of changes can I expect between original type board x upgraded VTA ST70 board with 3 tubes instead the 2 original 7199.

Also, concerning VTA upraded boards, what are the differences between classic 12AU7 board x octal 6SN7 board. Is it possible to hear any real audio change when comparing both?

PS: I live in Sao Paulo-Brasil and current outlet voltage is 127Vac. Any transformer change is needed if I decide to go with 6SN7 octal board?

All help is welcome, thank you very much!!

I think you can use the original config with just as much sound quality.
The picture , is this your amp ?  
Don't you have any tubes ? 7199 is scarce and expensive but can be replaced by ecf80/6bl8 ecf82/6u8
with a simple adapter that changes the pin-out.  You may also cut and reconnect 3 pins on the
circuit board to get tha same affect. Or get a new board that has all component and fits exactly that is already adjusted for these tubes. One example is on ebay :
https://www.ebay.com/itm/403193083801?hash=item5de02e3799:g:~hwAAOSwKldhUb~T

You also need a matched quad of EL34 tubes and a 5AR4 rectifier.

/PS
just listening on a record by merlina mercuri ( who by the way created my tinnitus )
/DS

peterh wrote:I think you can use the original config with just as much sound quality.
The picture , is this your amp ?  
Don't you have any tubes ?

/PS
just listening on a record by merlina mercuri ( who by the way created my tinnitus )
/DS

Hi Peterh,
Tks for your reply.
Yes, this is my amp. Sorry, I did not mentioned before, but it cames with all tubes and they appear to be in good condition.

Rgds

NSacchetti wrote:

peterh wrote:I think you can use the original config with just as much sound quality.
The picture , is this your amp ?  
Don't you have any tubes ?

/PS
just listening on a record by merlina mercuri ( who by the way created my tinnitus )
/DS

Hi Peterh,
Tks for your reply.
Yes, this is my amp. Sorry, I did not mentioned before, but it cames with all tubes and they appear to be in good condition.

Rgds

Fine, then you only have to restore it to dynaco configuration( if it differs). Forget about "mods" a fresh set of matched power tubes makes wonder if the tube is tired.

Regards

I'm not a tech but shouldn't a Variac be used here since there's 127 volts coming out of the wall? That's way high I'd think for an amp designed to run at 110v or 115v.

I recommend the VTA-70 driver board. With all new parts you’ll have a better measuring and sounding amplifier. Save the old driver board in case you ever feel like selling it. It looks like you have the original power transformer so I wouldn’t recommend the 6SN7 driver board as they draw more current than the smaller tubes. If you really want the 6SN7 driver board then you need to get a new production PA-060 which is taller and has higher current secondaries.

As for your high AC, you have a couple options. One would be to get a variac and the other would be to get a 6 or maybe even 12VAC transformer and wire it as a bucking transformer which reduces the AC.,

The power transformer in the picture appears to be quite a bit taller than the original Dyna supplied transformers as well as the offerings from Dynakit parts for use in their more recent ST70 kits.
It looks like there might be  a label on the top, but my guess is that it's sized to allow use of 6550 output tubes.
I wouldn't worry about using it with the 6SN7 circuit boards.
The circuit board itself looks like the original Dynaco part that has had new capacitors  resistors and sockets installed.
Since you have a good set of tubes that came with it, I'd suggest spending some time listening to it as it is for a while, to get to know what it sounds like, and then if your curiosity needs satisfying, installing either or each VTA board to see if you regard them as an improvement.
The circuit board installations are easily reversible if you want to go back to any of them.
There isn't a lot of cost involved, and it's a hobby so have some fun and form your own opinions.

Welcome from South Carolina ! I would certainly reduce that line volts as others have suggested. Also, in your 1st photo, it appears the amp is powered up (green lite) with no input? Never run a tube amp without both input and outputs loaded. Just a thought. Dale

I've done a ST70 with the 6SN7 board and I think it sounds great!
I used some NOS Sylvania 6SN7s that I had.
I first restored the amp with the original 7199 board and listened to it for a while
Then I installed the taller power transformer and the 6SN7 board.
I retained the 5AR4 tube rectifier and run it only in ultra-linear (never installed UL/triode switches)

By comparison the original 7199 version sounded a bit dark and bass was a little loose and bloated.
With the 6SN7 board the top end is more extended and more open and airy. Bass was tighter and better defined.
In both versions vocals in the mid range were great but in the 6SN7 version they seem to pop out in front more.
Overall sound is smoother and less grainy.

My regular amps are a pair of restored and updated McIntosh MC40 amps. I would describe them as more accurate with better bass and provides a more impressive sound stage.
But the modified ST70 has a magical quality to the midrange that is great with acoustic music and vocals.

I can't say how it compares to the 12AU7 version, I've never heard one with that board.