Bret Ludwig wrote:
WindsorFox wrote:
Bret Ludwig wrote:
Sal Brisindi wrote:
WindsorFox wrote:
Sal Brisindi wrote:
I knew one day I would own one of these, well that day happened,
today. I know some of you are not fans of this amplifier but pound for
pound, I don't think you can buy a better amplifier for the price.
Photos at http://www.tuberadios.com/temp/dynaco13.jpg
Provided you paid less than about $35 for it, that's true.
And far more people disagree with that opinion than agree with it I'm
afraid.
Yes, but they aren't buying it for quality of sound. It's a retro
style piece. That's all. It works, but most are only powered up
occasionally. Few run them all the time.
I want an amplifier to be really good, and I want to use it every day
for many hours. I want it not to eat tubes, crap up the sound, or give
off a level of heat that makes me think it will erupt in the near
future. Those things CAN be achieved with a Dynaco, but you wind up
spending several hundred dollars for a lot of stuff you are going to
throw most of in the trash.
Do we need to Air the Grievances against the ST70 again? I think so,
and I enjoy it even if we don't.
1. The pentode-triode circuit is no good. It uses the expensive and
crappy sounding 7199.
The trouble is that the pentode anode load is bootstrapped off the
following
concertina phase inverter so it has enough gain so that the amp
is still sensitive when NFB is applied. Pentodes have more distortions
than a dedicated
all triode input stage using say a pair of 6CG7 twin triodes.
But most PPL would never be able to tell the difference at the average 1
watt levels.
2. Said circuit is on a lame PCB that desperately needs to be smashed
and thrown in the recycle bin because it will cause a great amount of
trouble if used. I love busting them up. Sadly, I have approximately
196,300 boards yet to smash.
Yeah, I ain't a fan of the pcb used either; re-placing with a
sheet of Al with 4 x 9pin sockets for 6CG7 is better practice.
3. The power transformer is too small, leading to poor regulation and
high heat dissipation. Since its regulation stinks, they turned the
quiescent current (and the Class A power point) up, making it run even
hotter. And since the primary was wound for 110 volts nominal, all the
voltages are high, including the heaters. More heat yet!
The samples of ST70 I have seen do have hottish PTs, and highish B+,
especially when using solid state rectifiers, but there is room for a
larger tranny,
and one could do worse than use my schematic at
http://www.turneraudio.com.au/dynacost70mods.html
4. Bias circuit is sorely deficient, using a single odd value resistor
in the cathode circuit for a reason making no sense today. You cannot
individually bias each tube, meaning distortion can never be minimized.
Agreed!!!
5. About the best thing about the ST70 is the output transformers.
They are the one thing that's even adequate about it. But they are not
ideal: they achieve perfect AC balance at the expense of serious DC
unbalance. The copper losses will never be symmetrical.
I have found the OPTs are better overall than Quad or Leak,
but the circuit must be carefully damped to prevent oscillations,
and thse issues are addressed in my schematic.
If you want a naturally better amp than ST70, get an ST80.
But after removing the tube rectifier, a large variety of octal
output tubes can be used; even KT90 in triode, WOW!
Given all that where do you start the fix? When you could not buy
output transformers anywhere and used ST70s could be had for a few
dollars, the ST70 was a good place to start. Now, it's far better to
put one on ebay and use the funds to buy new stuff.
Fun is fun for sum I guess....
Real estate agents describe an old run down falling apart weatherboard
house
as a "renovator's delight filled with potential"
ST70 is a tinkerable old toy from which music may be had....
It looks just right on the bench inside the old weatherboard house
on a wet cold winter's day, and a cello can warm the cockles of your
heart.
Remember to turn the amps off when not in the room lest they burn down
the house
if you are so lazy to omit the active protection circuitry.
Patrick Turner.