I've completed a partial rebuild of an SCA-35.
basically stock circuit, with new parts, and
bigger power supply caps. 2 JJ 50uf/50uf caps
fit nicely where the old cans were. I added
A panasonic 220 uf cap between the JJ's and the
transformer. This is the 2nd cap in the B+ chain.
No tone or balance controls yet, just selector and
volume.

It sounds pretty good. Similar in character to my
ST-70, without quite as much authority.

Two questions:

Anyone who's heard the SCA-35 and ST-35, how much
better is the ST-35? If it's a big difference,
then maybe I'll change it over. And how about
the "dyna low power" at http://www.triodeel.com/dyna6bq5.gif?

This amp will be on my computer system, (no phono) so I've
got two tube sockets free to play with. Currently I have
resistors there to draw about what the original phono circuit
did. I guess the obvious thing to put in there would be
a tone circuit, maybe the PAS tone circuit would be
an ok starting point. Ideally I'd like it to be unity
gain, and switchable.

Oh yes, for those that are upset about we lurkers not
contributing, here's two things I learned:

Solen caps will melt in this amp.
2 series 22pf mica caps in place of the 12pf ceramic
in the feedback from the 6BQ5 screen doesn't work.

fetko

Hi,
Both sound the same to me. But the SCA-35 is more versitile. There
are some wiring enhancements you can make for channel to channel sonic
balance, but the amp is essentially an ST35 with passive tone controls,
a selector, a volume control and a nice phono stage. I do not feel it
performs as good as a ST-70 with a PAS-2, but it is still a pretty nice
little amp. I had one for years. Dynaco amps are quite nice sounding.
You can bring it alive on the high end with upgraded coupling capacitors
like Auricaps. They may be $10.00 a coupling capacitor, but they will
really bring out the detail in the sound of this amp. Then make sure all
wiring in the signal path is of equal length in both chanels, even if
some of the wiring seems too long in the one channel. This will greatly
improve the stereo soundstage since that amp has a couple wires in the
signal path that are very long in one channel and short in the other.
Replace any twisted pair type wires in the signal path with low
capacitance shielded wire.
As for the feedback, try changing the resistor network at the phase
splitter to reduce the feedback, then try a 22pf. This may work better.
From that point, you can then tinker with lowering the value of the
feedback resistor to make up for the changed values of the resistors at
the phase splitter / driver. The 12Pf is more for anti ringing in the
high frequencies. But, if you add too much capacitance, you always run
the risk of creating a nice hf oscillator if there is any phase shift
from the phase splitter to the output transformer. There are circuits
you can add to the plate feed of the stage which receives the inverse
feedback to counter this phase shift to a point. You can research the
Mullard 5-20 amp to learn about this. But, I am amazed your series pair
of 22Pf capacitors did not work. It has worked for me before. All I can
figure is that one caused a slight phase shift and the next one enhanced
this. Phase shifts of only a few degrees can cause real headaches in
feedback loops. You have to be careful of the type of capacitor used
and it's physical proximity to metallic objects and in some cases, the
feedback resistor, especially if you replaced it with a metal film type.
Bill B.

Fetko wrote:

I've completed a partial rebuild of an SCA-35.
basically stock circuit, with new parts, and
bigger power supply caps. 2 JJ 50uf/50uf caps
fit nicely where the old cans were. I added
A panasonic 220 uf cap between the JJ's and the
transformer. This is the 2nd cap in the B+ chain.
No tone or balance controls yet, just selector and
volume.

It sounds pretty good. Similar in character to my
ST-70, without quite as much authority.

Two questions:

Anyone who's heard the SCA-35 and ST-35, how much
better is the ST-35? If it's a big difference,
then maybe I'll change it over. And how about
the "dyna low power" at http://www.triodeel.com/dyna6bq5.gif?

This amp will be on my computer system, (no phono) so I've
got two tube sockets free to play with. Currently I have
resistors there to draw about what the original phono circuit
did. I guess the obvious thing to put in there would be
a tone circuit, maybe the PAS tone circuit would be
an ok starting point. Ideally I'd like it to be unity
gain, and switchable.

Oh yes, for those that are upset about we lurkers not
contributing, here's two things I learned:

Solen caps will melt in this amp.
2 series 22pf mica caps in place of the 12pf ceramic
in the feedback from the 6BQ5 screen doesn't work.

fetko

Bill,

Thanks for replying. Maybe I'll make the change to the ST-35
circuit sometime down the road when I'm bored or short of 7199's.
It could be an ugly wiring job..

The 7199 is giving *lots* of gain without any balance or tone controls
in the circuit, so that's another reason I was thinking of making the
change.

When I had the series pairs of 22pf (silver mica) in place of the 12pf
ceramic feedback cap, I was getting noticable distortion on cymbals
and sibilances. Returning to the 12pf ceramic, the problem went away.
Maybe a difference in the characteristics of the caps?

The rest of the parts are ordinary metal or carbon film resistors,
orange drop 716p's for the .1 coupling caps, Panasonic film for
the power module input cap, and the 1uf between pins 6 and 8 of the
7199 is currently solen. One of those seems to be getting ready
to melt now too, so they're the next to be replaced.

John

In article , Wbittle wrote:
Hi,
Both sound the same to me. But the SCA-35 is more versitile. There
are some wiring enhancements you can make for channel to channel sonic
balance, but the amp is essentially an ST35 with passive tone controls,
a selector, a volume control and a nice phono stage. I do not feel it
performs as good as a ST-70 with a PAS-2, but it is still a pretty nice
little amp. I had one for years. Dynaco amps are quite nice sounding.
You can bring it alive on the high end with upgraded coupling capacitors
like Auricaps. They may be $10.00 a coupling capacitor, but they will
really bring out the detail in the sound of this amp. Then make sure all
wiring in the signal path is of equal length in both chanels, even if
some of the wiring seems too long in the one channel. This will greatly
improve the stereo soundstage since that amp has a couple wires in the
signal path that are very long in one channel and short in the other.
Replace any twisted pair type wires in the signal path with low
capacitance shielded wire.
As for the feedback, try changing the resistor network at the phase
splitter to reduce the feedback, then try a 22pf. This may work better.
From that point, you can then tinker with lowering the value of the
feedback resistor to make up for the changed values of the resistors at
the phase splitter / driver. The 12Pf is more for anti ringing in the
high frequencies. But, if you add too much capacitance, you always run
the risk of creating a nice hf oscillator if there is any phase shift
from the phase splitter to the output transformer. There are circuits
you can add to the plate feed of the stage which receives the inverse
feedback to counter this phase shift to a point. You can research the
Mullard 5-20 amp to learn about this. But, I am amazed your series pair
of 22Pf capacitors did not work. It has worked for me before. All I can
figure is that one caused a slight phase shift and the next one enhanced
this. Phase shifts of only a few degrees can cause real headaches in
feedback loops. You have to be careful of the type of capacitor used
and it's physical proximity to metallic objects and in some cases, the
feedback resistor, especially if you replaced it with a metal film type.
Bill B.

Fetko wrote:

I've completed a partial rebuild of an SCA-35.
basically stock circuit, with new parts, and
bigger power supply caps. 2 JJ 50uf/50uf caps
fit nicely where the old cans were. I added
A panasonic 220 uf cap between the JJ's and the
transformer. This is the 2nd cap in the B+ chain.
No tone or balance controls yet, just selector and
volume.

It sounds pretty good. Similar in character to my
ST-70, without quite as much authority.

Two questions:

Anyone who's heard the SCA-35 and ST-35, how much
better is the ST-35? If it's a big difference,
then maybe I'll change it over. And how about
the "dyna low power" at http://www.triodeel.com/dyna6bq5.gif?

This amp will be on my computer system, (no phono) so I've
got two tube sockets free to play with. Currently I have
resistors there to draw about what the original phono circuit
did. I guess the obvious thing to put in there would be
a tone circuit, maybe the PAS tone circuit would be
an ok starting point. Ideally I'd like it to be unity
gain, and switchable.

Oh yes, for those that are upset about we lurkers not
contributing, here's two things I learned:

Solen caps will melt in this amp.
2 series 22pf mica caps in place of the 12pf ceramic
in the feedback from the 6BQ5 screen doesn't work.

fetko