Dynaco
SCA-35

A New Look at an Old Friend

Think the popular mods to your Dynaco
SCA-35/ST-35 achieve it’s best performance? Think again. Here’s a new mod
that really does improve performance, and has the documentation to prove
it. See the article here:

Dynaco
SCA-35/ST-35 mod article
(1.4MB)

Power increases about
30% at low frequencies where it’s needed. Distortion at 1kHz is
reduced 5X !

EFB™ circuit at the
heart of the Dynaco SCA-35 mod.

Complete instructions
for an easy mod!

Reader Comments

Posted
by Dave

September 28, 2014 - 07:29
am

Hi Stephen -- Congrats on your
successful installation of EFB! Your perceptions have been shared by
virtually everyone who has installed this modification in the small
Dynaco amplifiers. The improvements are both readily audible, and
easily demonstrated in the lab as well.

The conversion to
fixed bias operation changes the fundamental operation of the output
stage, while EFB action basically makes it immune to any
fluctuations in power supply operating voltages. With the improved
match this creates between the output tubes and output transformer,
more power can now be developed, and at notably less distortion as
well.

In addition, with the lower quiescent current your
output tubes now operate at, they will last longer, and the unit
will operate cooler as well.

Adding EFB to the small Dynaco
amplifiers really is one of those few modifications where the
results belie the simplicity of the modification, and the
improvements gained are had without compromise. Congrats on your
success!

Dave

Posted
by Dave

September 28, 2014 - 07:12
am

Hi Joe -- From the symptoms you have
now described, it is obvious that the negative feedback circuit is
now producing positive feedback. There are only two things that
could cause that to happen in this amplifier:

1. The leads
from your Z-565 OPTs are no longer connected the way they used to
be. This includes both the connections to the output tubes at pins#
7&9 (now pin# 8 with 100 ohm Screen Stability resistors
installed), and the connections to the output terminal speaker
connections. If ANY ONE of the leads from these transformers is not
connected properly, it will cause the problem you are
having.

2. The connection to pin #2 of the output tubes have
become reversed from their original connecting points. This two will
cause the problem.

It is a virtual certainty that the problem
will be found in one of these two areas. And whatever it is, has
been duplicated in both channels. You said you have already verified
the secondary connections of the OPT, and the proper grounding of
the black leads, so I would now concentrate on the output tube
socket connections.

Dave

Posted
by stephen zaneteas

September 28, 2014 - 04:24
am

Plugged the sca-35 into a variac and
gradually brought up the juice. Next, plugged in the DMM and
adjusted bias to .267 volts. Using a very loud and busy mix from an
xbox360 racing game plugged into an 24bit/96k upsampler...well...the
sound was awesome. S U P E R WIDE freq response. Basement
solid bass with a sweet upper high end. Compared to the pre-mod
Dynaco version there was significant reduction in distortion. Even
at full power. Lots of space and air around the instruments.
Superior channel tracking. I don't know why but I can discern the
crappy audio channels from DISH and the superior
ones.....Compression is SO obvious on many digital channels. They
are not FULL DYNAMIC RANGE nor do they-in my opinion-compare to a
true direct blu-ray playback. Popping in some 24bit transferred
source material-ssimply stunning. Detail and a deep soundstage. Im
happy......very.........

Posted
by Joe Janus

September 28, 2014 - 12:00
am

Dave
I would compare the noise to
the sound of feed back, as in the day when a mic or guitar got too
close to its amp, more like a howl ,, siren like without the rising
in pitch.. and fairly loud thru both channels with the pc-10a inputs
shorted. Substituting different tubes does not change this although
pulling the 6U8's does quiet it completely.

Joe

Posted
by Dave

September 26, 2014 - 08:51
pm

Joe -- It would be helpful to have more
detailed information. For example, is the noise in both channels or
just one? Is it loud or very soft in nature? You call it a high
pitched hum, which is somewhat confusing: Hum implies a low
frequency signal, but high pitched is a very wide open description.
More accurate information then would be a big help.

Normally,
connecting the 16 ohm feedback connection would not cause the
production of any noise, unless the leads at pins 7 and 9 of the
output tubes are connected incorrectly. If your modifications
included disturbing those, you might check for accuracy of these
connections.

The only other item would be lead dress. Input
and output connections that come too close together can cause
feedback problems as well.

As a test, place a direct short
across the inputs to your PC-10A boards. If the noise is still
present, then at lease it has been confined to the power amplifier
section.

Dave

Posted
by Joe Janus

September 26, 2014 - 05:50
pm

After redoing the ground and 16 ohm
connections, the oscilating hight pitched hum remains. It would be
nice to have a 2nd sca35 to compare readings at various undocumented
points... but... it will teach me not to make to many changes at
once... which in this case i did...(replace origial selector switch
and mod to pc11, new output binding posts, 3 prong ground which I
removed) but even with input to pc10 removed and tubes changed it
persists... wondering if I did not cause harm with 3 prong AC
attempt.. the hum does disappear when pc10 connections to 16 ohm
output taps are removed which leads me to suspect the pc10 board
components. ;Perhaps there are some readings and or method for
isolating the fault(yea mine) here rather than by disconnecting and
testing individual pieces and/or purchasing and substituting a new
pc10.... If not, then so be it and let the work begin...

1 - 2
- 3
- 4
- 5
- 6
- next>
- last>>

Add your comments
here...

Name:

Message:

_