EFB mod
board installed in the Dynaco ST-35

An
Installation of the EFB™ Bias Circuit
in a Dynaco ST-35

By
Arthur Grannell

The Dynaco ST-35 Power Amplifier has been the center
of my music system since I discovered it in 1982. I used one to replace my
pair of Dynaco Mark IVs after performing lengthy comparative listening
tests, and deciding that the smaller, cheaper ST-35 provided inherently
superior sound. Being a “tinkerer,” I have always looked for ways to
improve the circuitry further, but have found little, other than the well
known power supply and individual component improvements.

Dave
Gillespie’s EFB bias regulator is the first “true” circuit modification
that promised to provide genuine, measurable results, that I’ve found,
since taking an interest in this amp. I incorporated this simple circuit
in a completely refurbished and upgraded ST-35 amp project that I recently
completed, and performed power output testing with one and both channels
driven. The results were very similar to Dave’s published results, except
that this amp has a considerably increased power supply “reserve”, so the
difference in power output was not quite as dramatic. My next project was
then to add the EFB bias regulator to a completely “stock” ST-35. The
amplifier described below is a “factory built” ST-35 with no circuit
changes, other than a rectifier diode upgrade to UF4007s, the elimination
of input capacitor C1, and the changing of capacitor C2 from a ceramic to
a polystyrene type of the same value. Here is a photo of the bottom
side, before modifications:

The SCA35 that Dave describes and the ST-35 share
similar circuitry in the output and power stages, but have numerous
physical differences. Adding additional components to the ST-35 is
challenging due to the lack of available “real estate.”  The chassis is less than 3/4”
deep, so component size is limited. I started by adding the necessary grid
resistors as shown in the photo at right. The 1K resistors were
installed in place of the jumper wire from eyelets 8 and 9 to pin 2 of
each output tube. The 100 ohm resistors were bridged across terminals 8
and 9, and the output transformer screen lead attachment was moved to pin
8 of each output tube.

I fabricated a small board (at
left) from 1/16” phenolic punchboard that I had lying around. The grid
spacing on this board is
0.1875”, so it’s a product that seems to be
no longer available. An alterative would be to drill your own holes in a
piece of phenolic or FR4 board. The cut board dimensions are 1-13/16” x
1-13/16”, with a 7/16” x 13/16” cutout to clear the output transformer
leads. A 3/16” mounting hole was drilled to allow support with an additional #8 nut on the output
transformer mounting screw as shown.

I favor the use of turret
terminals for construction of small boards. The ones that I used are
Keystone #152, which are available from major electronics suppliers.
However, the staking tools are expensive and difficult to find. Therefore,
I attached the turrets into the board holes with a dab of “JB Weld” (JB
Quick), an epoxy that will withstand the heat from soldering. An
alternative to turrets could be small pieces of 14 AWG solid wire cut to
½” length, placed into appropriate sized holes drilled in the board, and
also held in place with JB Weld. JB Weld is electrically conductive,
however, so watch for “bridging” between components. Of course,
fabricating an etched pc board would work equally as well, keeping in mind
that attachment points such as eyelets or terminals would still be needed.

I used 5 ohm, 0.1% cathode resistors, rather than
paralleled 10 ohm resistors and mounted them directly on the board. A
Bourns 5K trimmer was mounted in a position to allow adjustment from an
area away from high voltage. A dab of common yellow wood glue was placed
under the trimmer to help secure it to the board. I decided to also mount
the output capacitor for the LM337 on the board, rather than use the aging
one enclosed in the original multi-section can. The value I had on hand is
33mfd, 25VDC, which will work fine in this circuit. One needs to watch the
polarity, here. The negative lead is grounded.

Referring now to the
photo at the top of article, I removed the original 95 ohm, 5 watt
resistor from the “can capacitor” as well as the two wires from the #4
terminal leading to pin 3 of the output tubes. I also temporarily removed
the black wire from the output transformer to the common terminal of the
right speaker terminal strip to facilitate board installation. I also
clipped off the ground wire from the can capacitor tab at the same speaker
terminal.

After temporary installation of the circuit board, I
located the LM337 on the chassis support and marked the mounting hole,
drilling it with a 1/16 inch bit. For convenience, I decided to mount a
ring terminal to the tab for the input signal  (ground, in this
situation), rather than the center pin on the bottom which I clipped off.
Two wires were soldered into the ring terminal. One lead goes to the bias
board ground, the other to the can tab ground. I felt this was necessary
as the chassis support is spot welded to the chassis, and could be a
marginal grounding point.

After soldering an extension wire and heat shrink
tubing on the LM337’s output terminal, I then installed it on the support
along with the ring terminal, using thermal compound, but without a mica
insulator and hole spacer. I then shortened the previously removed speaker
terminal ground wire and attached it to the “side” can capacitor ground
lug. This will, in theory, improve the grounding scheme, rather than
relying on grounding through the chassis or aluminum capacitor can. I ran
a new speaker ground wire to the output strip on the other side of the
chassis support. I then soldered the LM337 leads to the board terminals,
and shortened and attached the wires from pin 3 of the output tubes to
each 5 ohm resistor—one from each channel.

The installation was completed by attaching a wire
from the can capacitor (marked with a square) to the 360K resistor. This
resistor was mounted slightly above the board as it is a 2 “watter”, and gets
warm. Finally, I added a #8 screw with two nylon nuts, in a “sandwich”
arrangement, one above and one below the board in the corner of the notch
to stabilize the board. This screw, plus the connection to the LM337 on
the other corner, makes the board physically stable without any additional
chassis drilling. For extra insurance, I placed a small piece of tubing
over the black output transformer wire, as it now contacts this
stabilizing screw. The installation was complete with the drilling of only
one 1/16” hole as the only physical chassis alteration.

Some
of the essential parts that I purchased from Mouser—

Mouser #  511-LM337SP
Linear Regulators -
Standard 1.2-37V Adj Negative

Mouser # 71-CPF15R0000BEE14
Metal
Film Resistors - Through Hole 1watt 5ohms 0.1%

Mouser #
652-3296Y-1-502LF
Trimmer Resistors - Multi Turn 3/8 5Kohms Sealed
Vertical Adjust

Addendum

Since the above article appeared, I’ve been asked a
number of times for my opinion of any sound changes apparent in the
amplifier due to the inclusion of the EFB Bias regulator. This is a valid
question, since it’s well known that better “specs” in an audio circuit
don’t necessarily equate to better sound. A few months ago I did a
personal evaluation, comparing Dynaco cathode bias vs. EFB Bias. For those
who haven’t seen this report before, it is included here.

Listening Observations
Dynaco ST-35 Cathode Bias
compared with EFB Bias

Music Source:
Yamaha DVD-C940 DVD-SACD-CD Changer. CD’s upsampled at 176Kb/s.

Preamp:
Dynaco PAS-2 with modifications to the heater supply, power supply and
line stage.

Amp:
Essentially stock ST-35 with EFB mod incorporating an external switch that
changes between cathode and EFB bias. Tubes: Outputs: JJ EL84 (matched
Quad) Voltage Amp/phase inverter: 7247s–one marked “Dynaco”, one marked
“United Electron” both are marked “Made in Gt. Britain” and both believed
to be “Mullard”.

Speakers:
Altec-Lansing 890C (Bolero) LF: 10’ active (approximately a 604 with 10”
cone), 10”passive. HF: Small horn made by “Foster”. Estimated system
efficiency: 95-98 dB (1W, 1m).

I selected three specific
CD’s and one SACD for the tests. I played the tracks listed below using
one amplifier bias setting, then switching the amp off, changing only the
bias switch position, and immediately relistening to the tracks, going
back and forth between the two settings in no particular order.

1/ Stereophile Test CD2
I concentrated on
the first two tracks for evaluation. These were recorded by connecting a
Fender Bass guitar directly into the mixing console. Anyone who has heard
this, may agree that the effect is rather startling! My opinion is that
the sound has even more realism and “punch” with EFB bias.

2/ George Wright Playing the Mighty Wurlitzer Pipe
Organ, Vol. 3
I chose this CD figuring that if anything would
demonstrate an increase in amplifier power, this would be it! Anyone who
has listened to theater pipe organ, live or recorded, knows the floor
shaking, window rattling sound that emanates from the 16 and 32 foot
pipes. The lowest frequency produced is 16 Hz, and I tend to think that
was Mr. Wright’s favorite note!

Oddly, after listening
extensively to this CD, and switching back and forth, I could detect no
difference at all between the two bias settings. I could only guess the
reason for this, and that guess is that it has something to do with the
fact that my speakers do a poor job at 16 Hz. Altec rated them down to
40Hz, and I suspect response falls off quickly below that point. The test
tones down to 20Hz on the “Stereophile” CD confirms this suspicion. I have
heard this (organ) CD on a system that does reproduce 16 Hz fairly well,
and the sound is definitely different. Nevertheless, I’m still a little
puzzled by my lack of ability to hear a difference between the two
settings.

3/ Telarc Classical SACD Sampler 2

Track 2 and one or two other tracks were probably included on this
recording for the reason that the closely “miked” orchestral side drum
literally lifts you out of your seat, sounding much like thunder, at
certain points. Here, the difference between the two bias settings was
definitely noticeable, as I came further off the couch when the EFB bias
setting was used. I also think that other differences were apparent using
EFB bias including an improvement in the sound of cymbals and some brass
instruments. I tend to enjoy the music more with the latter bias setting
in ways that I find a little difficult to define.

4/ Bela Fleck–Double Time
I
consider the acoustic quality of this CD to be the highest of any
recording that I have in my collection; better than any SACD or DVD I’ve
heard. Consisting mostly of music played on banjo, mandolin, guitar and
bass, it defines the meaning of “transient response”.

To my surprise, the
difference between the two bias settings was more noticeable with this
one, than on any of the other disks. I asked another person to listen to a
track played both ways, and to tell me if she heard any differences, and
if so, if she could describe what she heard. She stated that on position 2
(EFB) the very subtle finger and pick sounds on the strings were
noticeably more defined and apparent. She preferred this setting, overall.
Her opinions matched mine, but I found another generally noninvolved,
non-critical listener’s opinion to be quite valuable, since she didn’t
know in advance what she was going to hear, and has no knowledge of the
technical difference.

In
summation, I’d say that, yes, there is a “sound” difference between Dynaco
cathode bias output, and Dave Gillespie’s EFB bias. The improvement is
measurable on the bench as increased power output and reduced distortion.
But, the effect on the final sound is apparent in ways that I for one,
would not have expected. The increase in power output in improved bass
would be expected, but it’s interesting that the modification shows up
with improved transient response and superior “musicality”,
overall.

Reader Comments

Posted
by Steve L.

June
12, 2013 - 01:59 pm

Hi James, I have replied on Facebook
with Dave's email address. Very nice FB page, by the way.

Posted
by James Fisher

June
12, 2013 - 09:37 am

Is there anyway I can get ahold of Dave

Gillespie. I would love to show him how I'm using his design.
And I ensure every purchaser is well a where of his design
influence. I even did a write up for Georgia Tech on it.

https://www.facebook.com/FisherAmps

Posted
by Dave

March 08, 2012 - 09:18 am

Hi Lydia --

It is actual circuit
design that has the lions share of influence on the sound a
particular unit will have. The best designs will emloy design
techniques and specifications that will inherently minimize the
influence that any external devices can contribute to the
performance achieved. The design of the SCA-35 encompasses these
techniques. Therefore, such external items as cabling, dampeners,
and the like become items of personal preference, as few of these
items have any true scientific double blind testing behind them to
allow support of any absolute recommendations. That is not to
disparage them in any way, but simply to say that it is safest to
assume that items marketed by subjective claims become items of
personal preference by definition. Therefore, only you can be your
own guide with such ancillary items.

As for the tone control
and input modifications for the SCA-35, check out the diytube site
under the ST-35 section. There you will find various threads to
address your questions regarding these issues. Answers to the rest
of your questions can be found in the Dynaco owner's manual for the
SCA-35, which is readily available on the web if you do not have
one.

Good luck with your restoration!

Dave

Posted
by Lydia

March 08, 2012 - 04:14 am

Hi Dave, Art,
I am in the process of
purchasing a restored SCA-35, and I was wondering if there is a lot
of difference in N.O.S. tubes, and new ones? If new, which ones do
you recommend? The 7199's I will have RCA's NOS. Only 6bq5/el84's I
have to figure out the ones I am to go with.... It is a stock unit
but he said that he added 5k trimmers to each output tube so that
each one can be biased individually. I have built a pair of Jon
Risch's Cross-connected speaker cables and I have already compared
them to the Kimber 4pr, and the Risch's CC89269, is so transparent
clearer, detailed mids and highs are more naturally present,
especially jazz drum solos. It does lack in thiness in the bass, but
maybe breaking it in will improve!? How do you think this will sound
with my Advent 6003's they are really "Maestros" from 1982. probably
the last year for the big heavy 3-way speakers with the gray donut
ring midrange. any advice on where to proceed once I get the unit?
Audioregenesis sells a neat board that fits in place of the can caps
and comes with the EFB.
is this a good idea or since I have
almost finished my EFB,, board I really want to use mine. What about
vibration? I am going to use Rubber Stoppers glued to the bottom
giving a little firm cushion raised up about an inch on my wood
cabinet, is this going to be good enough?
any other
recommendations are welcomed. Oh
by the way, some people have
bypassed the tone controls and sounded better do I have to do this
since I am going to do the EFB.? also I want to know if I can run my
CD/dvd Player and what input am I going to use? Can I make another
input, or change the "head in" to plug another input device say Tv
audio? can you run
parralel speaker wires 2-pair going to left
and right speakers? what about connecting 2-pairs of speakers
running in series each side?

Posted
by Art Grannell

February 29, 2012 - 07:53
pm

Bill,
1/2 watt resistors should be
fine. I used whatever I had on hand, which in this case, was 100ohm,
1 watt carbon composition on the screens, and 1Kohm 1/2 watt metal
film on the control grids.

Posted
by Bill

February 29, 2012 - 01:58
pm

I have checked this and other sites
about this mod and can't seem to come up with the wattage for the
new 100 ohm screen stability resistors. I have 1/2 watt, but it
looks like 1W in the picture. On Dave's installation, it looks like
1/2W size.

I have all the other values. Thanks.

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