st35mod5A low

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:st35mod1A low

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.”st35mod2A low  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.

st35mod3A lowI 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 st35mod4A lowwith 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. ring terminalFor 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 nylon screwthe 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


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.

Stereophile low1/ 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.

George Wright low2/ 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.

Telarc Sampler-2 low3/ 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.

Bela_Fleck-Double_Time_ low4/ 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 David Gillespie January 02, 2024 - 11:26 pm
Hi David -- It's my understanding that our friend Art is no longer with us, so I thought I would step in to answer your question. The heater circuit needs to reference ground to help ensure low noise operation, but elevating the heater circuit above ground with a + bias voltage of from about 10 to 75 vdc (or so) helps to produce the lowest possible noise operation when AC is used to power the heater circuit. Such a positive bias on the heater system usually ensures that the heaters always operate somewhat above that of the cathode potential relative to ground. This retards the migration of electrons from the heater to the cathode elements (across the h/k insulation) that might otherwise exist if the cathode elements operate at a greater positive potential than the peak positive potential that the AC powered heater circuit exhibits at any point in time when any portion of that circuit is directly grounded. Such migration would manifest itself as 60 Hz hum in the output, if it's occurring in an early enough stage for it to be sufficiently amplified. So, heater bias is an optional design consideration, and not a requirement when my EFB™ circuit is installed, but usually a good idea to consider when AC is used to power the heaters and lowest noise operation is desired. I hope this helps!


Posted by David January 01, 2024 - 04:35 pm
Hey Arthur! Thanks for this excellent article. Im currrently building a stereo dual EL84 Amplifier and want to adopt your EFB idea. It also will include a regulated phase inverter, which will drastically improve the symmetric performance of the amp. (Compared to the never symmetric katodyn / LTP) This is the starting point from where I came to "hey why not even regulate the power tubes bias"... and google helped me figure out your article! Thanks again! But there is one thing I am unsure about: Does in this configuration still the heater has to be connected to the EL84 cathodes? Because I cannot find this connection in the schematic. (Compared to another version, where someone has done this to his SCA35.) Hopeefully there has not too much time went by for an answer :-) Take care, David

Posted by Art Grannell November 19, 2018 - 11:31 pm
If you can temporarily solve the issue by rotating the switch, then there's a good possibility that it's just dirty contacts on the switch. While it isn't difficult to bypass the "Blend switch", as Dyna called it, I prefer to leave the switch in, as the single "A" and "B" channel function can be useful, particularly when troubleshooting.

Whenever I have my PAS opened, I routinely clean all the switches and controls by spraying contact cleaner into them while rotating the knobs. If you decide to try this, I'd strongly recommend that you also at least clean the input selector switch. This 3-gang switch is more complex, and more critical, overall than the other switches in the preamp.

Dirty switches and controls are a very common and usually easily solved issue with older audio equipment.

Happy Thanksgiving!

Posted by Brian November 19, 2018 - 06:32 am

Thank you so much for the suggestion! Yes, what started me on this repair journey was a loud hum that I traced back to a failed quad-cap in the PAS-3. I ended up getting this board to replace the selenium rectifier and quad capacitor:

The instructions left some to be desired, but I figured it out. (That's why the ST-35 instructions here were such a breath of fresh air!)

Since the last update I've been enjoying the system almost daily, but I did discover another problem... The left channel sometimes cuts out. I traced the issue back to the stereo/mono switch in the PAS-3. I've read it's common to take that switch out, as the novelty of showing mono vs stereo has long since waned and it's no longer needed. Next time I open up the PAS-3 I'll probably bypass that switch. In the meantime it's sufficient to rotate it clockwise and then back to full stereo and it resolves the issue.

Otherwise, the system is sounding awesome and performing flawlessly!

Hope all is well with you, and hope you and Dave have a great Thanksgiving!

Posted by Art Grannell November 05, 2018 - 08:10 pm
A previous reply seemed to indicate that you had worked on your PAS-3, before you repaired the ST-35. A suggestion: If you have not already done so, replace the selenium rectifier in the PAS heater circuit, with a pair of silicon diodes. Overheated old selenium rectifiers can emit poisonous gas, so this thought is related more toward safety than audio quality, although the change could improve the later.

Other than that, sit back, prop your feet up and enjoy the "fruits of your labor"!

Posted by Dave November 04, 2018 - 02:21 pm
Hi Brian -- Sounds like a plan, and glad the current ghost has been eliminated! When you do need new boards, I'll likely be here, as I have no plans at this point to stop their sale.

The bigger issue for you to remember is that "through hole" components (those with leads attached) -- which the boards I sell use -- seem to be drying up slowly one by one on a seemingly daily basis. I would bet that like so many vacuum tube specific products, there will always be a source somewhere for such parts, but like other vacuum tube specific components, the selection will be limited to only those that are most popular and profitable to produce.

For now however, enjoy the upcoming holidays and all the music you grew up with throughout them!


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