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Those who have read through the lengthy post regarding Dave Gillespie's EFB Bias circuit are aware of the potential to improve the performance of the SCA-35. Dave's article details the installation of his circuit only in this particular integrated amp at this point. I don't own an "SCA", but I do have 3 ST-35s, two of which now have this improvement. The first one that I did had extensive circuit and layout revisions, so that the way that I added the circuit wouldn't apply to most ST-35s. I have used it for testing, though, and I've found power output results that are very similar to the ones presented in Dave's article.

I've now completed the second installation; this time in a completely stock, factory built ST-35. Although the power output and power supply areas in the two amplifiers have similar circuits, the physical layout is much different. The small size of the chassis makes the addition more of a challenge.

For anyone that would like to see a step-by-step description of the way that I implemented the EFB bias circuit, the link below will take you to a short article about it, complete with some pictures.

http://www.tronola.com/html/efb_in_a_dynaco_st-35.html

So does it sound any better to you?

At this point, I haven't listened to it. It's been out of my system for this modification and some changes to my PAS preamp.
However, I'm working on a switching system that will allow me to alternate the ST-35 back and forth between the EFB bias and conventional cathode bias, so that I can compare the two directly. I have a couple other projects underway, but I hope to have this completed and installed in a couple weeks, at which time I'll be able to offer an opinion.

After a longer delay than expected due to unrelated issues, these are my findings:
First ---my system:

Music Source: Yamaha DVD-C940 DVD-SACD-CD Changer. CD’s upsampled at 176Kbs
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 (essentially a 604 with 10” cone), 10”passive. HF: Small horn made by “Foster” Estimated system efficiency: 95-98 db.

I selected 3 specific CD’s and 1 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. Wrights’ 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 can 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. It was originally released as a “direct–to-disk” record, and 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, 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 non-involved, 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.

Arthur, thank you for taking time to post your results with Dave's EFB bias circuit. It is now time to add it to my ST-35 and diytube stereo 35. Your step-by-step instructions will also be of considerable help to the conversion.

Cordially,
Lee Hankins

I would also like to thank Art for making such a detailed comparison, and taking the time to post his findings for the group. His results basically mirror my own, but since his tests and review are independent and unsolicited, it gives those contemplating the EFB modification additional confirmation, while the installation/use experience he has posted provides further practical information for consideration as well.

Thanks again Art for an outstanding job!

Dave

Art, great write-up. Easy to grasp the benefits of the EFB. I will incorporate the mod for sure.

Are my wattage figures right for the resistors?

(1) 360K 2 W B+

(1) 15K 1/2 W trimpot

(4) 1000R 1/2 W grid

(4) 100R 1/2 W screen

(2) 5R 1 W or (4) 10R 1/2 W cathode

Stunch --

Your parts list is correct, noting that the 15K resistor is not the trim pot itself, but rather a fixed value resistor connected between the trim pot and ground. I think that's what your parts list implies, but I just wanted to confirm that point. The trim pot itself can be a very low wattage device, since it dissipates much less than .1 watt.

Good luck with your modification!

Dave

Thanks Dave for the confirmation. I didn't know what to call that fixed resistor so I just called it trimpot since it was hooked up to it. I have a Bourns 3296W-502 pot which I think is rated at 0.5 watts.