Comments for page: A New Look

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Posted by André Bancsó January 25, 2013 - 07:13 am
Hi Dave,
When are your sca 35 pc kits ready?
Best regards,
André Bancsó

Posted by Joseph July 23, 2012 - 10:28 pm
Okay, my mistake - and what a doozy! I was looking at the SCA-35 Cap/EFB boards only. Just eye-balling it versus the ST-35 Cap/EFB board makes it look like the SCA-35 board is way too wide - I was thinking about removing the cap on the "right" side of the ST-35, then installing two SCA-35 boards bilaterally. It looks like a no-go unless I nudge the output transformers back to the rear edge of the chassis, and the SCA Cap/EFB boards still may not fit. And I'm NOT a fan of boards hanging off the edge of the chassis. On to Plan B.

Starting to write Plan B...

Posted by Dave July 22, 2012 - 11:28 pm
Joseph --

While I do not have any "official" dimensions of the PAC board, I do have my own copy of the board, which measures 8 13/32" long, by 1 19/32" wide, and 2 5/16" tall, from the top of the heatsink to the bottom of the standoffs underneath the board.

Other than literally stacking these boards (which would certainly preclude mounting the cover in place), I cannot see how two of these boards could be mounted on the ST-35 chassis. The PAC board mounts on top of the chassis along side the transformers on the can cap side (with a cut out to accommodate the power transformer), in between the two amplifier boards. Of course, the PAC board also replaces the can cap with new components, and uses the hole where the can cap was to accommodate the leads to the new board.

You could certainly use one PAC board for one channel, and then hand wire the second EFB regulator and power supply components underneath the chassis for the other. You can reference Art's article on this site to see an excellent example of how this can be done.

It sounds like a great project you are undertaking. I'd enjoy seeing how it turns out!

Thanks again.


Posted by Joseph July 22, 2012 - 09:07 pm

First, thank you for the reply.

My plan is to have one power transformer, two sets of diodes - one for each channel - a separate power supply for each channel, with everything that follows kept separate, including the EFB(TM) controls for the output tube bias. I want to avoid spurious coupling/cross talk between the channels. Do you know if two Pacifica Audio Regenesis cap board/EFB modules would fit on the ST-35 chassis? I didn't see any measurements for the dimensions.

Thanks again!

Posted by Dave July 22, 2012 - 07:20 pm
Hi Joseph --

To use the EFB(tm) modification as presented with a single channel of the ST-35 would required no component changes at all. In fact, to illustrate that point, in a standard SCA/ST-35 that has been modified with EFB, you could simply remove the output tubes from one channel at will, and then operate the unit as a single channel mono unit with no other changes what so ever. The B+ would rise somewhat due to the reduced load on the power supply, but the EFB circuit would automatically adjust the bias on the remaining two tubes to compensate for their removal -- just as it compensates for any changes in B+ due to changes in line voltage or power output conditions when all four tubes are installed. This action is the very essence of how the "Enhanced" portion of EFB operates, and is one of the reasons that the performance improvements it provides in these amplifiers is so effective. You certainly couldn't just remove two output tubes at will with the original single cathode resistor design. The remaining two tubes would be seriously under biased, and red plate rather immediately.

For your other questions, I would encourage you to reference the "Housekeeping" section at the end of my article, as there I discuss alternate ways to implement EFB to allow the control of individual tubes or channels, while still using a single 337 regulator device. Of course, you could certainly use two separate EFB circuits for each channel if you're using two separate power supplies, but with a common supply, I just wanted to make sure you are aware of the other options available to you.

Also in that section, I make mention of using EFB in other applications with other tubes. While the concept can certainly be applied to other designs, in the form presented here, it is best applied to designs that require a relatively low bias voltage, so as to minimize the voltage drop across the regulator, and therefore limit its maximum voltage, dissipation, and heat sinking requirements.

Thanks for your interest in EFB(tm), and good luck with your project!


Posted by Joseph July 22, 2012 - 02:41 am

What part value changes are needed in the EFB(TM) to have it control a single channel on the ST-35? I plan to make my ST-35 into (more-or-less) a dual mono unit, so one EFB(TM) unit per channel seems in order. May I presume that the EFB(TM) is applicable to the other Dynaco mono amps - Marks 2,3, and 4 - with appropriate parts value changes?

Thanks for your research and your willingness to share it with the rest of the DIY community.

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