Welcome to Dave’s Lab, where improved sound comes from improved performance. Here, audible and measured performance go hand in hand to achieve the highest measure of fidelity.

Even the best vacuum tube equipment from audio’s golden age contains compromises to allow for the economy of mass production. I take some of the best and most loved pieces of this equipment, search these compromises for worthwhile performance opportunities, and offer appropriate modifications to achieve them. These mods always come complete with an explanation behind the changes made, and full documentation as to before and after performance.
 
Please be sure to check in often. More great mods from the lab will be posted here in the future.

Short Subjects from the Lab

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:

Reader Comments

Posted by Dave

November 09, 2011 - 10:20 pm

Hi Hrudu -- The EFB(tm) mod is just as appropriate for the ST-35 as it is for the SCA-35. On the ST-35, R-42 is located near the bottom of the can cap, and connects between one of its main terminals, and one of its ground lug terminals. If the original R-42 component is still installed in your amplifier, it is a 95 ohm 5 watt resistor. While the article itself shows installation in an SCA-35, there are numerous examples on both the diytube.com and audiokarma.org sites showing how folks have installed the modification in their ST-35 amplifiers as well. If you go to those sites and search under "EFB", you will find plenty of discussion and very good pics showing different ways the EFB mod has been installed in these amplifiers. You can use them and the discussion to help with your installation. If I can help anymore, don't hesitate to ask. Good luck with it! Dave

Posted by Hrudu

November 09, 2011 - 11:41 am

Can I do this mode on a ST-35? Where is the R42? :)

Posted by Bill

February 09, 2011 - 10:17 pm

I thought I noticed the tie between the resistors before but wasn't sure. It makes perfect sense now. Thanks

Posted by David Gillespie

February 09, 2011 - 07:20 pm

Hi Bill -- You got it correct in your last post. I tried to maintain the original Dynaco color code with the modification, so in the pic with the 5 ohm resistors, the green wire on the left goes to the cathodes of the left channel, the red wire on the right goes to the cathodes of the right channel, and the red wire in the middle goes to the output of the EFB regulator. In the pic of the filter cap wiring, the old cathode bypass cap is the terminal at 11:00, and has three wires connected to it: a red wire identified as "to EFB output" which comes from the 337 regulator, a black wire which goes to the two hum balance controls, and an additional red wire which is the red wire coming up between the two 5 ohm resistors. It is not easy to see, but careful inspection will show that the two center terminals where the 5 ohm resistors are mounted are tied together to make for a common connection to both resistors at that point. I apologize for the confusion, and hope this helps! Dave

Posted by Bill

February 08, 2011 - 07:37 pm

Or is the middle pink wire common for both resistors (coming from the output of the EFB) and you used green for one channel and pink for the other maybe? This would make sense, but hung me up for a while. Or maybe I'm getting too old!

Posted by Bill

February 08, 2011 - 06:28 pm

I have been studying the EFB mod and am wondering if there is a wire missing on one of the 5 ohm resistors going to the cathodes of the output tubes. I'm assuming that the pink wires are EFB output, but I don't see a second green wire going to the cathodes of the other channel. Or am I missing something maybe?

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