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In Reply to: RE: The Dynaco SCA-35 is back! posted by mark.korda@myfairpoint.net on April 13, 2014 at 11:35:38
That simple modification makes quite a difference in the sound of an SCA-35 or ST-35.
I had to do a bit of searching to see that they're using 6GH8 as the input tubes on the amp board, since the ebay ad doesn't say. That's a good thing, because as Eli says, the 7199 is "unobtainium"
Follow Ups:
Hi Shovel, on the post I mentioned the volume controls. I'm sorry I meant the tone controls. With such a project wouldn't you think they would leave those out and style the faceplate accordingly? Loudness and filter too. Also, why would the SCA-35 have the 2 hum pots on the back if they got rid of the noisy 7199 tubes in the Gillespie mod. My ST-35 amp (stock) does not use the 7199 and has no hum pots. Just a thought Shovel...Mark K.
Hum pots are on the SCA-35 for the phono stage, due to the AC on the filaments. This is why there are no hum pots on the ST-35.
Hi SteveO and Flyquail56, thanks for the great answers. I had no clue and learned a good lesson. Thank you so much...Mark K.
I should have said "Nice active link to ebay" in reply to your original post! Thank you.Steve O and Flyquail have explained the hum pots nicely!
As for omitting the tone controls, etc. Some people want a re-issue to be as close as possible to the original. Changing too much might cost them sales, since it wouldn't be considered an SCA-35 anymore.
If you have a ST-35, I strongly suggest you consider the EFB modification. It doesn't eliminate the 7199 in the SCA-35, or the 7247 in your ST-35. What it does is change the cathode bias of your amp to an "enhanced fixed bias". By doing so, it improves the power output, lowers measured distortion, and the amp sounds much better.
You don't have to buy some kit, or boards, etc. The few parts necessary should cost you about $5, and greatly improves the sound of the amp. I haven't read one negative comment from anyone who's done the EFB mod. Edit: Which includes me, hearing the improvement to my SCA-35 after switching to EFB.
The specs shown are from a SCA-35, but the improvement will be the same for the ST-35.
Edits: 04/14/14
The hum balance pots in the SCA 35 are for the low level preamp stages and re not related to the power amp driver tube. The ST35 is a power amp and has no need for hum balance pots...if nec they'd be in the outboard preamp.
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