If anyone has a Dynaco SCA-35, here is a pretty easy mod you can do to it by rewiring the filter switch as a tone control bypass switch. The slide switch to the left and the tone controls are in circuit, to the right they are bypassed and the volume doubles as the tone circuit attenuates the signal.

I was going to permanently bypass the controls then I found some instructions using the filter switch. Its posted here for your modding pleasure and a photo of a SCA-35 I did today. I did this to 3 SCA-35 this year.

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SCA-35 Tone Control Bypass Function

Step-by-step instructions:

1) Desolder and remove everything from the filter switch (wires, capacitors, etc.) so that the lugs on the switch are completely empty. Leave the two wires that come from the volume control attached to the volume control.

2) Rewire the two wires coming from volume control lugs 3 and 6 (that used to attach to the filter switch) to the stereo/mono switch lugs 4 and 5 (i.e. where the 2 leads from the filter switch used to connect). The green wire goes to stereo/mono lug 4 and the red wire goes to stereo/mono lug 5.

3) The wires that run from bass control lugs 2 and 5 to balance control lugs 1 and 6 need to be re-routed to filter switch lugs 3 and 6. At first, disconnect them from the balance control only and see if they are long enough to reach filter switch lugs 3 and 6. (Red goes to filter lug 3; green goes to filter lug 6.) If they are, solder them there. If they are not long enough, disconnect them altogether and run new, longer wires to make the connection.

4) Run a new set of wires from balance control lugs 1 and 6 to filter switch lugs 2 and 5. Balance lug 1 goes to filter lug 5; balance lug 6 goes to filter lug 2.

5) Run a new set of wires from volume control lugs 2 and 5 to filter switch lugs 1 and 4. Volume lug 2 goes to filter lug 4; volume lug 5 goes to filter lug 1.

This completes the re-wiring. Now, when the filter switch is in the ‘off’ position, the tone controls will be out of the circuit. When the filter switch is in the ‘on’ position, the tone controls will be engaged. Note that the gain will now be much higher when the tone controls are out of the circuit. If you have been listening to the unit with the tone controls engaged, remember to turn the volume down before disengaging them as there will be a significant jump in volume when this is done.
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Hello Sal im in the process of upgrading / modifie a Dynaco-sca-35. I will try to make your tone-mod.
But Im a little confused by the numbering of the lugs on the slider switches, that you mentioned.. Could you please
tell / show how the numbers refers to the lugs in your instruction?
Thanks a lot - Peter, Copenhagen, Denmark

Ive tried to make the mod, very carefully following the instructions. When I turned it on = no sound - or a tiny allmost "not there" signal.
Is there anybody else who can explain the numbering of the slider switch with 6 poles/lugs .
I have just made a big upgrade to the Powersection of the amp and is was working ok, before attempting the mod.
- Peter

Hi Peter,
I just saw your post, did you ever figure out how to wire the tone bypass switch?

Sal

This is really timely as I just bought (haven't gotten yet) an SCA-35. This is the most promising page I've found so far. Not letting me post it for spam purposes, far enough. Search for Dynaco Doctor. My third is the one I mean. The first two look interesting.

KO

This is an extremely useful modification. The tone control circuits on the SCA-35, PAS 2/3, PAT4, are generally just awful, hair-trigger to adjust, and usually not matched very well between the channels. As I recall, all of the later preamps and integrated amps have a tone defeat switch for exactly this purpose.

Brett