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Old 30th April 2009, 10:42 PM   #1
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Default Simple SE cathode feedback...

Ok, this is the first issue that I've had with my Simple SE that I can't solve by thinking about it...

I wired up my cathode feedback switch like so:

Switch:
1 - OPT +
2 - input to SE PCB (not the gnd side)
3 - gnd

same for both channels. I'm running KT88's at the moment. When I switch to cathode feedback mode - I get barely any output to my speakers... turned all the way up I can barely hear it.

When I switch to non-feedback mode everything is the way it was before the switch mod (UL and non UL mode work fine)...

any hints?

Oh - and while the wiring diagram shows the gnd connection to the second post on the SE - I made the assumption that it was already grounded in the system... I'm thinking that this is probably the issue - ???

Steve
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Old 30th April 2009, 11:10 PM   #2
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My guess? Your switch does not have the pole where you think it is, and you are inadvertently shorting the OPT secondary to ground.
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Old 30th April 2009, 11:24 PM   #3
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Then I should get nothing... gnd to gnd.

And I did check the switch when I was wiring it.
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Old 30th April 2009, 11:40 PM   #4
chrish is offline chrish  Australia
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I wired as per the diagram and it worked. Maybe if you "changed something" - that is your problem. The only change I had to make was that the Edcor transformer was wired "backward". I ended up with positive feedback. Just swapped the secondaries around and fixed.

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Old 1st May 2009, 12:28 AM   #5
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It does have the feel of a missing ground - especially since I am getting output - just really low volume.

I'll add a ground wire to each channel to the grounding posts (damn things are pretty crowded now) tomorrow and see if that fixes the problem.

BTW - The KT88's I got from ebay - http://stores.ebay.com/SRS-Webstore for $56 for a pair shipped.

Makes the amp look even beefier with the bottles.
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Old 1st May 2009, 01:46 AM   #6
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You shouldn't need to add an extra ground wire to the speaker binding post. The Phoenix terminal on the board provides the ground connection.

I believe these composite photo/sketches are correct. It is important to note that my Simple SE is built using the "under chassis" mounting style. If your board is mounted above chassis, then all the connectors on the board will be on the other side (mirror imaged, if you like). Also keep in mind that some people will need scheme A, and others will need B. It depends on how the output transformer is wound. Note the connections to the switch, board, and speaker remain the same - only the polarity at the output transformer needs to get flipped.

Click the image to open in full size. Click the image to open in full size.
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Old 1st May 2009, 11:05 AM   #7
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Thanks Ty, you guys go the extra mile sometimes - I appreciate that!

From your diagram, what I have done is to not connect the gnd side of the screw (phoenix?) terminal on the PCB. In one position the switch shorts the CF terminal to gnd, in the other it connects the OPT to the PCB ...

My thinking is that there are two other gnd connections on the board at the inputs, I shouldn't need another pair on the output.
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Old 1st May 2009, 12:58 PM   #8
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If you are not using feedback, you can leave the secondary of the output transformer floating. It does not need to get referenced to ground.

If you are using feedback, one end of the secondary winding gets connected to the feedback circuit (the Phoenix terminal on the board) and the other end of the winding MUST be attached to ground. Otherwise it doesn't work, as you have discovered. The secondary winding is not inherently ground referenced somewhere inside the transformer - it is completely insulated.

I would encourage you to use the ground terminals available on the Phoenix blocks (terminal O1 at T1-SEC and T2-SEC), even if it requires running an extra piece of wire to your CFB switch or speaker binding post. George is usually careful to ensure the grounds on his boards are correctly routed and do not induce hum. If you start running your feedback circuit grounds through some other ground plane (or worse yet, the chassis) I'd be concerned you're running the risk of forming a ground loop and causing unwanted hum.
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Old 1st May 2009, 01:00 PM   #9
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crap - I know what I did... I used the wrong terminal on the PCB... I saw the cap negative going to the screw terminal and thought - that has to be the ground side.

Duh.
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Old 1st May 2009, 07:51 PM   #10
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I see your point Ty. It would be cleaner to have the speaker wires come from the board.

Just moved the wire over - all fixed! I must now go listen

Thanks for the help
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