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PostPosted: Fri Apr 19, 2019 3:10 pm 
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Could someone verify for me how the bias switch for the EL34 mod is supposed to function and how you can check to verify that it's wired properly before actually operating the amplifier. I see that one side of the switch has no wires going to it. Am I correct in my understanding that when the switch is in the EL34 bias position both bias resistors are in the circuit to provide the correct bias to V2 because they are wired to the end and middle terminals of the DPST switch. The EL34 bias resistor is wired to both of the end terminals and the 6V6 bias resistor is wired to both of the center terminals. Then, with the switch in the opposite position for 6V6 bias, only the one 750 ohm resistor is in the circuit because that resistor is wired to only the center terminals of the DPST switch. The lead that is wired to V2, Pin 3 is soldered across both of the center terminals to provide the bias needed. Hope I've explained myself sufficiently.

Thanks!


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PostPosted: Fri Apr 19, 2019 4:10 pm 
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Draw an imaginary line from toggle of switch thru the switch to the terminals. For 6V6 operation the line should point to empty, unused terminals. For 6L6 the imaginary line should go thru the terminals that are wired to the 560R (or 1K) resistor. So, yes you are correct. I found this file somewhere on the forum and saved it, and am going to put it somewhere on my bad girl.
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PostPosted: Fri Apr 19, 2019 4:48 pm 
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Thanks kindly for that explanation. Can you test for continuity through the switch from each wire that's connected to the terminals? I was trying to do this but I don't seem to be getting readings that make sense.

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PostPosted: Fri Apr 19, 2019 5:41 pm 
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You should be able to get continuity between between components (resistor and pin). Set meter to continuity, put one probe on pin 8 and touch other probe to each resistor eyelet on board. With switch in 6V6 position, you should have continuity on 750K resistor and none on 560 or 1K (depending on which you used). In the 6L6 position you should get continuity on both resistor eyelets.

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Last edited by Hankules on Fri Apr 19, 2019 5:50 pm, edited 1 time in total.

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PostPosted: Fri Apr 19, 2019 5:47 pm 
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Hope you are not checking continuity under power. If using ohms setting, you get a resistance reading and if you get OL (Open Lead) or infinity reading, then you have an open circuit. Continuity setting on my meter has beep tone if connection is good. Hope this is not more information than you need! I tend to over explain, sometimes.

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PostPosted: Sat Apr 20, 2019 9:39 pm 
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Not to worry. I appreciate your thoroughness and usually need things explained to me at least 3 times. :D I definitely did not check anything with the circuit live. Used my ohmmeter set to resistance to make sure I didn't have an open circuit. Don't have my resistors installed yet but just wanted to make sure the soldering joints were good. I'll re-check after I get the wires connected to the resistors on the board. I tend to be extra cautious and check and re-check everything. Good thing to, I found I had two of the filter capacitors installed with the incorrect polarity! Whew! You know the saying, measure twice, cut once. It certainly applies in this case.

Thanks Again.


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PostPosted: Sun Apr 21, 2019 4:05 pm 
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Know what you mean. Got my Tramp done last week and was getting crazy voltage readings on V1 (12AX7). Kept checking this and that, swapping tubes, checking voltages again. Today, Easter, I checked for continuity from the jumpered ground at top right of board to the jumpered grounds at bottom right that run to the preamp star ground. Continuity on the next to last eyelet , but none on the last! Added a jumper on top of board between eyelets, got a continuous ground across eyelets and to chassis (preamp) ground. Now voltages are within spec.

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PostPosted: Mon Apr 22, 2019 8:13 pm 
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The bias switch simple puts a second 5w power resistor in parallel with the 6V6 cahthode resistor thereby decreasing the total cathode ressistance and therefore increasing the bias voltage to the higher power tubes. You should be able to measure the change in resistance in both positions.

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PostPosted: Mon Apr 22, 2019 11:09 pm 
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Thanks Stephen. Appreciate your input. :thumbsup:


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