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Old April 8th, 2003, 02:45 PM   #1 (permalink)
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External Output Tube Matching Pot

I have a Fender PA 135 head (c. 1978), and it has both "Hum Balance" and "Output Tube Matching" external (screwdriver) adjustments on the rear. I think I've figured out the "hum matching" thing, but I don't know how to adjust the "Output Tube Matching" thing. I have 4 new so-called "matched" 6L6's", and I want to ensure that I'm getting the most out of the amp. I have 8 ohms worth of speakers into each of the 2 speaker jacks, so that's not a probelm. Any help on how to set/optimize/use the "Output Tube Matching" external adjustment???
Thanks,
The Ragman

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Old April 8th, 2003, 03:08 PM   #2 (permalink)
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The hum balance is on the filaments and should just be adjusted so the hum is minimal. The tube matching matches both sides of the p/p output section by adjusting the grid voltage on one side. This is the same as the bias adjustment on other SF amps (there are a few variations). By moving a couple wires around you can adjust the grid voltage to all tubes, thus having a real bias control.
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Old April 8th, 2003, 04:41 PM   #3 (permalink)
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As Tim mentioned....To use these controls; turn both channel volumes to ZERO. Fire up the amp and let it get stable. With the B+ on (not in STBY), put your ear to the speaker, and adjust the Output tube matching pot until you hear minimal hum. On my 135W TR, the hum will go completely away (Matched JJ's).

Next, turn on of the channel volumes to 10 (nothing at the input jacks). Put your ear to the speaker, and adjust the hum balance for minimal hum.

Be careful with the output tube matching control, as you can get the bias WAY out and cook a tube pair.

My personal feeling about these amps is that the output tube matching control is a valuable adjustment...due to the relatively high amount of plate currents that 2X2 6L6's can draw. IMO, If you want coltrol of the bias (not so critical on UL amps), add an additional control to the bias circuit.
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Old April 8th, 2003, 05:15 PM   #4 (permalink)
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The value of the tube matching control is diminished if you use matched/burned in tubes.
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Old April 8th, 2003, 06:55 PM   #5 (permalink)
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Tim, I agree that it's kind of a 'set and forget' type control/feature.

It's been my experience that tubes don't necessarily age at the same rate, and some tubes can change for some unknown reason. I've also had the opportunity to buy off the shelf tubes from an old electronics distributor (cheap)..and even have broken one or two accidentally by whacking one with the end of a guitar cord.

It akin to a spare tire. You never expect to need it, but..... :)
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Old April 9th, 2003, 08:23 AM   #6 (permalink)
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Thanks, Tim.

I never knew how to use these controls.
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Old April 9th, 2003, 08:25 AM   #7 (permalink)
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Thanks, eman!

I meant to say.
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