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Thread: Crazy Bias Issue

  1. #1
    Forum Member Michael Smith's Avatar
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    Crazy Bias Issue

    1977 Fender Super Twin Reverb, "converted" by a tech in 1997 from 180 Watts RMS to switchable 25/100 Watts. V6 and V11 tube sockets had the plate and grid wires removed. By adding some additional resistors, it looks like some sort of voltage divider was installed in conjunction with the 25/100 Watt switch being added to the rear panel.

    Well, today I got out my multimeter and measured the resistance from the plate of each tube to the power transformer center tap, then measured the voltage drop from each plate, and the plate voltage. Plugged these figures into my spreadsheet, and adjusted the "output tubes matching" pot on the rear panel to get the closest plate dissipation for all 4 power tubes. This was all done at the "25 Watt" setting. So at "25 Watt" setting, I have about 61 Watts of plate dissipation at idle.

    Then I put the amp into standby, switched to 100 Watt mode and again took the same measurements, without touching the output tubes matching pot, and got about 126 watts of idle plate dissipation, way above the maximum for 6L6's. If I adjusted the bias to get more reasonable values in the 100W mode, then the plate dissipation in 25 W mode varied a lot from tube to tube, and they were biased too cold.

    So I set the bias for the 25 Watt mode to around 50% and will not use the 100 Watt mode. 60 Watts is plenty clean and loud for this amp. I'm sure this is not the most complicated amp circuit ever built, but it is above my pay grade, lol.

    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

  2. #2
    Forum Member phantomman's Avatar
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    Re: Crazy Bias Issue

    Fender's six-tube output stages were always a bitch to set and adjust -- I'm surprised you got it that close. It was even worse (and more critical) for the 400PS.
    "When injustice becomes law then rebellion becomes duty."

  3. #3
    Forum Member Michael Smith's Avatar
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    Re: Crazy Bias Issue

    Quote Originally Posted by phantomman View Post
    Fender's six-tube output stages were always a bitch to set and adjust -- I'm surprised you got it that close. It was even worse (and more critical) for the 400PS.
    The owner's manual reads as follows: "Output tubes matching, This control makes it unnecessary to use matched power output tubes for optimum performance. Best balance is obtained by setting the Output Level to "1" and adjusting for minimum hum level. Ordinarily, no adjustment should be required unless the output tubes are changed".

    But I was impacting the plate current significantly by adjusting the pot (measured on each power tube). I'm glad I decided to take measurements. Perhaps a previous tech converted it to a true bias adjustment pot. So much wiring has been changed, the schematic is not very useful.
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

  4. #4
    Forum Member phantomman's Avatar
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    Re: Crazy Bias Issue

    Time-consuming as it may be, you might have to draw an all-new schematic based on your observations. You'll be able to document any and all changes made to the original circuitry.
    "When injustice becomes law then rebellion becomes duty."

  5. #5
    Forum Member Michael Smith's Avatar
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    Re: Crazy Bias Issue

    I think I'll leave well enough alone and put the chassis back in the head cabinet. All of the original electrolytic caps in the power supply have been replaced, along with some of the caps on the eyelet boards. It sounds good, and I've got a safe bias setting now. I also made a label for the rear panel, so I or a future owner will know to leave the amp set on the 60 Watt output setting, or to re-bias it if they plan to use the 120 Watt setting. This would probably entail replacing the 2.7K bias resistor, as it measures only about 1.7K currently, which is fine for the 60 Watt setting.
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

  6. #6
    Forum Member phantomman's Avatar
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    Re: Crazy Bias Issue

    Let sleeping Bassett Hounds lie.

    "When injustice becomes law then rebellion becomes duty."

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    Forum Member Michael Smith's Avatar
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    Re: Crazy Bias Issue

    Quote Originally Posted by phantomman View Post
    Let sleeping Bassett Hounds lie.



    Are you implying the Super Twin Reverb is a "dog" of an amp? I probably wouldn't disagree, and either would the sellers of them on Reverb.
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

  8. #8
    Forum Member phantomman's Avatar
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    Re: Crazy Bias Issue

    Frankly I never saw the need for such a temperamental behemoth even when there was a need. When I bought my '78 UL Twin Reverb the store also had one of those Super Twin Reverbs on hand (for about $180 more). I took a fast look at the chassis and politely declined it. It was easily about 20 lbs heavier than the standard TR, even with the Twin's pair of D120F JBL's and the rat's nest wiring looked like a nightmare to service.

    So in response to your whimsical comment I offer but one word......

    WOOF!

    "When injustice becomes law then rebellion becomes duty."

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    Forum Member Michael Smith's Avatar
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    Re: Crazy Bias Issue

    Now that mine has been "tamed" and biased correctly for 60Watts output, it has tons of clean headroom. Mine was a pawn shop purchase over 20 years ago, and since I put the chassis in a head cabinet it's much more manageable. I still use a small dolly to transport it around the studio from room to room. I added an additional label to the rear, so the next "lucky" owner won't red-plate the power tubes. "The Rascal" has given it the mandatory cat scan, and it's been approved for use.

    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

  10. #10
    Forum Member phantomman's Avatar
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    Re: Crazy Bias Issue

    When the cabinet of my UL TR finally crumbles into dust I'm gonna build a head cabinet for the chassis and throw the JBL's in my '67 silverface drip-edge Bandmaster enclosure. Currently in the BM are a matched pair of Concert Series Jensen C12Q's from 1966.
    "When injustice becomes law then rebellion becomes duty."

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