'79 SFDR Tube Bias

Discussion in 'Glowing Bottle Tube Amp Forum' started by Neener, Jul 31, 2021.

  1. Neener

    Neener Tele-Holic

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    This is my first time attempting to bias my own amp, my knowledge is super limited so bare with me!

    Installed a new set of tubes yesterday in a (heavily modified) '79 Deluxe Reverb, and it actually sounds really nice.

    I took a plate voltage reading with chassis on stand. I also am using a tube probe
    Tube 1 (V7) - 471v - (13.6-13.8 bias current)

    Tube 2 (V8) - 472v - (12.8-13 bias current)

    According to other threads this is totally wrong, but it sounds really nice. Are these functional numbers or is something way off? There is a chance the probe may be inaccurate. It's a cheapo from amazon.

    I also noticed that V8 (Tube 2) seems to be glowing a tad bit hotter. Is this normal?


    1.jpg

    Here is where I am taking readings for plate voltage

    2.jpg
     
    Last edited: Jul 31, 2021
  2. Neener

    Neener Tele-Holic

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    FYI:

    Bias cap is newer 50uf Sprague

    Filter caps are newer 20uf 475v

    (I have as set of 16uf filter caps as well as 25uf bypass caps on order)
     
  3. Paul G.

    Paul G. Friend of Leo's

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    EDIT: I JUST READ YOUR OTHER POST SAYING YOU'RE A TOTAL NOOB. I'M NOT COMFORTABLE ADVISING YOU TO GO INSIDE YOUR AMP. FOR A MINIMAL BENCH CHARGE, WHY NOT LET A TECH CHECK YOUR BIAS FOR YOU? You've made a big investment in a nice amp, and it should be operating properly. "It sounds good" doesn't mean it sounds its best.

    Below is my original post, for information only. I don't recommend a total noob does any of this without doing some reading, research and at least watching some videos for newbies. Thank you.

    I've pretty much given up on all those bias probe whatits. Some are ok, some are not, and some can damage your amp.

    You've got the plate voltage, good. Now, warm up your amp good, then shut it off. Discharge the caps. Set your meter to ohms and measure resistance from the center tap of the output transformer to each plate. You can find the CT at the standby switch. Write the resistances down.

    Turn the amp on, set your meter to the highest range DC Volts and measure the voltage at the CT (careful not to short out to the chassis) and at each plate. Write them down. Now it's time for arithmetic.

    Subtract each plate voltage from the CT voltage -- this is your voltage drop. Divide the voltage drop by the resistance you measured before. This is your plate current. Multiply that by the plate voltage and you will get the plate dissipation in watts.

    As always, be careful around a live chassis, hand in pocket, don't touch anything except with your probe and be extra careful not to short anything live to ground.

    Bonus: since your amp is pre-PCB, you can replace the cathode to ground jumper with a precision 1-ohm (1 Watt or better) resistor. Then you can measure the voltage drop across the resistor, and this is the same as the cathode current (Thank you Mr. Ohm). Then you can use a bias calculator such as RobRob's which will compensate for screen current and give you a quick result: https://robrobinette.com/Tube_Bias_Calculator.htm
     
    Last edited: Jul 31, 2021
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  4. schmee

    schmee Doctor of Teleocity Ad Free Member

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    What rectifier is in it? Your voltages are even higher than my BF's!
    Seems to me you are in about the right place with your ma for that high voltage. It doesn't have to be complicated. I hope you got JJ power tubes!.....
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes. The plate voltage is measured on pin3 were the blue and brown wires are terminated.

    Ime, the bias probe is probably (reasonably) correct.
    Someone has installed one hundred ohm resistors on pin8 of each power tube. These are acting as sort of a quasi cathode bias cap. I would replace them with a wire, or if you wish, a precision 1 Ohm 1% resistor (an alternative is 10 Ohm 1%). The precision resistors can be used to measure the plate dissipation as Paul G wrote.

    Get rid of the 100 Ohm resistors. It is not a mod or anything useful. Do not try to adjust the bias while those 100 Ohm resistors are in place. If you do not have a large soldering iron capable of soldering to a chassis, clip the resistor lead close to the resistor so you have a bit of the lead left on the chassis. You can then solder a wire or the precision resistor to that lead rather than having the hassle of soldering to the chassis.

    Try swapping positions of the power tubes to see if the heater glow changes. If the brighter glow stays with the tube *socket* the heater wiring/solder joints needs to be addressed. If one tube is always brighter than the other, I wouldn't be too concerned, as long as the tubes seem fairly well matched for plate dissipation.

    Unless there is a reason to replace the newer caps, don't bother. The uF rating is close enough. Your plate voltage will come down as you adjust for more mA through the tubes.
     
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  6. Ten Over

    Ten Over Tele-Holic

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    Those resistors are almost certainly one Ohm. The bands are brown-black-gold. 100 Ohm resistors would have burned-up a long time ago.
     
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  7. Neener

    Neener Tele-Holic

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    Yes, JJ Rec and Power Tubes!
     
  8. Neener

    Neener Tele-Holic

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    I know the basics of amp safety,
    No worries! I always discharge before entering chassis
     
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  9. Neener

    Neener Tele-Holic

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    When you say replace with wire, do you mean a wire from pin 8 to chasiss/ground?
     
  10. Wally

    Wally Telefied Ad Free Member

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    +1 with scheme on wanting to know what rectifier is 8n that amp. It should be a 5U4. The bias is cool…about 43% of max plate dissipation. Increasing the current draw would bring the voltages down, but I am going to think that you will still have high plate voltages. If you had 22ma of current draw and the plate voltage was down to 450vdc, the plate dissipation would be around 71%.
    So….what rectifier is in the amp at this time.
    I also do not see any reason for not using the bias probe.
    FYI, that glow you see in those power tubes is heater filament glow. With the plate voltage up around 471, it would be of interest to know what the heater filament voltage is.
    Caution…live circuits present dangers. Do not start taking voltage measurements on a live circuit unless you are aware of the dangers and the safety precautions.
     
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  11. Ten Over

    Ten Over Tele-Holic

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    Don't go changing anything until you know what is actually in there. With the amp off and cold, measure the resistance of the resistor that connects pin 8 to ground. Your meter may not accurately measure a resistance like one Ohm, but it will tell you the ballpark you're in.
     
  12. Neener

    Neener Tele-Holic

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    Wally it is a 5U4GB JJ rectifier.

    I'm going to set current a bit higher next time I fire up and report back.
     
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  13. Neener

    Neener Tele-Holic

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    Chassis photo dump

    Someone commented earlier it is a "nice" amp. I agree but some may not! :D

    As you can see, someone installed mid pots on both channels. I don't know what else has been done but it appears heavily modified.


    3.jpg
    4.jpg 5.jpg 6.jpg 7.jpg 8.jpg 9.jpg
     
  14. Wally

    Wally Telefied Ad Free Member

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    I remember it now, Neener!
     
  15. Wally

    Wally Telefied Ad Free Member

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    I would be looking for a 5R4 to drop some voltage there. It will be interesting to see what you get with more current draw. What 6V6s are in that amp?
     
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  16. Neener

    Neener Tele-Holic

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    I swapped power tube positions and the glow followed the tube, not the socket. On immediate power up the tip lit up

    10.jpg
     
  17. Neener

    Neener Tele-Holic

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    JJ 6V6 as well
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ten your eyes are better than mine. My old eyes see brn-blk-brn. If the colors are brn-blk-gold-gold it is a 1 Ohm resistor. In that case it can stay. It is not a precision resistor with that 5% tolerance code, so using them for bias readings may have some variance between tubes and the mA calculation may be off a bit. It will give a ballpark result and there is no problem leaving them in.
     
    Last edited: Jul 31, 2021
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  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes. That is how it was stock from fender. There is no problem with 1 Ohm resistors from pin8 to ground. 1R is hardly any resistance. It is only 1 Ohm more than a piece of wire after all.

    As Ten Over wrote. Make certain what you have there. I think Ten Over is right, the colors are probably brn-blk-gold-gold.
     
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  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @Wally and I are curious, What is the wall voltage and what is the 6.3 VAC heater voltage?
     
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