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Questions About 5F11 Biasing

Discussion in 'Amp Tech Center' started by DennisM, Nov 22, 2020.

  1. DennisM

    DennisM Tele-Meister

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    Thank you very much! I'm learning tons here. The red wire is connected to the b+1. I'm comfortable measuring on the plate pins. I did it with the cathode biasing on the 5e3. I know there's a ton of voltage there. I'm extremely careful. I've never done fixed bias...had no idea how to get these numbers from the OT. So, when it get's to operating temperature, I can set the meter to read ohms instead of VDC without messing up the meter with high voltage?
     
  2. DennisM

    DennisM Tele-Meister

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    Double post
     
    Last edited: Nov 23, 2020
  3. kingofdogs1950

    kingofdogs1950 Tele-Afflicted

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    My Mojotone 5f11 came set at ~58% and I haven't changed it.
    I did buy the bias kit which I will install one of
    these days, but for now I'm a happy camper.

    M

    pxlekcbcjx6r0fmevh6f.jpg
     
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  4. NTC

    NTC Tele-Meister

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    I must bow to Lowerleftcoast - I hadn't considered a change in resistance with temperature. Note that you CAN'T measure the resistance with the power on! You will still need to turn it off and discharge the caps before measuring the resistance. When taking thebias measurements, make sure the tremelo is off.

    I think another consideration is to have a pair of test leads with clips to make this safer. I just biased mine without such clips, but it is easy to short someting or blow up a meter if your probe slips - i have one with a blown ohms section and one with a blown current section due to such probe slips. It can be expensive and hazardous.

    Once you get used to this, try different bias settings to balance tone with the tremolo effect [vibrato? I never get this right]. Mine is sounding good at 70%, but perhaps I should back off some...
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @NTC has it right. Measure the resistance after you have shut it off. Once you have a good OT resistance measurement, that measurement should not change for the life of the OT. You can write the resistance inside the chassis for future bias adventures.

    It would also be good to have that resistance number should the health of the OT ever becomes suspect.

    You will get the hang of this fairly quickly. Don't get hung up on the numbers. The important thing is not to red plate the tubes.

    I suggest you find the 70% Plate Dissipation position on the bias pot, then play the amp at volume adjusting the pot to where the amp sounds the best. As long as the pot is not much above the 70% PD, that is where it should be set for your ears, with those tubes. As mentioned the trem on some amps becomes weak with higher bias settings, so listen for that as well.

    The original Fender amps were generally biased cold. I think your ear will lead you to somewhere around 60% PD. On High gain amps sometimes the 70% PD will sound best. Not always.;)

    So my advice is set it by ear and check to make sure it is not too hot.

    Another thing to know is that each tube is different. Some can handle a lot more than others. The companies have assigned wattage ratings that may not be accurate from one 6V6 to the next. For example a new JJ 6V6 is rated at 14W but can usually deliver much more. In contrast the new TungSol 6V6 is rated for 12W and sometimes can't deliver the 12W.

    And lastly, tubes last longer at lower bias settings. So, if your ear doesn't hear a difference, use the lower setting. On the other hand if the amp sounds great at 85% PD and it is not red plating, enjoy the sound but know the tubes will have shorter lives.
     
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  6. DennisM

    DennisM Tele-Meister

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    Will do. I am using TungSol 6v6's. JJ 5Y3. That's what was in the 5e3. I plan to use the Mojotone recommended 33K bias resistor from the pot to gnd. Before I fire it up, I will try to set the bias pot to read 56K (with the 33K) as on the originals and go from there. I think I can get the V drop, OT resistance and plate current now from all the help I've received here. I'm gettin fmart :D I was running an even 330VDC on the plates in the 5e3. Hope I can get that on the 5f11...eyes always peeled for red plating. One quick question: Is the 50V bias tap supposed to be AC? That's what my meter said when I tested it.
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    After the diode it should read VDC.
    The diode is what changes it. A rectifier tube is just two diodes in tube form rather than Solid State.
     
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  8. DennisM

    DennisM Tele-Meister

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    Got ya! Thanks tons and have a Great Thanksgiving.
     
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  9. DennisM

    DennisM Tele-Meister

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    @jsnwhite619 @Lowerleftcoast Fired it up. I started with the 10K bias pot with the 33K resistor. It was a diaster. Red plating, 84 mA, 266 VDC on the plates. Took it out this morning and just wired in the recommended 56K. MUCH better. Plates 400VDC, 20mA current. Ended up with -39VDC bias voltage. The amp sounds stellar and the tremolo is really strong. I also changed out the voltage dropping resistor from B+1 to B+2 from 4.7K to 470 Ohms as per the original schematic. Still a bit confused on how to calculate PD. Here's what I have:

    Plates: 400VDC
    Plate current: 20mA
    Voltage drop brown lead: 3.2
    Voltage drop blue lead: 2.06
    Primary resistance red to brown: 220 Ohms
    Primary resistance red to blue: 216 Ohms
    Bias voltage: -39VDC

    Still might bring the 20mA's up to 23/24 to get the plate voltage down a bit. Sounds pretty wonderful like it is though.
     
    Last edited: Nov 28, 2020
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Rob Robinette has a bias site. It makes finding the PD easy. Go to his site.

    First you choose the tube wattage you are using. For 6V6 either pick 6V6 12w or 6V6 14w. (Choose 12w for TungSol 6V6 ReIssue tubes. Choose 14w for most every other 6V6.)

    Next you put in your Plate to Cathode Voltage. (On the stock Harvard 5F11 that is the same as plate to ground.)

    Press *Calculate*. (Next to the *Class AB fixed bias* you will see the 50% 60% and 70% values calculated.)

    Now you will use Ohm's Law to find the current. Use a calculator and divide the voltage drop of the OT by the resistance to find the current.
    3.2/211= 0.01517 = 15.17mA
    2.06/207= 0.00976 = 9.76mA

    Compare the mA with the Bias Calculator figures. With the bias calculator set for 6V6 12w tubes and 400 Plate to Cathode voltage it shows 15mA is 50% PD. You have one tube right about there with 15.17mA the other tube is less than 50% PD at 9.76mA.

    That is it. You have a cool setting.

    The tubes hardly ever match perfectly. Some discrepancy comes from the tubes and some comes from the amp. I always swap the tubes into the other sockets to see if the bias can become more even out of curiosity. You may or may not want an evenness for sonic sake. Listen and see.

    If you like math you can figure all of this out to more exacting numbers. Jon Snell in an earlier post showed how.

    In this case we have a 400v plate to cathode divided by the 12w tube = 30mA for 100%PD.
    (Rob's calculater shows 30mA at 100% PD as well.)
    So let's continue and find the % when we have 15.17ma. 15.17/30 = 50.57% PD.
    When we have 9.76mA. 9.76/30 = 32.53% PD.
     
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  11. DennisM

    DennisM Tele-Meister

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    Crazy thing is those are Apex matched tubes...the Tung Sols. Oh well. I just changed the bias resistor to 48K which brought the bias voltage down to 38VDC and raised the pc. According to the calculations you just gave me, I raised the mA to 18 and 12, but on my bias probe I bought (tube goes in to probe and probe plugs in to socket and hooks to meter) I'm getting 23.8mA. Didn't bring down the plates though. Sounds really great here, so I'll leave it. Thanks much!
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The probe is probably finding the total current going through the tube. Plate current and Screen current. The screen current would need to be subtracted.
    When using the OT resistance method only the Plate current is being measured. That probably explains most of the difference in the measuring.

    Tube matching is not perfect. A tolerance of 5% is fairly standard.
    That said the amp is also responsible for some mismatch of the waveform which may be by design. To see how well your tubes are matched swap their tube socket positions and get another reading. If say, socket V3 is still at 18mA and socket V4 is still at 12mA no matter which tube is in each socket then the tubes are very well matched.

    With 5f11 type of bias, I think you have it near the sweet spot. I don't doubt it sounds good.
     
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  13. DennisM

    DennisM Tele-Meister

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    I absolutely love this amp. I did not like the 5e3....too much amp...and that's why I converted it. I have been playing around with the biasing. I'm very pleased with what I'm getting on the 6v6 plates: 343VDC. I had to reduce the 33k bias resistor to a 22K, and adjusted the bias pot all the way up to read 32K and slowly worked down until I got where I wanted to be. I ended up with 27.2 VDC bias voltage which brought the plate current up and the plate voltage down. I'm using Tungsols (but no more). I did swap out the tungsol V3 for a Groove tube I had laying around because I was having too big a difference in voltage drop between them. The Groove Tube made it way closer to V4, still lower...but better. I ended up with 20.8 mA on V4 and 18.8 mA on V3. 7.15 watts on V4 and 6.45 watts on V3. 60% on V4 and 54% on V3. That's where it sounds so good. Tremolo is luscious.

    I won't be buying TungSols in the future. Too high a failure rate and they act weird.

    I just wanted to thank you all for teaching me how the fixed biasing works, and for coaching an idiot through it. I couldn't have done it without you. I LOVE this 5f11 circuit!!
     
    Last edited: Dec 4, 2020
  14. NTC

    NTC Tele-Meister

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    I must have missed it somewhere - what power transformer did you use?
     
  15. DennisM

    DennisM Tele-Meister

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  16. NTC

    NTC Tele-Meister

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    Thanks. Similar to the Hammond 290BX I have been looking at.
     
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  17. DennisM

    DennisM Tele-Meister

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    The culprit, if anyone is interested. I love this amp. 5E3 converted to 5f11. I built the cabinet too. Only put in two inputs.

    vibrolux 001.JPG vibrolux 002.JPG
     
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