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EF80 PP amplifier with adjustable fixed bias

Discussion in 'Shock Brother's DIY Amps' started by reggiepe, Mar 9, 2021.

  1. reggiepe

    reggiepe TDPRI Member

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    Is it possible? I'd like to experiment with making an EF80 amplifier with bias wiggle tremolo. I know that we can do bias wiggle with a cathode biased amp, but it is not nearly as strong as it would be with a fixed bias output section. I have searched hi and lo for an example of EF80's in a fixed bias circuit and can't seem to locate anything.

    Thanks
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    It should be possible but the bias voltage and range will be small due to the small grid voltage swing of the little tube. You'll probably have to tweak the bias circuit resistors to get the voltage and range correct.
     
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  3. mrriggs

    mrriggs Tele-Meister

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    It appears that the EF80 does not have the suppressor grid internally tied to the cathode. You could inject the LFO signal there (Pin-9) and not have to mess with the bias signal at the control grid.
     
    Last edited: Mar 9, 2021
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  4. reggiepe

    reggiepe TDPRI Member

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    So, it could remain cathode biased then. Very nice find! Thanks!

    Thanks Rob! I had seen some 12BH7 circuits that were fixed bias....IIRC they were somewhere around -15
     
  5. mrriggs

    mrriggs Tele-Meister

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    Well, you'll have to try it and let us know how it works. The EF80 wasn't designed as a "dual-control" pentode so the suppressor grid will need a much larger signal swing than the control grid to affect the output. You may end up with the coolest tremolo ever devised or it may not work at all. Chances are you will end up somewhere between these two extremes. Never know til' you try.
     
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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    A 12BH7 fixed bias circuit should be very close to what an EF80 needs.
     
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  7. reggiepe

    reggiepe TDPRI Member

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    I have some work to do. I will return with results....good or bad. This thought process comes from one of my favorite circuits to build.....the 6G3. I have built three to date with one of them having an EL34 power section, but I wanted to get something in a lower power with a pentode. @robrob 's research gave me a push to try it with this Brownface circuit. I believe this one won't part your hair at 30 paces.
     
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  8. reggiepe

    reggiepe TDPRI Member

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    Built it and it needs some tweaking....well, I built the cathode biased version. I wanted to go off proven design with the EF80 first. I also disabled the tremolo in the circuit as I wanted to work on tone first. She goes into overdrive too quickly as wired and I'm thinking that may be due to voltages being too low in the preamp. Current droppers are at 510R, 10K, 27k

    I am going to convert to fixed bias today to maybe tighten up the output, but I have a question. I have gone back and forth with standard octals with grid bias and cathode bias but the pins on this one confuse me a bit. EF80 Pinout attached.

    Filaments (4,5), plates (7), and screens (8) are a given. Should I tie 1,3, and 6 to ground and then bring in the signal for the grids on 2 and 9 together, or just to pin 2?

    EF80_Pin_Out.jpg
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If you don't use a voltage divider to dump some signal into the little EF80 they will overdrive very early in the volume pot rotation.
     
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  10. reggiepe

    reggiepe TDPRI Member

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    thanks @robrob that did it! I had to dump 90% to ground to get the same onset of overdrive as it's big brother. Any recommendations of the pin wiring for fixed bias on the EF80?
     
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  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Socket wiring will be standard EF80 except the cathode will be grounded. Tie the cathode, suppressor and shield together.
     
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  12. reggiepe

    reggiepe TDPRI Member

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    Thanks @robrob

    It looks like I need a negative bias voltage around -2 to -4vdc with plates at 250vdc. One leg of the secondary off of the PT is reading at 204vac. I figured I would need a voltage divider to dump about 98% of that to ground. This seems to me to be super inefficient and will need some very large wattage resistors (15+ watts?) in that divider. Am I right thinking this way or is there a more efficient way of getting my negative bias voltage?
     
  13. reggiepe

    reggiepe TDPRI Member

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    Sorry, my mistake...I was running the numbers with dropping using the current capabilities of the transformer. Forgive my previous ramblings regarding the resistor wattage needed.
     
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  14. reggiepe

    reggiepe TDPRI Member

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    Did it! Negative Bias voltage @ -3.5vdc. Tubes are seriously mismatched with one running at 66% PD and the other at 86%. It is probably the reason that I am also chasing my tail with the bias wiggle tremolo. That 220k at the .1uF junction that goes to the intensity pot has me all the way up to 2.2M and 3.3M. At 220k it is killing those little EF80's. I took a decade box and dialed it up to the 2.2M selection and it was great with single coils, but would distort with buckers. 3.3M works for all, but it has lost it's bias wiggle characteristics (don't feel the pulse anymore) Anyone know of another place to dial it back some to try? I have some more EF80's coming in the hopes that I can get a closer matched set. My other option is to install a bias balance pot to get them a little closer.
     
  15. reggiepe

    reggiepe TDPRI Member

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    I can honestly say that this works. A NOS set of Mullard EF80's with a closer match than the ones I had sound a bit better. I was able to get a voltage divider to get the signal knocked way down on the low powered 6G3 circuit (dumping 97% to ground)...even further than I mentioned before. The 220k resistor in the tremolo circuit was replaced with a 6.8M to tame the swings down (I'd still like to hear if there is another way to do that instead of that big resistor) I will get a 6.8M in my next order to put that in permanently as it has a decade box connected for that resistor right now.

    When it overdrives about halfway into the volume dials, I hear a top end fizz that I need to get rid of (changed speakers 3 times, so I know it is in the circuit) I can get rid of it with the tone control, so it seems it may be a cap change or addition somewhere in that circuit.

    Thanks to all that helped in this experiment.
     
  16. Nickfl

    Nickfl Friend of Leo's

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    Could you share your schematic as currently built? I've been working on an EF80 JCM 800 and I've also noticed that these tubes seem prone to fizz. I've pretty well got it beaten in my build after a couple weeks of tinkering and I may have some ideas if you're interested, but my circuit is pretty different from yours and I'd probably be able to make more relevant comments if I could see what you've got going on first.
     
  17. reggiepe

    reggiepe TDPRI Member

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    Thanks @Nickfl I'd love to hear your recommendations on the fizzy overdrive.

    It's pretty much a 6G3 up to the power section. A few changes made to knock down the signal for the EF80's. My power section differs from yours as it is fixed bias. I built it with cathode bias first as that had been a proven output design, then once I had that working, I converted to fixed bias.

    I have not made a complete schematic of my build yet as I am still tweaking a few things. I have edited a 6G3 schem with some of my changes and attached.

    deluxe_6g3_schem.gif
     
  18. reggiepe

    reggiepe TDPRI Member

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    @Nickfl I have finished the schematic as built. I have a NOS set of Mullard EF80's in it now that are a closer match and had to adjust the bias circuit a bit. The voltages around the trem are a little off I presume as I took them without the 6.8M resistor in the circuit. With that missing resistor it seemed as if the oscillator wasn't firing. However everything else has been updated on the schem with voltages and changed or added part values. Would love to hear what you did regarding the reduction of the fizziness in the EF80's.

    Low_Power_Deluxe_6g3_Schem.gif
     
  19. reggiepe

    reggiepe TDPRI Member

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    Here are a couple of build photos......

    20210404_193008.jpg 20210404_193032.jpg
     
  20. jmp81sc

    jmp81sc Tele-Meister

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    Thanks for posting the schematic and build photos. Looks great. What is the perceived volume in comparison to say a 5 watt amp?

    I am also a fan of the 6G3 and am currently working on a 12Au7 Micro version. I will be interested in your impressions of the EF80 version and how it relates to the original tonally.
     
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