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Bias-Vary Vibrato Ticking with 6L6's

Discussion in 'Amp Tech Center' started by NTC, Nov 27, 2020.

  1. Nov 27, 2020 #1
    NTC

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    My "5F11" has much too high B+ for 6V6's, so I removed them and the 5Y3, replaced the with a 5AR4 and 6L6's. Now the vibrato ticks. I can try turning up the bias to see if it improves, but is there something else I should look at?
     
  2. Nov 27, 2020 #2
    Jon Snell

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    A 5Y3 has a 60volt drop under load but a 5AR4 has only a 10volt drop. That will increase your HT!
    https://jonsnell.co.uk/page/
    A 6V6 has a 450mA heater draw but a 6L6 900mA draw. Is your mains transformer big enough to stand the extra current?
    With tremelo off, is the bias set at or about -31volts?
     
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  3. Nov 27, 2020 #3
    NTC

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    This pt is not having an issue with voltages. Heater vtage is 6.54vac with the 6L6's. The 5AR4 is because a 5Y3 doesn't have the current rating for 6L6's. Bias viltage is about -51v and the plate dissipation of the 6L6's average 68.4٪. -31v would cook the tubes. With the 6V6's in, the bias voltage was about the same and I had over 460v on the platea and about 13W dissipation - cooking.
     
  4. Nov 27, 2020 #4
    Jowes_84

    Jowes_84 Tele-Meister

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    Hi NTC -
    no advise on the ticking.
    But
    my first Super Champ which I converted from US to EU voltage Power Supply had around 460V on the plates in quiescent mode (is that the correct term)- as soon as I turned up the amp and played, the voltage went down further below 450 - around 400/410 when distorting heavily and around 430/440 when playing mid-volume cleanish / crunch. I had no issues with my tubes and the sound was amazing.
    Ok, the Super Champ is a very different amp but if you liked the sound of the 6v6s and you dont just use super old, very valuable NOS tubes, you might be fine.

    Cheers.
     
  5. Nov 27, 2020 #5
    NTC

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    I had JJ 6V6's in it. It was starting to sound a bit dull, so I tried it with a different speaker cab and it still sounded dull. I think the JJ's were on the way out. Plugged the internal speaker into the wrong socket. Figured that out, plugged into the correct speaker jack and the tubes lasted about 5 minutes! Dumb mistake. The set I was using were some JAN Philips tubes and the results were as I stated. 460V is 110V above the rating. I know people (and Deluxe Reverbs) do this, but it isn't a recipe for tube longevity.

    BTW, I tried increasing the bias current on the 6L6'S and the ticking stays.

    I tried putting the 6V6's back in with the 5Y3. There was a pop sound that I also saw in one of the tubes - just not going to risk 6V6's without changing the PT.
     
    Last edited: Nov 27, 2020
  6. Nov 28, 2020 #6
    Wally

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    Ticking...no idea. How is the tremolo functioning at tht 68% of max plate dissipation? I have seen bias vary circuits that would not function at that level of dissipation...and could the ticking be related to that?
    460pv is high for 6V6s. I have never had a problem with JJs and higher voltages. As for vintage 6V6s, the Jim Kelley FACS amp runs 6V6s at a plate voltage higher than that. Kelley suggested a certain U.S. tube for the amp...GE or Sylvania..it escapes me which. All BF/SF Fenders run higher PV than the 6V6 spec sheet calls for....even the tweeds went beyond the specs, iirc. With modern tubes, one might be concerned with the screen grid voltGe. I know certain Russian tubes have screens that have trouble with higher voltage.
     
  7. Nov 28, 2020 #7
    Wally

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    One might want to try a 5V4 or even a 5R4 and go back to 6V6s.????
     
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  8. Nov 28, 2020 #8
    Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    There's no low pass filter on the oscillator output. Put a .022uF cap across the 100k plate load.
     
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  9. Nov 28, 2020 #9
    NTC

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    I'll have to look at the data sheets again. I thought 5Y3's were the highest resistance. I have one of those gigantic 5R4's. My 5V4 has one diode out (THAT would lower the voltage). I have a bunch of copper caps but I am afraid of them. They all check ok, but I used one in a Sunn 2000S and it was blowing fuses.

    I will have to try the 0.022uf across the plate load. The ticking isn't too bad when you are playing. Note yhat I have the same issue in another 6L6 amp. Since the amp didn't make this sound with 6V6's, I wonder if it has something to do with the way 6L6's bias.
     
  10. Nov 29, 2020 #10
    Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    There are a number of reasons why this could happen with a change of output tubes. The preamp B+ has probably dropped some, and the oscillator's load line may have changed slightly causing harmonics that weren't there before. The bottom line is that the 5F11 circuit doesn't have any filtering on the oscillator output to deal with any unwanted harmonics. Look at the AA964 and AA1164 circuits. There's a reason why a low pass filter was added to the oscillator output. A plate load bypass should clean it up also.
     
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  11. Nov 29, 2020 #11
    Wally

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  12. Nov 29, 2020 #12
    Charlie Bernstein

    Charlie Bernstein Poster Extraordinaire

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    Or you could just get a tremolo pedal. My Demeter Tremster sounds a lot better than any onboard trem (a.k.a. vibrato) I've ever had. Smooth, quiet.
     
  13. Nov 29, 2020 #13
    NTC

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    Not buyin' no stinkin' pedal:D.

    Wally, thanks for the table link. I tried the 5R4 and, with the 6L6's it only dropped 14 V vs. the 5AR4, so the drop would be a lot less once the 6V6's went back in.

    Phrygian77, I haven't measured the downstream voltages but I would think they sre highrr since the plate voltagevis now higher. When the original JJ 6V6's were in there, the plate voltage was about 430v. (I don't know where I had them biased as the original build was 15 years ago. I guess it was never really finished). I will look into the filter-I haven't tried the plate load bypass yet either.

    Thanks for all of the help. I am seriously considering a new Hammond pt and then buying a ot to make a sort of 5D8 twin out of with this pt, but probably with a single 12 or 15.
     
  14. Nov 29, 2020 #14
    Wally

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    Note that the 5D8 runs 380 volts on the plates according to the old Fender Schematic/layout. A 5D8 with 460pv is not a 5D8 imho. but then...many builds are not clones of the originals, right? It all depends on whether or not one wants to experience a fresh tweed circuit’s sound or hear something quite a bit different. When Marshall cloned the 5F6A for the first Marshall amp, they held true to the schematic numbers. Marshall moved to higher voltages later when they went to solid state rectification with EL-34s and started to modify that Bassman circuit and therefore the sound...bigger and punchier than the two big Fender Tweeds which gave Marshall the pathway. The Pv went from 430vdc to 560vdc. EL34’s are rated to 800 VDC on the plates. 6L6GCs have a limit of 500, iirc.
     
  15. Nov 29, 2020 #15
    NTC

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    For me, it is just a matter of using up parts, not exactness. It would be a 5D8 "inspired" amp. It would be really nice to have octal preamp tubes, but I can't make my own chassis. This is just a pipe dream at the moment. For now I have a 5F11 with 6L6's.
     
    Last edited: Nov 29, 2020
  16. Nov 29, 2020 #16
    Wally

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    I understand...with a PT that makes that kind of I would be looking at a later Fender circuit tha5 calls for that kind of voltage. Ommv.....
     
  17. Dec 30, 2020 #17
    NTC

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    Just as a follow up, I put the .022uf cap across the 100k plate resistor and tgat has cleaned up the ticking. The Vibrato is not as 3D nor as strong, but that is to be expected. I think the same result would have occurred if I had put in the series Low Pass Filter like in the AA964.

    I have also replaced the Power Transformer and can run 6V6's again. Voltages are like a BF Princeton. This is a very clean machine.
     
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