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6G3 Build Blocking Distortion?

Discussion in 'Amp Tech Center' started by 72_Custom, Oct 24, 2020.

  1. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    Howdy Folks,


    I am finishing up a build of a 6G3 brownface deluxe, but am having an issue that is vexing me. So I humbly ask the gurus of TDPRI for some input. When using the tremolo, I am hearing some faint distortion that almost sounds as if it’s “behind” the main guitar sound. The more I increase the intensity, the more prevalent it is. I believe this to be blocking distortion of the power tubes.


    I think the issue is rooted in the common problem that seems to plague almost every brown deluxe: all the voltages are way over written spec. The specified -26V bias voltage was measuring -36V off the bat. I was having some tremolo low frequency thump (the other common 6G3 problem) until I put a diode across the intensity pot. This made the thump 90% better, but it had the side effect of bumping the bias voltage to -42V.


    I replaced the stock 22K bias resistor with a 20K fixed resistor and a 10K pot. This allows me to dial the grid voltage down to about -38V with proper 6V6 bias, but with the tremolo fully engaged, it can swing to about -62V.



    Some troubleshooting steps I’ve done:


    • Adjusted the bias

    • Used a variac to lower input voltage down to 110V.

    • Removed the diode on the intensity pot. A slight reduction in distortion, but obviously the thump returns.Added grid stopper resistors. I tried various values from 1.5K up to 5K to no avail

    • Reduced grid leak resistors down to 100K

    • Changed the 100k resistor in the bias supply (tried various values from 100-150K). I’m not sure if this is typically done, but it was a wild idea that I had. With 150K, I could get the grid voltage to -28V. It seems to be improving the distortion some, but it throws the bias of the power tubes off enough that I’m unable to bring them into the correct range.

    • Because my plate voltages were about 445V with a GZ34, I’ve been experimenting with 5U4GB and 5Y3 rectifier tubes in an attempt to get the bias set correctly.

    • Changed speakers and cabinet just for a sanity check



    Am I on the right track? Am I way off base in thinking that the wild voltage swings of the bias wiggle tremolo are to blame for the issue? What else can be done about it? I can post some pictures or measurements if need be.
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    If 6V6s swing to -62v on the grids, you are hearing crossover distortion.
    Reduce the bias to the correct value for the HT, about -30volts and check the tremelo does not drive too hard. There should be only a variation of a maximum 10volts from the tremelo.
    The early 6G3s had a very limited tremelo effect and because the tremelo is driving the bias voltage it is easy to over drive the output valves and they don't like it too much and go to give a very saggy sound as the screen grids overheat.
     
  3. RadioFM74

    RadioFM74 Friend of Leo's

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    That's TDPRI. Awesome place.
     
  4. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    @Jon Snell Thanks so much for the input. That makes a lot of sense now!

    With the decreased bias voltage, the issue for me becomes getting the plate dissipation down to an acceptable level. Any advice on also reducing the plate current? Is it just modifying the values of the bias resistors or should I be setting my sights elsewhere?
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    What power transformer are you using? The best solution would be one that gives you under 400V B+ without band-aid fixes.
     
  6. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    I’m using a Classictone 40-18016. It’s a 330V-0-330V (like the schematic calls for), but in retrospect I could see how a 315 or even a 300 would be a better fit.

    I have a vintage 5Y3 in there at the moment to help get the plate voltage down, but the plate current can get a little out of control.
     
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  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Yeah, I don't know how the math worked on the original 6G3, either. I wonder sometimes if the PT secondary markings on Fender schematics were for unloaded voltages (note how they're usually nice round numbers instead of what one would expect for measurements from a real live amp on the bench), and modern transformer makers are unwittingly specifying loaded voltages for their "clones"... that might explain why so many people's builds have excessive voltages right out of the gate, even though they tried to get everything correct.

    If you're willing to dig into the pockets again, I'd recommend the Weber W25130 PT. The 540VAC secondary might seem like it would be way too low, but I have one in a 2x6V6 cathode-biased build, and with a 5AR4 I get 340V-350V depending on the wall voltage. A little short of the 365V on the 6G3 schematic, but not a bad "zone" to be in for a small brown Fender amp IMO.
     
  8. dan40

    dan40 Tele-Afflicted

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    Reducing the negative bias voltage to pin 5 of the power tubes should result in a hotter bias (higher ma reading) of the power tubes which will cause the plate voltage to fall because of the increased current draw. Raising the bias voltage will result in a cooler bias and higher plate voltage.
     
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  9. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    I think @ThermionicScott hit the nail on the head with the power transformer assessment. Another model does seem like the best solution.

    I’m not sure how clear I was about the issue. When setting the bias voltage to a more appropriate level (-30V), the plate current increases so much that even with the drop in plate voltage, my total plate dissipation is too high.

    I guess I was wondering if there were any “tricks” to help burn some B+ power. A crazy idea I had would be to add some cathode resistors and make it a mixed bias. Not sure how well it would work, though.

    Thanks everyone for the input and the advice, by the way.
     
  10. D'tar

    D'tar Friend of Leo's

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    IMO.... -30vdc says nothing to the bias of the output tubes. The number you are concerned with is the plate dissipation. Set bias at idle to 50-70% or however you like it sonically. As long as the -ve voltage allows you to hit your target in plate current it is correct. if you have too much or too little -ve vdc then adjust the bias circuit appropriately.
     
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  11. David Barnett

    David Barnett Doctor of Teleocity

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    Agree. The - voltage from the bias supply is a means to an end (plate current), not an end unto itself.

    Do what it takes to get the plate current right. With bias-wiggle trem it may need to be toward the cold side of the acceptable range.
     
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  12. dan40

    dan40 Tele-Afflicted

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    Like the fellas said above, you don't need to worry about hitting that number (-26v) for the amp to be working properly. Just get the dissipation in the 50-70% range and you should be fine. As David mentioned, these circuits do like to be run a little cooler for proper trem operation. Many vintage Fenders are running with slightly higher B+ voltage these days, but as long as you dial in the proper dissipation level they will run just fine. You may also just happen to have a pair of 6v6's that require a slightly higher bias voltage to run in the correct range.
     
  13. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Not 6G3's , so not the same Tremelo... but my old BFDR and BFD run close to 460v on the plates. They sound great, but yeah, voltages are way over spec in these amps. I don't think it's just line voltage differences either....
     
  14. _Steve

    _Steve TDPRI Member

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    To be fair I think @72_Custom already said he's measuring current. I think he was referencing the voltage because someone recommended he lower it to solve the problem. Also the schematic has a -26V label on the bias and he's a long way from there and with his bias voltages the current is too high and therefore exceeding the recommended plate dissipation.

    @72_Custom - I have a homebuilt 6G3 with a 330-0-330 PT (from a Hammond organ). If you want me to run any comparisons or test any voltages let me know. From memory my bias wasn't anywhere near -26V either.
     
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  15. D'tar

    D'tar Friend of Leo's

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    Yes. I realize he is measuring current. To me it seems as if he is trying to get the current and the -ve voltage to all line up as per the schematic. how are we measuring current here? There is no mention of plate dissipation %, only the bias voltage readings.

     
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  16. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    Sorry for all the confusion. I was indeed measuring plate voltage, but since that is unaffected by the changes to the bias section, I was not reporting those numbers and just stating the total plate dissipation. Perhaps the actual numbers will help shed some light on the issue.


    These numbers are with JJ 6V6S tubes, so I’m using 14W as the max. Measurements listed are with the bias pot in the same position and are measured on the same tube in all instances (the other one is flowing about 2mA less current).



    GZ34

    Bias Voltage: -38V

    Plate Current: 19mA

    Plate Voltage: 447V

    Percentage: 60%



    5U4GB

    Bias Voltage: -38V

    Plate Current: 21.6mA

    Plate Voltage: 429V

    Percentage: 66%


    5Y3 (and changing 100k resistor to 120k in bias section):

    Bias Voltage: -35V

    Plate Current: 23mA

    Plate Voltage: 402V

    Percentage: 67%



    With any of these settings, the amp is problem free with no/minimal tremolo. Unpleasant distortion gets introduced between 4 and 6 on the intensity dial.


    I was indeed trying to do what @_Steve had mentioned. The suggestion was made to get the bias voltage to -30V. When I lowered the voltage down to -30V, this would increase my plate current to 40mA and I would get PDs in excess of 90%. So my question was “how do I get the plate current and/or plate voltage down with the new bias setting?”. Hence the experiment with different rectifier tubes.


    If I’m now understanding it correctly, the bias voltage itself isn’t the answer. The answer is a lower plate dissipation by whatever means necessary (within acceptable limits). I’ll take another whack at it and report back the results with biases less than 60%. Looking at it now, the GZ34 does seem like a good option. But I was a little scared of the 447V because the spec sheet max screen voltage for those tubes is 450V and there aren’t any screen resistors.


    And again, I really appreciate everyone’s time and input. I’m an enthusiastic amateur in the DIY amp game, but I’m fully immersing myself in this world and trying to soak up all the knowledge that I possibly can.
     
  17. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    I don't want to beat a dead horse here, but I thought I'd provide an interesting update: I’ve been playing with it for a while now. I adjusted the bias all the way down to 30% and the distortion was still. In fact, as the bias got hotter, the problem decreased (presumably because the tremolo weakened).

    I found a post on another forum where someone had the same issue. He tried all the standard and logical things that were suggested to no avail. Eventually, he realized that the output transformer he had in there was not right for the job (it had a 27:1 turns ratio instead of 30:1). Swapping the OT instantly fixed the issue.

    A light bulb went off in my head. The 4 Ohm tap on the OT was not working, but I had just dismissed it as a wiring problem and had not considered that the OT could be the issue. I have a ClassicTone 40-18038 OT, which has 4 and 8 Ohm taps, but otherwise is supposed to be correct to vintage specs. Measuring the winding ratio revealed that instead of 30:1, it was actually almost 33:1. In other words, my 6K6 Ohm primary impedance is actually 8K5 Ohms!


    With the sudden demise of Classictone, that makes getting a replacement impossible. I'll have to order another transformer, but I may keep this one for an eventual Princeton Reverb build (if only the mounting patterns lined up!)
     
  18. theprofessor

    theprofessor Poster Extraordinaire

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    Would this work? (Though I wonder if the 330-0-330 might give you voltages that are too high.)

    https://reverb.com/item/6905938-cla...-40-18016-deluxe-style-power-transformer-120v
     
  19. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    That’s exactly the power transformer that I have!
     
  20. theprofessor

    theprofessor Poster Extraordinaire

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    Ha! Well, goes to show I didn't read the post closely enough. Sorry to be a dolt!
     
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