School Me on Blocking Distortion (Bias Wiggle Tremolo Help)

Discussion in 'Shock Brother's DIY Amps' started by 72_Custom, May 20, 2021.

  1. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    Last year I built a 6G3 Brown Deluxe clone. When cranking the intensity of the bias wiggle tremolo up, it has what sounds like a rather unpleasant blocking distortion. I wasn’t able to get it under control and I decided that now’s the time to take another shot at it. My guess is that the issue stems from some rather high voltages. My bias voltage at idle is around -40V and with the tremolo engaged, can swing all the way to -60V.


    I’ve read all I can on several sites about the subject, but I’m afraid that I’m totally stumped.


    Here are some of the steps I’ve tried:


    -Adjusting the bias (all over the place)

    -Lowering coupling caps to .022uF

    -Adding grid stopper resistors (values all the way up to 100K)

    -Reducing grid leak resistors to 100K

    -Disconnecting NFB

    -Increasing the size of the bias cap

    -Changing tubes (including 5881s and various types of rectifier tubes)

    -Changing the Output Transformer (I found a post on the Hoffman forums where someone had a similar problem and it didn’t go away until he changed from a 6.6K primary to an 8K. It turned out my “6.6K” OT was actually only 6.1K. I swapped it for an actual 6.6K. But even using the 4 Ohm tap with an 8 Ohm speaker, thus bringing my impedance well over 8K, didn’t help).



    I can provide more details and voltage readings if requested. Any assistance would be greatly appreciated. I’m just hoping for some helpful suggestions that can get me going in the right direction, because I’m completely stumped. Is it possible that it’s not blocking distortion at all?
     
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  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Not familiar enough with the 6g3 for specific advice for it, but I make it a habit to reduce the .022uf phase inverter input cap on all my tweed stuff now with a .01uf, and that makes a really big difference on blocking distortion and tighter overdrive.

    That would be here on the 5e3, not sure about the 6g3 though.

    upload_2021-5-20_22-49-36.png
     
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  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Also, if you have a youtube or vimeo account or similar, take a quick clip and post it. Sounds are worth a thousand words. ;-)
     
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  4. sds1

    sds1 Tele-Afflicted

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    Blocking sounds like the tubes are getting hard clipped, it's pretty easy to distinguish because you can hear the interruption in signal, as brief and repetitive that it is.

    I don't know a lot about what's normal with tremolo and what's not, but a high negative DC bias will send the tube into cutoff and the result is blocking distortion. That is just like, science and stuff.

    So, just thinking this thru aloud with you, it makes sense that you are hearing blocking distortion, and I don't see how your trem circuit would perform any better or worse than any other bias wiggle tremolo out there.

    The only thing that occurs to me is if you can lower the max negative DC bias you will stay further out of blocking territory. But that's what the intensity knob does, isn't it? :)

    I dunno man, I am anxious to see other replies because this sounds possibly kinda normal to me. All the troubleshooting work you've already done supports that notion.

    This is solid advice, but only in the context of input signal overdriving the power tubes, where it is the grid excursion that is sending the DC on the input grid more negative and causing blocking. The lower value cap is one way to reduce the grid excursion. In the tremolo use case, the oscillator is solely responsible for the extreme DC offset at the grid, and the coupling caps won't have any effect on this.
     
    Last edited: May 21, 2021
  5. sds1

    sds1 Tele-Afflicted

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    I think I muddied up some concepts there, and less sure that the tremolo wouldn't cause bias excursion like any other waveform introduced at the grid.

    Rather than edit the post I will just take my lumps as they come. :)
     
  6. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    Thanks for the responses folks. I appreciate the interest. See video link below.



    I’ve spent a good deal of time trying to get the idle bias voltage from -40V down closer to -30V (the schematic calls for -27V) while maintaining a proper plate dissipation, but I can’t seem to get it balanced. Hence the use of different types of power tubes and rectifiers. I’m starting to think I should replace my 330-0-330 power transformer with a 315-0-315 or even a 300-0-300.
     
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  7. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    Forgot to add:

    My line of thinking is that the problem must be in the power section (and not the preamp or phase inverter) because that’s where the tremolo happens and there are no problems without the tremolo.
     
  8. Jowes_84

    Jowes_84 Tele-Meister

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    Unsure - but isnt the distortion there also without the trem? I think I can hear it in the whole clip also towards the end - but could also be your recording device clipping.
    Maybe the issue is elsewhere?
    Did you add screen-grid resistors to the power tubes?
    If yes which value and have you verified it? (I once had 470k instead of 470R on the screen-grids… which resulted in very ugly distortion - oops)
     
  9. sds1

    sds1 Tele-Afflicted

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    Hard to say for sure from the video but my ears say no, that's not blocking. Sounds more like fizzy preamp distortion, or an oscillation.
     
  10. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    I’ll make a better quality recording instead of just using the phone’s internal microphone and send the link once it’s be re-uploaded.

    Yes, I do have screen grid resistors in there. I just doubled checked. They are indeed 470R.
     
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  11. NTC

    NTC Tele-Holic

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    6G3 has a long-tailed pair pi.

    Thread hijack alert: Jsn, does lowering that cap value reduce the fizzyness of the cathodyne pi? I had (and removed) the Paul C. mod in my 5E7 and I think it is more buzzy without. That cap change still leaves the low frequency cutoff in the sub audible range (under 20 Hz). On could even go a bit lower in value.
     
  12. jsnwhite619

    jsnwhite619 Friend of Leo's

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    From this similar thread: BUT, in spite of none of this making a difference, I did have a breakthrough with the distortion--after digging through some EL34 "tremelo" posts I found a suggestion from Steve Luckey to swap out the 220k resistor preceding the .1uf cap before the tremolo intensity control with a 470k to reduce the intensity of the signal. Cleaned it right up, with very little perceptible loss of depth.

    https://www.tdpri.com/threads/6g3-bias-tremolo-distortion-problem.1036803/page-2
     
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  13. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    So I did some more experimenting last night and this morning. Went through all of the above mentioned items to no avail. I had high hopes for the 470K resistor. Even tried bumping that up to 1Meg.

    I did, however, have some degree of success by putting the amp on a Variac to get the secondary output down to 300V (I ended needing to be a little over 100V on the Variac), swapping in a less efficient rectifier tube, and then adjusting the bias to get the PD back in range. With that bias voltage idling in the 26-30 range, I'd say it was about 90% better. Around -33V or so, things would start to get hairy again. I think I actually liked the tone better with the lower voltages, too.

    Now I really wish I would have gone for the ClassicTone PT with the 300 and 330 secondaries! Now I think I may have to drop in some big ol' resistors to get that voltage down.
     
  14. Cruisin Home

    Cruisin Home Tele-Meister

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    I would give a try at changing your bias divider resistors to lower just your bias voltage. Could also add zener.

    Just finishing vibrochamp type build( w/ extra power) and found nice transformer Hammond 290Ax (since our classictone boys are no more). I really like it especially for the $60. 275vac and 325vac secondaries (@120vac in). Running the lower secondary, with wall voltage little higher and a rectifier such as GZ34 it gives nice option for you.
     
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    There's something wrong with your bias or tremolo circuit. The tremolo is driving the power tubes into cutoff, not blocking distortion. Until you figure out why you can't get the bias right don't modify the rest of the amp trying to fix it.
     
  16. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Wow, that's ugly. Much more than I expected. These tremelo's can give you a "splatt" sound a little bit for sure, but that sounds like something else than the normal sound.
     
  17. schmee

    schmee Doctor of Teleocity Ad Free Member

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    I have plenty of deluxe and princeton builds with super high voltage and not that issue. So I doubt that is your problem.
     
  18. 72_Custom

    72_Custom Tele-Meister Silver Supporter

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    Thanks for the input everyone. I’ll go over everything again with a fine toothed comb and see if I can find something amiss. I’ve built several more amps since I first put this one together and have learned a great deal along the way. So hopefully I’ll be able to get to the bottom of it.
     
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