Screen stoppers on ultralinear amps?

Discussion in 'Amp Tech Center' started by timfred, Jan 22, 2020.

  1. timfred

    timfred TDPRI Member

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    I'm converting a 1950s Harman-Kardon C-100 mono hi-fi amp for guitar use. This is a total teardown and remake into something 18-wattish.

    The amp is originally an ultralinear design with 2 6CM6s (small bottle 12 watt 6V6s) in PP, cathode bias. B+ will be around 350-375V. I contemplated ditching the UL taps but I think instead I might try it as UL with little or no NFB and see how that sounds.

    From the factory this had no screen stopper resistors, but it was also designed to be never overdriven with B+ around 305V (screens idling at 310V). When I look at Fender UL schematics, it looks like they put 470 Ohm 1W resistors on 6L6 screens (500V B+).

    Any advice? Add screen resistors? If so, what value?
     
  2. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    You could use 470 as well. Here is a schem I drew, using a 3P3T switch (S3A, B & C) to give the options .........1: UL on - FB off...........2: UL off - FB on.........3: UL off - FB off.

    There is also a second switch (S2) where the bias can be switched between fixed and self (cathode) bias. - Maybe not a bad idea for your build, seeing how the HT is fairly low.

    The build itself was more of an ideas test for me, that I hadn't used much other than for function tests, before starting and getting stuck on the complicated chassis building that I intend to finish one day......:rolleyes::oops:
    upload_2020-1-22_8-50-6.png
     
  3. Jon Snell

    Jon Snell Tele-Meister

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    470R is ideal for a small amplifier, most larger amplifiers use 1k or 1k5 and sometimes 2k2. Don't be tempted to use wire wound as these resistors act as fuses if there is a short circuit in a valve.
     
  4. DougM

    DougM Friend of Leo's

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    I have an EL84 powered stereo amp made by Jolida. I removed the screen taps from the output transformers to the screen grids on the tubes, connected 100 ohm 2 watt resistors from the screen grids to the plates to make it triode operation, and removed the negative feedback. It puts out less power now, and would no doubt measure higher THD, but it sure sounds better.
     
    Last edited: Jan 22, 2020
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  5. Jon Snell

    Jon Snell Tele-Meister

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    It should sound nice in triode mode. As I say, the screen resistors help to limit the screen current and of course to act as a fuse link in case of a catastrophic failure.
     
  6. Commodore 64

    Commodore 64 Friend of Leo's

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    I didn't use any screen stoppers when I repurposed UL Bassman 10 iron. One channel was Dr Z Route 66 inspired, the other was Marshal JCM800 inspired. All with UL taps. Sounds awesome.

    [​IMG]
     
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  7. printer2

    printer2 Poster Extraordinaire

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    The main point of us using stoppers there is when we drive the amp to full output the plate voltage gets dragged down below the screen supply voltage. The electrons then are attracted to the screen rather than the plate and the screen being less massive overheats and burns up. In less than severe duty the stoppers reduce high frequency oscillations. With UL operation the screens are supplied from the same source as the plate, so when the plate voltage is dragged down so is the screen voltage. So there is less need of a resistor. The size of resistor can act on the dynamics of the amp though.
     
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  8. FenderLover

    FenderLover Poster Extraordinaire

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    Just another reference, HiFi amps often use 47-100 ohms for UL screen connections primarily to suppress the chance of oscillation. Guitar amp schematics often don't show them at all.
     
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  9. timfred

    timfred TDPRI Member

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    Thanks all, your input has enabled my laziness. I'll do nothing to start (leave out any screen stoppers) then try resistors, pentode, and triode modes based on sound or if I'm burning up tubes. Good news - 6CM6 tubes are cheap and readily available.
     
  10. timfred

    timfred TDPRI Member

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    Well, that was a bust. When I got my PS made I hooked it up and checked voltages. The tube plate/screen voltages were idling way off - #1 at 342/349 (about like I expected), #2 at 315/342.

    I checked the DCR of the primaries from the center tap. #1 was 175/17 Ohm, #2 was 2.43K/2.5K (very high resistance). Even weirder is that the DCR between the #2 plate and screen taps is 3.8K (instead of 2.43+2.5 = 4.93K), which should be impossible unless there are multiple current paths between them.

    So I figure this OT is trash, and likely why the amp came non-working with only one power tube. I was excited to hear it but if I have to buy one I'm not spending money on a wacky oversized UL transformer experiment. I ordered a Deluxe Reverb replacement.
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I recommend 470 ohm screens for your amp because of what they do to the overdrive tone.
     
  12. trobbins

    trobbins TDPRI Member

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    timfred, the history of what wasn't working, and the apparent OT primary problem is a reasonable indication of what can happen if you don't add in some protection with old amps. Even your considered aim of running the amp without screen stoppers for guitar use puts the amp in the same vulnerable position for collateral damage of the OT or PT if a valve fails. Damaged transformers are a much bigger pain to deal with than a bad valve.
     
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