Dealing with the screens on a 5E7 (a question or two)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Sep 9, 2008.

  1. brad347

    brad347 Member

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    So I'm waiting on the last of the parts to build one of these. I'm doing a few little tweaks to the original circuit, nothing major, just stuff like adding an adjustable bias, and using a switching pot for the presence to have the option of defeating the NFB loop by 'clicking' the presence control all the way left.

    The other thing I want to do is to do something about the dubious screen grid situation on the power tubes.

    [​IMG]

    As you can see, there are no screen resistors, and the screens are connected to the B+ voltage, so they would probably have nominally the same potential as the plates, which would not be good for tube life.

    So what I want to do is move the plate supply to the spot between the first filter and the choke, and leave the screens where they are, after the choke. Sorta like on a 5F6-A bassman.

    I also was planning on fitting 470Ω screen resistors (I've got both big 2 watt carbon comps or 5w wirewounds laying around).

    My questions are:

    1) will the screen resistors change the sound? I've heard varying opinions on this. I assume 470Ω is the appropriate value, but some people like to go lower and others like to go higher.

    2) Since I'm moving the B+ supply to a spot before the filter choke, should I up the value of the first filter cap to, say, 30µf? Or will it even make any difference to have the B+ before the choke? I notice the 5F6-A has two 20µf in parallel before the choke, while the 5E6 is configured like a 5E7 with a single 16µf on either side of the choke and the screens/plates tied together and connected after the choke. What do you recommend I do here?

    3) Is there anything else I should be considering with this little tweak?

    Thanks to anyone who can respond. It would be especially great if someone knowledgeable like Aiken (made you look! haha) could respond.

    Thanks.
    :knitting :knitting :knitting
     
  2. abro163

    abro163 Member

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    Hi I actually just built the exact same thing, have the presence clickable to defeat the NFB. + bias pot. (and a few other things like switchable paul c mod etc)

    The screens are at higher potential than the plates on these, even on the fender schematics, you can put in screen grid resistors, I just put in 100ohm, so the effect should be small as the resistance is only equal to the choke.

    I made the OT centre tap switchable between after the choke and before the choke. But I left the 16uF(22uf) in place (didn't double up filter caps like the bassman). ALthough in both modes the screen voltage is still higher than the plate as the choke has lower resistance than the output transformer hahah :crazy. I havent tested this switch yet but I imagine there will be a difference with it cranked, as you will get sag and de-regulation through the choke with the plates fed off it.

    Screen resistors will affect tone, I suppose you could check your self just short out the 470ohm with a jumper wire to compare. 470 should be hardly noticeable. You are esentially adding resistance to what the screen sees, so you get more sag.
     
  3. mooreamps

    mooreamps Senior Member

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    Screen grid resistors "do" affect the tone. Installing a high enough value will add a certain element of touch responce to the amp. The value I use for the 6L6 is 1.2K ohms ; 2 watt ; flameproof.

    -g
     
  4. Trout

    Trout Member

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    If Memory Serves, I have 1K ohmite wirewounds in mine,(1 per tube) without them, I had been getting an odd internal glow that changed when you hit cords.
    Dig in on a cord and it got brighter, at idle it was fine.

    No problems since.
     
  5. hasserl

    hasserl Member

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    Funny, I don't recall hearing about this being a dubious situation, certainly not a problem. How many of these amps are out there? If it was a problem you'd think there would be some talk about it. I've got a 5E5A which is the same circuit, I've not noticed it as a problem. Nothing wrong with tweaking the circuit to fit your needs or wants, but I wouldn't worry about it being a problem.
     
  6. brad347

    brad347 Member

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    Well there IS some talk about it and has been for a long time, just maybe not in the places you're reading/chatting. :)

    For example, in Gerald Weber's first book there is an article that discusses it. (I know some people don't like GW, but just saying, there is talk about it).

    In any case, I'm sure it wasn't a problem when people did not overdrive their amps, played them at relatively low volumes, and when quality tubes were plentiful and relatively inexpensive.

    But since I plan on cranking this amp and am putting a set of fairly pricey old-stock Tung Sol 5881s in it, I'm concerned about anything that makes the screens light up like Trout was getting.
     
  7. brad347

    brad347 Member

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    This is interesting. So if you get around to test-driving it, it would be interesting to know if there was enough filtering with the single 16µf stage of filtering with the OT CT connected before the choke.
     
  8. VikingAmps

    VikingAmps Member

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    It will change the tone and response of the amp, but you will prolong your tube life.
     
  9. Trout

    Trout Member

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    I have tired it, now not on an actual or vintage 5E7. This was in a clone using Pacific Transformers.

    The single 16uf was borderline at best. At low volumes it had a very minor hum, but cranked up to 9/10 it was rather noticable.

    The screen glowing while cording at moderate volumes was an issue, the resistors took care of it the also.

    I later added the choke and moved the OT CT to the second cap as in the stock circuit. Got rid of the minor hum, but still needed the screen resistors. Everything in the amp was stock values and voltages were only 10-15V higher than on schematic.

    I finally ditched the 5U4, put in a GZ34 and raised the 3 caps to 22UF and the 4th cap to 16uf,Then swapped out the original OT for a bigger bassman type OT.
    Interestingly, the standard sized OT caused the screen glow issue whereas the larger Bassman did not.

    This may actually be an inductance issue related to specific brands of transformers. My test case used Pacific Transformer replacements which sound great, but might not be as good as others.

    Either way, the increased voltage with the GZ34 combined with the larger OT made it into a real monster.
    Just for safety sake, I have the screen resistors installed in the final tweaked amp.
     
  10. brad347

    brad347 Member

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    Trout, that's helpful info.

    So, hmm... maybe if I increased the first stage of filtering to 30µf it would help smooth out that hum.

    When you fitted screen resistors and moved the OT centertap back to its original position after the choke, was the screen still at a higher voltage than the plate? Or did the screen resistors drop it enough?
     
  11. Trout

    Trout Member

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    The screen ran just above the plate, around 5-7V. which IMO is within a less than 10%+/- spec

    I would not raise the first cap, my person experience was the rectifier tube (5U4)would arc. I went through several brands before I found some Sylvania 5U4's that seems to hold up. The GZ34 allows for a tad higher 1st cap, but also raises the B+.

    The Screen resistors only dropped the voltage a couple volts, not enough to say it was a cure, but they do help protect from the current.
     
  12. brad347

    brad347 Member

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    Thanks for that.
     
  13. 909one

    909one Member

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    Hey there,
    I also have a 5e7 clone.
    I bought the Ceriatone clone as a low cost alternative to a real bandmaster, but also in an attempt to try and learn a little about tube amp circuits.

    Should I be worried about this issue of no screen resistors? I run the amp pretty loud.

    Also, I installed a switch to cut the NFB loop. But I just put in a switch right after the loop leaves the speaker output. Is this a bad way of doing it? Will it harm anything?

    Also, what are your feelings about the Paul C mod? I am thinking of trying that Mod to smooth out a little f the distortion at higher volumes.

    And lastly, a rebias is necessary when replacing the rectifier with something different, right? With this help to increase headroom?

    Sorry for all the questions. Still learning.
     
  14. brad347

    brad347 Member

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    The way you did the NFB-defeat switch is fine.

    I have no opinion on the Paul C mod. Yes, a rebias is in order when changing the rectifier tube to a different type. Moving from a 5U4G to a 5AR4/GZ34 will raise plate voltage and, in turn, possibly give a slight increase in headroom.

    As for your screen resistor question, I'm not sure. As someone pointed out above, many of these amps have been run for years without them. But it is enough of an issue to get some people talking about it now and again.
     

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