tube amp - tech question - need advice

Discussion in 'Amps [BG]' started by pfschim, Nov 29, 2013.


  1. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Joined:
    Apr 26, 2006
    Location:
    SF Bay Area
    my cousin, who is a very skilled EE and who has experience with audio amplifier circuits, and some MI amp experience, was looking at the schematic for my Fender Studio Bass amp and suggested a mod to the amp.

    My best attempt to describe what he is suggesting is to sever the screen grid connection for the 6L6GC power tubes. On the schematic - its the #4 pin connection (blue - white) with the 470 resistor (470, 1W (3)) on it that runs from the 6L6GC at V6,V7,V8 to the output transformer.

    His point is that this mod will allow much more even order harmonics and will not otherwise impact the functioning of the amp.

    So, before I consider trying this, I thought I might ask some of the tech experts here weigh in on this question.

    Thanks
  2. nashvillebill

    nashvillebill

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    I would not advice removing screen grid resistors. They are there to protect the tube by limiting current to the screen. They also can help eliminate parasitic oscillations.

    470 ohm is commonly used and most folks feel that value does not affect the "tone" of the amp. Indeed, some of the classic amp designs use the 470 screen grid resistors.
  3. B-string

    B-string Supporting Member

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    Are you using this for guitar not bass duty? Why drop to lower power "triode" operation? Is he planning on adjusting the NFB and bias also?

    His reasoning is very flawed, these "changes" will have wide effects on operation many of which result is very poor performance if not damage.
  4. nashvillebill

    nashvillebill

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    Ooops, I read too hastily, he wants to change to triode connection?

    If less power and a more compressed sound is your goal, why not just go from fixed bias to cathode bias?
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  6. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    Apr 26, 2006
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    SF Bay Area
    this is why I am seeking input.
  7. Chainsaw Willie

    Chainsaw Willie

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    Google the words "triode pentode switch". It is a common mod. The connection between the 470 ohm resistor and the screen bias supply is cut, then the 470 ohm resistor is tied to the plate. Thar ya go, sorta triode mode with the screen grid tied to the plate through 470 ohms.

    This is done on guitar amps to get "cranked tube tone" at lower volumes.

    It will reduce your output power, and it will change the voice of the amp. It is also easily reversible if you don't like it. Also it can be wired with a switch so you can change back and forth between pentode and triode mode.

    I have done it three times. I don't really care for the tone, and think in general that if you want to mod your amp for more distortion at lower volume - you should just get a lower power amp. Instead of a quad of 6L6s, go for an amp with a pair of 6l6s, or even step down to 6v6's.

    However, I built an amp for a guy that loves the triode mode feature. Maybe you will like it.
  8. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Joined:
    Apr 26, 2006
    Location:
    SF Bay Area

    well, I did not start with a goal at all. I like the sound of the amp just fine. My cousin, who does generally know what he is talking about - at least from an purely engineering perspective - made the suggestion to me while he was at my place for Thanksgiving.

    I am not committed to doing this mod, and he said that we could do it, then undo it immediately if I did not like it. He also said that there would be no harm to the amp at all.
  9. Passinwind

    Passinwind Charlie Escher Supporting Member

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    His amp uses six 6L6s, at ~500 volts on the plates. I'd guess that his cousin is actually proposing changing from ultralinear mode to the more common beam tetrode mode with screen grid resistors tied to plate voltage, not to triode operation.
  10. B-string

    B-string Supporting Member

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    Gotcha :). Even if his thinking was triode operation leaving the screens "floating" is not a great idea, pentodes do not like operating that way. The screens should be tied to the plates, the trouble with that is the plate voltage will exceed the screen's voltage rating in class A/B.
  11. Mr. Foxen

    Mr. Foxen

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    Taking stuff out of ultralinear is a commonly known mod. But being in ultralinear is unusual, so its modifying something unusual to be the same as most stuff, and that is fail.
  12. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    ok, thanks for all the insight, I appreciate it.

    I definitely do not want to reduce the power of the Studio Bass, at 180-200w, it has just the right amount of power for the gigs I use it on (with a Fender 1x15 or LDS 2x15).

    Interesting to consider a switchable situation to be able to flip back and forth between the std circuit and the suggested mod.

    I typically prefer very clean tone, but for some old school/lo fi stuff I have been doing lately, a little grit can be very appropriate.

    I'm going to have to give this some thought before I decide.

    thanks again TB experts
  13. BassmanPaul

    BassmanPaul Gold Supporting Member

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    The Guitar amps I build have such a switch to change to Triode mode. There is a second that switches between Cathode and Fixed bias. These allow four differing tonalities of the basic amp. Works very well for guitar but I'd not consider them as part of a bass amplifier.
  14. jeff62

    jeff62 Supporting Member

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    Why? :) A Mesa Boogie Mark 4 that I used to own sounded wonderful when switched in either mode (triode/pentode). Real distinct difference in feel between the two modes. Always wondered why this wouldn't work for bass too.
  15. BassmanPaul

    BassmanPaul Gold Supporting Member

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    As a designer I work to achieve specific tonal goals. As a bass player I again aim to get a tone that I like. It's one of the reasons I built my own pre-amp for my rig. While I like tubes, I consider a SS power section a more elegant way to achieve matching to the load.
  16. beans-on-toast

    beans-on-toast Supporting Member

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    The amp, with the screen transformer taps, is designed to be clean at high volume. It is a characteristic of amps like this. Some people find that they are sterile sounding because they are so clean. Modifying the screen connection will allow it to distort sooner. Guitar players want this in their amps. When people perform this mod on Fender amps, they often change the output transformer as well. That can get costly.

    You can always try a mod. Connect the screen to the plate through a resistor, disconnecting it from the transformer tap. If you don't like it, the mod is fully reversible.
  17. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    yes, my cousin was very clear about that. This is a key reason I would consider trying it - that it's easily reversable - AND - it won't hurt the amp in any significant way to try it.

    we'll see. He'll be back at my place during Christmas, so I have until then to decide.

    cheers
  18. Steve Dallman

    Steve Dallman Supporting Member

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    As an amp tech, modder and builder, I've done quite a few pentode/triode switches. With 6L6's and 6V6's the sound of the triode wiring is muffled and sounds to me like someone put a cardboard box over the amp. With EL34's and EL84's the sound is closer to the stock pentode, just softer and a bit lifeless.

    I don't see the value for a bass player. I have taken a pair of tubes off the UL taps in a couple 135 watt Twin Reverbs, to warm the amps up, and knock 'em down to a more traditional TR tone. If all tubes are to be taken off the UL taps, a choke would have to be added to the power supply. Fender went to the UL designs to save the cost of a choke in these amps.

    For bass, I would leave the UL wiring, perhaps blackface the PI section although I don't remember the PI circuit in your amp, and would add a bias adjust, keeping the bias balance. I might be tempted to double the B+ capacitance to increase headroom (If I was gigging with it.) But it's quite a great bass amp as designed.
  19. T-Bird

    T-Bird

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    Finland (Northern Europe)
    Hi.

    As a someone who dislikes UL in MI applications I'd guess that as well.

    As a matter of a fact, getting rid of UL was the first thing that came to my mind when I read the op.

    To me triode mod for bass/kbd use doesn't make sense.

    For vocals or PA duty it makes much more sense.


    Regards
    Sam

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