Fender TRRI Half-power mod

Discussion in 'Amps and Cabs' started by Lolasaurus, Jan 25, 2012.

  1. Lolasaurus

    Lolasaurus Member

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    I just found this online and it's kinda freaking me out:
    "Half power mod – Pull out two 6L6 and disconnect 1 speaker.

    Pull out the two inner 6L6 tubes and you have a ~40-50W amp. You should/can also disconnect one of the speakers to not have an impedance mismatch. The Twin reverb output transformer is designed to work most efficiently with the following impedances/loads and tube power configurations:

    4 x 6L6 -> 4 ohm speaker load. And then comes the standard Fender +/- 100% tolerance.
    2 x 6L6 -> 8 ohm speaker load. And then comes the standard Fender +/- 100% tolerance.
    The plate voltage will increase a little when two of the tubes are removed. Ideally you should have the amp re-biased to not run the two remaining tubes too hard.

    The voltage levels on the plus- and minus-sides of the output transformer’s primary circuit remains the same whith two and four 6L6 tubes. But the delivered current is doubled with four tubes. Hence, the power is also doubled. Power (Watts) = Current (Amperes) x Voltage (Volts).

    This mod is a must-know survival trick for all Twin owners."
    via http://fenderguru.com/amps/twin-reverb

    Has anyone tried this / can recommend it? I never heard of it before, but I was planning on retubing and getting new speakers for mine, so this would be a nice way to save money and accommodate to the volume i play at.
     
  2. GearHeadFred

    GearHeadFred Member

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    It's all true.. and it all works!

    Small print: You will find that 1/2 power is not as much of a reduction as you think it will be.
     
  3. rockonomics

    rockonomics Member

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    This isn't new. It's been happening as long as there have been 4 output tube Fender amps and possibly other brands as well.
     
  4. barron

    barron Member

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    I agree with all 3 above posts...
     
  5. Lolasaurus

    Lolasaurus Member

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    doesn't matter which speaker?
    what should I do with the loose ends? cover with electrical tape?
     
    Last edited: Jan 25, 2012
  6. Lolasaurus

    Lolasaurus Member

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    this, but it does sound great a lower levels now.
    + is it normal for the bias to be unchanged after doing the mod?
     
  7. Leftyman

    Leftyman Silver Supporting Member

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    I have been doing this for years. You can pull either the 2 inner tubes or the 2 outer. For what it is worth, I have never changed anything speaker wise and I have never had any problems. I have the stock Jensen speakers in there hooked up as they were from the factory. I do realize pulling 2 tubes changes the impedience however, I had an amp tech a number of years ago tell me I should be ok and so far he has been right. If I had to guess I would say it has been 8 years now. I also agree with the above post that going from 85 to 40-50 watts is really not going to do a whole lot for the overall volume. I pulled 2 tubes back in the day to make the amp a little quieter. Now I have 2 tubes in there because it is just less tubes I have to purchase. Remember that a typical 50 watt amp is only about 4db less than a 100 watter.
     
  8. smolder

    smolder Gold Supporting Member

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    2 tubes, one speaker and the volume is cut substantially. It makes all of those twins now selling for 5-600 bucks a great bargain. Great cleans... and many ways to get the dirt.
     
  9. Lolasaurus

    Lolasaurus Member

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    So i did it, and idk but I think it successfully cut the volume (not by a whole lot, but no small amount).
    But after about a half hour of playing I started getting bad clipping.
    I'm not sure, but I think it was usually on notes fading out, like the amp couldn't handle the more quiet range.
    Any ideas on what could have caused this?
    I took out the speaker that had only two wires connected to it instead of four; i put electrical tape on the ends of the loose wires.
    When I first took out the tubes and unplugged the speaker, I used a bias probe and it read 39mA, which is what I had it set to before. Could it be that after adjusting to the new set-up the bias changed?

    PS. Should also note that the power tubes I ordered were matched "hard." Reading up on the half-power mod it said that puts double the load on the remaining to power tubes, hence the sweeter tone at lower volumes - could it be the setback of hard clipping? Time to change to a new style if I wanna keep it like this (which I do)?
     
  10. GearHeadFred

    GearHeadFred Member

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    Woah.. you got a lot goin' on there...

    First, it's MOST LIKELY won't cause a problem to keep both speakers connected. Yes, there is a mismatch, but you're also running 1/2 the power, so it's not as serious as a full power mismatch.

    If you are going to disconnect 1 of the speakers, taping off the ends is a good idea. Make sure you tape each wire separately..., then fasten them to something so they can't move around.

    It only puts "double the load" on the remaining power tubes if you leave both speakers connected.

    39ma is a slightly hot bias but should be OK.

    Bad clipping is not good. Make SURE those loose speaker wires can't touch each other or anything else. Make sure the tubes are seated well. Make sure the speaker connections to the one speaker are solid. Try the other set of tubes to see if the problem stays with the tubes. Good luck.
     
  11. Lolasaurus

    Lolasaurus Member

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    At low volumes it's crackling and popping sounds, not loud or overwhelming the guitar sound, but they're there in the background.
    I turned up the volume to like 3 1/2, and it became a rumbling overdrive - which would be cool if it weren't happening on a twin reverb.
    So I went back to 4 tubes, 2 speakers. On the lowest notes (i play a baritone) it distorts - about a quarter of the time.

    I read somewhere (trying to find it) that the reissues can't handle the this mod.
    Possible? Unlikely?
     
  12. GearHeadFred

    GearHeadFred Member

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    There is nothing magic with pulling 2 output tubes. The tubes are wired in 2 parallel circuits.. You're simply reducing the power by 1/2 on each side of the "push/pull" circuit. The reason for using 1 speaker is to keep the transformer happy as it's matching the impedance from the tubes to the speakers.

    The re-issues are no different. If anything, this "mod" should reduce the stress on the amp components as the power is substantially reduced.

    You've got an unrelated problem in the amp.. Perhaps a microphonic pre-amp tube (easy fix), a bad/intermittent connection, or a failed component (resistor, cap)
     
  13. Solideogloria

    Solideogloria Member

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  14. GearHeadFred

    GearHeadFred Member

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    Well, it's one thing to say that you may not like the results (subjective)... but I don't agree with his points about not being 1/2 the power (OK, maybe it's 55%), or that the mismatch creates tone problems (there is no mismatch if you disconnect one speaker).
     
  15. Solideogloria

    Solideogloria Member

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    In your opinion, does this "mod" make a difference in volume output?
     
  16. GearHeadFred

    GearHeadFred Member

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    Yes.

    However, 1/2 power (or 55%) results in a 3 dB reduction in volume. Noticeable, but not dramatic.

    I agree with him that if you're looking for crunch from a Twin, you should be looking at a pedal or a different amp. They are clean, clean amps.. Even at 45 watts..
     

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