Strange doings with my Deluxe Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by teleman55, May 26, 2015.

  1. teleman55

    teleman55 Member

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    Strange. Go to swap tubes. Bias meter reads almost tweed levels. Put a 5ar4 in place of the 5u4gb and get readings like i should with the 5u4gb. I'm hoping the bias probe just went south on me but it seems to be functioning properly. Any thoughts on the cause? I've been using another amp a lot lately but used this old faithful twice recently, including Saturday night. It was fine but seemed a little stiff. That's why I thought I'd change tubes and speaker after that. To re voice the amp. It's been fine for a long time and nothing done except that it got weird a year ago and one of the caps under the doghouse thang was loose on one end. I re-soldered it. That fixed that. Nothing else done to it.
     
  2. pdf64

    pdf64 Member

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    Could you put some numbers to this?
    An old, failing original bias supply cap might cause this.
     
  3. Blue Strat

    Blue Strat Member

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    Yes, numbers please. Also, have you replaced your meter battery lately? If not, do that first.

    What are the readings with the old tubes and the new tubes?
     
    Last edited: May 26, 2015
  4. zenas

    zenas Member

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    5u4 so I'm guessing it's a silverface and it sounds like it still has the original electrolytics.
    If that's the case change all the electrolytics and what the other guys said.
     
  5. teleman55

    teleman55 Member

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    Checked the battery. (my first thought). Some numbers:
    PV/ma
    390/24
    395/20.4
    400/15.5
    410/9.7
    420/3.9
    This with the EH 6v6 that were in there.
    This is all off (like stepped down) from where it was before.
    Tried JJs and also different 5u4 and got a similar difference.
    Yeah it's a silverface. Caps are about 8 years old. Put a 5ar4 in just to see and got readings similar to what I'd normally get with a 5u4. Wish I still had my Pro reverb to cross check the meter. Other amps now are el84. Any way to check this bias rite with a multimeter? I know they had problems with these but this one's been fine for a long time.
     
  6. zenas

    zenas Member

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    You could open the amp up with a multi meter that reads MA to double check those numbers.

    Doing so puts you inside the amp live where the highest voltages are though. One wrong move pops the fuse inside the meter or gives you one hell of a shock. Probably could kill you too but so far it's just been the fuse and one hell of a shock for me.

    Dose your probe read off both sockets or just one?
     
  7. pdf64

    pdf64 Member

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    Why use such a hazardous procedure, when it seems far simpler and safer to measure the OT primary resistance (CT to each plate) and the voltage across those sections?
    Safety being relative when dealing with an open, energised tube amp chassis.

    Sorry, I don't understand what all the readings represent; is how the conditions on one tube seem to be changing over time?
     
  8. zenas

    zenas Member

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    When you think about it the transformer shunt method of bias measurement gets called unsafe. But if you work on amps at all your going to be inside checking voltages.
    Put the one ohm resisters in and you gotta go inside.

    The one key difference is with a meter set to measure miliamps the voltage goes through to the other probe. So if you stick that probe in your hand whilst grabbing a pen you get one hell of a shock.

    You only do that once though. :)
     
  9. teleman55

    teleman55 Member

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    That's the plate voltage and ma at various stops of the bias pot. That's what I meant by strange. It's in the 60-70% range with a plate voltage in the low 390s. Whereas that's always been at a PV of around 410 to 420 depending on the tubes. I've tried several different known good pairs of power tubes and several 5u4s and and get more or less the same thing. Tried a 5ar4 just to see and got readings like I used to get with the 5u4. Whis is just about what I'd expect. A 5ar4 used to run about 440 in this, not that I used one. Probe reads off both sockets, tubes are matched pairs. both read more or less the same. So I guess its either the amp or the probe, or both. Amp worked fine Saturday night. I'm gonna crank it and see. Any thoughts on what could cause this?
     
  10. pdf64

    pdf64 Member

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    Maybe your line voltage is running low?
     
  11. TimmyP

    TimmyP Member

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    390/24
    395/20.4
    400/15.5
    410/9.7
    420/3.9

    I'm surprised at how much the B+ changes for a given bias current change. I'd do this again, and measure the ripple on the B+ at each bias setting (if your meter can accurately measure AC with a big DC offset - some cannot). While you are at it, measure the ripple on the bias too.
     
  12. teleman55

    teleman55 Member

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    Thanks never would have thought of that but just tested it and it's 118.8, so unless it was low for two nights and is back up??
     
  13. zenas

    zenas Member

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    Has the amp worked right since that filter cap lead under the dog house was resoldered?

    Speaking of that when new Fender put a piece of weather stripping inside the dog house to hold the caps. That needs to get replaced when new caps go in. The original stuff gets hard and new caps are smaller.
    Ace has it in 10 ft rolls about enough to do about 20 Fender recaps.
     
  14. teleman55

    teleman55 Member

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    Yah worked fine and biased fine since then. Weatherstrip is new enough. Gonna crank it now.
     
  15. teleman55

    teleman55 Member

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    Yeah that's pretty weird. That's what originally struck me, plus that the B+ reading for a given bias current had suddenly dropped 20+ volts. Can't measure all you talked about, but both readings I can take fluctuate a little more. Not wildly but a little more than usual. Cranked the amp. Great crunch @6 on channel 1. Channel 2 (no bright cap) a little quieter and tinny at same level. On ten volume is same on both but harsher distortion on 2. Quieter cleans are nice. But, seems to lack a little DR smoothness. Reminds me of a valveking a little, with that cold Peavey bias. But that could be the amp or because I biased it with a bad meter. Channel 2 had that same thing Sat night but I was trying a 5751 in there then so I figured that was it. Maybe not. I gigged a zillion gigs with this amp the past 7 years or so but haven't used it much lately with the band. You'll laugh at what I've been using, A Crate V30 made in beautiful Yellville, AK. But love that amp. But gotta get this DR right. It's still me real #1. Especially when I do my own stuff..Any ideas would be mucho helpful.
     
    Last edited: May 27, 2015
  16. teleman55

    teleman55 Member

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    Could a bad filter cap cause the b+ voltage drop?
     
  17. teleman55

    teleman55 Member

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    Wow! You called it. As I posted before, I checked it at your suggestion and it was 118, a little low but not out of range. Checked it again tonight. It was 121.something. Decided to check everything on the amp again and it's as it should be. Must have been some wild power fluctuation going on here the past few days. Haven't been any outages. Just learned a who'd a thunk it lesson. Weird readings? Don't forget to check what's coming out of the wall. And do it more than once. When I checked it first i must have got it a the top of a fluctuation that was bottoming out who knows where.
     
    Last edited: May 29, 2015
  18. Steppin' Wolfe

    Steppin' Wolfe Member

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    Telemann55, re: li ne voltage fluctuation......it is of value to set bias accordingly, ime. A Variac is handy to have for this purpose......and it is interesting to take note of the difference in sonics when the line voltage is at different levels. A 9 volt swing on the plates of the preamp will make a big difference in the sonics....even when the power tubes are rebiased for the same plate dissipation. This is a big argument for controlling the power that he amp is seeing from one setting to the next.
     
  19. teleman55

    teleman55 Member

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  20. mxvin

    mxvin Member

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    I use a variac to keep the voltage at a more steady state. House VAC fluctuates more in the summer months due to high use of air conditioners.
     

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