DRRI NOS pre amp tubes?

Discussion in 'Amp Central Station' started by Crazy neddie, Dec 21, 2014.

  1. Crazy neddie

    Crazy neddie Tele-Meister

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    I have a 1990 DRRI I picked up recently and it came to me completely stock. It doesn't have a mark on it and I suspect it hasn't seen much action. Anyway, I did replace the original power tubes with some Tung Sol RI's and bias the amp, and replaced the reverb tube with NOS JAN 12ayz. The power tubes were labeled with Fender, Groove Tubes, and Electro Harmonix. I think the pre amp tubes are groove tubes. So I was going to swap some tubes on the pre amp side and see what sounds the best. I do have a couple old stock 12ax7's. A Raytheon black plate and what I think is a Fender branded RCA. Then I also have a slew of CP 12ax7's to try too. Any experience you guys can share before I start would be great.
    Oh, I should mention that the amp sounds great but gets a touch raspy at volume. This may be the speaker but I wonder if a tube swap might help?
     
  2. muchxs

    muchxs Doctor of Teleocity

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    It's like solitaire. Use whatcha got and swap it around until you get the best of a bad lot.

    IMO V1 and V2 are most important while your reverb driver (V3) and trem oscillator (V5) are least important.

    Try 5751s in V1 and V2 with a 12AV7 in V6. If 5751s are too expensive try 12AT7s.
     
  3. Wally

    Wally Telefied Ad Free Member

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    crazyneddie, for my purposes...and if you are talking about V3 when you say 'reverb tube', I would go to a 12AU7 there. IT is a lower gain driver tube....same design as a 12AT7 but not as much output.
    Try what you want in V1--Normal ch--and V2--Vib channel. V4 has one triode that is a gain stage for the Vic ch, so there is a soicn pathway there to listen to.
    Raspiness can come from a variety of causes. What speaker? I don't like to listen to modern Jensens. And....I have to ask...at what plate dissipation..bias point...is the amp operating?
     
  4. Crazy neddie

    Crazy neddie Tele-Meister

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    Thanks for the replies guys. Ah let's see, I actually can't remember exactly (I should note this stuff!) but I used a amp head bias probe and measured the plate voltage for a reference and then I think I ended up at about 28 mv and I think the plate voltage was around 440? I could be off a shade with my numbers, but I spent some time doing it and that was where it sounded best to my ears.
    The speaker is a Jensen reissue C12K. Also I do have some 5751's around too. And I have used 12au7 in a reverb on a RI amp before and I liked that a lot. I just don't have one handy at the moment. But I agree as the reverb sounds like garbage turned up above 3.
     
  5. Crazy neddie

    Crazy neddie Tele-Meister

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    I tried a bunch of tubes in V2 and ended up liking the one that came out of it the best. Some kind of fender branded groove tube.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Okay...IF those numbers are accurate....28ma at 440PV...then that is one hotly biased DRRI, imho. For a 14 watt tube, that current draw and plate voltage yield a plate dissipation factor of 88%....quite a bit higher than most of us would want to run in a fixed bias Class A/B circuit. Most of us put a limit at 70%..
    Also, that is some stiff plate voltage, ime. But....at 440Pv, one would be looking for a curretn draw of about 22.3ma. IF I had a DRRI with that type of plate votlage, I would run JJ's....but whadahey.....I have run TungSol RI in one with no ill effects so far.
    Fwiw, teh schematic calls for 391VDC on those plates. Bias will cause that to 'adjsut' to teh curretn draw demands...but 440VDC on the plates is high, ime.
     
  7. viking

    viking Friend of Leo's

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    It is the norm that they are higher than the 391 , no ?
    420 volts keep echoing in my brain reg Dr´s
     
  8. Wally

    Wally Telefied Ad Free Member

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    The AB763 schematic calls for 415PV. With modern wall votlages, that number is usually higher than that....somewhere around 440-450??? I would expect the DRRI to have B+ vltlage that is more in keeping with the schematic...390-400. An amp on the bench will tell its own story, though.
    The problem with that 440PV...IF that was the plate voltage at that 28ma of current draw...is that when the biasing puts the plate dissipation down where most of us would run it, one is going to see more than that 440VDC on the plates..because that Pv will rise as the current draw is decreased.
     
  9. Crazy neddie

    Crazy neddie Tele-Meister

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    Guys I really don't remember what the PV was. I could be way off! I was paying attention when I did it though and it's not that far from 70%, just a couple mv hotter is where it sounded best to me.
     
  10. DaveKS

    DaveKS Friend of Leo's

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    From that era more than likely one of the good old chinese tubes. Only way to know would be to compare plates/build with know tube brands.
     
  11. uriah1

    uriah1 Telefied Gold Supporter

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    5751 bring warmth to V2
    imho
     
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