Hot Deluxe Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by aeolian, Nov 2, 2004.


  1. aeolian

    aeolian Member

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    Questions for all the rocket scientist's here who've played with these.

    I'm reworking an 80's (I think, it has the orange stripe in the grilecloth) Deluxe Reverb to blackface.

    The B+ is kind of high. I'm getting about 450V where the schematic says 415 and almost 220 where the schematic says 180. The dividers on the PS measure correctly. Is this okay? Should I change the resistors to drop the B+ in the preamp down? Add one in the first stage to get the voltage down in the output?

    The 1K5 resistors to the grids were bad and the BF schematic didn't have them so I took them out. Are these necessary for the hotter PT's in the silverfaces? I am running a 5U4.

    Given these higher B+ voltages, should I lower the bias? How hot is reasonable? I have low cost Russian Mesa tubes in there while I'm playing with it, but eventually want to put some NOS Phillips or some such. The EH and JJ literature say that they are better at high voltate amps than the old tubes. Any opinions?

    Thanks,
     
  2. Stringrazor

    Stringrazor Member

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    The buzz about the JJ 6V6 is excellent. I just put a set in my '70 SFDR and like 'em. This USENET post suggests they can [edit fixed "can't" typo - sorry] take whatever you give 'em.
     
  3. aeolian

    aeolian Member

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    According to Lord Valve, the JJ 6V6s can handle 6L6 like current. I wrote him about the excessive B+. I changed the PS network to drop the preamp down to 190 but since the output plates come right off the rectifier, they're still up around 450.

    As I work though this amp, I found that the bias is offset. I matched a pair of the 220K bias resistors but still get 2ma offset with V8 being higher.

    This amp also had a three wire cord with the ground switch disconnected. Should I capacitively couple the neutral to chassis gnd like when the ground switch was in there?
     
  4. Wakarusa

    Wakarusa Member

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    1) The JJ 6V6 will tolerate all kinds of abuse and sounds nice, but it doesn't really (to my ears) sound like a 6V6. More like 1/2 way 'tween a 6V6 and 6L6.

    2) You probably want to put the 1K5 grid stoppers back in on the power tubes. They help prevent parasitic oscillation in the power section and also help with blocking distortion. You can use larger values without significant affect on the tone too. Aiken has done a nice write up on grid stoppers that's worth reading (or you can just go through the more dense RDH4 ;))
     
  5. Blue Strat

    Blue Strat Member

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    What Todd said about the grid stoppers. Consider them a valid upgrade in later models.
     
  6. aeolian

    aeolian Member

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    The amp saturates really quickly and has no volume. I read Aiken's article and wonder if there is some offset or load up happening that doesn't cut in and out like blocking distortion but is saturating the OT. So, should I go for 2.2K? 4.7K? I don't want an excessively bright amp (I already disconnected the bright cap) but still want a Fendery sparkle (which is all distorted now). Reading Randal's article, I could calculate the stopper resistance if I new the impedance of the OT, but I don't. Should I just measure it's DC resistance (which would give a small margin in my favor in the calcs)?
     
  7. Wakarusa

    Wakarusa Member

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    Huh.. unless the amp is oscillating I wouldn't imagine that the lack of grid stoppers would cause the early distortion/no volume you're seeing (and even then I ain't so sure). Is B+ where it is supposed to be? Throughout the amp? Problems with the preamp or PI?

    If you have an o-scope now's the time to hunt through and see where the nasty distortion is coming from.
     
  8. aeolian

    aeolian Member

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    Thanks Todd,

    As I said at the top, the B+ is abnormally high. I'm thinking about putting a 5R4 rectifier in it to bring it down to normal BF levels but my initial thought is that the high B+ ought to make it cleaner, not more distorted.

    I did try a few at a store last night for comparison. Of the two real BF's, one was really clear and the other broke up a little with the volume on 3. A pull boost vintage SF broke up almost as much as mine. All seemed louder though. I can see if I can borrow a scope from work (don't have one at home). Before I invest in one, I'd dump the amp and buy the good one at the store (and be money ahead). I got into this as a kind of experiment, and to see if I could tweak a simple amp to my liking. But it's not behaving the way I expected. There's a breakthough there somewhere I'm sure. ;)
     
  9. John Phillips

    John Phillips Member

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    Is it tube-independent? (ie, if you swap the power tubes, V8 stays higher.)

    If so, you've probably got a DC leak in the coupling cap from the PI. Not a serious one or you'd get more imbalance than that, but it's not good.
     
  10. Wakarusa

    Wakarusa Member

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    Could also be some imbalance in the OT (center tap ain't quite dead center) combined with some mismatch in the tubes once they get up to operating voltage.
     
  11. aeolian

    aeolian Member

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    10 points for the OT cluprit. Got a scope on it and both tubes behaved really differently. Discovered that one side of the OT is open. It was feeding everything back though one side. Basically had a single ended 12W amp! :)

    At least the preamp looks clean with the high plate voltages.

    Now the thing is replacing the OT. Antique sells one for $24 while Mojo sells one for $42. Anyone have any idea what the differences are? Any other good sources for magnetics?
     
  12. aeolian

    aeolian Member

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    Started thinking about the fact that this amp has the 022749 export power transformer with multiple taps. I'm wondering if they're hooked up wrong. Trying to find a diagram of this PT. Right now it's hooked to the black and white wires. There is a blk/red and blk/wht along with a grn, org, and yel wire. The existing connector has the lowest resistance of any combination of the white and black-something wires. This would imply to me, less turns and a higher primary to secondary ratio, therefore higher output voltage. At the moment the secondary is putting out 385 a side compared to the 330 it's supposed to have.
     
  13. Wakarusa

    Wakarusa Member

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    Test the heater voltage.

    If the heater voltage is correct then you're on the right PT winding -- always been a better indicator for me than B+ when it comes to these things.
     
  14. drbob1

    drbob1 Silver Supporting Member

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    I'm surprised no one has mentioned a variac? The voltage all across the board are going to be high running on 120 vs 110, no? My Bassman that I was having trouble biasing dropped into the textbook range when I ran it at 110... Perhaps not practical if it's a gigging amp but fine for studio/basement warriors.
     
  15. aeolian

    aeolian Member

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    I want to use this amp for recording and coffee shop gigs, so a Variac is an extra bit of cobble I don't want to deal with.

    Turns out the OT is bad on one side and I'm thinking there is a short somewhere in the primary of the PT that changes the ratios, so I ordered a new set from David Allen. For what I've got into this thing, I could've got an Accomplice kit. Maybe next time. At least now I can put a GZ34 in there and have a real BF going.

    Back to the soldering iron this weekend.
     
  16. aeolian

    aeolian Member

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    Got the new iron from David Allen (wonderful guy as everyone suggested) and put it in this weekend. With a JJ GZ34, plate voltage came right down to 415 like the schematic says. This is the upgrade PT with enough moxie to run a Vibrolux if wanted. I couldn't fit his rotated Vibro OT in my silverface cab so I got his thicker core stock spec tranny. With JJ 6V6's at 24ma I'm getting 24 watts out of this sucker and it sounds awesome. Read some recent articles about orange drops sounding sterile but this sounds good to me. Might put in Mallory's sometime down the road. The NOS Mullard PI helped with the warmth and a NOS Phillips 5751 made the front end come alive. Want to get a couple more and try them in V4 and the front end of my Fuchs.

    Man, this thing sounds so good now, I can't stop playing though it. And that's what matters. :)
     
  17. ROKY

    ROKY Member

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    That's what I love about 'em awesome tubes, they are :YinYang
     

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