Rectifiers question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jzb, May 19, 2004.


  1. jzb

    jzb Member

    Messages:
    572
    Joined:
    Sep 1, 2003
    Location:
    West of Boston
    I have a Victoria 518T that doesn't break up until you dime it. Even with a Bob Sweet Booster in line.

    I'm gonna change the 12ax7 NOS tube for a new GT 12ax7M.

    The question is this: If I sustitute a RCA 5AR4/GZ34 for the RCA 5Y3 will I blow the 6v6gt tube to pieces and melt the transformer?

    -j
     
  2. Troy Baer

    Troy Baer Member

    Messages:
    320
    Joined:
    Jan 6, 2002
    Maybe. I tried putting a Weber GZ34 equiv. Copper Cap in place of the 5Y3 equiv. I normally use in a homebrew of mine which has a tweed Deluxe style power section (cathode-biased 2x6V6), and while it sounded great, it caused the 6V6s to red-plate after about a minute. You would not want to run it that way for very long IMHO.

    --Troy
     
  3. jzb

    jzb Member

    Messages:
    572
    Joined:
    Sep 1, 2003
    Location:
    West of Boston
    Interesting, but still doesn't answer my question though.

    Is it wrong to match a GZ34 with a 6v6GT in a Victoria 518-T (Fender Champ tweed clone)?

    -j
     
  4. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    I don't think it will hurt the 6V6 - not directly anyway.

    Yes, it will raise the B+ voltage. Tweed Champs have reasonably low B+, and Fender themselves raised it drastically in the later BF and SF ones. But you may need to rebias it - Fender didn't, and later Champs usually run the 6V6 very hot, often with plate glow. This isn't too good for them obviously, but it can be easily cured by increasing the cathode-resistor value (and the cathode-cap voltage rating).

    Another thing to be careful of is that the filter cap voltage rating is not exceeded. It won't be if they're 450s, but worth checking that they are.

    BUT - I really don't see why you want to do this, if the problem is that the amp doesn't break up soon enough. Increasing the B+ will just give even more headroom and make overdriving it yet harder.

    In fact, not much you do short of drastically lowering the B+ will make that much difference - Champs really don't have a lot of gain, and don't overdrive the preamp much, if at all, before the power tube. I actually fitted one with a Master Volume once (in place of the 6V6 grid resistor) and it did almost nothing.

    You could try replacing the plate resistor for the first tube stage with a 220K, but even then you're not likely to get it to break up at lower volume - it will just have more gain.

    What you probably need is an attenuator. You don't really need a high-power one either - you're only dealing with 5W or so. I built a simple one that worked reasonably well with a 25-ohm 10W rheostat and a 4.7-ohm 10W resistor (I think).
     
  5. jzb

    jzb Member

    Messages:
    572
    Joined:
    Sep 1, 2003
    Location:
    West of Boston
    Thanx... this confirmed what I thought. It did seem strange though that the boost pedal did *nothing* but raise the volume of the amp. It seem counter intutitive to me. I expected it to cause an overdrive of the preamp and a compression of tone resulting in a drop in volume.

    Oh well... should'a stayed awake more in Solid State Physics...

    Back to the books....

    -j
     
  6. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    For earlier breakup in a tweed champ (5F1) add a 25V 25uF bypass capacitor around the 1.5K preamp cathode resistor.

    For crazy early breakup (though not that great a tone IMO) add one around the second stage cathode resistor too.
     
  7. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    That's interesting - my 1959 5F1 came with that cap, stock. I'm quite certain it was stock too, even though the schematic doesn't show it - it was identical to, and identically dated to the other caps in the amp, and the soldering looked original. That particular one did have a much fiercer sound! Nice amp, I wish I hadn't sold it now...

    I wonder if tweaking the values of the two caps would help? I agree, using a 22(25)uF makes the sound really saturated and 'too much'.

    You're still never going to get distortion at a lot lower volume though - basically all you're doing is increasing the gain and making the amp more distorted at full volume.
     
  8. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    Yeah, I've also seen 5E1's that don't have the bypass cap (though the schem. shows it as present). No idea if they got clipped or left out intentionally.

    I agree that the bypass cap value is a good place for experimentation and that it primarily changes the gain of the stage (though IIRC it affects freq. response of the stage as well). Agree too that there shouldn't be much difference in volume at clip, just earlier clipping.

    However, in practice, the perception is that breakup is occuring at a lower volume, particularly with both stage cathodes bypassed. I suspect that this has something to do with the change in freq. response. Embarrassed to admit that I always kinda took it for granted instead of doing the analysis to discover exactly why it sounded that way.

    For lower volume distortion I've also done triode mode and substitution of inefficient speaker with reasonable results, though the triode mode favors additional power supply filtering.
     
  9. Blueser

    Blueser Member

    Messages:
    6,677
    Joined:
    May 21, 2002
    Location:
    Long Island, NY
    Why not just drop a good pedal in front of the amp, if you want distortion?
     
  10. jzb

    jzb Member

    Messages:
    572
    Joined:
    Sep 1, 2003
    Location:
    West of Boston
    I'm not a big fan of distortion pedals. I feel they lack a certain charm and nuance that an overdriven preamp circuit or clipping power tube stage can give.

    It might just be me.... I'm a little off center on many issues and thoughts.

    Some think the glass is half full. Me, I'm still looking for the right glass.
     
  11. hunter

    hunter Supporting Member

    Messages:
    3,730
    Joined:
    Mar 3, 2002
    Out of curiosity, you didn't happen to plug into input 2 instead of input 1. Input 2 sends less signal to the tube input stage. Caveat, I don't know how the Victoria is wired (although Victorias are typically pretty true to Fender designs). I have a 5F1 Champ and the grit starts about 4 on the dial with single coils in input 1.

    Regarding your rectifier question, I would not put a GZ34 in there without checking tube voltages (plate to cathode not just plate to ground) and currents to get an idea of the load on the tubes and transformers. And, as pointed out, this substitution won't help your too much headroom problem.
     
  12. jzb

    jzb Member

    Messages:
    572
    Joined:
    Sep 1, 2003
    Location:
    West of Boston
    Yeah, I read the schematic and saw that the Fender circuit puts a resister inline with the 2'd input.

    I didn't use the GZ34 after all. I'll flip the 518 in favor of another amp for the sound I'm going for instead.

    Thanx for the input though!

    -j
     
  13. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    The typical Fender setup (pair of 68K resistors, 1M tie to ground on jack 1, tip of jack one connected to shunt of jack 2) is supposed to yield a 6dB cut on channel 2 with respect to channel 1.
     

Share This Page