Is 100uf too much for GZ34?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 8nthatK, Dec 19, 2004.


  1. 8nthatK

    8nthatK Supporting Member

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    I typically use no more then 50uf/500v filtering but I'm working on a high gain circuit and would like to hear what the greater capacitance sounds like. I like what I've got, but curious...

    Do you think 100uf would be too hard on the rectifier? (JTM45 type PS). Would there be choke considerations/replacements in going to the higher value cap or can I get a good rectifier and just change the cap? I'm using 40uf to the power tubes and 20uf for each pre.

    THX for the insight!
    -K
     
  2. KLB

    KLB Member

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    The 5AR4 is rated at about 60mf max.

    You can probably safely use both sides of a dual 32mf.

    100mf might be too much of a hit for it.

    Just curious, what are your plate volts under load?

    Cheers,
    Ken
     
  3. Wakarusa

    Wakarusa Member

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    Kind of curious what you're shooting for here... less ripple or stiffer power supply or both? Also, what kind of amp is it? How willing are you (and how much room is there) to hack on the power supply?
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Yes,

    I use 68uf with out a problem, though they are rated
    at 60uf max.

    After the choke fill your boots.

    Whispering: (Todd, he said JTM45 type PS).
     
  5. 8nthatK

    8nthatK Supporting Member

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    Mostly to tighten up the supply... it may be too stiff, but I'd like to turn over all stones.
    The amp is an original pre-design (not that I don't think it hasn't been done before by someone) into what is pretty close to a JTM45 PS.
    It's built on a mule chassis right now, so room or originality is not an issue.
    There's 422v on the plates now, but the PT that I want to use on the final chassis will put 452v on the plates, I want to hear both though before deciding on the PT for sure.

    Ampnerd; Was your "Yes" meaning as, it's too much, or yes as I can use the 100uf?

    Thanks for all the suggestions!
    -K
     
  6. 8nthatK

    8nthatK Supporting Member

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    I was hoping the 60uf was conservative...
     
  7. 8nthatK

    8nthatK Supporting Member

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    Hmmm...maybe I need to keep the plate voltage down and go with a 5U4 if using the 100uf. Though I didn't really want that sag.
    I need to check the current the rectifier is seeing again I guess, I know it was under 250ma...
     
  8. KLB

    KLB Member

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    http://www.fust-electronica.nl/rectifiertubes.htm

    The max first filter cap load for a 5U4GB is 32mf.

    Have a look at the chart on the above weblink.

    Why not try some UF5408 3A 1000v ultra-fast diodes instead of a tube rectifier. You can use the 100mf first stage with no worry. If the voltages are too high later in the amp, use larger dropping resistors, say 12-15K instead of 10K.

    I have a Germino LV55 that I setup with a 5U4GB and 6V6, with a 12AU7 phase inverter. This nets about 420v on the plates. I put the impedence selector one step below the speaker load, a 16 ohm Cannabis Rex in a combo.

    Note that the plate volts could approach 500v on some original JTM-45 running KT-66.
     
  9. 8nthatK

    8nthatK Supporting Member

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    Thanks for the link, I had that backwards in my head for whatever reason?...
    I've thought about going with diodes, but I kinda like what the tubes plate offer, besides it looks purtty and is already set up : )
     
  10. TheAmpNerd

    TheAmpNerd Member

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    If you are worried about having a too stiff supply then I
    wouldn't even think about approaching 60uf, let alone
    100uf.

    If you are going to try on different rectifiers,
    I'd keep that first cap 20 - 22uf.

    From this link (which is accurate): http://www.fust-electronica.nl/rectifiertubes.htm

    You can try out any of the following:
    5AU4
    5R4GB
    5U4G
    EZ90
    5AR4

    Then, you can beef up the supply after the choke
    with an additional PI filter.

    You should already have 150uf after the choke,
    if that is your guide.
    Try them and see.

    NOTE: If you still want a large cap (100uf) to start with and want the "sag" assciated with a tube rectifer, then
    buy the various Weber Copper Cap solid state plug in
    rectifiers.

    http://www.webervst.com/

    Good luck and let us know what you choose and the
    results.

    Happy Building : )
     
  11. Wakarusa

    Wakarusa Member

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    Oops. shoulda seen the comment early about "JTM 45".

    Couple of thoughts - I'd say the ratings in the chart linked to earlier are conservative for some of the rectifiers. The empirical evidence is all of those Fenders with 16uF next to a 5Y3. I've also run 5R4 with significantly more than 4uF as the first filter.

    All of that said, I've also fixed a BUNCH of amps where someone has slugged in 100uF next to a 5AR4 and the rectifier kept melting down. Go figure.

    For a tight and responsive power supply, the suggestion to go solid state is, well, solid. If ripple rejection isn't the issue then the extra capacitance you're talking about would tend to work against the sag you'd see in the tube rectifier.

    Another thing to keep in mind when looking at power transformers: pay attention to not only B+ but % regulation at rated load. Short story is that a weak PT will show more sag than any rectifier. A PT with better regulation will hold B+ like a rock.

    The mention of 150uF after the choke is a little misleading because it's split up with isolating resistors -- which, btw, is another way to get a tighter feel in the amp: separate out more of the stage power supplies with additional isolating resistors and shunt caps.

    Overall, I'd suggest you're working at crossed purposes -- a tighter power supply that has to use a tube rectifier seems like kind of a waste of a tube to me.
     
  12. PaulC

    PaulC Member

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    There's 3 things to look at - how much sag do you want, how "fast" do you want the sag to respond, and how much ripple do you want. Sag is coming from the transformer regulation and diode setup. Ripple from the amount of filtering. The filtering will effect the envelope of the sag. Ripple will effect ghost notes, and fizzy pwr tube distortion. All 3 interact.

    Circuits with a lot of sag, and then alot of filtering can sound really sluggish. that's because there's a big change in voltage under load that takes awhile to drop and recover - sometimes inbetween notes. Circuits with alot of sag and low filtering can have a big snap to the attack because the voltage can charge and discharge giving an attack to the notes, but you can have some fizzy sounds and ghost notes. Really stiff regulation can have a very "fast" sound because there's little sag, so everything stays the same.

    If you want more punch in the bottom you might need better regulation instead of more filtering. More filtering will help out the first note, but it might not recover fast enough for the second one. there's a ton of things to dink with in that area.

    PaulC
     
  13. 8nthatK

    8nthatK Supporting Member

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    Thanks gentlemen, this helps a lot. I'll experiment with some 50uf and under filtering for the first stage; I'll also try some of the other excellent suggestions.

    Great explanation that makes since to me PaulC...thanks.

    -K
     
  14. ArchtopIIg

    ArchtopIIg Guest

    In their day, GZ34s were rated for a 60uf +/- the 20% which was common.

    I assure you you will not notice much above 60uf, if you do you will not like it unless you are building bass amp.

    if you need the johnny holmes stiffness, go solid state. the GZ34 is a close as your gonna get with a tube plus 60uf, this is already a stiffy.

    THe JTM45 will sound alot LESS musical with 60 or even 50 uf.

    ArchtopIIg (Dr.KT88-Ampage)
     
  15. scottosan

    scottosan Supporting Member

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    That chart was confusing. I'm building my first tube rectified amp. The list said max 10H. Does that mean you shouldn't use more than a 10H choke? or is tha only using an input choke vs. input cap?
     
  16. Amp Nut

    Amp Nut Member

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    The Komet 60 uses 80mf with a GZ34 rectifier.
     
  17. rmconner80

    rmconner80 Cantankerous Luddite Silver Supporting Member

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    I agree with the "cross purposes" post. A high gain amp with a stiff PS... and a tube rectifier?

    Just use SS diodes.

    Or if you really want the fireworks and bravado of a glowing tube rectifier (especially the fireworks...), then go for a mercury vapor 83 rectifier. Of course you'll need to change the socket, and you'll need to be sure you don't drop it :rolleyes:
     
  18. markdurham

    markdurham Member

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    There is another 80uF cap under the power supply board that is in series with that 80uF cap for a total of 40uF on the GZ34.
    markd
     

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