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Lowering B+

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jzucker, Aug 4, 2003.

  1. jzucker

    jzucker Supporting Member

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    With my new Elizabeth amp, I'm getting 479v on the plates of the power tubes. I'd like to be able to use svetlana or EH 6L6 tubes and for those, I want to get the plate voltages down to 450ish...I'm currently running a 5U4GB rectifier which does the trick but I'm wondering about changing the dropping resistors in the power supply? Anyone else do this?

    I'm also considering using one of the webber copper caps...I assume it's very similar to my idea with the dropper resistors...

    I'd appreciate any feedback/comments...

    Of course, I could try a 350-0-350 PT instead of the 375-0-375 one that's in there but that's an expensive solution...
     
  2. John Phillips

    John Phillips Member

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    Not meant as a criticism, just curious...

    Why not just use good 6L6s that don't need the lower B+? Or why not stick with the 5U4 if that's doing the job? Seems unnecessary to modify the amp...
     
  3. jzucker

    jzucker Supporting Member

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    Both good points. I probably will buy a set of NOS 7581 tubes but lately I've been spending like it's going out of style and don't feel like spending $100 on a pair of tubes! I'd like to be able to use cheap svetlana and EH tubes. The Svetlana EL34 handles the higher voltages and those sound really good at that voltage (which is what I currently have installed) but I'd like to use 6L6 tubes too...

    Regarding the 5U4, I'll probably keep that in for the time being but I get very little sag with that in there. I'd like to at least get GZ34 level sag...

    Thanks for your help. I love this place and how so many members (you especially) are so knowledgeable.

    Jaz
     
  4. Fuchsaudio

    Fuchsaudio Member

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    I used the New Sensors with an approximate 3% failure rate on the last 500 tubes. This is in the ODS-100 and 50's, running approx. 500V B+. The Svetlanas in the same amps have had '0' percent failures and produce a tad more power. I wouldn't sweat either. The 7581's are a nice tube, but sonic's are a little different, and yup, they have gotten pricy.
     
  5. jzucker

    jzucker Supporting Member

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    I'm not sure if I understood your post completely. Are you referring to the 6L6EH tube running at 500VB+ with the 3% failure rate? If so, that's great news for me since I have a couple pairs of those...The only issue is the reverb driver tube which may be running too hot with the GZ34 rectifier. I forgot to measure it with the GZ34 but it measured 440v or so on the plates of the reverb driver with the 5U4GB...

    Thanks. You guys are very generous with your help and advise! :)
     
  6. PaulC

    PaulC Member

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    "Regarding the 5U4, I'll probably keep that in for the time being but I get very little sag with that in there. I'd like to at least get GZ34 level sag..."

    Actually the 5u4 will have more sag than the GZ34. The GZ34 will raise the voltage and sag less. If you are seeing little sag with the 5u4 that would lead me to think the tubes are biased pretty hot - little current change from idle to full power.

    The voltages with the 5u4 are fine if the tubes are biased for it. If you want to try dropping the voltage and add a little sag then add a resistor between the output of the rect tube and the first filter cap. 50 to 150 ohms would be good starting points. Take your total current draw and times it by the resistor value to get a close idea of the idle voltage drop. The entire current load for this tap will be going through this resistor, so you want it to have a hefty wattage rating. Mouser sells some nice 25-50 watters in cool heatsinks that mount to a chassis.



    If you want to drop voltage without the sag then something like a zenor string on the center tap of the B+ winding works nice. another way is something called a "bucking" transformer on the secondary winding. Myself I don't like to rig things up. If I really have a problem with the voltages I'll just swap out the tranny. But like Andy I've run a ton of Svet 6L6's at your voltages without any problems. Later - PaulC
     
  7. jzucker

    jzucker Supporting Member

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    Can you explain that in more detail? Lord Valve explained to me that the 5U4GB will sag less due to the higher current rating it carries over the 5AR4...

    Thanks for your advice regarding B+! :)

    Jaz
     
  8. PaulC

    PaulC Member

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    The 5U4 can source more current than a GZ34, but it's internal resistance is higher than a GZ34 dropping more voltage. At idle you'll see something like 20 to 40 volts different depending on you bias points/idle current. At full pwr you'll see a little more voltage drop with the 5U4, but it really depends on your bias points as to how much. The closer you get to constant current the less drop you'll see with either tube.

    The main thing to watch for is the current ability of the 5vac winding. The 5U4 draws a little over 1 amp more than the GZ34, and alot of the trannies made for a GZ34 will not have the winding over rated enough to run the 5U4 without burning up. PaulC
     
  9. ericb

    ericb Silver Supporting Member

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    Hey , I'm not a tech like John and Paul, but 1 thing I learned about 5 years back is 5u4's sag more than gz34/5ar4's! I have quite a few amps that I tried them both with (temporarily in some circumstances due to the excess draw of the 5u4's) and the difference was always obvious... I have a 5r4 that is in between both and very nice.

    Eric
     
  10. AlNelson

    AlNelson Guest

    I agree with PaulC. It is all about the internal resistance of the rectifier, in this situation. The GZ34/5AR4 would sag less and make for a higher B+.

    Why not contact Winnie and ask him to suggest a simple mod to drop a few volts? He is a nice guy and I am sure he would want you to be happy with the amp.

    Though, I also agree that most 6L6GC's should work at 500VDC since that is the listed rating. Or, stick in a pair of KT66's.
     
  11. Ayan

    Ayan Member

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    I read the replies you got thus far, and believe all of them offer good suggestions. As for the webber copper caps, I have no idea what that is.

    However, if you truly want to reduce the B+ regardless of what rectifier you use in the power supply, what you can do is use zener diodes on the ground side of the mains transformer secondary. You can cascade say 10V 5W zeners to achieve a 10V, 20V, etc. drop.

    Note that the zener will just be dissipating power in the form of heat, so it has to be sized accordingly and placed such that it will get some heat sinking action (great to chassis mount them). Also beware that the power used up in the zener will come right out of your amp's available power. So if your zener arrangement uses up say 5 W, a 50W amp will probably be able to put out 45W, unless the mains transformer is overspecced in the first place, before the power supply starts sagging.

    Good luck,

    Gil
     
  12. Fuchsaudio

    Fuchsaudio Member

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    Gil is referring to the center tap of the High Voltage secondary . It almost sounded like he meant the AC primary of the transformer there. This point is normally grounded on most tube amps. In this case a zener and/or string of zeners in installed at the point in the circuit to knock down the rectified DC voltage by the approximate value of the zener string. Randall Aiken has a tutorial about this on his website. The wattage is very important. A stud-mount high wattage zener makes the most sense, can be chassis mounted, and the chassis will provide a heat-sinking for the diode to stay cool as possible. Just be careful where you mount it, because sometimes in will make some additional noise in the amp via the ground.

    TIDBIT: The filter choke on a Deluxe (Bandmasters and other Fenders too) can be relocated (I put it where the rectifier tube formerly was in my mods), and a small fan can be installed behind the power tubes blowing out of the amp. You wanna see how nice and cool that amp can be when your done ! It will run all night and the chassis will be barely warm. If you buy a 12 V surplus computer fan, run it off the filament line with one diode and a 6800 16 V cap, you get about 8 volts. The fan runs at a reasonable speed, and is quiet. Why more manufacturers don't do this (especially with tube upside down) is baffling to me.

    The Weber Copper Cap is a solid state tube rectifier replacement with a built-in sag. It's on the www.Webervst.com website. I've not used them. THD also made something called the "Reactive Rectifier" which was similar, but required installation inside the amp, while the CC is just a plug in.

    Again, I wouldn't be so hung-up on the whole voltage issue, but suit yourself Jack.
     
  13. Ayan

    Ayan Member

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    I wonder why you would think that, Andy. It seems you replied to my post before you had a chance to catch my explicit reference to the secondary all together... :cool: What I wrote was, quoting my previous post:

    However, if you truly want to reduce the B+ regardless of what rectifier you use in the power supply, what you can do is use zener diodes on the ground side of the mains transformer secondary.

    By the way, in Jack's case, yes, it corresponds to the center tap of the mains transformer. However, if the mains has no centertap, or has a centertap that's NOT meant to be grounded (like on some Marshall mains, where the CT goes between the first two totem-poled power supply caps, and therefore is connected to about 250VDC, as opposed to ground) the zener goes on the negative end of the bridge rectifier. That is why I chose to say "the ground end of the secondary," as opposed to the CT, since it covers both possible scenarios.

    Cheers,

    Gil
     
  14. Blue Strat

    Blue Strat Member

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    Actually, a 5R4 rectifier offers more series resistance and MORE sag (not a lot more though) than a 5U4 rectifier. Using one of these will also lower you plate voltage somewhat.

    BTW, we have JAN Philips 7581As for $74 per pair....just a little over 2X the price of Svet or EH 6L6GCs with about a 5X life expentancy increase.
     
  15. jzucker

    jzucker Supporting Member

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    WOW ! Great tip Andy. Thanks. I'm definitely going to do that. I assume the fan's amperage requirements are negligable? I'm a little concerned about the power transformer's amperage rating since it runs REALLY hot and I've heard so many horror stories about transformers.com units...It's rated at 6A off the heater tap and I'm running a pair of EL34s (2) 12AX7 and (2) 12AT7 so the power tranny ought to be running nice and cool but it's not.


    Did you say you are running your 6L6EH tubes at 470V (or higher) ?

    I'm going to order a couple sets of those.

    Thanks again,

    Jaz
     
  16. Fuchsaudio

    Fuchsaudio Member

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    If your not running a rectifier tube, you can use the 5 V winding and build a little voltage doubler with two caps and two diodes. If you are using the 6V winding, most of these fans pull about 25 ma, which is negligible. Although I would not count on it for cooling the tranny, the tube and transformers (and overall chassis temps) are noticibly lower in the amps when they have a fan in them. Some models do not have enough space. Most amps (I'd say 95%) end up with fans and run very cool.

    I run the 6V6's in my ODS-30 at 400 volts. Actually, I played with 500V B+ (like the Jim Kelly) and a separate 400V screen voltage source, and it sounded awesome, but the tube life and reliability were not very good.
     
  17. jzucker

    jzucker Supporting Member

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    OK, dumb question...I was looking through the mouser catalog and all the fans looked like they needed some kind of bracket to mount them to the chassis bottom. Is there a bracket to do that which I couldn't find?

    Another dumb question? Do I use a bridge rectifier or a pair of diodes or does it matter?

    Oh, I was talking about running the 6L6 tubes at 470V. Didn't you say you did that with the 6L6EH tubes in your amps?

    Thanks again Andy.

    Jaz
     

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