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Thread: Subed with GZ34 Rectifier, Slower DC Voltage Climb

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    Member HaroldBrooks's Avatar
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    Subed with GZ34 Rectifier, Slower DC Voltage Climb

    Recently installed a GZ34 into my 6L6 project amp, and after my usual fashion of obsessing about startup voltages, I measured the ascent of the B+ voltage to see if it came close to my new 600vdc first filter cap.

    I was pleasently suprised to find the GZ34 rectifier in this amp ramps up voltage to more or less the stable operating B+ of 473vdc. The prior rectifier tube was a 5U4GB, and that allowed the B+ to go all the way up to 540vdc or so, exceeding the original 450vdc filter cap by a good margin. I attributed this large 90vdc overage to modern higher wall voltages (the amp is from the 1940's).

    It seems I got more protection out of the new Rectifier tube in having a slow startup. Just to be safer though, I also installed two series CL-80 3 amp Thermistors on a primary AC transformer lead, to limit current inrush, as I am using a large first filter capacitance (94uf) to reduce hum. I want the new JJ GZ34 Rectifier to last a while and it's rated to 60uf for first filter capacitance. The thermistors in steady state only trim off perhaps 10vdc from B+ so that is fine by me.

    Question: What is different about the GZ34 vs the 5U4GB that slows the B+ voltage rise so much ? Thanks !

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    It should be the other way round. A typical GZ34 drops less voltage than a typical 5U4GB. A GZ34 dropping that much more voltage is suspect. It must get very hot.
    Without load (no tubes inserted) the voltages should be essentially the same with both rectifiers.

    What B+ does the schematic specify?

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    Last edited by Helmholtz; 01-25-2020 at 02:54 PM.
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    Agreed, that's a lot of voltage drop for a GZ34. Modern production tubes do have a larger voltage drop than older ones, though, but not as much as you're getting.

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    Supporting Member TomCarlos's Avatar
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    This comes from another thread here on MEF

    gz34 vs 5y3 vs 5u4 - The Dynaco Tube Audio Forum

    Take a look at post #3.

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    Quote Originally Posted by HaroldBrooks View Post
    I was pleasently suprised to find the GZ34 rectifier in this amp ramps up voltage to more or less the stable operating B+ of 473vdc. The prior rectifier tube was a 5U4GB, and that allowed the B+ to go all the way up to 540vdc or so, exceeding the original 450vdc filter cap by a good margin.
    I'm guessing that the GZ34 being indirectly heated warms up slower at switch on than the power tubes so the B+ rises to the operating voltage without overshoot but with the faster warm up 5U4 the B+ rises to 540V before the power tubes start conducting to pull it down. What is the stable (after warm up) operating voltage with the 5U4? It should be lower than with a GZ34.

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    Good point!
    So you think that the 5U4 heats up faster the the power tubes and the 540V is just a transient overshoot (not quite clear from the OP) settling at a voltage lower than with the GZ34?

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    Quote Originally Posted by Helmholtz View Post
    So you think that the 5U4 heats up faster the the power tubes and the 540V is just a transient overshoot (not quite clear from the OP) settling at a voltage lower than with the GZ34?
    Yes, that's right.

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    Quote Originally Posted by Helmholtz View Post
    It should be the other way round. A typical GZ34 drops less voltage than a typical 5U4GB. A GZ34 dropping that much more voltage is suspect. It must get very hot.
    Without load (no tubes inserted) the voltages should be essentially the same with both rectifiers.

    What B+ does the schematic specify?
    Sorry, I wasn't clear and mixed two concepts together. Thu 5U4GB had a lower operational steady state B+ voltage, from memory around 440vdc, but the GZ34 steady state is around 473vdc. The big difference is the climb in voltage on startup. The 5U4GB allows the B+ to temporarily jump to 540vdc, the GZ34 more or less climbs and settles on the steady state 473vdc voltage, and doesn't over-shoot it.

    Unfortunatly, no schematic to be found anywhere.

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    Last edited by HaroldBrooks; 01-26-2020 at 02:39 AM.
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    Quote Originally Posted by Dave H View Post
    I'm guessing that the GZ34 being indirectly heated warms up slower at switch on than the power tubes so the B+ rises to the operating voltage without overshoot but with the faster warm up 5U4 the B+ rises to 540V before the power tubes start conducting to pull it down. What is the stable (after warm up) operating voltage with the 5U4? It should be lower than with a GZ34.
    It is lower, and I wasn't clear about that in my OP, 5U4GB around 440vdc, and the GZ34 around 473vdc. I wanted a higher steady state B+ by using the GZ34 and got it, I also got a welcomed slower voltage ramp up on start up.

    Wasn't sure why, but the indirect heating makes sense ! Somehow I falsly equated indirect heating with an older design, but I was mistaken. Thanks for the help.

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    Last edited by HaroldBrooks; 01-26-2020 at 02:42 AM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by HaroldBrooks View Post
    It is lower, and I wasn't clear about that in my OP, 5U4GB around 440vdc, and the GZ34 around 473vdc. I wanted a higher steady state B+ by using the GZ34 and got it, I also got a welcomed slower voltage ramp up on start up.

    Wasn't sure why, but the indirect heating makes sense ! Somehow I falsly equated indirect heating with an older design, but I was mistaken. Thanks for the help.
    Indirect heating on the gz34 was my first thought when I read the first couple of posts. All I can say is,.. Enjoy your cake! You can eat it too I understand about obsessing over pedantic matters like input surge voltage and capacitor ratings. And I've taken measures in builds to ensure against the problem. Major manufacturers commonly ignore this, some of our more experienced techs notice this without wincing and the filter cap failure in amps that stress this matter is actually very low. But I'm with you and have taken above industry measures to circumvent it. Maybe it's a non issue because capacitors are typically very good with surge voltages.?. But I also know that I've had marginal luck with newer design caps going outside their rating and older design caps seem to be up for anything until they're not. I guess what I'm saying is that I know the materials are at least a little different than they use to be. So the old standard of relying on repeated surges not being an issue didn't sit well with me building new amps. I see to it that all my filter circuits are rated over the unloaded voltage. (<period) Since we really don't know what modern capacitor architecture will do in these conditions compared to the golden age stuff it gives me solace. The reality is that it's probably a non issue. But after the era of the gray Illinois caps (late 80's to late 90's) failing in everything I just don't take chances.

    YMMV and rant over.

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    I always thought the 5AR4/GZ34 had controlled warm up but looking at the RCA data sheet I'm not seeing any reference to that.

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    Just my thoughts: perhaps while the tube may take longer to warm up, it may not be the result of a design for a controlled interval. If I recall, the controlled warm-up was like 11 seconds. Especially important in the old days of series heater strings in radios and particularly TV sets.

    All acaps have a surge voltage rating, even if it is not readily apparent on the specs. As to large numbers of Illinois failures and similar, without having hundreds of them to analyze, we can only rationalize the reasons. It may be simply crappy caps rather than specifically overshoot at turn-on. The 450v ones might just as easily have failed at 300v. We won't know.

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    Quote Originally Posted by loudthud View Post
    I always thought the 5AR4/GZ34 had controlled warm up but looking at the RCA data sheet I'm not seeing any reference to that.
    Same here, I looked at several GZ34 data sheets but didn't see "Controlled heater warm up" It's mentioned in the link below for the "Mullard" reissue but they say it's directly heated which can't be right.

    GZ34 reissue

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    Last edited by Dave H; 01-26-2020 at 11:56 AM.

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    Perhaps the slow warm up was just expected to be taken as read, given that it's indirectly heated? A 'controlled warm up' would imply a specified characteristic parameters, which a manufacturer might not wish to be held account to.
    Even with the mil versions of regular directly heated rectifiers that were indirectly heated, eg 6087 for 5Y3, the warm up time isn't specified, just 'approximately the same as other tubes' https://tubedata.altanatubes.com.br/...083/6/6087.pdf

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