1st filter with GZ34, is 100uf to big - diyAudio
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Old 25th October 2012, 09:04 PM   #1
ivan H is offline ivan H  Australia
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Default 1st filter with GZ34, is 100uf to big

Hi, I built like a transitional JTM50 & have been using with a SS rec & 2x 50uf main filter. To use a GZ34 would I need to use a smaller value filter? I read on the Vintage Amp forum that the GZ34 can have up to 60uf, but it said thats only a recommendation for the tubes life, & that going 2x over with the filter size was no big deal. Wot do u guys think. Any input appreciated. Cheers
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Old 25th October 2012, 09:18 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

AFAIK you should never exceed the maximum capacitor value
and a bit less is a good idea for life so you should only use 50uF.

rgds, sreten.
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Old 25th October 2012, 09:37 PM   #3
ivan H is offline ivan H  Australia
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Thanks 4 ur reply. I'm no tech but I kinda thought that, & I know that 2x EL34 is loading up a GZ34 as it is when cranked. I can just remove the link between the 2 halves of the dual 50uf can, but would I be better to go to, say 32uf for the 1st filter? Screens, PI & both Pre's r 32uf. Cheers
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Old 26th October 2012, 03:22 PM   #4
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Hammond H100 organs have a 47 uf cap as the first filter on two GZ34's. But they have a "40 ohm" choke between the tube and cap to cut the surge current. One of my H100's made in 1967 and used in a home has two hammond brand rectifiers in it. The other made in 1968 and used in a church had Sylvania 5Y4's in it. Don't talk to me about hiring a pro to repair your organ. Cheapskate. For more volume he should have changed the 1967 build cap.
One cheaper thing you can do to cut surge current is put 47 ohm 5 watt resistors between the transformer and the plates. This both cuts surge current, and protects the transformer from a shorted rectifier tube. Those are more common now, according to net lore, now that the RCA and GE and Mullard factories are dead. I used solder terminal strips from tubesandmore.com or triodeelectronics.com to mount them. I bought a little roll of 600 v rated tinned wire at the same time from the same place, for the jumper to the tube.
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Last edited by indianajo; 26th October 2012 at 03:31 PM.
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Old 27th October 2012, 12:48 AM   #5
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my pre amp filter/supply cap is
now 100uf on my moded 5e3 with a JJ gz34 and no probs so far, its fixed the poping I used to get with the amp running all out.
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Old 30th October 2012, 02:53 PM   #6
Struth is offline Struth  Canada
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Hi Guys

If you look at the tube manufacturer's data for tube rectifiers, they recommend placing series resistance for each plate. This acts to limit the initial charging current of a noncharged capacitor.

A cap looks like a dead short at the instant power is applied. The only thing that limits the charge current is circuit resistance, comprised of the DC winding resistance of the plate winding of the PT, resistance of wiring and connections, dynamic resistance of the rectifier, and any deliberately added resistance used to create low-pass filtering for the supply.

Suppose B+ is 400V and there is 1R of total resistance in the charge path. The initial charge current is 400A. If you add a 10R resistor feeding the cap, initial current is limited to 40A. The data sheets for most tube rectifiers suggest values of 135R, which for the example limits the current to just under 3A. The problem with such a high value is that it also limits the operating current for the amp and introduces serious supply sag under load.

However, adding just a few tens of ohms will greatly improve tube rectifier life.

A common alternative to a single large first filter is to split the first position in two, separated by a low-value R. Again, if you have RCRC, the two sections act as low-pass filters reducing noise that may come through the PT and recto noise from getting to the amp. The Cs can be moderate values and the Rs 10-33R each

Have fun
Kevin O'Connor
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Old 30th October 2012, 08:59 PM   #7
ivan H is offline ivan H  Australia
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Thanx 4 replies guys, there's some really good info there. I've been considering doing the "Ceriatone GZ34 mod" of placing diodes between pins 7 & 6 & also 5 & 4 (cathodes to 6 & 4) & a .01uf/3kV disk cap across 6 & 4 (diode switching hash), with PT's HT secondry leads moved to pins 7 & 5. Does anyone have any thoughts on this? Reccomended or no. All input appreciated. Cheers
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Old 30th October 2012, 11:23 PM   #8
Struth is offline Struth  Canada
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Hi Guys

Don't mount things on the unused pins of the recto socket - they are open to provide voltage isolation. The voltage between the plates is the full end-to-end voltage of the PT plate winding. The voltage between either plate and cathode is the peak voltage of one half the winding.

Adding solid-state rectifiers does nothing to alleviate your fears of the the too-high filter cap value damaging the tube UNLESS you place the solid-state diodes in parallel with the tube. In such a wiring, two diodes must be series-connected for each arm of the half-bridge. This wiring allows the tube to soften the overall turn-on of the diodes en masse while the SS diodes handle the bulk of current. Sag due to the recto will be nil.

Placing diodes in series with the tube does nothing with respect to handling surge currents. More diode noise is added this way, but...

Rectifier noise in tube amps - especially guitar amps - is extremely over-blown. It is a non-issue. Other problems in amp wiring swamp real and theoretical diode noise by a large margin. Even with all those other things addressed, diode noise is still not of concern.

Caps across the diodes should be avoided.

The way to reduce diode noise if you believe it to be necessary, is to add 1R to 10R between the diodes and the first filter. This filters all the high harmonics of the rectification. On the PT side of the diodes, add a small series R and a small C rated for AC voltage use to suppress noise that might couple into the PT. It actually supresses noise bilaterally.

See TUT3 for more details.

Have fun
Kevin O'Connor
londonpower.com
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Old 30th October 2012, 11:35 PM   #9
ivan H is offline ivan H  Australia
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Ok, thanx heaps 4 that, I really appreciate ur advice. This forum is such a great "learning" place. Cheer
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Old 31st October 2012, 02:34 AM   #10
ivan H is offline ivan H  Australia
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BTW, the Ceriatone mod is not 4 the 1st filter, rather, its supposedly 2 protect the OT & tubes from AC in the event of a recto failure. Would I b right in thinking that as the PT's HT secondry's CT is fused with a 500mA slow blow to earth, in the event of the GZ34 failing the fuse would blow pretty well immediately, b4 damage to other major components occured, so protection shouldnt b an issue? Cheers
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