GZ34 anode load resistor calculation - help? - diyAudio
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Old 30th January 2007, 05:08 AM   #1
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Default GZ34 anode load resistor calculation -help

I'm a bit rusty on calcualtions.

Hammond transformer 374BX (240VAC primary)
Secondary winding 375V-0-375V, DC resistance measured about 105 Ohms DCR.
GZ34/5AR4 tube rectifier used here.
Current load measured at output to amp circuit = 130mA
Rectification is RC filtering 32uF > 75ohm > 32uF > output = 130mA @ 440VDC

Can any guru calculate what's the optimum value resistors to install at the GZ34 anodes?.
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Old 30th January 2007, 12:12 PM   #2
SY is offline SY  United States
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I'm not clear on why you want to put resistors in that spot. Can you explain further?
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Old 31st January 2007, 12:23 AM   #3
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Hi coolmaster ,

To calculate the exact value , I need to know :

1 ) The primary DC resistance ( 240 VAC winding )
2 ) If the DC resistance measured ( 105 ohms ) is
for the total secondary winding ( 750 V ) , or is
for each secondary’s half ( 375 + 375 V )

With these informations , I will calculate the EXACT
resistor value , you need to connect at EACH ANODE
of the GZ34 .

I am waiting .......

Carlos
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Old 31st January 2007, 04:29 AM   #4
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I've made the measurements again.

Primary 0-240VAC tap in use = 8ohm DCR (total winding)
Secondary 375-0-375 = 99ohm DCR (total winding DCR)

The second winding on each half from 0-375 is not exactly equal, slightly off by 4-5ohms.
My input voltage is exactly 230VAC in operation, incoming supply being regulated by a huge AVR.

Eversince new, its after half hour the tranny becomes a little too hot for my liking and giving me the jitters its bad for the tranny on the long run. I'm sure the circuit hasn't exceeded the current capacity ratings for other windings. The amp is typically a Mullard 5-20. As mentioned earlier, the current draw is only 130mA. Tranny rated at 175mA at HT winding. The DC is C-R-C filtering, 32uF-100ohm-32uF.

Thank you.
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Old 31st January 2007, 10:38 AM   #5
Merlinb is offline Merlinb  United Kingdom
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Here's the formula (you might have to wait 10 seconds for the advert to vanish so you can view the page)
It's near the bottom under "Minimum Limiting Resistance"
http://www.freewebs.com/valvewizard/fullwave.html
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Old 31st January 2007, 02:31 PM   #6
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Hammond transformers tend to get hot. The 3XX series are cooler running than the 2XX series because they are rated for 50Hz operation. You are in a 50Hz country, so they will get warm.

I am using Allied 6K7VG and 6K56VG transformers (made by Hammond, but cheaper than the same Hammond TX) near their maximum ratings. They get hot enough that you don't want to put your hand on them for more than a few seconds. I have been using these for over 10 years (dozens of HiFi and guitar amps) with zero failures.
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Old 31st January 2007, 04:04 PM   #7
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Merlinb: Thank you for the link. Have calculated out the resistor required. Just installed in 75 ohm (2X150ohm in parallel,what I've got in hand) and so far so good after listening to some music for he past hour.

tubelab.com : Yes, its 50hz here and I just didn't feel too good about it being hot enough not to touch for more than few seconds, but its been like that for 2 years now, just itching to see if I could make it run cooler. Had no idea Hammond mains tranny run hot. I always thought its similar to any other brand or make, when properly sized and implemented. Other than that, everything else is just fine.
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Old 31st January 2007, 08:37 PM   #8
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Default Limiting resistors

Hi all,
Mullard quote in their maintenance manual fro a GZ34 400v on the anodes and a capacitor input smoothing filter a total resitance of 125R per anode. So for your transformer which is 50R per half you need to add 75R to make the 125R which is exactly what you have done. The reason for the resistors is to limit the current as the GZ34 is a fairly low impedance rectifier, so if you hot switch it, it could draw excessive current and either flashover or strip the cathode.

John Caswell
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Old 31st January 2007, 08:40 PM   #9
SY is offline SY  United States
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Remember, that's with a capacitor of 60uF, right at the tube's limit. One would expect 1/3 less peak current into a 40u cap. That's why I don't think those resistors are needed.
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Old 31st January 2007, 09:19 PM   #10
kevinkr is offline kevinkr  United States
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My experience parallels SY's pretty closely, I don't pad the resistance and never use more than 50uF input capacitance.

I have found that adding a 1N4007 in series with each plate is quite effective in preventing flashover whether the tube is hot or warming up - and use these religiously in any application where the secondary voltage is greater than 750VCT.. I'd do that in lieu of the 75 ohm resistors which needlessly degrade regulation in a class a/b amplifier.
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