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Old 7th June 2014, 06:20 PM   #1
teemumm is offline teemumm  Finland
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Default Mullard 5-20 power supply

I have been planning to build an 2-channel amp based on Mullard 5-20. As this will be built in single chasis, I have been planning to use single mains transformer (and then separate chokes and GZ34s for each channel).

I have been running some LTSpice simulations of the power supply using 3k3 resistor as a load. The original schematics of 5-20 specify 410V transformer and using two wirewound resistor before GZ34. The resistor values are not mentioned in the schematics and these depend on transformer winding resistance. Nor the total resistance (transformer + resistors) are mentioned.

I have been thinking that maybe Hammond 378CX would be ideal transformer for this (400-0-400V, 465mA). According to Hammond documentation the secondary winding resistance would be around 20 ohms. In order to get the voltage ratios correct, I would need to add 220ohm resistors between the transformer and GZ34.

Do you think there would be some problem if I used instead lower secondary voltage in transformer and skip the resistors or use lower values? Hammond 373EX (325-0-325V, 345mA) for example?
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Old 7th June 2014, 07:21 PM   #2
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I used 400-0-400, 200mA (for each channel) and ended up using two 100 ohm 10 watt resistors in series with the plates of the GZ34 and the voltages are just about spot on to Mullard specification.
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Old 7th June 2014, 07:53 PM   #3
teemumm is offline teemumm  Finland
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Quote:
Originally Posted by famousmockingbird View Post
I used 400-0-400, 200mA (for each channel) and ended up using two 100 ohm 10 watt resistors in series with the plates of the GZ34 and the voltages are just about spot on to Mullard specification.
Do you have some idea why there are such resistors? Why GZ34 cannot be connected directly to the transformer secondary?
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Old 7th June 2014, 08:14 PM   #4
SY is offline SY  United States
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Quote:
Originally Posted by teemumm View Post
Why GZ34 cannot be connected directly to the transformer secondary?
http://frank.pocnet.net/sheets/035/5/5AR4.pdf
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Old 7th June 2014, 08:31 PM   #5
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Mullard states in my book they are "mainly" there for voltage control. The book I have said 410-0-410 for the mains transformer and if a transformer with a low winding resistance is used 400-0-400 should be adequate. So I went with Edcor XPWR011 400-0-400, 200mA and the voltages were still high. I was going to use some 5U4GB's to bring the voltage down but I like the GZ34 for it's slow voltage ramp up time. I ended up with the two resistors which worked perfect.
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Old 7th June 2014, 08:37 PM   #6
teemumm is offline teemumm  Finland
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Originally Posted by SY View Post
Thanks. This gave me some idea why they are there.

Actually I found this GE datasheet that explains the situation even better:

http://www.drtube.com/datasheets/5ar4-ge1959.pdf
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Old 7th June 2014, 08:43 PM   #7
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Ah yes datasheets, thanks SY


Looks like limiting resistor per plate is 125 for 400-0-400 transformer.
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Old 7th June 2014, 08:43 PM   #8
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Old 7th June 2014, 08:56 PM   #9
hpeter is offline hpeter  Europe
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Originally Posted by teemumm View Post
Do you have some idea why there are such resistors? Why GZ34 cannot be connected directly to the transformer secondary?
yes if secondary has required resistance
if not it may die sooner
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