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Old 20th September 2005, 08:29 AM   #1
diyAudio Member
 
Join Date: Nov 2002
Location: Athens, Greece
Arrow GM70 Power supply

Good day to all. Iím planning to build a SE GM70 amplifier with self bias.

I have all the parts to build a power supply from surplus parts consist of:

OPTION No1
One 2500V C.T. 500mA old military potted transformer
Two 2B28 Xenon tube rectifiers
One 2 uF /1500V oil capacitor
One 10mH /500mA 70 Ohm RDC choke
One capacitor bank of 100uF total

This PS gives 1290V DC with 260 mA or 1440V DC with 145mA load.
This is going to power both L and R channel
I do believe that the voltage is too high for the GM70 even if the resistor on the cathode will deduct about 80 to 100 volts (or not???).

OPTION No2
The second option is to use two other transformers that I have in hands:

One 740V 300mA old military potted transformer
One bridge rectifier consist of one 5R4 tube and 2 silicon diodes
One 47uF cap
One One 10mH /500mA 70 Ohm RDC choke
One capacitor bank of 100uF total
This PS deliver about 940V DC @ 140mA load
In this case I will use 2 diff power supply units on for l and one for R channel.

Is there someone who builds a GM70 to help?
Any suggestions regarding the above project?
FYI the driver stage (for the moment) will be a 5842 DC coupled to a 6BG4 IT (1:1) GM70 7K OPT.
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Old 20th September 2005, 09:16 PM   #2
Fuling is offline Fuling  Sweden
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Join Date: Aug 2001
Location: Sweden (Mora)
Are those chokes really 10mH?
10H seems more realistic I think.

How much experience do you have of building tube equipment?
2500V transformers and Xenon rectifiers might not be the easiest stuff to start with if you have limited experience.
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Old 21st September 2005, 08:17 AM   #3
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Join Date: Nov 2002
Location: Athens, Greece
Sorry the chokes are 10H.
I have a big experience with HV due to I'm a radio amature and I have already build many KW RF power amplifiers before.

I have already build on breadboard the mentioned PS and it woks pretty good.

Thanks
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Old 22nd September 2005, 01:23 PM   #4
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Location: aberdeen, uk
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Lots of info about GM70 applications here You need to join first but its free.
There is a data sheet here which gives a max plate voltage of 1500 for the gm70 so your first circuit would probably work nicely.
Never seen or heard xenon rectifiers in action so please let us know how it all sounds (and post some pics)
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Old 22nd September 2005, 05:49 PM   #5
amperex is offline amperex  United States
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Join Date: Feb 2005
Location: MI
The 2500 volt CT transformer looks best to me. Try choke input if you need lower voltage. A power supply simulator indicates about 1100 volts DC with a choke input, MV type (3B28) rectifiers & 250 ma load. The choke input with 100uF is 'on the edge' for lower ripple. A pair of smaller current second chokes (one to each GM70) would be better even with a 2uF cap on the output.
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