480-0-480vac To Power A SE GM70!?? - diyAudio
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Old 8th December 2010, 12:52 AM   #1
Zekk is offline Zekk  Singapore
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Default 480-0-480vac To Power A SE GM70!??

Hello Everyone Out There!

I've a power transformer of 480-0-480vac @ 400mA and am planning to build a SE GM70 with it.

Question is, what's the max vdc achievable and how to do it?

Preferably around 750vdc would be adequate without scourcing for heavy duty componants.

Thanks, Zekk.
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Old 8th December 2010, 12:53 AM   #2
Zen Mod is offline Zen Mod  Serbia
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Duncan's Amp Pages

Software

http://www.duncanamps.com/psud2/psud2_setup.exe
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Old 8th December 2010, 01:00 AM   #3
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max VDC is 1.414*VAC, so around 670VDC

Unless you get jiggy with a doubler.

Going to depend a bit on your total current draw of course....

You could full wave rectify across the entire secondary ie 960VAC at half the rated current, and choke input to reduce the VDC

Many ways to skin the cat.

...edit - and what Zen Mod sez too...
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Old 8th December 2010, 01:10 AM   #4
kevinkr is offline kevinkr  United States
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My design will use 1040VCT full wave rectified with either choke input or more likely a HV regulator as that is my thing.. Dangerous stuff..

Friends locally who have built with the GM70 all state that 800V is the point where this tube really starts to shine which ought to be easily achievable with 960VCT and choke input.. Special chokes would be required to keep noise under control and should probably be placed on the negative rail to slightly dielectric stress.
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Old 8th December 2010, 01:18 AM   #5
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dangerous to be sure! There should be a special caveat pop-up that automatically displays when posts mentioning high voltages are made...

As I've noted before, once you are over 750V the electrons have a nasty trick of coming looking for you rather than waiting for your approach.

THere is a lot to be said for lower votages and higher currents!
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Old 8th December 2010, 01:37 AM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by aardvarkash10 View Post
dangerous to be sure! There should be a special caveat pop-up that automatically displays when posts mentioning high voltages are made...

As I've noted before, once you are over 750V the electrons have a nasty trick of coming looking for you rather than waiting for your approach.

THere is a lot to be said for lower votages and higher currents!
That's a very good idea actually.. Some email clients detect words like "attach[ed]" or "attachment" in the text and then ask if you want to attach something to that particular email. (Current Linux versions of Thunderbird do this - I was quite surprised the first time it happened.. Wow.. )

The forum software could put the "Caution High Voltage" symbol in the header or something like it. Probably not supported in the current version of the forum software.

Safety is very important, and as I think about it I wonder whether the OP has given this sufficient thought.. The question makes me wonder a bit...

@ OP: Much care and thought is required around high voltages. A read of the high voltage thread here is mandatory!
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Last edited by kevinkr; 8th December 2010 at 01:39 AM.
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Old 8th December 2010, 01:53 AM   #7
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I used a 480 volt industrial control transformer feeding a voltage doubler made with 5AR4's. I get 1100 volts with no load and 1050 volts with both Chinese 845's cranked up to about 90 mA each. I run the old RCA's at about 70 mA. The SE amp makes 40 WPC in A2 either way, but does rock music better with the higher current. I don't want to push the old tubes too hard. The Chinese tubes have been to 110 mA but don't sound any better than 90 mA. I am afraid that the higher current might cause a 5AR4 to explode.
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Old 8th December 2010, 02:31 AM   #8
djn is offline djn  United States
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You might want to join the GM70 yahoo group. Lots of good info there as well. When I first started thinking about a GM70 amp, I found a lot of writings about how the GM70 really needs a very minimum of 800v B+ to sound good. Below that and they sound lifeless. I am running 880v B+ on mine.
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Old 8th December 2010, 04:00 AM   #9
Zekk is offline Zekk  Singapore
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Quote:
Originally Posted by tubelab.com View Post
I used a 480 volt industrial control transformer feeding a voltage doubler made with 5AR4's. I get 1100 volts with no load and 1050 volts with both Chinese 845's cranked up to about 90 mA each. I run the old RCA's at about 70 mA. The SE amp makes 40 WPC in A2 either way, but does rock music better with the higher current. I don't want to push the old tubes too hard. The Chinese tubes have been to 110 mA but don't sound any better than 90 mA. I am afraid that the higher current might cause a 5AR4 to explode.
WOW! That's like Santa have dropped in early for me! Delighted that it can be done. But how?

For a start, using a single 6EM7 direct coupled to GM70 operating at 750vdc@85mA. KISS! Up-grade later.

Ya, I'm a newbie and and am aware that HV mishap have no second chance!
I've built a SE TT21 running at 670vdc using a pair of 816 mercury ratifier.
Perhaps, I've barely qualified to join this elite class of HV tube amp diyer ... or have I ?
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Old 8th December 2010, 04:04 AM   #10
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everyone's welcome, but funny questions raise concerns about your health!

At those working voltages you don't often get a chance to do it again properly - its once or dead.

In the end, IF you take EXTREME care and understand fully what you are doing before you do it well... its your call!
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