480-0-480vac To Power A SE GM70!?? - Page 2 - diyAudio
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Old 8th December 2010, 04:19 AM   #11
Zekk is offline Zekk  Singapore
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Quote:
Originally Posted by djn View Post
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.
Jim Dowdy's proven and well known SE GM70 used 735vdc on anode!
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Old 8th December 2010, 09:54 AM   #12
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hi,
with your 480-0-480V transformer, if you wish to go for high voltage, you could use a full wave solid state rectification between both 480V taps first and ignore the CT (giving you 960VAC). After rectification , this will be around 960x1.414=1350V DC. If you pass this through a tube rectifier, you will lose around 30-80V depending on the tube used, then passing through a CLCLC, you can reach a voltage of around 1200V DC with no load.

However, make sure your capacitors, chokes and output transformers and wires can take these voltages. Double insulate everything!!. Sparking is a very real and dangerous possibility. Make sure you also incorporate bleeder resistors in the power supply to drain the capacitors after the thing is switched off. Also, place the amp safely out of reach of any curious creatures, your child, your cat or your dog. Best would be an earthed Faraday cage.

This is the voltages which my Brother in law runs his FU-13 amplifier and my brother runs his 845 amp. :-) and soon maybe my soon to be built GM-70. We like living on the edge!!
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Old 8th December 2010, 10:19 AM   #13
piano3 is offline piano3  United Kingdom
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I was going to make the same suggestion as quickie except to use a hybrid bridge with 2 SS diodes and 2 damper diodes which are possibly a bit more robust than double rectifier tubes at these voltages. If you then use a low value-a few uF- high voltage film cap before your choke you can reduce the voltage a bit without wasting power.
I can only reiterate everyone else's safety concerns; there is no way I would ever attempt an amp at these voltages.
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Old 8th December 2010, 02:06 PM   #14
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Zekk View Post
Jim Dowdy's proven and well known SE GM70 used 735vdc on anode!
I've heard one and I would still stand by the recommendation to use a minimum of 800V - that hundred volts or so makes a significant difference. IIRC the one I heard used a little over 800V..
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Last edited by kevinkr; 8th December 2010 at 02:08 PM.
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Old 8th December 2010, 03:00 PM   #15
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The GM70 amp in question has a plate-to-ground voltage of 735 vdc, but approx 55 of those volts are used for cathode bias. So, the actual plate-to-cathode voltage is 685 vdc.
This amp uses the copper-plate tubes, which I am uncomfortable running at more than about 75 W dissipation.

Jim
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Old 8th December 2010, 06:26 PM   #16
Reegal is offline Reegal  Italy
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Hi Zekk,
here the schematic of my amp with GM70 S.E. maybe can be helpful for you...
To work well with low THD you need high voltage! take care, it is dangerous...
GM70-II.pdf
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Old 9th December 2010, 04:43 AM   #17
Zekk is offline Zekk  Singapore
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Reegal ... thanks for that educational, full dress schematic!
I like the simple, no frill, 20vdc / 1250vdc power supply.
I beleives many guys out there will be equally grateful.

I'm not savvy enought to come out with a hybrid rectifier drwg.
I can only interpret schematic drawings and electronic terms is a no no for me.

Attached is my hand-drawn, copied from various designs.
Guys ... kindly correct me ya ..... thanks!

Greatly appreciates it all.

Zekk
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Old 9th December 2010, 07:09 AM   #18
Zekk is offline Zekk  Singapore
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Default Here's it!

<script src='http://img844.imageshack.us/shareable/?i=gm70hvps001.jpg&p=tl' type='text/javascript'></script><noscript>Click the image to open in full size.</noscript>
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Old 9th December 2010, 09:18 AM   #19
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Originally Posted by Zekk View Post
<script src='http://img844.imageshack.us/shareable/?i=gm70hvps001.jpg&p=tl' type='text/javascript'></script><noscript>Click the image to open in full size.</noscript>
This looks alright. But I would serialise another set of solid state rectifiers with the rectifiers you have there. I would also use a slightly higher cap value for the first cap... maybe around 10uF 1.6kV to reduce the losses in the choke and add another LC stage after. Make sure your rectifiers, caps and chokes can take the voltages present!!

AS I mentioned earlier, the centre tap need not be connected, so you can safely insulate it.
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Old 9th December 2010, 09:36 AM   #20
piano3 is offline piano3  United Kingdom
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Have you a particular reason for using mercury rectifiers? With your value of 0.25 uF for the first cap the supply is a choke input supply with output voltage~0.9 V in (RMS). If you follow quickie's advice and use a 10uF cap then it is a cap input supply with much higher voltage output. I would agree completely with advice about series diodes(maybe use 3 with equalising resistors) and about second filter. The query about centre tap-you just leave it floating. Also the low frequency resonance of the filter is about 11 Hz. with a Q of between about 3-6 depending on choke DCR; you may want some additional resistance in series with the choke to damp this peak.
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