• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

480-0-480vac To Power A SE GM70!??

Zekk

Member
2009-03-16 9:56 am
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.
 
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...
 

kevinkr

Administrator
Paid Member
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.
 
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!
 

kevinkr

Administrator
Paid Member
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.. :D)

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! :D
 
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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.
 

Zekk

Member
2009-03-16 9:56 am
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 [email protected] 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 ?
 

Zekk

Member
2009-03-16 9:56 am
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! ;)
 

quikie22

Member
2009-09-24 1:53 pm
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!!
 
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.
 

Zekk

Member
2009-03-16 9:56 am
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
 

quikie22

Member
2009-09-24 1:53 pm
<script src='http://img844.imageshack.us/shareable/?i=gm70hvps001.jpg&p=tl' type='text/javascript'></script><noscript>[IMGDEAD]http://img844.imageshack.us/img844/613/gm70hvps001.jpg[/IMGDEAD]</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.
 
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.