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Old 26th July 2011, 05:49 AM   #1
SanderW is offline SanderW  South Africa
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Join Date: Jul 2011
Default Valve power supply - How to size transformer?

Hi all,

I have the following valve line up:

2 x GZ34 (1.9A ea @ 5v)
2 x 6SL7GT (0.3A ea @ 6.3v, 2.3mA @ 250v)
4 x 807 (0.9A ea @ 6.3v, 140mA @ 400v)

So with a capacitor filter I need a transformer that can do the following:

5v AC RMS @ 3.8A Continious
6.3v AC RMS @ 4.2A Continious
290v AC RMS (just over 400v DC, unregulated) @ 565mA intermittently.

The B+ will only approach max very intermittently, but the heater supplies will be running constantly whether there is signal or not.

So do I give these specs to the transformer supplier or do I just order a thumbsuck like:

5v 4A
6.3v 4.5A
290v 600mA

Should I build in headroom for voltage sag under load conditions? I see these unregulated supplies everywhere, does anyone build a supply like this that is fully regulated?

Thanks!
Sander
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Old 26th July 2011, 12:31 PM   #2
DF96 is offline DF96  England
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One HT line or two? If two, then you need two separate 5V windings as the GZ34 has its heater connected to the cathode.

Application? Duty cycle?
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Old 26th July 2011, 12:41 PM   #3
SanderW is offline SanderW  South Africa
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Hi DF96,

One HT line, with the two GZ34's in parrallel to rectify. The application is a stereo audio amp that will in all likelyhood never be driven hard, just playing at living room levels. The design intent was to make it glow, not to raise the roof...

So I don't know about duty cycle, heaters 100%, B+ also 100% while it is on, but not at max levels. Amp will have push pull on each channel, so mode AB1.

Actually, being push pull, both tubes will never have full B+ simultaneously, so each has a duty cycle of 50%, or the two together has a 100% duty cycle, but only at 140mA not at 280 like I originally thought? Unless I am totally missing the point.

The B+ is also a centre tapped winding, so I assume 300-0-300, or 290-0-290... How important is 10 or 20 volts either way on something like this?

Thanks!
Sander
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Old 26th July 2011, 03:38 PM   #4
DF96 is offline DF96  England
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OK. Was 140mA for one channel or two? In normal hi-fi use the DC current consumption won't rise very much, because signal peaks are only present for a small fraction of the total time.

Your best bet is to use PSUD2 to model the HT supply. This will tell you what the secondary RMS current will be. Alternatively, just guess that it will be 3-4 times the DC current.

10 or 20 volts either way will not matter.
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Old 27th July 2011, 10:33 PM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by SanderW View Post
.
Actually, being push pull, both tubes will never have full B+ simultaneously, so each has a duty cycle of 50%, or the two together has a 100% duty cycle, but only at 140mA not at 280 like I originally thought? Unless I am totally missing the point.
Yes. correct but you want a power transformer that has a max AC current rating that is at least 2X the max required DC current. This is provide a certain amount of load regulation.

Some will say to go even larger than 2x but if the intent is low "bedroom" level sound levels you don't need the headroom. Go larger than 2X if you intend to fill up a 20 foot square room with normal music volume levels.

You are right that a few volts one way or the other on the B+ does not mater to much. The way to think about it is in "percent". Certainly a 5% difference in B+ does not matter much. However I've played around with a Variac transformer where I can adjust the B+ as I listen and mostly "more is better" as long as do don't go past limits.
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Old 28th July 2011, 05:06 AM   #6
SanderW is offline SanderW  South Africa
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Thanks Chris,

This is the 'rule of thumb' advice that I was looking for. I was just concerned that if I build the transformer to specs it may turn out to be too small. Does the same 2x approach also hold for the heater supplies? Being constant draw I assume it is easier to design.

I have also thought of the variac approach, but that would imply splitting the 5v & 6.3v transformer from the B+ transformer? What is the normal practice here?

Also if I only need 140mA per channel, I should be OK with just one GZ34 rectifier instead of two in parallel. This is a circuit for an old commercial amp that I was trying to reproduce and they had two GZ37's in parallel. I have already changed that to GZ34's as they are more readily available and the specs seemed fine.

DF96, I have downloaded PSUD2 and am trying to model my power supply, but this will apparantly take a few extra days to figure it out!

Thanks!
Sander
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Old 30th July 2011, 12:45 AM   #7
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Location: Melbourne
Quote:
Originally Posted by SanderW View Post

I have also thought of the variac approach, but that would imply splitting the 5v & 6.3v transformer from the B+ transformer? What is the normal practice here?
On consumer equipment heaters are usually supplied from the HV transformer.
On commercial and industrial equipment the heaters are usually supplied from their own power supplies. Heater voltage and correct application time are critical in obtaining good tube life so applications with expensive tubes can afford some increased power supply complexity, most tubes at work turn on the heaters 10 minutes before the HV. All heater supplies have adjustable voltage.

If you are building from scratch then there is little reason to use the same transformer for heaters and HV, this was done in the consumer equipment of yesteryear for economy but it might work out cheaper to use separate transformers these days due to availability
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Old 30th July 2011, 12:31 PM   #8
DF96 is offline DF96  England
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Running heaters for 10 mins with no HT may be fine for industrial valves with special cathodes and special getters, but ordinary valves may develop cathode interface. Best to apply heater and HT more or less together - a few seconds either way will do no harm.
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Old 30th July 2011, 02:09 PM   #9
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The Tubes have large indirectly heated Oxide cathodes and ion pump getters, enabling the tubes to be held on standby with reduced heater voltage for long periods of time. Tube life 10k - 50k hrs depending on the device. Thermal cycling and arcover are the main enemies of high power tubes, loss of cathode emission kills them all eventually.

I am not suggesting 10 minute warm up for audio tubes, just enough time to get some cathode emission before applying the HV, this is mainly to reduce any chance of arcover. If the equipment uses a choke input filter, the HV is going to go sky high before the output devices start drawing current. Older equipment had the delay provided by the thermionic rectifier which doubled as a soft starter not that it was good for rectifier life. Mercury vapour rectifiers required heater power first.

I may be a bit rusty on the small stuff AFAIR it was always preferable to have heater power then HV not the other way around also control grid bias needs to be applied before HV.

Other advantages of separate supplies is the ability to soft start the heaters (reduced thermal shock) and apply correct voltage independent of any HV load, maybe this is overkill for audio where the tubes are cheap. The Older ARRL handbooks and the RCA transmitting tube manual are good references.
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Old 30th July 2011, 03:24 PM   #10
DF96 is offline DF96  England
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High power (e.g. transmitters) usually means high voltage and then the advice about delaying HT is valid. For normal domestic audio voltages arcing is unlikely. A choke input PSU will only go 40-50% over normal, so provided the capacitors can cope the valves will probably be OK. If not, a string of zeners can waste a bit of power until the valves warm up.

Valves which are intended to run for significant periods with full heaters and no current (e.g. early computers and other logic) have special cathodes which do not form interface.
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