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Old 11th April 2015, 07:03 PM   #1
brickj is offline brickj  United Kingdom
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Question Power Transformer Rating?

Hi,

Can someone please help guide me in how I should think about the current rating for power transformers in valve amplifiers.

I'm putting together a supply for a 7591A based Baby Huey. According to the data-sheet, I believe I need 184mA idle and 330mA peak for the plate and screen for a stereo amplifier. Is there a rule-of-thumb for the power supply current rating?

Using PSUD2 with a bridge rectifier and a single smoothing capacitor, the current drawn from the transformer is ~2x the load current i.e. designing for a 330mA load the transformer needs to be rated at ~660mA. This seems a lot higher than many designs I've seen, so I'm assuming that in practice people typically don't specify the power supply to run at full-power continuously

Am I doing something wrong and/or is there a rule-of-thumb that I should use for the power supply current rating based on the idle/peak current consumption of the output valves?

Thanks.

Justin.
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Old 11th April 2015, 08:05 PM   #2
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The power rating for the power transformer should be 3x maximum power consumption of the circuit.
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Old 11th April 2015, 08:54 PM   #3
brickj is offline brickj  United Kingdom
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If I understand your proposal correctly, that would suggest a ~400VA transformer (400 * 0.33 * 3). That's 50% higher than what I thought would be conservative by specifying for peak current consumption. I feel I must be missing something e.g. this Dynaco ST70 replacement transformer is rated at 720V CT @ 300mA i.e. 216VA.
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Old 11th April 2015, 09:03 PM   #4
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by brickj View Post
I'm putting together a supply for a 7591A based Baby Huey. According to the data-sheet, I believe I need 184mA idle and 330mA peak for the plate and screen for a stereo amplifier. Is there a rule-of-thumb for the power supply current rating?
What matters is the maximum average current demand, not the peak current. Also, 'valve transfomers' usually take into account a typical power factor, so when they say "200mA" or whatver, they mean 200mA DC.

I'm not familiar with the Baby Huey, but if the average current at max power happens to be 230mA say, then you'd be OK with a 230mA transformer deliberately designed for valve applications. On the other hand, if its a modern off the shelf transformer not designed for valve applications then you should multiply the figure by about 1.5, implying a transformer current rating of 345mA.

But, how long will the amp be driven to max power? If it's a hifi amp, not much. If that's the case then you can reasonably derate the current rating a bit. Say, 200mA (valve transformer) or 300mA (modern transformer).
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Old 11th April 2015, 09:38 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by brickj View Post
If I understand your proposal correctly, that would suggest a ~400VA transformer (400 * 0.33 * 3).
That's 50% higher than what I thought would be conservative by specifying for peak current consumption.
I feel I must be missing something e.g. this Dynaco ST70 replacement transformer is rated at 720V CT @ 300mA i.e. 216VA.
Hey, that's exactly three times the 70W total output power rating!
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Old 11th April 2015, 09:48 PM   #6
brickj is offline brickj  United Kingdom
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Originally Posted by Merlinb View Post
What matters is the maximum average current demand, not the peak current. Also, 'valve transfomers' usually take into account a typical power factor, so when they say "200mA" or whatver, they mean 200mA DC.
ah, that explains a lot. Thanks.

I'm likely to use a custom made toroidal transformer and hence it looks like I should be using 1.5x as a rule of thumb, and then derate by up to 15% for hi-fi application.

I'm not sure what the average current demand will be, but clearly peak is an upper bound i.e. if I specify the B+ secondary AC current rating as 330mA * 1.5 * 0.85 = 420mA, that should be sufficient.

This is also now consistent with the Dynaco ST70 replacement transformer I referred to, assuming it is specified with an appropriate power factor i.e. 300mA * 1.5 = 450mA.
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Old 11th April 2015, 09:57 PM   #7
brickj is offline brickj  United Kingdom
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Originally Posted by rayma View Post
Hey, that's exactly three times the 70W total output power rating!
Yes, good point. That seems like a reasonable rule-of-thumb. I was thrown a little with Keruskerfuerst suggestion of 3x max power consumption.
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Old 11th April 2015, 10:10 PM   #8
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These guides from Sowter and Hammond might help?

How to Specify HT Windings

Design Guide for Rectifier Use
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Old 11th April 2015, 10:38 PM   #9
316a is offline 316a  England
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A few things to consider . If using choke input or with a small value capacitor after the rectifier , the mains transformer will typically run cooler than a comparative supply running capacitor input . Windings for full wave or valve rectifiers will have far greater losses than a single winding for a bridge rectifier , for this reason I prefer hybrid bridge rectifiers over valves . Better safe than sorry , always over rate the mains transformer , once the chassis is bashed to fit , you're stuck with it . Also avoid transformers with a 220V or 230V primary in the UK . Using multiple transformers , one for HT , one for heaters etc also adds flexibility if you have the space and funds

316a
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Old 12th April 2015, 12:48 AM   #10
AJT is offline AJT  Philippines
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Quote:
Using multiple transformers , one for HT , one for heaters etc also adds flexibility if you have the space and funds
i specially like this....i read somewhere that dc rectifying the filament supplies, use of a separate supply traffo is better,
haven't tried this but would like to try one day...
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