I could use some help with specs for a set of power transformers for my DCPP mono tube amps. I am struggling to get this spec'd right
What the amplifier needs based on the schematics:
Engineer's amp 50W monoblock
Hammond 290FX or equivalent
640VCT @ 200mA DC
60V mA DC bias tap
6.3V @ 5.5A filament
1) Forum members have advised me that because it's specified as DC current, 640VCT @ 300mA would be advisable
2) I received this through private message of a respected board member:
"When you spec a custom transformer usually the winder wants to know AC RMS current, not the DC B+ current. There are lots of rules of thumb. But in general you spec considerably higher RMS current than the DC current - as current is actually drawn in pulses. There are guidelines anywhere between 1.5x and 2x. I usually use 1.8x to 2x.
You are best off overspecing it. Specifying higher current will increase the cost a bit, but it will work better and run cooler.
I would probably spec 400mA RMS."
So far so good. I now understand the 300mA forum members were advising me to get. So i tried asking a custom winder for a quote:
640VCT @ 400mA
Bias tap at 60V
6.3V @ 5.5A
21V @ 1.7A (21v sweep tubes)
20V @ .2mA (typo, I should have specified 20V @ 0.12A for WE 408A)
3) This is where I get confused. One of the custom winders (will remain anonymous) replied with:
"The 640vct @ 400ma is a fairly large VA and will heat no matter, to have it
potted and you not knowing its actual loading is a disaster waiting to
happen. Having all those windings will assure its overheating.
Many multi-secondary transformers are a product of progressive design/s, cut
an try to work right.
To just design with a guess is what we do not do anymore.
Your request should be made into two or three separate transformers due to
heating and winding coupling distortion/s. best would be one HV trans and
one 6.3v/5.5A -21v/1.7A and one 60 and 20v bias whatever.
Also the RMS value is .707 of a peak value and has nothing to do with power
supplies except determining the wire size of a HV secondary. for 400ma use
.282 CM/ampere wire. We use this factor.
The actual running voltage you will get, is determined by your rectifier/s,
filter type, values used and idle and peak loading. Doubling the current
rating is not a real cure, it just makes the transformer bigger."
"Not much we can do for you, this is not buildable as stated."
What the amplifier needs based on the schematics:
Engineer's amp 50W monoblock
Hammond 290FX or equivalent
640VCT @ 200mA DC
60V mA DC bias tap
6.3V @ 5.5A filament
1) Forum members have advised me that because it's specified as DC current, 640VCT @ 300mA would be advisable
2) I received this through private message of a respected board member:
"When you spec a custom transformer usually the winder wants to know AC RMS current, not the DC B+ current. There are lots of rules of thumb. But in general you spec considerably higher RMS current than the DC current - as current is actually drawn in pulses. There are guidelines anywhere between 1.5x and 2x. I usually use 1.8x to 2x.
You are best off overspecing it. Specifying higher current will increase the cost a bit, but it will work better and run cooler.
I would probably spec 400mA RMS."
So far so good. I now understand the 300mA forum members were advising me to get. So i tried asking a custom winder for a quote:
640VCT @ 400mA
Bias tap at 60V
6.3V @ 5.5A
21V @ 1.7A (21v sweep tubes)
20V @ .2mA (typo, I should have specified 20V @ 0.12A for WE 408A)
3) This is where I get confused. One of the custom winders (will remain anonymous) replied with:
"The 640vct @ 400ma is a fairly large VA and will heat no matter, to have it
potted and you not knowing its actual loading is a disaster waiting to
happen. Having all those windings will assure its overheating.
Many multi-secondary transformers are a product of progressive design/s, cut
an try to work right.
To just design with a guess is what we do not do anymore.
Your request should be made into two or three separate transformers due to
heating and winding coupling distortion/s. best would be one HV trans and
one 6.3v/5.5A -21v/1.7A and one 60 and 20v bias whatever.
Also the RMS value is .707 of a peak value and has nothing to do with power
supplies except determining the wire size of a HV secondary. for 400ma use
.282 CM/ampere wire. We use this factor.
The actual running voltage you will get, is determined by your rectifier/s,
filter type, values used and idle and peak loading. Doubling the current
rating is not a real cure, it just makes the transformer bigger."
"Not much we can do for you, this is not buildable as stated."
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