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Old 22nd August 2009, 09:34 PM   #1
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Default Hammond PT power ratings

Can anyone confirm the power ratings on Hammond transformers? I was looking at the 369AX which is rated at 250VCT @ 100mA and 6.3V @ 2A. Hammond suggests that the center tap of the HT supply should be grounded, so I always assumed that this meant using full wave rectification, meaning ~125VDC at 100mA. But, it is rated at 40VA, which seems to mean that running with a bridge rectifier for 250VDC at 100mA is fine. Can anyone confirm this?
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Old 23rd August 2009, 02:49 AM   #2
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Hi dsavitsk ,

I have NO doubt , because Hammond wrote on the datasheet " 250 VCT @ 115 mA " what means that you can use the entire primary winding voltage ( 250 VAC ) with all
current rating ( 115 mA ) . The center tap earthed , is only if you need to use the bias voltage available ( 50 VAC ) , thus you only will have 125 V available at each side of CT.

Regards ,

Carlos
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Old 23rd August 2009, 02:56 PM   #3
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Quote:
Originally Posted by dsavitsk View Post
Can anyone confirm the power ratings on Hammond transformers? I was looking at the 369AX which is rated at 250VCT @ 100mA and 6.3V @ 2A. Hammond suggests that the center tap of the HT supply should be grounded, so I always assumed that this meant using full wave rectification, meaning ~125VDC at 100mA. But, it is rated at 40VA, which seems to mean that running with a bridge rectifier for 250VDC at 100mA is fine. Can anyone confirm this?
I have no doubt that Hammond published a design guide that answers these
questions with precise formulae. As you surmise, the available DC output current
is different for FWCT vs bridge rectifier, but it's not a straightforward factor
of the AC VA rating. It also makes a huge difference whether it's a cap input or
choke input filter, or a pure resistive load. Most Hammond PT current ratings are
normalized to the FWCT capacitor input filter case.

From the document:

for FWCT, cap input, I D.C. = 1.00 X Sec. I A.C. (115 mA DC for 369AX)
for FW bridge, cap input, I D.C. = 0.62 X Sec. I A.C. ( 71 mA)
for FW bridge, choke input, I D.C. = 0.94 X Sec. I A.C. ( 108 mA)

http://www.hammondmfg.com/pdf/5c007.pdf

(I just researched this for a guitar amp I'm building because I wanted to use
a bridge rectifier for higher output voltage with a L-input filter)

Cheers

Michael

PS I still apply a 1.5X to 2X derating factor power transformers, particularly
Hammond transformers, to reduce heat and buzzing problems.

PPS There are errors in the Hammond document. Average DC output voltage
is not the same for choke input vs cap input filters. The DC voltage for the
cap input cases is in error. Also there is rectifier loss to account for.

Last edited by Michael Koster; 23rd August 2009 at 03:18 PM.
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Old 23rd August 2009, 07:16 PM   #4
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Quote:
Originally Posted by Michael Koster View Post
for FW bridge, cap input, I D.C. = 0.62 X Sec. I A.C. ( 71 mA)
for FW bridge, choke input, I D.C. = 0.94 X Sec. I A.C. ( 108 mA)
Hmmm. I am working on a scrap box amp, and I happen to have a couple of these as well as some largish chokes (15H, 125mA). I need about 80mA and 200V across the output tube, and ~10mA and 100V across the driver (& 100V across it's load). I was hoping to DC couple which would require ~90mA @ 300V but it does seem like that is pushing things. If I use the chokes as quasi choke inputs, with a small input cap to eliminate some noise, then according to PSUD I end up around 200V which actually works quite well for AC coupling. That's probably more reasonable, especially since it doesn't require a 20W resistor to bias the output tube.


Quote:
Originally Posted by Michael Koster View Post
PS I still apply a 1.5X to 2X derating factor power transformers, particularly
Hammond transformers, to reduce heat and buzzing problems.
These are 300 series. My sense is that they can be derated a little less than the 200's as they seem less prone to heat and buzzing.
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Old 23rd August 2009, 08:23 PM   #5
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Quote:
Originally Posted by dsavitsk View Post
Hmmm. I am working on a scrap box amp, and I happen to have a couple of these as well as some largish chokes (15H, 125mA). I need about 80mA and 200V across the output tube, and ~10mA and 100V across the driver (& 100V across it's load). I was hoping to DC couple which would require ~90mA @ 300V but it does seem like that is pushing things. If I use the chokes as quasi choke inputs, with a small input cap to eliminate some noise, then according to PSUD I end up around 200V which actually works quite well for AC coupling. That's probably more reasonable, especially since it doesn't require a 20W resistor to bias the output tube.

These are 300 series. My sense is that they can be derated a little less than the 200's as they seem less prone to heat and buzzing.
It seems like at 200V DC you should be OK. he 300 series do seem more
sturdy than the 200 for the same VA rating.

My guitar amp is a scrapbox special also. I made a rule that it had to fire up
and make good tone without having to buy anything special.

I use a 500V CT 100mA Hammond with a 5U4/UF4007 hybrid bridge choke or
quasi choke input to run my 807 between 400 and 500V. My current is only
going to be 40-50mA on the 807 plate + maybe 2-3 mA for the screen and the
pentode driver.
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