ampage archive

Vintage threads from the first ten years

Power Trans

 

2/22/2000 2:34 PM
Nigel Brewer	Power Trans I am planning to build a fifty to sixty watt amp this summer, I am not Exactly sure which power transformer   to use. I am thinking of either   a Hammond 272HX or a 273BX. Has anyone here used these? Both transformers have about the same VA rating. I will be running 5 12aX7'S, and SS rectifier.

 

2/22/2000 4:49 PM

Benjamin Fargen

Nigel,     Either of those transformers will work good....you just have to decide on what your going for. With SS rectification the 273BX will end up giving you a B+ of about 450-460VDC while the 272HX will give you a B+ of 390-400VDC. Both current ratings are acceptable at the 175-200ma rating.   The difference in B+ is the difference between having a higher powered Marshall/Blackface type power tube plate voltage(440-450vdc) and a browner Tweed type power tube plate voltage(380-385vdc)   you will just have to decide what your need is and how much clean headroom you want.     Hope this helps,   Benjamin.

 

2/22/2000 5:04 PM

Mike Kach

I find Hammond transformers in question are adequite for no more than 2 pre amp tubes and 2 6L6-EL34 types, due to its limitted currient rating. The transformers when pushed to the max tend to overheat, and vibrate. I find that overrating the currient demand, two times is enough for a safe operation, I tend go with at least 1/2 amp rating on a HV secondaries for a 40-50watt amp. I feel that excessive heat is an issue in the transformer, at the same time an extra currient with a proper filtration scheme, tends to offer an extra bass response.

 

2/22/2000 5:30 PM
jason	The 272HX is rated at 200ma.   Let's give him 40mA per EL34, so that's 80 mA. Then we'll say probably no more than 1.2mA per 12AX7 for 5 tubes gives us 5 x 2 x 1.2=12mA for a total of 92mA. Sounds good there.   The 6.3v heater winding is rated at 6A.   1.5A per EL34=3A   .3A per 12AX7 (5 of them)=1.5A for a total heater current draw of 4.5A. Still sounds good to me.   I'd say go for it.   jason

 

2/23/2000 6:51 AM
mack	jason great reply im just learning   i was wondering how you come up   with 40ma for el34?

 

2/23/2000 7:00 AM
Bruce	I think Jason is assuming the 272HX tranny will not put out more then about 410vdc under a light load of 2 EL34s.   Since they are 25 watt tubes, a nice sounding safe idle current of 40ma will have the tubes sitting at about 15w to 16w at idle.   25w x .65 = 16w   16w/aprox 410vdc = 39ma     Bruce

 

2/22/2000 5:35 PM

Benjamin Fargen

Mike,     interesting.......     I have not had any problems with Hammond trannies regarding overheating and vibration(?)What configuration were you running and what particular model of tranny were you using?   Were you trying to run a high current class A configuration like Matchless or was it a normal push pull type amp?     I wouldn't think that the 3 extra preamp tubes Nigel wants to run(pulling maybe 4.5ma more) would be the difference between overheating those transformers and not overheating them.     Thanks for the input   Benjamin

 

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