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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

Either of those transformers will work 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,  
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

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.  
2/23/2000 6:51 AM

jason great reply im just learning  
i was wondering how you come up  
with 40ma for el34?  
2/23/2000 7:00 AM

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  
2/22/2000 5:35 PM
Benjamin Fargen

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  
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