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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. |
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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. |
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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. |
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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 |
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mack![]() |
jason great reply im just learning i was wondering how you come up with 40ma for el34? |
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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 |
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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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