Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tone Junkie on July 22, 2011, 01:18:09 am
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OK Im confussed maybe you guys can help most marshall transformers are rated 150 MA for 2 el34 tubes that push close to 50 watts most fenders PT seem to be at 200 MA for 2 6l6,s so do a pair of 5881,s need a 200 MA
what if I wanted to run a quad of 5881,s I have a nice 250MA PT would that run a quad of 5881,s with about 360vdc to the tubes. Its late Im tired hope this makes sence.
Thanks Bill :think1: :dontknow: :help:
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Check the data sheets first. Since you are talking 360VDC on the plates, this falls right into a textbook example for a 5881. Lets see... a pair from the data sheets claim:
Class A/B, 360V plate, 270V g2, 88mA Ia, 26W into a 6.6k load.
I am not 100% sure I trust those numbers, but hey lets go with that... 176mA for a quad. You have 250mA, so your
B+ will rise a little bit, but that gives you plenty of headroom when you hit it... depending on how you build your B+ supply, I would think you should be fine with that setup. It may need a bit of tweaking, but I would think it would be minor.
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Thanks Triode that what I didnt understand it 88ma i didnt know if that was per tube per 2 tubes or what.
Bill
:happy1: :happy1:
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OK Im confussed maybe you guys can help most marshall transformers are rated 150 MA for 2 el34 tubes that push close to 50 watts most fenders PT seem to be at 200 MA for 2 6l6,s so do a pair of 5881,s need a 200 MA
The Marshall tranny info for two el34s in pp doesn't appear to be correct? On the spec sheet, it looks like you would need twice as much as what you listed.
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See3 the marshall spec transformer here and you will understand everyone I have looked at no matter who makes it says the same thing
http://www.classictone.net/40-18054.pdf (http://www.classictone.net/40-18054.pdf)
350-0-350 150ma
Thanks Bill
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Actually these specs are what confused me, every 50 watt marshall transformer is the same. I tried to get the specs from dougs site I would have never linked to another site that sold things that he also sold. But he didnt give specs on his to show what I was getting at .
Thanks Bill
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150mA, 200mA, what's the difference?
The exact same PT could be rated 150mA for 24/7 100,000 hours -OR- 200mA 4/3 for 10,000 hours total.
> every 50 watt marshall transformer is the same
They probably all copy a number Jim wrote 40 years ago; likewise Fender PT clones copy Leo's numbers. US 1950 and UK 1967 had different assumptions about regulation, rating, and life.
Go by weight. If a "50W PT" is 10 pounds it is plenty hefty. Fender "50W PT" seem to be 7.7 pounds; Marshall "50W PT" seems to be 5 pounds. This may be improved materials in the 1960s, or the continued high cost of metal in the UK, or just Jim shaving corners.
The "50W EL34" amps will hew close to the data-sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL34.pdf) 400V 3.5K 54 Watt condition. This shows 269mA (plus preampery). This suggests Jim's "150mA" number falls short. Yet we KNOW these amps make 50W clean and over 70W not too bent. Therefore the PT will support 269+mA as long as your fingers will last.
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Thanks PRR some of these things play with my head untill you put it in plain english.
Bill
:think1: