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Old 2nd December 2007, 10:09 PM   #1
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Default el84 driver tube, with less the 5ma current

hello people,

i'm refurbishing an old amp.
it is a push pull el84. it was completely down, so i kept only the trannies and the chassis.

the only problem: the power trannie puts out : 140vdc at 0.19a dc; in the original scheme there was a voltage doubler, so you end up with something like 320vdc (or a bit lower) at 90ma.

i need 80ma for the el84, left with 10ma for the drivers.

in the orignial scheme there were 5 * ecc83, each probably at 2ma.
but i don't wanna use the phono stage or the ecc83 as a driver. Can anybody help me on choising a decent driver that can live with 5mA per channel.

thanx a lot
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Old 3rd December 2007, 12:41 AM   #2
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well people, i don't think this problem has to be solved, but if it is, that is nice too;


i have this transfo, and i could not believe it was capable of putting out this small amount of Amps.

i have found an old "simple rule". it states the power of an transfo.

it says that
the height (in cm) of the iron plates * the width of the iron leg that goes into the windings : this number has to be taken to the power of 2 and then you have the power.
and then multiplied by 0.64.

in my case:
hight of the plates : 4cm
width of middle leg : 3 cm

(4*3)*0.64 = 92watt.


having a
6v at 0.3 amp
6.3 at 4 amp
140v at ???? amp


the core will saturate at 140v with 0.463 Amp. which is a lot more then the 0.190 A which is written on it.

so i wonder: i have seen nice designs for a EL84PP, they use all sorts of drivers, but most of the time they need a bit more then a few mA, so even if this trannie is designed to give 140v at 190ma, how bad will it be to draw 220ma ?

i think a good measurment of the power rating of a trannie is the heat it is dissipating ?

am i a bit right on my thoughts ?
thanx
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Old 3rd December 2007, 01:12 AM   #3
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Default Re: el84 driver tube, with less the 5ma current

Quote:
Originally posted by then_dude
in the orignial scheme there were 5 * ecc83, each probably at 2ma.
but i don't wanna use the phono stage or the ecc83 as a driver. Can anybody help me on choising a decent driver that can live with 5mA per channel.

thanx a lot
greetings
If you're talking preamps, I haven't designed one yet that needed to pull 5mA. Even if you have to go over a few mA, it won't make that much difference.
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Old 3rd December 2007, 01:19 AM   #4
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Have a look at the RH84.

dave
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Old 3rd December 2007, 07:44 AM   #5
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thanx guys,

i'll just go ahead with something.

the RH48 is a SE design, i need a pp.

but there are a lot of designs

an eCC88, ECC81, ECC99,

but maybe i'll go with a poindexter (in stead of a 6V6 you can use a EL84),

i'm gonna check it out again.
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Old 3rd December 2007, 08:12 AM   #6
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Quote:
Originally posted by then_dude
the RH48 is a SE design, i need a pp.
I'll blame it on brain fade... yves & gingertube have done a PP variation thou.

dabe
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Old 3rd December 2007, 09:15 AM   #7
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Quote:
I'll blame it on brain fade... yves & gingertube have done a PP variation thou.
Yes I would say this could be a very good way to go. This design gets the best out of small output transformers and delivers very convincing bass. The Gingertubes amp uses ECL86's which is OK because the ECL86 contains a derated EL84. The triode section is half of a ECC83, so you should be able to build a nice little amp with the valves you already have. The amp is called the "Baby Huey".

Shoog
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Old 3rd December 2007, 08:49 PM   #8
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thanx a lot people, i'll look into it.
still don't know, if it is written on the transfo, i better pay attention to it, not dragging 140ma at 300v out of it for example. it will run as hot as Georgian asphalt.
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Old 3rd December 2007, 10:00 PM   #9
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hello,

i have red about baby huey,

still a few problems

1)i have old opt, so no ultralineair, it is gonna be triode or pentode;

2)maybe a bigger problem, the el84 are biased at 38ma, so i need around 160ma at 300v for this amp.

if i follow the instructions on the power trannie i can only get 95ma at 350V. (with a doubler). but that' is almost twice as much. maybe a tad to much.
could they be tammed down to 25ma ? or do i loose all the potential then ?
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Old 3rd December 2007, 10:13 PM   #10
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Default Re: el84 driver tube, with less the 5ma current

Quote:
Originally posted by then_dude
the only problem: the power trannie puts out : 140vdc at 0.19a dc; in the original scheme there was a voltage doubler, so you end up with something like 320vdc (or a bit lower) at 90ma.

Quote:
Originally posted by then_dude
if i follow the instructions on the power trannie i can only get 95ma at 350V. (with a doubler).
AFAIK, it is not correct to derate the transformer because of the use of a voltage doubler.
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