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Old 3rd April 2004, 10:33 AM   #1
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Default el84 quad schematic..something is wrong...?

i want to build some how a cheap amp using el84' and found this schematic...
is it ok? values are correct? i find it not reasonable that the OT impedance is 1.25k..mmm...

http://www.bonavolta.ch/hobby/en/audio/el84_4.htm

Click the image to open in full size.
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Old 3rd April 2004, 10:58 AM   #2
mig-ru is offline mig-ru  Russian Federation
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Can't see the schem, try posting it again. As for the OPT impedence being 1.25Kohms, it being a quad, each of the tubes "sees" it as 5Kohm impedence (1.25x4=5), so the impedence isn't low at all.
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Old 3rd April 2004, 12:17 PM   #3
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Default so...go to the transformer maker and..

ask for 1.25k ohm transformer?
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Old 3rd April 2004, 12:48 PM   #4
Fuling is offline Fuling  Sweden
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Hammond makes one. Look for transformers that are designed to fit 6C33C.
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Old 3rd April 2004, 01:45 PM   #5
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Default thank you:)

i think a local company can make it cheaper for me...
just sending 2 cheaper via mail vill be expansive
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Old 3rd April 2004, 02:28 PM   #6
Fuling is offline Fuling  Sweden
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Good idea. The Hammonds are bigger than necessary and they are quite heavy, too.

Winding a 1,25k/8R OPT shouldn´t be too hard, since winding capacitances etc. becomes less important at lower impedance ratios. With four EL84´s (Ok, 6BQ5´s) in // you won´t need an extremely high primary inductance, 10-15H or so should be enough for a good bass response.

A piece of cake compared to, say, a 10k/8R OPT with 100H prim. inductance.
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Old 3rd April 2004, 03:54 PM   #7
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hello DUD

1. just take in account, this is a Single Ended amp, the power
you can get of this arrangment is about 10 Watt.

2. the output of the driver stage would be better taken
from it's cathode rather than the anode IMHO.

robonibi
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Old 3rd April 2004, 05:05 PM   #8
Fuling is offline Fuling  Sweden
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Good point, robo7

If I were to build this circuit I´d change the input/driver stage completely.
Say, one ECC88 per channel with one section as gain stage and the other as cathode follower using direct coupling between the two.
No need for all the gain of a ECC83, but the current capability of a ECC88 would be at hand for driving the input capacitance of the four power tubes.
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Old 3rd April 2004, 08:38 PM   #9
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Default im a little lost im new in tubes circuits..

can any of you drag the jpeg into the "paint brush" and mark the
changes for me?
can you recommend a better circuit for the first stage?
thank you!!
David
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Old 4th April 2004, 11:56 AM   #10
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There is a simpler schem to build at bonavolta's site:

El84 push-pull 1

and here is attached my variation on the them. I raised the supply
a little and lowered the current on the o/p pair so you get bit
more power.
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File Type: gif el84_4.gif (31.4 KB, 348 views)
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