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Old 29th July 2012, 11:32 AM   #1
kjjeep is offline kjjeep  Venezuela
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Join Date: Aug 2010
Smile First post and a couple P.P. questions

Hi guys.

This is my first post, be patient, I cannot dedicate much of my time to amps 'cause
my other hobby is.. (hint: my nickname).
Until today, along the years, I've only built some EL32 SEs (actually british VT-52)
Yeah, akward, but those tubes seem to last forever..
Now I'm collecting parts to build a stereo EL84 PP and, obviously, doubts are rising.
First issue is the PSU. In order to maintain the 'look' I thought EZ81 would do,
but it seems like I might be pushing it to the limits, so I fear I'll have to go
the 5AR4 route, am I right? I'm not considering diode rectification and will employ
a pure LC filter (oops, hope the coil can withstand the current..).
Secondly, as I'll probably use a cathodine ph.splitter, how comes that some schemes
simply show identical plate and cathode resistors while others split the lower ones
into two, (almost like when You apply NFB), guess it's in order to correctly bias the
tube, but if it's so, why then some do and others don't (BTW, thinking of 12AT7s).
Would both layouts work equally well or is the 'three resistors' configuration better?

Will keep You posted, though it will probably take a while (and $$$).

PS: as I've got 'more than one...' I shall be using classic tubes such as AX7, AU7
and so on... even a BH7 is a bit non std to me, sorry, I know I'm obsolete...

Greetings to everyone and thank You.
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Old 29th July 2012, 02:48 PM   #2
DF96 is offline DF96  England
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The cathodyne, like the cathode follower it is based on, has two different ways of achieving bias: DC coupling from the previous stage, or cathode resistor bias. Which is 'best' depends on circumstances, such as the rest of the amp.
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Old 29th July 2012, 03:04 PM   #3
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A GZ34 is more suitable or use a semi conductor instead
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Old 29th July 2012, 03:41 PM   #4
kjjeep is offline kjjeep  Venezuela
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Join Date: Aug 2010
Thank You DF96, glad You're around...

Yeah, I very much suspected that, in fact I've never built a DC coupled stage, guess this shall be
my first time. I've just Read some useful info on a current thread about a ccs EL84 P.P., I'll just have to
calculate static conditions (IIRC) to find out values of resistors. I suppose that 'the rest of the amp' is
the input stage, well, almost...

What do You make of the rect. tube? I know it's only a matter of summing max current of all stages,
but as I've never built a P.P., I understand that the figure of max plate+screen current is for one tube, so
probably an EZ81, given it's max ratings, won't be enough if I got it right, too bad...
Also, as I have some mil type 12AU7 (6189), I know they are not very popular here, but would they sound
so awful in a cathodyne or better forget it and go straight for the 12AT7 or maybe a 6DJ8?

Thank You again.
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Old 29th July 2012, 04:28 PM   #5
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Location: Lindau
Hi!

Use TV Damper tubes as rectifier. They are cheap, robust and can handle lots of current. A pair of 6CG3 for example can easily deliver 500mA DC

Thomas
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Old 29th July 2012, 06:43 PM   #6
rmyauck is offline rmyauck  Canada
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If you want you could use 2 EZ81's like the Eico HF-81. Schematic & manual @ akdatabase.org & Yahoo Eico groups.

AK Database PDF Gallery - Eico/Eico HF-81 Owner's Manual

Randy
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