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Old 20th March 2005, 10:03 AM   #1
Cassiel is offline Cassiel  Libya
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Default naive questions

Hi,
I've just got some naive questions....i did search...

First, i miswire the triode/pentode switch which i had installed in my amp (grounded cathode, SE, single valve per channel-PCL86-)so both were connected at the same time, the screen going with a 330 ohm resistor to the plate and with a 2,2k resistor to the B+;
it did sound great ...no idea, clueless.
Now it's wired ok for fear the tube may go
Also, why in pentode mode the current should be higher?...i am using a 330 cathode resistor for both T & P.
I am happy with its sound just wondering basic things which i don't know.
Also, with the OT floating, the pentode mode needed feedback to sound right and the triode sound was darker, fatter and sweeter but didn't make me happy. When i grounded one side of the OT(hammond 125ESE) everything changed again,(WHY?) i then had to remove the feedback for now it didn't help much neither in P nor in T( sound was less sparkly) and the sound in the T mode went much better and it stays( the winner) and i only change to pentode when i do house cleaning(LOUD).
Also, I tried a partial feedback(read Alex Kitic posts) and connected both plates with a 100k resistor without changing anything else. Pentode sounds better with this arregement, can reach almost full volume without distorting, so better than before, but i removed it cause i don't listen often the P and like a lot the T so why bother? To those who need power i recommend the partial feedback ( read el 84 RH post).
Well this is all, i hope these questions are clear enough:
Why in pentode more current?
Why groundind the OT changes the sound?
Why the sound is ok with the screen grid connected to the plate and to B+ ?
Thanks in advance to whoever answers these questions, if someone does(hopefully).
Viva glowing tubes!!
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Old 20th March 2005, 11:34 AM   #2
Yvesm is offline Yvesm  France
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Hi !

Quote:
Why in pentode more current?
Perhaps because, in penthode mode, the screen voltage is a bit higher than the anode one wich suffer from some drop in the OT
Quote:
Why groundind the OT changes the sound?
Duh ? More parasitic capacitance to ground ?
But how do you apply NFB without having a side of the secondary grounded ?
Quote:
Why the sound is ok with the screen grid connected to the plate and to B+ ?
If I understand well, this just add a 2K2 resistor accross the primary of the OT.
Says it is happy to be a bit damped ? I don't really know !

Yves.
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Old 20th March 2005, 01:18 PM   #3
Yvesm is offline Yvesm  France
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Ho, and I've forgotten the essential: these tubes have very hi gain and are very prone to ultrasonic oscillations that can only be observed with an o'scope.
Neat wiring, getting outputs and inputs well away, grid stoppers and even shields may cure that.

Yves.
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Old 22nd March 2005, 09:50 AM   #4
Cassiel is offline Cassiel  Libya
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Thanks Yvesm...seems people found my questions silly rather than naive. Oh well...

Cheers!
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