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Pooh Bear
Guest
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 Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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kseely wrote:
| Quote: | Ian Iveson wrote:
Does it make any difference what kind of pot I use to control fixed
bias? What are the issues here please.
cheers, Ian
This pot has constant current through it and needs to be capable of
handling that without burning up. Your average 1/2 watt pot isn't up to
the challenge. I have an amp with a 2 watt cermet element (conductive
plastic, rather than carbon) 10 turn presision trimmer to adjust the
bias. You can get them from Mouser along with a little box thingy that
lets you chassis-mount them. Last I checked there were two brands like
this in their catalog.
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Cermets are actually metal film track on ceramic former. CERamicMETal film.
That's where the name comes from. Duh !
Do you *seriously* think a bias pot needs to dissipate 2 watts ? God
almighty !
If you think you need 10 turn, you designed the circuit really badly.
Graham
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Tim Williams
Guest
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| Posted:
Mon Nov 29, 2004 6:18 am Post subject:
Re: Pot |
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"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:41AA64D6.C3A3195B@hotmail.com...
| Quote: | You know the bias will be in the - 45 to - 55 region ( say ). Use endstop
resistors to restrict the range on the trimpot.
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Good for general use, but I like being in control and that means being able
to span several amplification classes in the swing of a screwdriver. ;)
Even so, most of my amps have had "endstop" resistors, simply because the
trimmers I mentioned are ALL that I have. ;-)
Tim
--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms |
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Pooh Bear
Guest
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| Posted:
Mon Nov 29, 2004 6:18 am Post subject:
Re: Pot |
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Tim Williams wrote:
| Quote: | "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:41AA64D6.C3A3195B@hotmail.com...
You know the bias will be in the - 45 to - 55 region ( say ). Use endstop
resistors to restrict the range on the trimpot.
Good for general use, but I like being in control and that means being able
to span several amplification classes in the swing of a screwdriver. ;)
|
That you will certainly achieve !
Class C ? Really ?
| Quote: | Even so, most of my amps have had "endstop" resistors, simply because the
trimmers I mentioned are ALL that I have. ;-)
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Hmmm. OK.
Graham |
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Patrick Turner
Guest
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| Posted:
Mon Nov 29, 2004 5:24 pm Post subject:
Re: Pot |
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Ian Iveson wrote:
| Quote: | Does it make any difference what kind of pot I use to control fixed
bias? What are the issues here please.
|
I can't resist saying that the natural stuff is the way to go;
don't be biased by that hydroponic rubbish grown in suburban houses
under lights.
Seriously, I use 10k x 3 watt wire wound pots, likely to stay together
for 50 years in a tube grid circuit, and survive the occassional
meltdown of a tube.
If carbon pots are to be used, let them also be lowish R,
about 20k max, and never with more than 1 mA of current,
and always with some taped and cardboard over the opening
for the connections that many pots have which allow dirt, dust, and
grime to get in,
and also the very dangerous entry of soldering fumes which contain
sticky muck that gradually fouls a track.
Have a 470k at least from the wiper of pots to the -ve end of the pot,
so that if
the pot goes open or intermittent, the grid has a bias applied that is
more -ve than
the wanted bias. Thus the tube is saved from bias failure.
Patrick Turner.
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Doug Schultz
Guest
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| Posted:
Tue Nov 30, 2004 12:23 am Post subject:
Re: Pot |
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"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:T%oqd.41595$F7.11945@fe1.news.blueyonder.co.uk...
| Quote: | Does it make any difference what kind of pot I use to control fixed bias?
What are the issues here please.
cheers, Ian
|
Obviously BC hydroponics are the best.
hehe Vancouver has to be famous for something.
Doug |
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Ian Iveson
Guest
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| Posted:
Tue Nov 30, 2004 12:23 am Post subject:
Re: Pot |
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Thanks all.
Can't see all this via my news server, dunno why.
What I can see makes sense though. I have used little 10-turn
cermets so far, with end stops, and a circuit that defaults low
(with a pull-down resistor in case of pot failure).
The 10-turns turn out to be wishful thinking. I can adjust to a
micro-amp, but there is no point because the valves drift more than
that anyway.
One issue I haven't seen mentioned, temperature drift of the pot...I
guess it is best to use the same type as the other resistors in the
dividers and keep them close together so they all vary by the same
amount. Cermet and metal film would go together OK, thanks Pooh I
didn't know that.
cheers, Ian |
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Tim Williams
Guest
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| Posted:
Tue Nov 30, 2004 6:23 am Post subject:
Re: Pot |
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"Doug Schultz" <Douglas_Schultz@hotmail.com> wrote in message
news:KtLqd.376883$nl.298109@pd7tw3no...
| Quote: | Obviously BC hydroponics are the best.
hehe Vancouver has to be famous for something.
|
Pffbbt....that's what *I* said! ;-)
Tim
--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms |
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Doug Schultz
Guest
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| Posted:
Thu Dec 02, 2004 5:38 am Post subject:
Re: Pot |
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"Tim Williams" <tmoranwms@charter.net> wrote in message
news:uKRqd.7338$NG7.4012@fe03.lga...
| Quote: | "Doug Schultz" <Douglas_Schultz@hotmail.com> wrote in message
news:KtLqd.376883$nl.298109@pd7tw3no...
Obviously BC hydroponics are the best.
hehe Vancouver has to be famous for something.
Pffbbt....that's what *I* said! ;-)
Tim
--
Hey Tim, |
Didnt see that part of your post. So we really are the North American
Amsterdam are we?
Doug |
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Tim Williams
Guest
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| Posted:
Thu Dec 02, 2004 6:10 am Post subject:
Re: Pot |
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"Doug Schultz" <Douglas_Schultz@hotmail.com> wrote in message
news:Wttrd.407244$%k.401665@pd7tw2no...
| Quote: | Didnt see that part of your post. So we really are the North American
Amsterdam are we?
|
Heh... well, you sure seem to be. Maybe that's why you missed it ;)
(For posterity-)
| Quote: | Does it make any difference what kind of pot I use
I'm sure Gregg can hook you up with some suitable BC export.. ;^)
|
Tim
--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms |
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Nothing40
Guest
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| Posted:
Fri Dec 03, 2004 6:19 am Post subject:
Re: Pot |
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"Doug Schultz" <Douglas_Schultz@hotmail.com> wrote in message news:<Wttrd.407244$%k.401665@pd7tw2no>...
| Quote: | "Tim Williams" <tmoranwms@charter.net> wrote in message
news:uKRqd.7338$NG7.4012@fe03.lga...
"Doug Schultz" <Douglas_Schultz@hotmail.com> wrote in message
news:KtLqd.376883$nl.298109@pd7tw3no...
Obviously BC hydroponics are the best.
hehe Vancouver has to be famous for something.
Pffbbt....that's what *I* said! ;-)
Tim
--
Hey Tim,
Didnt see that part of your post. So we really are the North American
Amsterdam are we?
Doug
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Yep,AKA "Vansterdam" ;-) |
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