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Message |
Ian Iveson
Guest
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 Posted:
Sun Nov 28, 2004 11:44 pm Post subject:
Pot |
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Does it make any difference what kind of pot I use to control fixed
bias? What are the issues here please.
cheers, Ian
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Sander deWaal
Guest
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| Posted:
Sun Nov 28, 2004 11:58 pm Post subject:
Re: Pot |
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"Ian Iveson" <IanIveson.home@blueyonder.co.uk> said:
| Quote: | Does it make any difference what kind of pot I use to control fixed
bias? What are the issues here please.
|
A *very* good quality one. Unless you put it in the circuit as
adjustable resistor in a voltage divider so that when the wiper has a
bad contact, full negative Vg will arrive at the grid.
Most circuits I see use it as a pot indeed, which is dangerous when
the wiper comes loose. And it *will*, after a few years.
That said, I use closed Pihers (the most common adj. resistor in
Europe) in a voltage divider circuit whenever I need adjustable grid
bias.
Other than that, I prefer cathode biasing in my own designs.
--
Sander de Waal
" SOA of a KT88? Sufficient. " |
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kseely
Guest
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| Posted:
Mon Nov 29, 2004 12:20 am 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.
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.
Karl |
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Tom Schlangen
Guest
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| Posted:
Mon Nov 29, 2004 12:31 am Post subject:
Re: Pot |
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Hi Ian,
| Quote: | Does it make any difference what kind of pot I use to
control fixed bias? What are the issues here please.
|
It should be a good capsuled one. Generally, tube circuits
with their "high" voltages seem to attract dust. Maybe
electrostatic issues kick in - not with voltages present
at a bias pot, of course, but maybe with the parts at
higher operation voltages around them.
So far, I have used '50s vintage open carbon trimmers,
capsuled nowadays trimmers and HQ "Cermet" (metalized
on ceramic substrate) spindle-operated trimmers in my
projects, and I think the simple and cheap capsuled
ones of nowaday production is all that is needed to
get the job done for decades to go.
The Cermet ones may have the longest stability over
time, but regarding this criterium any pot, except
maybe the old open-style ones, outperform the tubes
operation point wear shift anyway.
Chosing a really suitable combo of combined fixed and
on-fixed (pot) values to get a reasonable adjusting
voltage "window" for fixed bias is more important
than the actual type of pot used, I think.
Tom
--
Okay, maybe I am paranoid. But that doesn't mean
they are not out to get me. - unknown |
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Tim Williams
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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"Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:28dkq01ol4tnuirrclnji4o7pb034rtkov@4ax.com...
| Quote: | Why would a 1/2 watt pot burn up?
|
Bias voltage applied to it.
| Quote: | Surely the grids don't draw any current in a competent AB design?
|
Nope (unless class 2, but that element is handled differently anyway).
| Quote: | Vg is about 50 volts max, and there's no reason why the voltage
divider for Vg couldn't be designed with some 10- or even 100 k-ish
resistors and pots.
The dissipation will then be negligible.
|
10k pot for 50V bias setpoint, say 70VDC across the pot = 1/2W dissipated
(and 7mA per tube). Not exactly negligible. Using a larger pot, say 100k
reduces the grid leak that can be used as it adds to grid circuit resistance
(a 100k pot split midwise is 25k impedance, 1/4 the max. rating for a 6L6).
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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mick
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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On Sun, 28 Nov 2004 21:34:42 +0100, Sander deWaal wrote:
| Quote: | Jon Yaeger <jono_1@bellsouth.net> said:
On fixed-bias only PP amps I always put a 10 ohm 1/2 watt resistor
between the cathode and ground. That way, if you smoke the pot, you
won't get burn-out.
The resistor always comes in handy for measuring cathode current. But why
not wire up the pot just as an adjustable resistor to ground, and add
another resistor to -Vg to form a voltage divider with the grid at the
junction of both?
That way, if the pot fails, the tube automatically gets maximum -Vg.
I use that topology for years now, never failed me.
|
I've been doing that for the design I'm working on at present. Making the
raw -Vg about -24v for 6L6s and pulling it closer to ground with the pot.
You can make the slider ground too, which makes adjustment far less risky
by poking a screwdriver through a hole in the chassis! Keeping the -Vg at
-24 is good (for me) as it doesn't cut the valve off, just moves it into a
very safe area.
--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk |
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Sander deWaal
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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Jon Yaeger <jono_1@bellsouth.net> said:
| Quote: | On fixed-bias only PP amps I always put a 10 ohm 1/2 watt resistor between
the cathode and ground. That way, if you smoke the pot, you won't get
burn-out.
|
The resistor always comes in handy for measuring cathode current.
But why not wire up the pot just as an adjustable resistor to ground,
and add another resistor to -Vg to form a voltage divider with the
grid at the junction of both?
That way, if the pot fails, the tube automatically gets maximum -Vg.
I use that topology for years now, never failed me.
--
Sander de Waal
" SOA of a KT88? Sufficient. " |
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Sander deWaal
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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"kseely" <kseely@gmail.com> said:
| Quote: | 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.
|
Why would a 1/2 watt pot burn up?
Surely the grids don't draw any current in a competent AB design?
Vg is about 50 volts max, and there's no reason why the voltage
divider for Vg couldn't be designed with some 10- or even 100 k-ish
resistors and pots.
The dissipation will then be negligible.
Unless you're talking about cathode bias pots, of course.
--
Sander de Waal
" SOA of a KT88? Sufficient. " |
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Tim Williams
Guest
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| Posted:
Mon Nov 29, 2004 12:32 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
|
I'm sure Gregg can hook you up with some suitable BC export.. ;^)
I know Pat prefers wirewound but as long as it doesn't go scratchy and
open-circuit the grid, you're fine. So far I've used teensy 1/8W 10k
trimmers with fine success (just don't put more than 30V across 'em!).
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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Jon Yaeger
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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in article 1101669614.244984.23430@z14g2000cwz.googlegroups.com, kseely at
kseely@gmail.com wrote on 11/28/04 2:20 PM:
| 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.
Karl
|
I've used both the small precision trimpots as well as the sturdier Allen
Bradley types.
Bought a mess of 10K Clarostat pots from my local surplus store . . . The
kind with small slotted shats that mount in 1/4" holes for $2 each. They
don't need knobs and are unobtrusive.
The surplus store is Austin Electronics at:
http://www.austinelex.com/ae_001.htm. The parts are not in the on-line
catalog, but they are Clarostat RV6LAYSA103As. The contact there is Lloyd
Carver.
To Sander's point, there are some designs that incorporate fixed and cathode
bias. Ergo, if the pot opens up the output won't self-destruct.
On fixed-bias only PP amps I always put a 10 ohm 1/2 watt resistor between
the cathode and ground. That way, if you smoke the pot, you won't get
burn-out.
;-)
Jon |
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Pooh Bear
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
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Tim Williams wrote:
| Quote: | "Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:T%oqd.41595$F7.11945@fe1.news.blueyonder.co.uk...
Does it make any difference what kind of pot I use
I'm sure Gregg can hook you up with some suitable BC export.. ;^)
I know Pat prefers wirewound but as long as it doesn't go scratchy and
open-circuit the grid, you're fine. So far I've used teensy 1/8W 10k
trimmers with fine success (just don't put more than 30V across 'em!).
|
Cermet trimmers are typically good for 1/2W. The ceramic substrate is good
with heat.
Wirewound sound like an awful ( and very old fashioned ) choice. It feels
like the wiper is bouncing on and off the track when you adjust them. I
just hope it isn't !
Graham |
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Pooh Bear
Guest
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| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
|
|
Tom Schlangen wrote:
| Quote: | Hi Ian,
Does it make any difference what kind of pot I use to
control fixed bias? What are the issues here please.
It should be a good capsuled one. Generally, tube circuits
with their "high" voltages seem to attract dust. Maybe
electrostatic issues kick in - not with voltages present
at a bias pot, of course, but maybe with the parts at
higher operation voltages around them.
So far, I have used '50s vintage open carbon trimmers,
capsuled nowadays trimmers and HQ "Cermet" (metalized
on ceramic substrate) spindle-operated trimmers in my
projects, and I think the simple and cheap capsuled
ones of nowaday production is all that is needed to
get the job done for decades to go.
The Cermet ones may have the longest stability over
time, but regarding this criterium any pot, except
maybe the old open-style ones, outperform the tubes
operation point wear shift anyway.
Chosing a really suitable combo of combined fixed and
on-fixed (pot) values to get a reasonable adjusting
voltage "window" for fixed bias is more important
than the actual type of pot used, I think.
|
I agree 100% about restricting the adjustment range window. Nothing
worse than 'twitchy pot' syndrome - and one that can accidentally be set
to Vg1 = 0 !
Graham |
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Pooh Bear
Guest
|
| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
|
|
Tim Williams wrote:
| Quote: | "Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:28dkq01ol4tnuirrclnji4o7pb034rtkov@4ax.com...
Why would a 1/2 watt pot burn up?
Bias voltage applied to it.
Surely the grids don't draw any current in a competent AB design?
Nope (unless class 2, but that element is handled differently anyway).
Vg is about 50 volts max, and there's no reason why the voltage
divider for Vg couldn't be designed with some 10- or even 100 k-ish
resistors and pots.
The dissipation will then be negligible.
10k pot for 50V bias setpoint, say 70VDC across the pot = 1/2W dissipated
(and 7mA per tube). Not exactly negligible. Using a larger pot, say 100k
reduces the grid leak that can be used as it adds to grid circuit resistance
(a 100k pot split midwise is 25k impedance, 1/4 the max. rating for a 6L6).
|
Why put all the volts across the pot ?
You know the bias will be in the - 45 to - 55 region ( say ). Use endstop
resistors to restrict the range on the trimpot.
Makes accurate biasing easy too.
Graham |
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Pooh Bear
Guest
|
| Posted:
Mon Nov 29, 2004 12:32 am Post subject:
Re: Pot |
|
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Sander deWaal wrote:
| Quote: | Jon Yaeger <jono_1@bellsouth.net> said:
On fixed-bias only PP amps I always put a 10 ohm 1/2 watt resistor between
the cathode and ground. That way, if you smoke the pot, you won't get
burn-out.
The resistor always comes in handy for measuring cathode current.
But why not wire up the pot just as an adjustable resistor to ground,
and add another resistor to -Vg to form a voltage divider with the
grid at the junction of both?
That way, if the pot fails, the tube automatically gets maximum -Vg.
I use that topology for years now, never failed me.
|
I concur with that approach too. It makes perfect sense. I also use it ( but
not for tubes which I haven't designed with for a long time - semis are more
reliable for one ! )
Graham |
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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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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.
|
You want one that's reliable. Mainly meaning that the contact won't go
high resistance due to tarnishing since it'll rarely get moved.
If you were talking about a pcb mounting part, a cermet track part with
carbon tipped wiper would be perfect and not particularly expensive.
Graham |
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