| Author |
Message |
Phil Allison
Guest
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 Posted:
Fri Jan 14, 2005 3:41 am Post subject:
Re: Pots for Fixed Bias |
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"Pooh Bear"
| Quote: |
** The WW pots are specified for their high reliability of contact and
stability compared to carbon pots - it is hard to find ones with less
than a 2 watt rating.
So, given that the circuit is 40 years old, and taking into account
the quantum leaps which have been made in component technology,
a modern good quality carbon pot might do. I realise that a bias
failure means disaster.
A cermet pot will be far better than even any wirewound.
|
** What absolute crap - Pooh Face is off with the faries again.
| Quote: | The wiper tends to
'jump' on wirewounds IMHO since there isn't a flat surface for it to
travel on.
|
** Not relevant to the application.
A 10k or 20 k ohms WW pot has good resolution.
| Quote: | They certainly never feel very nice.
|
** Lack of a silky feel is not relevant either.
| Quote: | You can avoid 'disaster' by adding a sensibly valued resistor from the
negative
bias connection 'across' the pot from the negative bias connection to the
wiper.
If the wiper ever goes open, the valve gets nearly full negative bias.
100k
should do the job.
|
** The whole idea is to use a high reliability part in the first place -
which a WW pot already is.
.............. Phil
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Patrick Turner
Guest
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| Posted:
Fri Jan 14, 2005 4:29 am Post subject:
Re: Pots for Fixed Bias |
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Iain M Churches wrote:
| Quote: | Greetings Rats!
I have been looking at some schematics for fixed
bias power amps. As I understand it, the bias circuit
draws little or no current, but the potentiometers
specified seem to be 2W wirewound.
Can anyone explain?
|
Reliablity.
Never use a carbon track pot where there is any likelyhood of
even a small flow of DC.
To make bias adjustable, you must have about 15v dc across the pot
and the pot value is typically 10 to 20k, so you have perhaps
1.5 mA of dc flow, and you get some DC at the wiper when grid current
flows,
which will happen as the tube ages, even at idle conditions.
Be a man, use a wirewound rugged pot and be done with it and never ever
experience the distaster caused by bias failure.
The cheap Taiwanese carbon pots are not as good as ones made
properly 30 years ago in home countries, when a bias failure would bring
unruly mobs to the factory gates.
Patrick Turner.
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Patrick Turner
Guest
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| Posted:
Fri Jan 14, 2005 4:39 am Post subject:
Re: Pots for Fixed Bias |
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Iain M Churches wrote:
| Quote: | "mick" <mick@mixtel.co.uk> wrote in message
news:pan.2005.01.13.17.35.47.675344@mixtel.co.uk...
On Thu, 13 Jan 2005 19:08:33 +0200, Iain M Churches wrote:
Greetings Rats!
I have been looking at some schematics for fixed bias power amps. As I
understand it, the bias circuit draws little or no current, but the
potentiometers specified seem to be 2W wirewound.
Can anyone explain?
Could be the voltage across the pot and its value? It is the current
*through* the
pot, not the bias voltage tapped off it that is causing the heat.
Difficult to guess without seeing the circuit.
--
Mick
Hi Mick.
The transformer bias winding is 60VAC then rectified full-wave
bridge. The pots are 4x 25k in parallel, with the wiper of each
feeding the grid of each tetrode.
|
Is that all?
Perhaps you may find that 60 vac will make -84v, and that may be filtered down
to -60v, and then that is applied to one end of the pots.
The other end of the pots should not be grounded because otherwise you have
60v across the bias posts when it isn't needed since one would never need to
bias the grids at
less than -30v, so there should be a resistor of about 5.6k between 0V and the
ends of the pots.
There should also be a 470k from each pot wiper to the -60v end, then in case
the wiper-pot
connection goes open you will have some bias still applied to the tube.
30v across a 25k pot is 36 milliwatts.
The higher power rated pots are there for reliability, not power handling.
Patrick Turner.
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mick
Guest
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| Posted:
Fri Jan 14, 2005 4:51 am Post subject:
Re: Pots for Fixed Bias |
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|
On Thu, 13 Jan 2005 19:50:54 +0200, Iain M Churches wrote:
<snip>
| Quote: | Hi Mick.
The transformer bias winding is 60VAC then rectified full-wave bridge.
The pots are 4x 25k in parallel, with the wiper of each feeding the grid
of each tetrode.
|
The voltage is sqr2xVac = 1.414x60 = approx 85V
The power dissipation in each pot is Vsquared/R = 7225/25000 = approx 0.3W
So, no, they don't need to be 2W rated. 0.5W or, preferably, 1W
would be fine. However, as others have pointed out, they should be *very*
reliable and temperature stable so WW or cermet should be ok (carbon are
really too unreliable for this job). Also, you will probably have
difficulty in finding a WW pot with less than a 2W rating!
--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk |
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Phil Allison
Guest
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| Posted:
Fri Jan 14, 2005 8:46 am Post subject:
Re: Pots for Fixed Bias |
|
|
"Pooh Bear"
| Quote: |
** The whole idea is to use a high reliability part in the first
lace -
which a WW pot already is.
Do please advise what adverse issues you consider relevant with a cermet
type.
|
** Goal post moving and onus reversal is my pet hate:
" A cermet pot will be far better than even any wirewound.
** What absolute crap - Pooh Face is off with the fairies again.
The wiper tends to 'jump' on wirewounds IMHO since there isn't a flat
surface for it to
travel on.
** Not relevant to the application.
A 10k or 20 k ohms WW pot has good resolution "
** The onus is on YOU Pooh to substantiate the claims YOU made above.
| Quote: | Using old technology toobs doesn't require the use of old technology
passive
components too !
|
** WW pots have always been and still are inherently high reliability.
Precision pots are all multiturn WW.
Seems you have not made sufficient acquaintance with them.
............... Phil |
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Pooh Bear
Guest
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| Posted:
Fri Jan 14, 2005 8:46 am Post subject:
Re: Pots for Fixed Bias |
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|
Phil Allison wrote:
| Quote: | "Pooh Bear"
** The WW pots are specified for their high reliability of contact and
stability compared to carbon pots - it is hard to find ones with less
than a 2 watt rating.
So, given that the circuit is 40 years old, and taking into account
the quantum leaps which have been made in component technology,
a modern good quality carbon pot might do. I realise that a bias
failure means disaster.
A cermet pot will be far better than even any wirewound.
** What absolute crap - Pooh Face is off with the faries again.
The wiper tends to
'jump' on wirewounds IMHO since there isn't a flat surface for it to
travel on.
** Not relevant to the application.
A 10k or 20 k ohms WW pot has good resolution.
They certainly never feel very nice.
** Lack of a silky feel is not relevant either.
You can avoid 'disaster' by adding a sensibly valued resistor from the
negative
bias connection 'across' the pot from the negative bias connection to the
wiper.
If the wiper ever goes open, the valve gets nearly full negative bias.
100k
should do the job.
** The whole idea is to use a high reliability part in the first place -
which a WW pot already is.
|
Do please advise what adverse issues you consider relevant with a cermet type.
Using old technology toobs doesn't require the use of old technology passive
components too ! It may enhance the 'look and feel' aspect though. :-)
Graham |
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Choky
Guest
|
| Posted:
Fri Jan 14, 2005 1:54 pm Post subject:
Re: Pots for Fixed Bias |
|
|
"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:pTAFd.141358$Z7.6416@fe2.news.blueyonder.co.uk...
| I asked the same question not long ago.
|
| Thermal stability is one reason for over-rating.
|
| Wirewound and Cermets have both been deemed appropriate. You don't
| need 0-60V adjustment so you can use "end stop" resistors and reduce
| the pot value to suit. Or you could reconfigure to a more failsafe
| circuit which would use the pot in shunt so open-circuit failure
| would result in turnoff rather than meltdown. Be careful not to draw
| too much current or you may get unacceptable ripple from the supply.
|
| cheers, Ian
|
|
good (old ;) point ;
besides,always use fuse in cathodes or Patrick's scr implementation of
protection circ
--
--
..........................................................................
Choky
Prodanovic Aleksandar
YU
"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
............................................................................. |
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Iain M Churches
Guest
|
| Posted:
Fri Jan 14, 2005 4:52 pm Post subject:
Re: Pots for Fixed Bias |
|
|
"Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:l8ndu059t07uia7rsmq86c5qr2o2u3hrks@4ax.com...
| Quote: | "Iain M Churches" <tael@kolumbus.fi> said:
In most cases, the pots are wired as voltage dividers with the wiper
at the grid (best via a grid stopper of 1 kohm).
A better circuit would be that where the pot is used as a variable
resistor with the wiper to ground, and a fixed resistor to the
negative voltage.
That way, when the wiper accidentally comes loose, the grid voltage
will drop to the max. negative voltage and hence protect the tubes
from drawing excessive current.
The distortion that will appear may call your attention to a failed
pot :-)
--
Sander de Waal
|
Excellent, Sander. many thanks.
Iain |
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Iain M Churches
Guest
|
| Posted:
Fri Jan 14, 2005 7:59 pm Post subject:
Re: Pots for Fixed Bias |
|
|
<calcerise@hotmail.com> wrote in message
news:1105651955.788620.120360@c13g2000cwb.googlegroups.com...
| Quote: | The best soluution is a cathode resistor bypassed with a relay that is
normally open so that if you lose bias voltage it opens and fixed bias
is provided. Another good fix is simply use a cathode resistor that is
designed to burn out if the tube pulls grossly excessive current: it
also doubles as a shunt resistor for measuring quiescent current.
|
I have seen this on some GEC broadcast amps. When the rectifiers
on these amps were replaced with SS full wave bridges, a delay
relay was also added to switch the HT after 45 seconds warm up.
The relay was across the bias circuit, so that if the bias failed,
the relay would drop out and the HT would fall to zero.
I thought that was nifty:-)
Iain |
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Guest
|
| Posted:
Sun Jan 16, 2005 2:29 am Post subject:
Re: Pots for Fixed Bias |
|
|
GEC were a very capable bunch and in fact their very good book was
where I first saw this...it's been used elsewhere too.
AudioXPress/OCSL has reprinted this book. |
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Ruud Broens
Guest
|
| Posted:
Tue Jan 18, 2005 12:34 am Post subject:
Re: Pots for Fixed Bias |
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|
"Iain M Churches" <tael@kolumbus.fi> wrote in message
news:cs8mo4$5qo$1@phys-news1.kolumbus.fi...
:
: <calcerise@hotmail.com> wrote in message
: news:1105651955.788620.120360@c13g2000cwb.googlegroups.com...
: > The best soluution is a cathode resistor bypassed with a relay that is
: > normally open so that if you lose bias voltage it opens and fixed bias
: > is provided. Another good fix is simply use a cathode resistor that is
: > designed to burn out if the tube pulls grossly excessive current: it
: > also doubles as a shunt resistor for measuring quiescent current.
:
: I have seen this on some GEC broadcast amps. When the rectifiers
: on these amps were replaced with SS full wave bridges, a delay
: relay was also added to switch the HT after 45 seconds warm up.
: The relay was across the bias circuit, so that if the bias failed,
: the relay would drop out and the HT would fall to zero.
: I thought that was nifty:-)
:
: Iain
:
Exactly what i've used for a practice guitar amp :-)
Using about -20 V, when above -15V, the HT relay will drop
driven by a 2N2905A ,using a nominal 12V relay
HT on delay about 40 s,
cheers,
Rudy |
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