| Author |
Message |
Iain M Churches
Guest
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 Posted:
Thu Jan 13, 2005 10:08 pm Post subject:
Pots for Fixed Bias |
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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?
Iain
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mick
Guest
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| Posted:
Thu Jan 13, 2005 10:33 pm Post subject:
Re: Pots for Fixed Bias |
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On Thu, 13 Jan 2005 19:08:33 +0200, 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?
|
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
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk |
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Iain M Churches
Guest
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| Posted:
Thu Jan 13, 2005 10:50 pm Post subject:
Re: Pots for Fixed Bias |
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"mick" <mick@mixtel.co.uk> wrote in message
news:pan.2005.01.13.17.35.47.675344@mixtel.co.uk...
| Quote: | 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.
Iain |
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Phil Allison
Guest
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| Posted:
Fri Jan 14, 2005 12:43 am Post subject:
Re: Pots for Fixed Bias |
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"Iain M Churches"
Mick
| Quote: |
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.
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 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.
................. Phil |
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Iain M Churches
Guest
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| Posted:
Fri Jan 14, 2005 12:53 am Post subject:
Re: Pots for Fixed Bias |
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"Phil Allison" <philallison@tpg.com.au> wrote in message
news:34o1bkF4cqj62U1@individual.net...
| Quote: |
"Iain M Churches"
Mick
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.
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 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.
Iain |
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Phil Allison
Guest
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| Posted:
Fri Jan 14, 2005 1:29 am Post subject:
Re: Pots for Fixed Bias |
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"Iain M Churches"
"Phil Allison"
| 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.
|
** Carbon pots have not had any quantum leaps - except of the backflip
variety. Large size, high quality carbon pots will cost more and be harder
to find than 2 watt WW ones.
| Quote: | I realise that a bias failure means disaster.
|
** 25 kohm WW pots might be obscure, but 10 kohm ones are common enough -
just scale the fixed resistor/s to suit the voltage adjustment range needed.
............... Phil |
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Ian Iveson
Guest
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| Posted:
Fri Jan 14, 2005 1:30 am Post subject:
Re: Pots for Fixed Bias |
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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 |
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Iain M Churches
Guest
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| Posted:
Fri Jan 14, 2005 1:32 am Post subject:
Re: Pots for Fixed Bias |
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"Phil Allison" <philallison@tpg.com.au> wrote in message
news:34o41fF4d30bvU1@individual.net...
| Quote: |
"Iain M Churches"
"Phil Allison"
** 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.
** Carbon pots have not had any quantum leaps - except of the backflip
variety. Large size, high quality carbon pots will cost more and be
harder to find than 2 watt WW ones.
I realise that a bias failure means disaster.
** 25 kohm WW pots might be obscure, but 10 kohm ones are common
nough - just scale the fixed resistor/s to suit the voltage adjustment
range needed.
|
I can get the pots, I have access to a large broadcast workshop/service
dept.
But do they need to be 2W WW?
Iain |
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Iain M Churches
Guest
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| Posted:
Fri Jan 14, 2005 1:38 am Post subject:
Re: Pots for Fixed Bias |
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"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:pTAFd.141358$Z7.6416@fe2.news.blueyonder.co.uk...
| Quote: | I asked the same question not long ago.
|
Great minds? :-)
| Quote: |
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.
|
Yes. The schematic has both "top" and "bottom" resistors.
| Quote: | 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.
|
Can you be a little more explicit?
Cheers
Iain |
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Sander deWaal
Guest
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| Posted:
Fri Jan 14, 2005 1:39 am Post subject:
Re: Pots for Fixed Bias |
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"Iain M Churches" <tael@kolumbus.fi> said:
| Quote: |
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:34o41fF4d30bvU1@individual.net...
"Iain M Churches"
"Phil Allison"
** 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.
|
| Quote: | 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.
|
| Quote: | ** Carbon pots have not had any quantum leaps - except of the backflip
variety. Large size, high quality carbon pots will cost more and be
harder to find than 2 watt WW ones.
I realise that a bias failure means disaster.
|
| Quote: | ** 25 kohm WW pots might be obscure, but 10 kohm ones are common
nough - just scale the fixed resistor/s to suit the voltage adjustment
range needed.
|
| Quote: | I can get the pots, I have access to a large broadcast workshop/service
dept.
But do they need to be 2W WW?
|
Like Phil said, it is hard to find ones with less than a 2 watt
rating.
It is just the nature of wirewound pots.
You can use 25 watts types if you like, and fit them at the back -
SUV style :-)
--
Sander de Waal
" SOA of a KT88? Sufficient. " |
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Sander deWaal
Guest
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| Posted:
Fri Jan 14, 2005 1:45 am Post subject:
Re: Pots for Fixed Bias |
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"Iain M Churches" <tael@kolumbus.fi> said:
| Quote: | Yes. The schematic has both "top" and "bottom" resistors.
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.
Can you be a little more explicit?
|
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
" SOA of a KT88? Sufficient. " |
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Phil Allison
Guest
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| Posted:
Fri Jan 14, 2005 2:04 am Post subject:
Re: Pots for Fixed Bias |
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|
"Iain M Churches"
| Quote: | "Phil Allison"
** 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.
** Carbon pots have not had any quantum leaps - except of the backflip
variety. Large size, high quality carbon pots will cost more and be
harder to find than 2 watt WW ones.
I realise that a bias failure means disaster.
** 25 kohm WW pots might be obscure, but 10 kohm ones are common
ough - just scale the fixed resistor/s to suit the voltage adjustment
range needed.
I can get the pots, I have access to a large broadcast workshop/service
dept.
But do they need to be 2W WW?
|
** You just repeated your question which has been answered.
Use multiturn WW pots of you like - easy to get them in 20 kohms.
................ Phil |
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Ian Iveson
Guest
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| Posted:
Fri Jan 14, 2005 2:16 am Post subject:
Re: Pots for Fixed Bias |
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"Iain M Churches" <tael@kolumbus.fi> wrote
| Quote: | Can you be a little more explicit?
|
http://www.firebottle.com/ampage/schems/mpwr100.gif
If your adjustment circuit is not bypassed by a cap, you need to
watch you don't unintentionally change the input impedance to the
grid, and hence the frequency response of the stage. If it is
bypassed, and you use smaller value resistors, you may need a larger
bypass cap to suit.
cheers, Ian |
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Guest
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| Posted:
Fri Jan 14, 2005 2:32 am Post subject:
Re: Pots for Fixed Bias |
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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. |
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Pooh Bear
Guest
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| Posted:
Fri Jan 14, 2005 2:35 am Post subject:
Re: Pots for Fixed Bias |
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Iain M Churches wrote:
| Quote: | "Phil Allison" <philallison@tpg.com.au> wrote in message
news:34o1bkF4cqj62U1@individual.net...
"Iain M Churches"
Mick
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.
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 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. The wiper tends to
'jump' on wirewounds IMHO since there isn't a flat surface for it to travel on.
They certainly never feel very nice.
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.
Graham |
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