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Claus Misfeldt
 
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Default Is plate dissipation really plate current x plate voltage?

I'm about to bias one of my guitar amps. By now I used the "crossover notch"
method (...and it worked for me), but as this is said to be inaccurate I'm
willing to improve on that.

I've read

http://www.tonelizzard.com
http://www.aikenamps.com
http://aga.rru.com

on this issue, but still have a question. The reason on biasing output tubes
is afaik to operate the tubes in the appropriate range and not drive them
beyond their limits. The important parameter is the max. plate dissipation
of the tube which must not be exceeded. I've read that in a class AB amp
the maximum value of the plate dissipation is "normaly" reached at an
output of about 1/2 of the nominal wattage. So it is considered to be a
good rule of thumb to adjust the idle current so that about 70% of the max.
allowed dissipation is reached.

But: Is it really correct to multiply plate current and plate voltage to
determin that bias point?

In my case the amp has:
- dual 6V6GT tubes
- 305V Plate voltage
- 26mA Plate current per tube
- a kathode resistor of 270 Ohm

The date sheet for the 6V6GT says: max plate diss. 12W; idle current 70mA at
a plate voltage of 285V in a class AB amp (Bias at -19V). Now 70mA x 285V
are about 20W!?!

What I read in my amp is 26mA / Tube at -15V bias (26mA x 305V = 18,3W)

That confuses me...
Claus



--
Claus Misfeldt
www.frozenfrog.de

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Fabio Berutti
 
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The data You saw are correct: they are only referred to A COUPLE of tubes as
it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V, the
tube works actually with (305-19)=286V
- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for both
tubes. In this case, the current cannot be 26 mA per tube: being the cathode
bias given by V=RxI it follows 19=270xI - I=70.37 mA per pair as per tube
D/S.
Plate dissipation is consequently (70/2)*286=10W, which is correctly
conservative for guitar use.
As per Your basic question, dissipated power can be normally calculated as
V*I at the "base" point (zero signal) because the two halves of the signal
tend to compensate each other thanks to plate thermal inertia (the tube
heats up during the positive half-cycle and cools down during the negative
one). This is true for A1 and AB1 class as normally used for audio, while
for high-power tubes operating in B class or after the onset of grid current
(AB2 class) things are a bit more complicated and the mean value of the
integral of I*dV during the whole operating cycle should be calculated.

Ciao

Fabio


"Claus Misfeldt" ha scritto nel messaggio
...
I'm about to bias one of my guitar amps. By now I used the "crossover

notch"
method (...and it worked for me), but as this is said to be inaccurate I'm
willing to improve on that.

I've read

http://www.tonelizzard.com
http://www.aikenamps.com
http://aga.rru.com

on this issue, but still have a question. The reason on biasing output

tubes
is afaik to operate the tubes in the appropriate range and not drive them
beyond their limits. The important parameter is the max. plate dissipation
of the tube which must not be exceeded. I've read that in a class AB amp
the maximum value of the plate dissipation is "normaly" reached at an
output of about 1/2 of the nominal wattage. So it is considered to be a
good rule of thumb to adjust the idle current so that about 70% of the

max.
allowed dissipation is reached.

But: Is it really correct to multiply plate current and plate voltage to
determin that bias point?

In my case the amp has:
- dual 6V6GT tubes
- 305V Plate voltage
- 26mA Plate current per tube
- a kathode resistor of 270 Ohm

The date sheet for the 6V6GT says: max plate diss. 12W; idle current 70mA

at
a plate voltage of 285V in a class AB amp (Bias at -19V). Now 70mA x 285V
are about 20W!?!

What I read in my amp is 26mA / Tube at -15V bias (26mA x 305V = 18,3W)

That confuses me...
Claus



--
Claus Misfeldt
www.frozenfrog.de



  #3   Report Post  
Claus Misfeldt
 
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On Dienstag, 31. August 2004 11:22 Fabio Berutti wrote:

The data You saw are correct: they are only referred to A COUPLE of tubes
as it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V,
the tube works actually with (305-19)=286V


....agreed

- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for both
tubes. In this case, the current cannot be 26 mA per tube: being the
cathode bias given by V=RxI it follows 19=270xI - I=70.37 mA per pair as
per tube D/S.


-19V Bias is from the data sheet. I measured by two ways:
1.) 267 Ohm (measured) / 14,9V (mesured drop)= 56mA; 56/2=28mA
2.) Resistance of prim. leads of OT: 109 Ohm; Voltage drop on that, when
idle 2,78V = I= 26mA

Would you agree in saying, that this current seems to be a bit too low?

Regards
Claus

--
Claus Misfeldt
www.frozenfrog.de

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Patrick Turner
 
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Claus Misfeldt wrote:

I'm about to bias one of my guitar amps. By now I used the "crossover notch"
method (...and it worked for me), but as this is said to be inaccurate I'm
willing to improve on that.

I've read

http://www.tonelizzard.com
http://www.aikenamps.com
http://aga.rru.com

on this issue, but still have a question. The reason on biasing output tubes
is afaik to operate the tubes in the appropriate range and not drive them
beyond their limits. The important parameter is the max. plate dissipation
of the tube which must not be exceeded. I've read that in a class AB amp
the maximum value of the plate dissipation is "normaly" reached at an
output of about 1/2 of the nominal wattage. So it is considered to be a
good rule of thumb to adjust the idle current so that about 70% of the max.
allowed dissipation is reached.

But: Is it really correct to multiply plate current and plate voltage to
determin that bias point?

In my case the amp has:
- dual 6V6GT tubes
- 305V Plate voltage
- 26mA Plate current per tube
- a kathode resistor of 270 Ohm

The date sheet for the 6V6GT says: max plate diss. 12W; idle current 70mA at
a plate voltage of 285V in a class AB amp (Bias at -19V). Now 70mA x 285V
are about 20W!?!

What I read in my amp is 26mA / Tube at -15V bias (26mA x 305V = 18,3W)

That confuses me...
Claus


The idle dissipation in a tube is the anode to cathode voltage x anode current,
PLUS the screen to cathode voltage x screen current, which is usually
close to cathode current x anode to cathode voltage.

So for 300v a-k, the max total Ia and Ig2 idle current you can use is 12 / 300
= 40 mA

For guitar amp use, 25 mA is all I would use.
The only time the max allowable idle current would be
where the amp is intended for real class A use, with a high
value RL, and with 24 watts of idle dissipation for two tubes, all you get
at 40% maximum efficiency is 9.6 watts of pure class A.
That's never enough for musos, mostly only happy
when there is 30 watts from two such tubes when
sending a grossly overloaded signal with 50% THD to a forgiving speaker.

I have a Fisher amp here with 6BQ5 outputs rated at 12 watt each,
and with 350v for Ea and Eg2, I have to trim the idle current
down to 34 mA max, and this is for a hi-fi amp
which does not have to withstand the abuse and lower value load to get the
higher power.
If Ea was 250v, then Ik could be only 48 mA max for class A,
and lower for AB.

Patrick Turner.


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Patrick Turner
 
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Claus Misfeldt wrote:

On Dienstag, 31. August 2004 11:22 Fabio Berutti wrote:

The data You saw are correct: they are only referred to A COUPLE of tubes
as it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V,
the tube works actually with (305-19)=286V


...agreed

- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for both
tubes. In this case, the current cannot be 26 mA per tube: being the
cathode bias given by V=RxI it follows 19=270xI - I=70.37 mA per pair as
per tube D/S.


-19V Bias is from the data sheet. I measured by two ways:
1.) 267 Ohm (measured) / 14,9V (mesured drop)= 56mA; 56/2=28mA
2.) Resistance of prim. leads of OT: 109 Ohm; Voltage drop on that, when
idle 2,78V = I= 26mA

Would you agree in saying, that this current seems to be a bit too low?


28mA would be about right for Ik
at Ea = 285v.

Patrick Turner.



Regards
Claus

--
Claus Misfeldt
www.frozenfrog.de




  #6   Report Post  
Claus Misfeldt
 
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On Dienstag, 31. August 2004 15:50 Patrick Turner wrote:



Claus Misfeldt wrote:

On Dienstag, 31. August 2004 11:22 Fabio Berutti wrote:

The data You saw are correct: they are only referred to A COUPLE of
tubes as it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V,
the tube works actually with (305-19)=286V


...agreed

- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for
both tubes. In this case, the current cannot be 26 mA per tube: being
the cathode bias given by V=RxI it follows 19=270xI - I=70.37 mA per
pair as per tube D/S.


-19V Bias is from the data sheet. I measured by two ways:
1.) 267 Ohm (measured) / 14,9V (mesured drop)= 56mA; 56/2=28mA
2.) Resistance of prim. leads of OT: 109 Ohm; Voltage drop on that, when
idle 2,78V = I= 26mA

Would you agree in saying, that this current seems to be a bit too low?


28mA would be about right for Ik
at Ea = 285v.


....but by now I only get about 6W out of a pair of 6V6 while it should be
about 14W. Seems there is still something wrong.

Claus

--
Claus Misfeldt
www.frozenfrog.de

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Patrick Turner
 
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Claus Misfeldt wrote:

On Dienstag, 31. August 2004 15:50 Patrick Turner wrote:



Claus Misfeldt wrote:

On Dienstag, 31. August 2004 11:22 Fabio Berutti wrote:

The data You saw are correct: they are only referred to A COUPLE of
tubes as it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V,
the tube works actually with (305-19)=286V

...agreed

- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for
both tubes. In this case, the current cannot be 26 mA per tube: being
the cathode bias given by V=RxI it follows 19=270xI - I=70.37 mA per
pair as per tube D/S.

-19V Bias is from the data sheet. I measured by two ways:
1.) 267 Ohm (measured) / 14,9V (mesured drop)= 56mA; 56/2=28mA
2.) Resistance of prim. leads of OT: 109 Ohm; Voltage drop on that, when
idle 2,78V = I= 26mA

Would you agree in saying, that this current seems to be a bit too low?


28mA would be about right for Ik
at Ea = 285v.


...but by now I only get about 6W out of a pair of 6V6 while it should be
about 14W. Seems there is still something wrong.


You need the powers of observation from experience.
In time that comes along.

Patrick Turner.



Claus

--
Claus Misfeldt
www.frozenfrog.de


  #8   Report Post  
Fabio Berutti
 
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Yes indeed, I'd say that the tubes You've got in there are a bit "cooked".
I don't like very much shared cathode R, because if one tube goes low and
draws less current the overall bias decreases, the other tube is overloaded
and wears out sooner. If You want to tweak a bit the amp, provide separate
Rs with separate bypass capacitors as recommended by the manufacturers for
many high transconductance tubes. Being a guitar amp a 47uF/63V should be
OK as bypass cap.
If You want new 6V6-GTs, I built a HiFi amp using Russian 6P6S (the black
ones made by Reflektor). They can be found for about 2$ on the Net and I
found them as good as any other, but if You need a matched couple You need
to get at least some 5 or 6 of them and sort'em out... tolerances are not
according to German standards, unfortunately.
Anyway, it's quite an easy tube to get. I saw JJ-Tesla recently launched
its "new" 6V6, must be good.

Ciao

Fabio


"Claus Misfeldt" ha scritto nel messaggio
...
On Dienstag, 31. August 2004 11:22 Fabio Berutti wrote:

The data You saw are correct: they are only referred to A COUPLE of

tubes
as it was common practice for PP pairs.
As per the dissipation:
- first, the tube only sees V difference among cathode and anode:
therefore, if the feed ing voltage is 305V and the cathode bias is 19V,
the tube works actually with (305-19)=286V


...agreed

- second, when fitted with cathode bias the system is already
self-regulating. It seems Your amp has a common cathode resistor for

both
tubes. In this case, the current cannot be 26 mA per tube: being the
cathode bias given by V=RxI it follows 19=270xI - I=70.37 mA per pair

as
per tube D/S.


-19V Bias is from the data sheet. I measured by two ways:
1.) 267 Ohm (measured) / 14,9V (mesured drop)= 56mA; 56/2=28mA
2.) Resistance of prim. leads of OT: 109 Ohm; Voltage drop on that, when
idle 2,78V = I= 26mA

Would you agree in saying, that this current seems to be a bit too low?

Regards
Claus

--
Claus Misfeldt
www.frozenfrog.de



  #9   Report Post  
Sander deWaal
 
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"Fabio Berutti" said:

If You want new 6V6-GTs, I built a HiFi amp using Russian 6P6S (the black
ones made by Reflektor). They can be found for about 2$ on the Net and I
found them as good as any other, but if You need a matched couple You need
to get at least some 5 or 6 of them and sort'em out... tolerances are not
according to German standards, unfortunately.


I have a box full of them, and they're good for almost *everything*.
They sound good, too.

I'm happy to have a tube tester, but you can also match your tubes in
the amp itself. That's only a statical match, though, and you'll need
to insert separate cathode resistors as Fabio explained.

--
Sander deWaal
"SOA of a KT88? Sufficient."
  #10   Report Post  
Fabio Berutti
 
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Sorry, Claus, I forgot to specify that Mr. Turner is right when he says that
it would be better to use a lower dissipation ( - a higher, more negative)
bias in a guitar amp. In a push-pull pair of valves operating in AB1 class
You actually get MORE power out if You reduce the idle current (increase the
bias), because efficiency increases. The counterpart is an increase in
cross-over distortion, because the "overlap" between "push" and "pull"
phases becomes smaller and the output transformer has its troubles to tie
the 2 halves together without leaving a "notch" in between. This is why in
HiFi use the PP pairs work closer to class A, while Marshall amps dedicated
to heavy metal headbangers use nearly class B biasing.
A further advantage of a higher bias is that the tube runs cooler and last
longer.
Anyway, I'd plug in 2 new tubes, 'cause 26 mA @ -14V bias and 290V are a bit
low
By the way, are the tubes connected in UL or in pentode? And if they're
pentode, how high is the G2 voltage? If it is lower than specified it will
reduce dramatically the tube efficiency! Maybe there's only a higher R in
the G2 supply line...

Ciao again

Fabio


"Claus Misfeldt" ha scritto nel messaggio
...
I'm about to bias one of my guitar amps. By now I used the "crossover

notch"
method (...and it worked for me), but as this is said to be inaccurate I'm
willing to improve on that.

I've read

http://www.tonelizzard.com
http://www.aikenamps.com
http://aga.rru.com

on this issue, but still have a question. The reason on biasing output

tubes
is afaik to operate the tubes in the appropriate range and not drive them
beyond their limits. The important parameter is the max. plate dissipation
of the tube which must not be exceeded. I've read that in a class AB amp
the maximum value of the plate dissipation is "normaly" reached at an
output of about 1/2 of the nominal wattage. So it is considered to be a
good rule of thumb to adjust the idle current so that about 70% of the

max.
allowed dissipation is reached.

But: Is it really correct to multiply plate current and plate voltage to
determin that bias point?

In my case the amp has:
- dual 6V6GT tubes
- 305V Plate voltage
- 26mA Plate current per tube
- a kathode resistor of 270 Ohm

The date sheet for the 6V6GT says: max plate diss. 12W; idle current 70mA

at
a plate voltage of 285V in a class AB amp (Bias at -19V). Now 70mA x 285V
are about 20W!?!

What I read in my amp is 26mA / Tube at -15V bias (26mA x 305V = 18,3W)

That confuses me...
Claus



--
Claus Misfeldt
www.frozenfrog.de





  #11   Report Post  
Claus Misfeldt
 
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On Dienstag, 31. August 2004 18:05 Patrick Turner wrote:


You need the powers of observation from experience.
In time that comes along.


I know that I don't know - so I'm glad there are powerful people in that
sence who are willing to share their experience with others...

Thanks to them for their postings
Claus

--
Claus Misfeldt
www.frozenfrog.de

  #12   Report Post  
Patrick Turner
 
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Claus Misfeldt wrote:

On Dienstag, 31. August 2004 18:05 Patrick Turner wrote:


You need the powers of observation from experience.
In time that comes along.


I know that I don't know - so I'm glad there are powerful people in that
sence who are willing to share their experience with others...

Thanks to them for their postings
Claus


Well if we know we don't know it all, we start
with hope....

-
Claus Misfeldt
www.frozenfrog.de


Standing beside a triode will unfreeze a frozen frog,
but if you kiss it you won't turn into a handsome prince ;-)

Patrick Turner.


  #13   Report Post  
Claus Misfeldt
 
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On Mittwoch, 1. September 2004 11:15 Patrick Turner wrote:



Claus Misfeldt wrote:

On Dienstag, 31. August 2004 18:05 Patrick Turner wrote:


You need the powers of observation from experience.
In time that comes along.


I know that I don't know - so I'm glad there are powerful people in that
sence who are willing to share their experience with others...

Thanks to them for their postings
Claus


Well if we know we don't know it all, we start
with hope....


....I hope you hope :-)

Claus

--
Claus Misfeldt
www.frozenfrog.de

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