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at
 
Posts: n/a
Default Using power triode or power pentode wired as a triode as a split-load phase splitter tube?

Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio about
his concerns about the 250V driving voltage, as this amp is being designed
by a competent designer, I have trust that the desing will be workable, and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this amp,
because they're very personal and related to the planned use of the amp, and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info. Something
that caught my attention (especially as my amp will be using power triodes
as output tubes), was the mentioning that a power triode tube or a power
pentode wired as triode could be used as the split-load phase splitter tube
to get lower output impedance. Now, I'm asking you experts, is this idea
sensible, and beneficial, as the original author is claiming it might be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase splitters
(in terms of unloaded voltage balance), it has two serious difficulties,
namely that the output impedance of the two voltages is radically different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode. the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor. i.e., low
z out in the K circuit and high z out of the P circuit. If the high Z signal
is looking at the starving mouth a power triode grid represents, you can
easily imagine that it will be really easy to load it down. A typical power
triode might have 15pf of grid to plate C, plus the socket stray (5pf),
times the Miller number (mu times 20pf times the driver Miller, which might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not using
a cathode follower...). All in all, it could typically shift from 60pf at 0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load
down by 8khz at full swing. The bottom tube will not load down until 80khz
or there-abouts. Additionally, you will need about 400 volts B+ to swing 40
top and bottom, not to mention that the tube in front will have to provide
40 volts to the phase inverter! Weird! This is why Williamson used a combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits phase
with near perfect balance, but it drives the much easier grids of a small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn
have the same output z and easily drive pp grids. One further consideration
that few have considered is using a power triode as a P/K inverter. An EL-34
wired in triode, with 4k P+K resistors could have an output z low enough to
avoid problems in the audio range. The same 400v B+ would still apply...
I'm not pretending I did understand all the technical factors involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't needed,
but I doubt it, based on the high price tags associated with quality phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at


  #2   Report Post  
Fabio Berutti
 
Posts: n/a
Default

I'll suggest You a couple of books:

- Brian Jones, "Valve Amplifiers"
- the RCA-Radiotron Designer's Handbook (only used, printed in the 50s,
cost a lot but it's worth every penny)
- the books with the schematics developed by Mullard or General Electric
Co. of England (GEC)

You can get them from Antique Electronics Supply or from Amazon or from many
sources.

Ciao

Fabio

"at" ha scritto nel messaggio
...
Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio about
his concerns about the 250V driving voltage, as this amp is being designed
by a competent designer, I have trust that the desing will be workable,

and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this amp,
because they're very personal and related to the planned use of the amp,

and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info. Something
that caught my attention (especially as my amp will be using power triodes
as output tubes), was the mentioning that a power triode tube or a power
pentode wired as triode could be used as the split-load phase splitter

tube
to get lower output impedance. Now, I'm asking you experts, is this idea
sensible, and beneficial, as the original author is claiming it might be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase

splitters
(in terms of unloaded voltage balance), it has two serious difficulties,
namely that the output impedance of the two voltages is radically

different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode. the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor. i.e.,

low
z out in the K circuit and high z out of the P circuit. If the high Z

signal
is looking at the starving mouth a power triode grid represents, you can
easily imagine that it will be really easy to load it down. A typical

power
triode might have 15pf of grid to plate C, plus the socket stray (5pf),
times the Miller number (mu times 20pf times the driver Miller, which

might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not

using
a cathode follower...). All in all, it could typically shift from 60pf at

0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load
down by 8khz at full swing. The bottom tube will not load down until 80khz
or there-abouts. Additionally, you will need about 400 volts B+ to swing

40
top and bottom, not to mention that the tube in front will have to provide
40 volts to the phase inverter! Weird! This is why Williamson used a combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits phase
with near perfect balance, but it drives the much easier grids of a small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn
have the same output z and easily drive pp grids. One further

consideration
that few have considered is using a power triode as a P/K inverter. An

EL-34
wired in triode, with 4k P+K resistors could have an output z low enough

to
avoid problems in the audio range. The same 400v B+ would still apply...
I'm not pretending I did understand all the technical factors involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't

needed,
but I doubt it, based on the high price tags associated with quality phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at




  #3   Report Post  
at
 
Posts: n/a
Default


"Fabio Berutti" wrote in message
...
I'll suggest You a couple of books:

- Brian Jones, "Valve Amplifiers"
- the RCA-Radiotron Designer's Handbook (only used, printed in the 50s,
cost a lot but it's worth every penny)
- the books with the schematics developed by Mullard or General Electric
Co. of England (GEC)

You can get them from Antique Electronics Supply or from Amazon or from

many
sources.

Ciao

Fabio


Thanks, I'm gonna buy as many books as I can find.

But reading them takes many months and years, and right now I have this one
question of importance to my present situation, what kind of phase splitter
tube to use? So, can anybody give me a simple answer, or, must I wait until
I already have the answer, before I can finish this amp desing, and start to
build it?
Your answers are greatly appreciated.

Thanks.

-at


  #4   Report Post  
Fabio Berutti
 
Posts: n/a
Default

More to the point:

- I'm not concerned about the 250V p-p required to drive the 300B, I just
say it's not easy to provide them (BTW 180V can be enough)
- the cathodyne or concertina or distributed load phase splitter is OK for
driving pentodes, but You'd need 800V for driving a 300B out of it
- the interstage transformer is very difficult to wind correctly, often You
end up with a narrow bandwidth and with phase shifts leading to FB
instability. See the GEC design for a DA42 125W amp, if You want to use an
IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray
inductance.
- You can use the classic scheme of the Williamson amp
(gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a
stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain
RC coupling is the more reliable in terms of phase shift and does not
require using transformers made of Unobtainium.

A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about
30W at the onset of grid current with a low distortion even using no
feedback. At the good end You'll have one of the most beautiful amps that
ever played music over the Finnish Tundra...
As per the power supply You'll need 2x 5U4G to feed the power tubes with the
320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid
spinal injuries while moving it around.
A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with
no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a
pentode is needed to provide the extra gain, see the classic Leak schemes).
I keep on suggesting Octal types because they're MUCH easier to weld in.
TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher
B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and
lots of.
I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm
designing a similar unit with Svetlana's SV811, but I use a long-tail
splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way
to avoid FB while keeping about 1% 2nd h. and getting some 50W.

Ciao and.. mail us a picture of the working Beast.

Fabio

"at" ha scritto nel messaggio
...
Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio about
his concerns about the 250V driving voltage, as this amp is being designed
by a competent designer, I have trust that the desing will be workable,

and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this amp,
because they're very personal and related to the planned use of the amp,

and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info. Something
that caught my attention (especially as my amp will be using power triodes
as output tubes), was the mentioning that a power triode tube or a power
pentode wired as triode could be used as the split-load phase splitter

tube
to get lower output impedance. Now, I'm asking you experts, is this idea
sensible, and beneficial, as the original author is claiming it might be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase

splitters
(in terms of unloaded voltage balance), it has two serious difficulties,
namely that the output impedance of the two voltages is radically

different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode. the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor. i.e.,

low
z out in the K circuit and high z out of the P circuit. If the high Z

signal
is looking at the starving mouth a power triode grid represents, you can
easily imagine that it will be really easy to load it down. A typical

power
triode might have 15pf of grid to plate C, plus the socket stray (5pf),
times the Miller number (mu times 20pf times the driver Miller, which

might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not

using
a cathode follower...). All in all, it could typically shift from 60pf at

0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load
down by 8khz at full swing. The bottom tube will not load down until 80khz
or there-abouts. Additionally, you will need about 400 volts B+ to swing

40
top and bottom, not to mention that the tube in front will have to provide
40 volts to the phase inverter! Weird! This is why Williamson used a combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits phase
with near perfect balance, but it drives the much easier grids of a small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn
have the same output z and easily drive pp grids. One further

consideration
that few have considered is using a power triode as a P/K inverter. An

EL-34
wired in triode, with 4k P+K resistors could have an output z low enough

to
avoid problems in the audio range. The same 400v B+ would still apply...
I'm not pretending I did understand all the technical factors involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't

needed,
but I doubt it, based on the high price tags associated with quality phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at




  #5   Report Post  
Fabio Berutti
 
Posts: n/a
Default

In 2 words, I prefer the long-tail. I built both cathodynes and longtails,
but I personally hear that the cathodyne has a bit less vivid sound. The
longtail is more brilliant (IMHO), this is why I chose it for my will-be
flagship amp (6SL7 longtail + 6BL7 driver +SV811-10 power).
The same arrangement can easily drive a 300B. I understood You already had
chosen the cathodyne.
To make it even shorter: a 300B PP will ALMOST ALWAYS sing like the angels,
unless You make a gross mistake somewhere. Differences among components and
circuits (as long as they are good) are only a matter of taste.

Ciao

Fabio

"at" ha scritto nel messaggio
...

"Fabio Berutti" wrote in message
...
I'll suggest You a couple of books:

- Brian Jones, "Valve Amplifiers"
- the RCA-Radiotron Designer's Handbook (only used, printed in the 50s,
cost a lot but it's worth every penny)
- the books with the schematics developed by Mullard or General

Electric
Co. of England (GEC)

You can get them from Antique Electronics Supply or from Amazon or from

many
sources.

Ciao

Fabio


Thanks, I'm gonna buy as many books as I can find.

But reading them takes many months and years, and right now I have this

one
question of importance to my present situation, what kind of phase

splitter
tube to use? So, can anybody give me a simple answer, or, must I wait

until
I already have the answer, before I can finish this amp desing, and start

to
build it?
Your answers are greatly appreciated.

Thanks.

-at






  #6   Report Post  
Patrick Turner
 
Posts: n/a
Default



at wrote:

"Fabio Berutti" wrote in message
...
I'll suggest You a couple of books:

- Brian Jones, "Valve Amplifiers"
- the RCA-Radiotron Designer's Handbook (only used, printed in the 50s,
cost a lot but it's worth every penny)
- the books with the schematics developed by Mullard or General Electric
Co. of England (GEC)

You can get them from Antique Electronics Supply or from Amazon or from

many
sources.

Ciao

Fabio


Thanks, I'm gonna buy as many books as I can find.

But reading them takes many months and years, and right now I have this one
question of importance to my present situation, what kind of phase splitter
tube to use? So, can anybody give me a simple answer, or, must I wait until
I already have the answer, before I can finish this amp desing, and start to
build it?
Your answers are greatly appreciated.

Thanks.

-at


To learn how to design amps which make smoke, stray RF signals, lousy sound,
and which blow fuses and cost a fortune to repair takes 3 days.

The troubles you get are reduced with time spent reading, and applying what is
learnt
as you learn it, so when building an amp stage, you experience the electronic
principles along the way.

There is no easy fast instant route to wisdom or experience.

Patrick Turner.

  #7   Report Post  
Mark
 
Posts: n/a
Default

"Fabio Berutti" wrote in message ...
More to the point:

- I'm not concerned about the 250V p-p required to drive the 300B, I just
say it's not easy to provide them (BTW 180V can be enough)
- the cathodyne or concertina or distributed load phase splitter is OK for
driving pentodes, but You'd need 800V for driving a 300B out of it
- the interstage transformer is very difficult to wind correctly, often You
end up with a narrow bandwidth and with phase shifts leading to FB
instability. See the GEC design for a DA42 125W amp, if You want to use an
IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray
inductance.
- You can use the classic scheme of the Williamson amp
(gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a
stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain
RC coupling is the more reliable in terms of phase shift and does not
require using transformers made of Unobtainium.

A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about
30W at the onset of grid current with a low distortion even using no
feedback. At the good end You'll have one of the most beautiful amps that
ever played music over the Finnish Tundra...
As per the power supply You'll need 2x 5U4G to feed the power tubes with the
320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid
spinal injuries while moving it around.
A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with
no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a
pentode is needed to provide the extra gain, see the classic Leak schemes).
I keep on suggesting Octal types because they're MUCH easier to weld in.
TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher
B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and
lots of.
I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm
designing a similar unit with Svetlana's SV811, but I use a long-tail
splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way
to avoid FB while keeping about 1% 2nd h. and getting some 50W.

Ciao and.. mail us a picture of the working Beast.

Fabio


I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You
may be right with 30 watts. Since I have one source of information,
that is what I will have to use a guide for B+ and power output. I
would think that running them at 400 volts would be just as good with
about 35 watts output and longer life. The 5 watts will not be missed.

I agree with you about no feedback. I prefer to not use it.
Calculating it is very difficult because of the number of variables to
getting the right figures. It is easier to use an amplifier and put
feedback in and then determine the rest. Feedback is mainly used to
lower the effective plate resistance thus higher damping. Triodes do
not need feedback because they have high damping (low plate
resistance).

What B+ do you use and the resistor values for the cathode and plate
in the 6BL7/6BX7 drivers for a 300B or other similiar high drive
outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found
they will output the voltage needed for high drive outputs, 70 volts,
but are close to the maximum output voltage with 300 volts as B+,
51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm
load. Driver transformers will be costly for quality. The 6C5 works a
bit better than a 6SN7 because it will output a bit more voltage
before distortion sets in with the same load resistors.

I do hope to build myself a p-p 300B stereo amplifier when I can
afford it. I will use the 5AR4s to have B+ delay and fixed bias will
be used on the outputs.

"at" ha scritto nel messaggio
...
Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio about
his concerns about the 250V driving voltage, as this amp is being designed
by a competent designer, I have trust that the desing will be workable,

and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this amp,
because they're very personal and related to the planned use of the amp,

and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info. Something
that caught my attention (especially as my amp will be using power triodes
as output tubes), was the mentioning that a power triode tube or a power
pentode wired as triode could be used as the split-load phase splitter

tube
to get lower output impedance. Now, I'm asking you experts, is this idea
sensible, and beneficial, as the original author is claiming it might be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase

splitters
(in terms of unloaded voltage balance), it has two serious difficulties,
namely that the output impedance of the two voltages is radically

different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode. the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor. i.e.,

low
z out in the K circuit and high z out of the P circuit. If the high Z

signal
is looking at the starving mouth a power triode grid represents, you can
easily imagine that it will be really easy to load it down. A typical

power
triode might have 15pf of grid to plate C, plus the socket stray (5pf),
times the Miller number (mu times 20pf times the driver Miller, which

might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not

using
a cathode follower...). All in all, it could typically shift from 60pf at

0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load
down by 8khz at full swing. The bottom tube will not load down until 80khz
or there-abouts. Additionally, you will need about 400 volts B+ to swing

40
top and bottom, not to mention that the tube in front will have to provide
40 volts to the phase inverter! Weird! This is why Williamson used a combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits phase
with near perfect balance, but it drives the much easier grids of a small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn
have the same output z and easily drive pp grids. One further

consideration
that few have considered is using a power triode as a P/K inverter. An

EL-34
wired in triode, with 4k P+K resistors could have an output z low enough

to
avoid problems in the audio range. The same 400v B+ would still apply...
I'm not pretending I did understand all the technical factors involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't

needed,
but I doubt it, based on the high price tags associated with quality phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at


  #8   Report Post  
Fabio Berutti
 
Posts: n/a
Default

In fact in my project the 6BL7s should be choke-loaded and work at about
300V, 30 mA. Vout can be up to 500Vpp. Nevertheless, due to choke stray
inductance, I'm considering plain RC loading. A separated PS for the
driving stage is already provided, and it can go as high as 400V. I'll
fiddle a bit with Tubecad. I suppose a 10-15K load is OK, plate resistance
is only1K7 for the 6BL7.

Ciao

Fabio


"Mark" ha scritto nel messaggio
om...
"Fabio Berutti" wrote in message

...
More to the point:

- I'm not concerned about the 250V p-p required to drive the 300B, I

just
say it's not easy to provide them (BTW 180V can be enough)
- the cathodyne or concertina or distributed load phase splitter is OK

for
driving pentodes, but You'd need 800V for driving a 300B out of it
- the interstage transformer is very difficult to wind correctly, often

You
end up with a narrow bandwidth and with phase shifts leading to FB
instability. See the GEC design for a DA42 125W amp, if You want to use

an
IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray
inductance.
- You can use the classic scheme of the Williamson amp
(gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a
stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84).

Plain
RC coupling is the more reliable in terms of phase shift and does not
require using transformers made of Unobtainium.

A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide

about
30W at the onset of grid current with a low distortion even using no
feedback. At the good end You'll have one of the most beautiful amps

that
ever played music over the Finnish Tundra...
As per the power supply You'll need 2x 5U4G to feed the power tubes with

the
320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid
spinal injuries while moving it around.
A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity

with
no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a
pentode is needed to provide the extra gain, see the classic Leak

schemes).
I keep on suggesting Octal types because they're MUCH easier to weld in.
TRhe 6SN7 can drive a 300B, but it would be better using the GT type

(higher
B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap

and
lots of.
I'd keep with the "no-feedback" philosophy, to make a PP with triodes.

I'm
designing a similar unit with Svetlana's SV811, but I use a long-tail
splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this

way
to avoid FB while keeping about 1% 2nd h. and getting some 50W.

Ciao and.. mail us a picture of the working Beast.

Fabio


I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You
may be right with 30 watts. Since I have one source of information,
that is what I will have to use a guide for B+ and power output. I
would think that running them at 400 volts would be just as good with
about 35 watts output and longer life. The 5 watts will not be missed.

I agree with you about no feedback. I prefer to not use it.
Calculating it is very difficult because of the number of variables to
getting the right figures. It is easier to use an amplifier and put
feedback in and then determine the rest. Feedback is mainly used to
lower the effective plate resistance thus higher damping. Triodes do
not need feedback because they have high damping (low plate
resistance).

What B+ do you use and the resistor values for the cathode and plate
in the 6BL7/6BX7 drivers for a 300B or other similiar high drive
outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found
they will output the voltage needed for high drive outputs, 70 volts,
but are close to the maximum output voltage with 300 volts as B+,
51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm
load. Driver transformers will be costly for quality. The 6C5 works a
bit better than a 6SN7 because it will output a bit more voltage
before distortion sets in with the same load resistors.

I do hope to build myself a p-p 300B stereo amplifier when I can
afford it. I will use the 5AR4s to have B+ delay and fixed bias will
be used on the outputs.

"at" ha scritto nel messaggio
...
Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio

about
his concerns about the 250V driving voltage, as this amp is being

designed
by a competent designer, I have trust that the desing will be

workable,
and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this

amp,
because they're very personal and related to the planned use of the

amp,
and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a

split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info.

Something
that caught my attention (especially as my amp will be using power

triodes
as output tubes), was the mentioning that a power triode tube or a

power
pentode wired as triode could be used as the split-load phase splitter

tube
to get lower output impedance. Now, I'm asking you experts, is this

idea
sensible, and beneficial, as the original author is claiming it might

be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase

splitters
(in terms of unloaded voltage balance), it has two serious

difficulties,
namely that the output impedance of the two voltages is radically

different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode.

the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor.

i.e.,
low
z out in the K circuit and high z out of the P circuit. If the high Z

signal
is looking at the starving mouth a power triode grid represents, you

can
easily imagine that it will be really easy to load it down. A typical

power
triode might have 15pf of grid to plate C, plus the socket stray

(5pf),
times the Miller number (mu times 20pf times the driver Miller, which

might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not

using
a cathode follower...). All in all, it could typically shift from 60pf

at
0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will

load
down by 8khz at full swing. The bottom tube will not load down until

80khz
or there-abouts. Additionally, you will need about 400 volts B+ to

swing
40
top and bottom, not to mention that the tube in front will have to

provide
40 volts to the phase inverter! Weird! This is why Williamson used a

combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits

phase
with near perfect balance, but it drives the much easier grids of a

small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in

turn
have the same output z and easily drive pp grids. One further

consideration
that few have considered is using a power triode as a P/K inverter. An

EL-34
wired in triode, with 4k P+K resistors could have an output z low

enough
to
avoid problems in the audio range. The same 400v B+ would still

apply...
I'm not pretending I did understand all the technical factors

involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power

triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around

so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't

needed,
but I doubt it, based on the high price tags associated with quality

phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at




  #9   Report Post  
Patrick Turner
 
Posts: n/a
Default



Mark wrote:

"Fabio Berutti" wrote in message ...
More to the point:

- I'm not concerned about the 250V p-p required to drive the 300B, I just
say it's not easy to provide them (BTW 180V can be enough)
- the cathodyne or concertina or distributed load phase splitter is OK for
driving pentodes, but You'd need 800V for driving a 300B out of it
- the interstage transformer is very difficult to wind correctly, often You
end up with a narrow bandwidth and with phase shifts leading to FB
instability. See the GEC design for a DA42 125W amp, if You want to use an
IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray
inductance.
- You can use the classic scheme of the Williamson amp
(gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a
stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain
RC coupling is the more reliable in terms of phase shift and does not
require using transformers made of Unobtainium.

A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about
30W at the onset of grid current with a low distortion even using no
feedback. At the good end You'll have one of the most beautiful amps that
ever played music over the Finnish Tundra...
As per the power supply You'll need 2x 5U4G to feed the power tubes with the
320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid
spinal injuries while moving it around.
A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with
no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a
pentode is needed to provide the extra gain, see the classic Leak schemes).
I keep on suggesting Octal types because they're MUCH easier to weld in.
TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher
B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and
lots of.
I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm
designing a similar unit with Svetlana's SV811, but I use a long-tail
splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way
to avoid FB while keeping about 1% 2nd h. and getting some 50W.

Ciao and.. mail us a picture of the working Beast.

Fabio


I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You
may be right with 30 watts. Since I have one source of information,
that is what I will have to use a guide for B+ and power output. I
would think that running them at 400 volts would be just as good with
about 35 watts output and longer life. The 5 watts will not be missed.


8 watts from an SE 300B which idles at 30 watts of dissipation is about all
one should expect, unless you use beefed up versions of the 300B available from
emission labs, at ungodly prices.
So a pair in PP can only give 16 watts in class A, and I always thought
about 28 watts was about right for class AB into say 4 ohms,
so that with 8 ohms, its all class A and you get the 16 watts.




I agree with you about no feedback. I prefer to not use it.
Calculating it is very difficult because of the number of variables to
getting the right figures. It is easier to use an amplifier and put
feedback in and then determine the rest. Feedback is mainly used to
lower the effective plate resistance thus higher damping. Triodes do
not need feedback because they have high damping (low plate
resistance).


If you have a pair of 300B in PP, the anode to anode Ra is about 1.6k,
and if the OPt is 10k to 6 ohms, the Z ratio is 1,666 :1,
so the 1,600 ohms of Ra-a appears as 0.96 ohms,
but if the OPT winding losses total 10%, or 0.6 ohms,
then Ro = 1.6 ohms approx, which could ideally be lower.
12 dB of NFB would reduce this 1.6 ohms to less than 0.5 ohms.


What B+ do you use and the resistor values for the cathode and plate
in the 6BL7/6BX7 drivers for a 300B or other similiar high drive
outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found
they will output the voltage needed for high drive outputs, 70 volts,
but are close to the maximum output voltage with 300 volts as B+,
51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm
load. Driver transformers will be costly for quality. The 6C5 works a
bit better than a 6SN7 because it will output a bit more voltage
before distortion sets in with the same load resistors.


You don't need a costly IST, use a choke with a CT,
and a pair of additional R to isolate the stray shunt C of the choke,
and the inductance shunt at LF,
and you have a splendid driver, which can be CR coupled to the 300B grids, see:-
http://www.turneraudio.com.au/websch...ma550w335h.gif

Patrick Turner.

  #10   Report Post  
Patrick Turner
 
Posts: n/a
Default



Fabio Berutti wrote:

In fact in my project the 6BL7s should be choke-loaded and work at about
300V, 30 mA. Vout can be up to 500Vpp. Nevertheless, due to choke stray
inductance, I'm considering plain RC loading.


Using the choke *as well as the L* is the answer.
The choke has a high impedance for most of the band,
and if the R = 4 x Ra at least, then even at DC or at some HF,
the gain won't roll off to nothing.

The load seen by the triode with such a DC delivery path
is then a much higher value, and the following grid bias R for 300B
can be a lower value which still won't load down the driver triode/s
so the driver thd is *much* less, as loadline analysis will show
when you draw all this up before building anything.



A separated PS for the
driving stage is already provided, and it can go as high as 400V. I'll
fiddle a bit with Tubecad. I suppose a 10-15K load is OK, plate resistance
is only1K7 for the 6BL7.


If the choke has 400 k impedance at 1 kHz, which ie easy to get with a
non gapped choke with CT, then the tube never sees the 15 k load.
The load line is dramatically flatter, thd down about 10 dB at least,
and voltage swing a lot wider.

Patrick Turner.



Ciao

Fabio

"Mark" ha scritto nel messaggio
om...
"Fabio Berutti" wrote in message

...
More to the point:

- I'm not concerned about the 250V p-p required to drive the 300B, I

just
say it's not easy to provide them (BTW 180V can be enough)
- the cathodyne or concertina or distributed load phase splitter is OK

for
driving pentodes, but You'd need 800V for driving a 300B out of it
- the interstage transformer is very difficult to wind correctly, often

You
end up with a narrow bandwidth and with phase shifts leading to FB
instability. See the GEC design for a DA42 125W amp, if You want to use

an
IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray
inductance.
- You can use the classic scheme of the Williamson amp
(gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a
stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84).

Plain
RC coupling is the more reliable in terms of phase shift and does not
require using transformers made of Unobtainium.

A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide

about
30W at the onset of grid current with a low distortion even using no
feedback. At the good end You'll have one of the most beautiful amps

that
ever played music over the Finnish Tundra...
As per the power supply You'll need 2x 5U4G to feed the power tubes with

the
320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid
spinal injuries while moving it around.
A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity

with
no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a
pentode is needed to provide the extra gain, see the classic Leak

schemes).
I keep on suggesting Octal types because they're MUCH easier to weld in.
TRhe 6SN7 can drive a 300B, but it would be better using the GT type

(higher
B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap

and
lots of.
I'd keep with the "no-feedback" philosophy, to make a PP with triodes.

I'm
designing a similar unit with Svetlana's SV811, but I use a long-tail
splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this

way
to avoid FB while keeping about 1% 2nd h. and getting some 50W.

Ciao and.. mail us a picture of the working Beast.

Fabio


I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You
may be right with 30 watts. Since I have one source of information,
that is what I will have to use a guide for B+ and power output. I
would think that running them at 400 volts would be just as good with
about 35 watts output and longer life. The 5 watts will not be missed.

I agree with you about no feedback. I prefer to not use it.
Calculating it is very difficult because of the number of variables to
getting the right figures. It is easier to use an amplifier and put
feedback in and then determine the rest. Feedback is mainly used to
lower the effective plate resistance thus higher damping. Triodes do
not need feedback because they have high damping (low plate
resistance).

What B+ do you use and the resistor values for the cathode and plate
in the 6BL7/6BX7 drivers for a 300B or other similiar high drive
outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found
they will output the voltage needed for high drive outputs, 70 volts,
but are close to the maximum output voltage with 300 volts as B+,
51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm
load. Driver transformers will be costly for quality. The 6C5 works a
bit better than a 6SN7 because it will output a bit more voltage
before distortion sets in with the same load resistors.

I do hope to build myself a p-p 300B stereo amplifier when I can
afford it. I will use the 5AR4s to have B+ delay and fixed bias will
be used on the outputs.

"at" ha scritto nel messaggio
...
Ok, I've spent time designing my amp with an experienced designer.

For personal tastes and reasons, I've planned to build an integrated
push-pull power triode amp using 300B power tubes. (Thanks for Fabio

about
his concerns about the 250V driving voltage, as this amp is being

designed
by a competent designer, I have trust that the desing will be

workable,
and
I'll manage to build it so it'll work :-).
Also thanks go for all you good people here in this newsgroup who have
shared your experience, and answered my many questions in detail!

I'd rather save you from the details why I would like to build this

amp,
because they're very personal and related to the planned use of the

amp,
and
rather ask you about the following subject.

The amp is going to have either a transformer phase splitter, or a

split
load phase splitter. Well, that's what has been suggested to me.

I've searched the net about the subject, and did find some info.

Something
that caught my attention (especially as my amp will be using power

triodes
as output tubes), was the mentioning that a power triode tube or a

power
pentode wired as triode could be used as the split-load phase splitter
tube
to get lower output impedance. Now, I'm asking you experts, is this

idea
sensible, and beneficial, as the original author is claiming it might

be?

Here's the whole piece of text in question:
1.. split load inverter: Arguably the best balanced of all phase
splitters
(in terms of unloaded voltage balance), it has two serious

difficulties,
namely that the output impedance of the two voltages is radically
different,
and it has NO gain. in a split load, the plate and cathode loads are
identical resistors, and output is taken from the plate and cathode.

the
bottom part is functionally a cathode follower, and the top part is a
grounded cathode circuit with a large unbypassed cathode resistor.

i.e.,
low
z out in the K circuit and high z out of the P circuit. If the high Z
signal
is looking at the starving mouth a power triode grid represents, you

can
easily imagine that it will be really easy to load it down. A typical
power
triode might have 15pf of grid to plate C, plus the socket stray

(5pf),
times the Miller number (mu times 20pf times the driver Miller, which
might
easily be 20pf by the time you're swinging 30-60 vrms, if you are not
using
a cathode follower...). All in all, it could typically shift from 60pf

at
0
signal to about 400 pf at full output. If the driving Z is 50k
(approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will

load
down by 8khz at full swing. The bottom tube will not load down until

80khz
or there-abouts. Additionally, you will need about 400 volts B+ to

swing
40
top and bottom, not to mention that the tube in front will have to

provide
40 volts to the phase inverter! Weird! This is why Williamson used a

combo
cathodyne and a diff amp. It's a good solution. The cathodyne splits

phase
with near perfect balance, but it drives the much easier grids of a

small
dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in

turn
have the same output z and easily drive pp grids. One further
consideration
that few have considered is using a power triode as a P/K inverter. An
EL-34
wired in triode, with 4k P+K resistors could have an output z low

enough
to
avoid problems in the audio range. The same 400v B+ would still

apply...
I'm not pretending I did understand all the technical factors

involved.
That's why I would appreciate your expertise on the matter. So, for my
application, would there be any benefit for using a power

triode/pentode
wired as a triode as the phase splitter tube, to get that lower output
impedance? I've got spare EL-34 tubes and other pentodes lying around

so I
could use them, or buy more 300B tubes.

Of course, if I decide to go the transformer way this question isn't
needed,
but I doubt it, based on the high price tags associated with quality

phase
splitter transformers. If I'm wrong, and you know some good and
"inexpensive" transformers for this use, please correct me.

If you feel this question was complete spamm, my apologies!

Thanks!


-at



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