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Using power triode/pentode wired as triode as a split load phase splitter tube?



 
 
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  #1  
Old August 26th 04, 10:49 PM
at
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Default Using power triode/pentode wired as triode as a split load phase splitter tube?

Hi,

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. This is the
only designing question left, what kind of phase splitter to use.

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.
Or would some other phase-splitter configuration work better for my project?

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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  #2  
Old August 27th 04, 03:09 AM
Patrick Turner
external usenet poster
 
Posts: n/a
Default



at wrote:

> Hi,
>
> 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. This is the
> only designing question left, what kind of phase splitter to use.
>
> 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?


Yes, if well implemented.

>
>
> 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...


The Williamson drive amp is a good drive amp for 300B.

But I would try something like this at
http://www.turneraudio.com.au/websch...ma550w335h.gif

>
>
> 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.
> Or would some other phase-splitter configuration work better for my project?
>
> 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


Good luck,

Patrick Turner.

  #3  
Old August 27th 04, 04:02 AM
Engineer
external usenet poster
 
Posts: n/a
Default

"at" > wrote in message
...
> Hi,
>
> Ok, I've spent time designing my amp with an experienced designer.


(snip)

> 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.

This is the
> only designing question left, what kind of phase splitter to use.


Have you considered this phase splitter.

http://www.bonavolta.ch/hobby/en/audio/el84_7.htm

This was a very nice little amplifier in its day (I built two.)

Cheers,

Roger


  #4  
Old August 27th 04, 05:21 AM
Tim Williams
external usenet poster
 
Posts: n/a
Default

"Engineer" > wrote in message
...
> Have you considered this phase splitter.
>
> http://www.bonavolta.ch/hobby/en/audio/el84_7.htm


Only if you change R6 to a CCS. Otherwise, the plate resistors have to be
suitably unbalanced.

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms


  #5  
Old August 27th 04, 10:47 AM
Patrick Turner
external usenet poster
 
Posts: n/a
Default



Engineer wrote:

> "at" > wrote in message
> ...
> > Hi,
> >
> > Ok, I've spent time designing my amp with an experienced designer.

>
> (snip)
>
> > 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.

> This is the
> > only designing question left, what kind of phase splitter to use.

>
> Have you considered this phase splitter.
>
> http://www.bonavolta.ch/hobby/en/audio/el84_7.htm
>
> This was a very nice little amplifier in its day (I built two.)


Yes, this amp, which is very similar to a Mullard design
has a simple pentode input with 12AX7 LTP, and is ok where
a tiny drive voltage is needed, but the driver circuit is right out
of its depth for a pair of 300B.
6SN7 or trioded 6V6 or EL84 are then required for the 300B.

Or possibly a couple of 6SH7 of 6BX6/EF80 for a circuit like the Quad
40.

Patrick Turner.

>
>
> Cheers,
>
> Roger


 




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