I'm pretty sure I understand the concept of a push pull tube amp, but
today one of the tubs in my Leslie 147 (6550) died most 4th of
julyishly. When I pull it out of the socket and replace the blown
fuse, the sucker still generates good tone.

Question of course is why does it still work? Probably really bad for
the amp and speakers so I have no intention of using it this way, just
curious as to why it actually generates any tone yet alone good tone.

Thanks,

Rob

leftofthedial wrote:
I'm pretty sure I understand the concept of a push pull tube amp, but
today one of the tubs in my Leslie 147 (6550) died most 4th of
julyishly. When I pull it out of the socket and replace the blown
fuse, the sucker still generates good tone.

Question of course is why does it still work? Probably really bad for
the amp and speakers so I have no intention of using it this way, just
curious as to why it actually generates any tone yet alone good tone.

Thanks,

Rob

most push pull amps are biased for Class AB which means they are
operating class A at lower levels and only really operate class B at
higher levels. It only needs one tube to operate in Class A. With
only output tube, the output power avaialbe without distortion should
be much lower.

Mark

Mark wrote:
leftofthedial wrote:
I'm pretty sure I understand the concept of a push pull tube amp, but
today one of the tubs in my Leslie 147 (6550) died most 4th of
julyishly. When I pull it out of the socket and replace the blown
fuse, the sucker still generates good tone.

Question of course is why does it still work? Probably really bad for
the amp and speakers so I have no intention of using it this way, just
curious as to why it actually generates any tone yet alone good tone.

Thanks,

Rob

most push pull amps are biased for Class AB which means they are
operating class A at lower levels and only really operate class B at
higher levels. It only needs one tube to operate in Class A. With
only output tube, the output power avaialbe without distortion should
be much lower.

I wouldn't worry to much about damage to the speakers.
The output transformer by nature will protect the
speakers from most types of damage. Unless the bias is
very upset by the missing tube (probably isn't) the
remaining circuitry can go about it's business as usual.
Does the plate glow red-hot ?

Even if the bias is set to fully class B you still get
half the waveform. The hotter the bias is set toward
class A the more waveform is shared by both tubes (and
both halves of the output transformer primary).

What you have is the ultimate in 'vintage warmth' ;-

rd

Interesting answers. I guess looking at the schematic, it looks like
an open circiut without the second tube though, which I guess is what
ultimately is puzzling. I guess the schematic doesn't show a
continuity path without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

It shows a loop with one 6550 at the top and another at the bottom with
the transformer in between and a phase inverter 12au7 tube dividing the
signal down both paths. Wish I had it in front of me right now but
it's down at the studio.

Okay then, thanks.

leftofthedial wrote:
Interesting answers. I guess looking at the schematic, it looks like
an open circiut without the second tube though, which I guess is what
ultimately is puzzling. I guess the schematic doesn't show a
continuity path without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

It shows a loop with one 6550 at the top and another at the bottom with
the transformer in between and a phase inverter 12au7 tube dividing the
signal down both paths. Wish I had it in front of me right now but
it's down at the studio.

The B+ power supply connection is at the center-tap
of the output tranny, and likely isn't drawn out fully,
just referenced as "B+". The current flows through one
half of the output primary for each tube.
Literally "Push-Pull".

rd

"leftofthedial" wrote in message
oups.com...
Interesting answers. I guess looking at the schematic, it looks like
an open circiut without the second tube though, which I guess is what
ultimately is puzzling. I guess the schematic doesn't show a
continuity path without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

It shows a loop with one 6550 at the top and another at the bottom with
the transformer in between and a phase inverter 12au7 tube dividing the
signal down both paths. Wish I had it in front of me right now but
it's down at the studio.

Not open; you've got the B+ going into the center tap of the output
transformer, with current flowing to and through the remaining tube, then
into the ground through the cathode resistor and (maybe) capacitor. If you
had the other tube plugged in you'd have an identical circuit in parallel to
this one, except that it's handling signals of opposite polarity. The
schematic makes it look like one big loop, but it can also be considered as
two smaller loops.

Peace,
Paul

With both tubes in the DC part of the anode currents will cancel with one
tube this will not happen at best the output transformer will saturate early
but with only one tube in the amp is well below proper operating conditions
at worst you will damage the output transformer expensive :-(
John D

"spud" wrote in message
news
The heater voltage might also be higher with one output tube out of
the string. Probably not good. Interesting that it sort of works as
half a single ended amp though.

On Tue, 18 Oct 2005 06:16:54 GMT, "Paul Stamler"
wrote:

"leftofthedial" wrote in message
roups.com...
Interesting answers. I guess looking at the schematic, it looks like
an open circiut without the second tube though, which I guess is what
ultimately is puzzling. I guess the schematic doesn't show a
continuity path without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

It shows a loop with one 6550 at the top and another at the bottom with
the transformer in between and a phase inverter 12au7 tube dividing the
signal down both paths. Wish I had it in front of me right now but
it's down at the studio.

Not open; you've got the B+ going into the center tap of the output
transformer, with current flowing to and through the remaining tube, then
into the ground through the cathode resistor and (maybe) capacitor. If
you
had the other tube plugged in you'd have an identical circuit in parallel
to
this one, except that it's handling signals of opposite polarity. The
schematic makes it look like one big loop, but it can also be considered
as
two smaller loops.

Peace,
Paul

leftofthedial wrote:
Interesting answers. I guess looking at the schematic, it looks like
an open circiut without the second tube though, which I guess is what
ultimately is puzzling. I guess the schematic doesn't show a
continuity path without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

The B+ is supplied to the center tap of the output transformer. So if
only one tube is in place, that tube will modulate half the transformer,
BUT the transformer will have massive amounts of DC that isn't balanced
out, so the low end will be very poor.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Scott Dorsey" wrote in message

leftofthedial wrote:

Interesting answers. I guess looking at the schematic,
it looks like an open circiut without the second tube
though, which I guess is what ultimately is puzzling. I
guess the schematic doesn't show a continuity path
without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

The B+ is supplied to the center tap of the output
transformer. So if only one tube is in place, that tube
will modulate half the transformer, BUT the transformer
will have massive amounts of DC that isn't balanced out,
so the low end will be very poor. --scott

I suspect that the perception that this one-tubed broken
push-pull amp is anything but totally broken, is based on
the power levels being really low and/or people tolerating a
lot of distortion.

Arny Krueger wrote:
"Scott Dorsey" wrote in message

leftofthedial wrote:

Interesting answers. I guess looking at the schematic,
it looks like an open circiut without the second tube
though, which I guess is what ultimately is puzzling. I
guess the schematic doesn't show a continuity path
without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

The B+ is supplied to the center tap of the output
transformer. So if only one tube is in place, that tube
will modulate half the transformer, BUT the transformer
will have massive amounts of DC that isn't balanced out,
so the low end will be very poor. --scott

I suspect that the perception that this one-tubed broken
push-pull amp is anything but totally broken, is based on
the power levels being really low and/or people tolerating a
lot of distortion.

The whole POINT of the guitar amp is to sound totally broken. That
is where much of the guitar tone comes from. I assure you that if a guitar
amplifier were built that was accurate and neutral-sounding, nobody
would want it.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Scott Dorsey" wrote in message

Arny Krueger wrote:
"Scott Dorsey" wrote in message

leftofthedial wrote:

Interesting answers. I guess looking at the schematic,
it looks like an open circiut without the second tube
though, which I guess is what ultimately is puzzling.
I guess the schematic doesn't show a continuity path
without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

The B+ is supplied to the center tap of the output
transformer. So if only one tube is in place, that tube
will modulate half the transformer, BUT the transformer
will have massive amounts of DC that isn't balanced out,
so the low end will be very poor. --scott

I suspect that the perception that this one-tubed broken
push-pull amp is anything but totally broken, is based on
the power levels being really low and/or people
tolerating a lot of distortion.

The whole POINT of the guitar amp is to sound totally
broken.

Yes but that begs the question why they put the second tube
in! ;-)

I think the answer to that is that they put the second tube
in so the amp could play louder.

The OP is then just not noticing the reduced output for any
of a number of good reasons.

Scott Dorsey wrote:

Arny Krueger wrote:
"Scott Dorsey" wrote in message

leftofthedial wrote:

Interesting answers. I guess looking at the schematic,
it looks like an open circiut without the second tube
though, which I guess is what ultimately is puzzling. I
guess the schematic doesn't show a continuity path
without the tube in place. Must just be assumed if you
know what your looking at from 40-50 years ago.

The B+ is supplied to the center tap of the output
transformer. So if only one tube is in place, that tube
will modulate half the transformer, BUT the transformer
will have massive amounts of DC that isn't balanced out,
so the low end will be very poor. --scott

I suspect that the perception that this one-tubed broken
push-pull amp is anything but totally broken, is based on
the power levels being really low and/or people tolerating a
lot of distortion.

The whole POINT of the guitar amp is to sound totally broken. That
is where much of the guitar tone comes from. I assure you that if a guitar
amplifier were built that was accurate and neutral-sounding, nobody
would want it.

Many guitar amps run their output valves some way into grid current for
a significant proportion of the time.

I once got caught out with a design that was fine until the output stage
went into grid current. The balancing of the push-pull took place at
the grid resistors of the output valves and the sudden DC shift fed back
into the previous stage and had the most horrible side effects.

I redesigned the amplifier with the driver stage doing the balancing and
the sound was totally different. It was a 12 watt amplifier, but it
could easily sound as loud as a 30 watt transistor one because it
allowed 'nice-sounding' compression.


When one output valve dies, the distortion will be predominantly second
harmonic, this will sound an octave above the fundamental and will still
be tolerably musical. If the feedback around the output stage is
relatively light (as may well have to be to maintain stability with a
'budget' output transformer), the driver stage won't try too hard to
compensate for the faulty output stage and the higher harmonics will be
far less prevalent than if you similarly overloaded a 'better'
amplifier.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

I suspect that the perception that this one-tubed broken
push-pull amp is anything but totally broken, is based on
the power levels being really low and/or people tolerating a
lot of distortion.

The distortion is principally even-order in this case.

I think the answer to that is that they put the second tube
in so the amp could play louder.

Does that mean that with only one tube, the amp can only get to 5.5?

On Tue, 18 Oct 2005 17:19:20 -0700, William Sommerwerck
wrote:
I think the answer to that is that they put the second tube
in so the amp could play louder.

Does that mean that with only one tube, the amp can only get to 5.5?


5.5 it still more than 5