Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

TIA

Rob (UK)

wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

//Walt

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.

James. )


"Walt" wrote in message
...
wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

//Walt

James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.


"Walt" wrote in message
...

wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.


--
I hope that in a few years it [Wikipedia] will be so bloated that it
will simply disintegrate, because I can't stand the thought that this
thing might someday actually be used as a serious reference source.
Because in its current form, it's not to be taken seriously at all.

- Horst Prillinger (see
http://homepage.univie.ac.at/horst.p...06/000623.html)

In article , David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

Heard that ALL the time way back. How about arching of the plates?
or even turning red, or damaging the output transformer. Is that enough?

greg


"Walt" wrote in message
...

wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

GregS spake thus:

In article ,
David Nebenzahl wrote:

James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

Heard that ALL the time way back. How about arching of the plates?
or even turning red, or damaging the output transformer. Is that enough?

Sorry, still sounds like urban folklore to me. What are the exact
mechanisms that cause these problems? "Arching" (sic) plates? I doubt it.


"Walt" wrote in message
...

wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.


--
I hope that in a few years it [Wikipedia] will be so bloated that it
will simply disintegrate, because I can't stand the thought that this
thing might someday actually be used as a serious reference source.
Because in its current form, it's not to be taken seriously at all.

- Horst Prillinger (see
http://homepage.univie.ac.at/horst.p...06/000623.html)

David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.


Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

I have encountered gear that will fry itself if you attempt to run it
into an open circuit, but every example I can think of is RF, not audio.

I remember having it drilled into my head at an early age never to run
an amp into an open circuit, but I've never fried anything that way,
tube or transistor, even though I've disregarded that admonishment.

My best conjecture is that it's an old wive's tale left over from the
days when gear had interesting "features" like tieing the chasis to the
neutral. With features like that, one had to be verrrrry careful. I'd
be surprised if anyone is still making amps that fry when they encounter
an open circuit. But, I'm open to being told I'm wrong.

//Walt

In article , wrote:
David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.


Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

I have encountered gear that will fry itself if you attempt to run it
into an open circuit, but every example I can think of is RF, not audio.

I remember having it drilled into my head at an early age never to run
an amp into an open circuit, but I've never fried anything that way,
tube or transistor, even though I've disregarded that admonishment.

My best conjecture is that it's an old wive's tale left over from the
days when gear had interesting "features" like tieing the chasis to the
neutral. With features like that, one had to be verrrrry careful. I'd
be surprised if anyone is still making amps that fry when they encounter
an open circuit. But, I'm open to being told I'm wrong.

Acording to one web page I just read, a resistive dummy load is
not necessarily the best thing to use, or easiest on the amplifier.
I may impose a bit more load than a standard speaker. I think its
clear that an unloaded amplifier will have higher p-p voltages
on the primary of the transformer.

greg

GregS spake thus:

In article , wrote:

David Nebenzahl wrote:

James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

I have encountered gear that will fry itself if you attempt to run it
into an open circuit, but every example I can think of is RF, not audio.

I remember having it drilled into my head at an early age never to run
an amp into an open circuit, but I've never fried anything that way,
tube or transistor, even though I've disregarded that admonishment.

My best conjecture is that it's an old wive's tale left over from the
days when gear had interesting "features" like tieing the chasis to the
neutral. With features like that, one had to be verrrrry careful. I'd
be surprised if anyone is still making amps that fry when they encounter
an open circuit. But, I'm open to being told I'm wrong.

Acording to one web page I just read, a resistive dummy load is
not necessarily the best thing to use, or easiest on the amplifier.
I may impose a bit more load than a standard speaker. I think its
clear that an unloaded amplifier will have higher p-p voltages
on the primary of the transformer.

Possibly, but surely not higher than the rated maximum for the
output-stage tubes, no? (Unless the designer was skating on very thin
ice...)


--
I hope that in a few years it [Wikipedia] will be so bloated that it
will simply disintegrate, because I can't stand the thought that this
thing might someday actually be used as a serious reference source.
Because in its current form, it's not to be taken seriously at all.

- Horst Prillinger (see
http://homepage.univie.ac.at/horst.p...06/000623.html)

In article ,
GregS wrote:

My best conjecture is that it's an old wive's tale left over from the
days when gear had interesting "features" like tieing the chasis to the
neutral. With features like that, one had to be verrrrry careful. I'd
be surprised if anyone is still making amps that fry when they encounter
an open circuit. But, I'm open to being told I'm wrong.

Acording to one web page I just read, a resistive dummy load is
not necessarily the best thing to use, or easiest on the amplifier.
I may impose a bit more load than a standard speaker. I think its
clear that an unloaded amplifier will have higher p-p voltages
on the primary of the transformer.

I also wonder about stability. A tube amplifier tends to operate at
high impedances, and this may increase the effect of parasitic
inductances and capacitances which might introduce unwanted
feedback between the output and input of one or more stages in the
amp. Depending on the stage's gain, bandwidth, and the type of
parasitic impedances in the circuit, it's possible that conditions for
oscillation might exist if the stage's output doesn't see some amount
of resistive loading. A resistive load connected to the output would
"spoil the Q" of the parasitic feedback loop, reducing the loop gain
and restoring stability.

An amp which broke into an unchecked RF oscillation could certainly
exhibit some of the symptoms which have been described - tube elements
overheating due to high current flow, or arcing over due to high
voltages.

I've recently been dealing with a related problem having to do with an
amateur-radio transmitter - part of a repeater. The amplifier stage
(MOSFET-based) is supposed to be stable. It's stable when operating
into a 50-ohm load, no question. However, when it's hooked up to a
duplexer over certain lengths of coax cable, it begins to oscillate
spontaneously (even with the exciter stage turned off - there's no
transmit input to the amp stage!).

My guess is that the diplexer's impedance at that frequency (close to
its notch, and thus quite reactive), when transformed by the coax
cable, ends up being very high at the transmitter's output terminals.
There's apparently enough parasitic coupling in the amp and enough
loop gain to satisfy the conditions for oscillation at this frequency.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

While there is no "universal" answer, in my experience, MOST
tube amplifiers and MOST reasonably contemporary (last 30
years?) solid state amplifiers have no problems with no output
load.

If it were me, however, I'd just make sure by switching both the
input and the outputs of the amp, so the amp that's not connected
to the speaker is also not being driven as well.

It is definitely a good idea to switch both the ins and the outs for another
reason... There may be a lot of noise coming from the input of one amp that
will go right into the input of the other amp. Another way to get rid of
this is to make a simple unity gain op amp to buffer the input of both amps.
A simple y splitter is not a good idea.

James. )


wrote in message
ups.com...

David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

While there is no "universal" answer, in my experience, MOST
tube amplifiers and MOST reasonably contemporary (last 30
years?) solid state amplifiers have no problems with no output
load.

If it were me, however, I'd just make sure by switching both the
input and the outputs of the amp, so the amp that's not connected
to the speaker is also not being driven as well.

In general... output transistors are high current devices and can drive
speakers directly. Tubes are high voltage devices and require an output
transformer to match the impedance and to more or less exchange voltage for
current. If you do not properly load the secondary of the transformer, then
the primary isn't really a primary anymore. It's just a coil of wire with a
lot of resistance and reactance. It no longer exhibits the right load for
the tube(s) at all.

James. )



"David Nebenzahl" wrote in message
.com...
GregS spake thus:

In article ,
David Nebenzahl wrote:

James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.

Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

Heard that ALL the time way back. How about arching of the plates?
or even turning red, or damaging the output transformer. Is that enough?

Sorry, still sounds like urban folklore to me. What are the exact
mechanisms that cause these problems? "Arching" (sic) plates? I doubt it.


"Walt" wrote in message
...

wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.


--
I hope that in a few years it [Wikipedia] will be so bloated that it
will simply disintegrate, because I can't stand the thought that this
thing might someday actually be used as a serious reference source.
Because in its current form, it's not to be taken seriously at all.

- Horst Prillinger (see

http://homepage.univie.ac.at/horst.p...06/000623.html)

There is something else you might want to consider... For the cost of a
decent active crossover, you could bi-amp your system. Although this would
require altering your speakers to eliminate one of the passive crossovers
points. If your speakers are three way, then you would drive the woofer
directly with the transistor amp and drive direct to the midrange with the
passive crossover to the tweeter with the tubes!

Ta. James. )




wrote in message
oups.com...
Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

TIA

Rob (UK)

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

TIA

Rob (UK)

James Lehman wrote:
There is something else you might want to consider... For the cost of a
decent active crossover, you could bi-amp your system. Although this would
require altering your speakers to eliminate one of the passive crossovers
points. If your speakers are three way, then you would drive the woofer
directly with the transistor amp and drive direct to the midrange with the
passive crossover to the tweeter with the tubes!

Ta. James. )

Interesting :-)

I don't think I'm going to get a screwdriver out to my lovely Dynies!
Anywho, the point of the exercise is to use the valve amp for 'proper'
listening (vinyl, cds and so forth), and the Yamaha for video sources
and mp3s and the like. The Yamaha would act as the pre in both cases.

Rob

Walt wrote:
wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

//Walt

Those are expensive switches, and I really wanted something less
elaborate. I think I'll either make my own, or just plug the wires in
when needed. Thanks all the same - good advice.

Rob

wrote:
David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp unloaded.
So get a pair of 8 ohm dummy loads.
Does anyone here know, *for certain*, whether this would be a bad thing
or not? Not just idle speculation: what are the effects of running a
tube amp (at normal volume, say, not cranked all the way up) with no
load. Any effect at all? Any harm done? I've been curious for a while.

While there is no "universal" answer, in my experience, MOST
tube amplifiers and MOST reasonably contemporary (last 30
years?) solid state amplifiers have no problems with no output
load.

If it were me, however, I'd just make sure by switching both the
input and the outputs of the amp, so the amp that's not connected
to the speaker is also not being driven as well.


The Yamaha would act as preamp in both cases. Curiously, while it has a
preamp out for the main speakers (L-R), there is no option in the menus
to disable them. I would loosely assume that the Yamaha is therefore
safe in open circuit.

Rob

Walt wrote:
David Nebenzahl wrote:
James Lehman spake thus:

I don't think it is a good idea, in general, to leave a tube amp
unloaded.
So get a pair of 8 ohm dummy loads.


Does anyone here know, *for certain*, whether this would be a bad
thing or not? Not just idle speculation: what are the effects of
running a tube amp (at normal volume, say, not cranked all the way up)
with no load. Any effect at all? Any harm done? I've been curious for
a while.

I have encountered gear that will fry itself if you attempt to run it
into an open circuit, but every example I can think of is RF, not audio.

I remember having it drilled into my head at an early age never to run
an amp into an open circuit, but I've never fried anything that way,
tube or transistor, even though I've disregarded that admonishment.

My best conjecture is that it's an old wive's tale left over from the
days when gear had interesting "features" like tieing the chasis to the
neutral. With features like that, one had to be verrrrry careful. I'd
be surprised if anyone is still making amps that fry when they encounter
an open circuit. But, I'm open to being told I'm wrong.

//Walt

I`ve seen plenty of guitar amps that have flashed across valve bases or
arced internally in the output transformer when the speakers load has
become disconnected. Anyone who worked on group gear back in the 60`s &
70`s will tell you likewise - certainly here in the UK


Ron(UK)

--
Lune Valley Audio
Public Address Systems
Hire Sales Maintenance
www.lunevalleyaudio.com

Rob wrote:
Walt wrote:
wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.


A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

Those are expensive switches, and I really wanted something less
elaborate. I think I'll either make my own, or just plug the wires in
when needed. Thanks all the same - good advice.

OK. How about a panel with binding posts and banana jack jumpers? I
was going to suggest that at first. Easy cheap home project,
electrically equivalent to replugging except you don't have to fish
around behind the amps to do it. Power down both amps before you
repatch and you're golden.

//Walt

"James Lehman" wrote in message
.. .
In general... output transistors are high current devices and can drive
speakers directly. Tubes are high voltage devices and require an output
transformer to match the impedance and to more or less exchange voltage
for
current. If you do not properly load the secondary of the transformer,
then
the primary isn't really a primary anymore. It's just a coil of wire with
a
lot of resistance and reactance. It no longer exhibits the right load for
the tube(s) at all.

James. )




Running a valve amp without a load will result in very high voltages (much
higher than the internal HT supply) being fed back into the valves by the
transformer. This can easily exceed their rated voltage, and indeed that of
the tranformer primary. It will not last very long at all.



Gareth.

Walt wrote:
Rob wrote:
Walt wrote:
wrote:

Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.


A quick Google search turned up this:
http://www.smarthome.com/9725780.html
and this:
http://www.nilesaudio.com/product_se....php?catcdID=7

There's lot's of other options. It's not really rocket science: it's
just a switch. With the valve (tube) amp, just make sure that it can
operate safely into an open circuit.

Those are expensive switches, and I really wanted something less
elaborate. I think I'll either make my own, or just plug the wires in
when needed. Thanks all the same - good advice.

OK. How about a panel with binding posts and banana jack jumpers? I
was going to suggest that at first. Easy cheap home project,
electrically equivalent to replugging except you don't have to fish
around behind the amps to do it. Power down both amps before you
repatch and you're golden.

//Walt


Cheers Walt - that's just about it, I'll give it a go. My thinking was
to provide something that was idiot-proof, and that's not just me on
this occasion(!) - visitors etc. But your plan looks straightforward
enough, thanks.

Rob

"James Lehman" wrote in message
.. .
In general... output transistors are high current devices and can drive
speakers directly. Tubes are high voltage devices and require an output
transformer to match the impedance and to more or less exchange voltage
for
current. If you do not properly load the secondary of the transformer,
then
the primary isn't really a primary anymore. It's just a coil of wire with
a
lot of resistance and reactance. It no longer exhibits the right load for
the tube(s) at all.

I suppose it all depends on the amplifier design. I have used several solid
state amplifiers that have speaker switches on the front panel. If you turn
off the speaker, the amp runs totally unloaded. Since audio amplifiers are
constant-voltage devices (basically discrete op-amps) the feedback simply
compensates, and you get practically the same voltage with or without
speakers.

Tube/valve amps are somewhat different, but many of them also use feedback,
so unless the phase-shift in the transformer goes awry, a well-designed tube
amplifier should be self-limiting also.

In the 1970's and 80's, I did contract engineering work for several radio
stations. Many of them still used a lot of tube equipment at that time,
including monitor amplifiers. The microphone switch in the studios was
invariably connected to a mute relay on the amplifier speaker outputs. When
the microphone was on, the speaker was disconnected. No attempt was ever
made to dump the power to a dummy load. The DJs liked the monitors LOUD. The
amplifiers were in continuous use, 24x7 and I cannot recall a single failure
that was attributable to unloaded operation. Just normal wear and tear.

Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.

That's only because OTL amplifiers behave entirely differently. If a solid state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

Graham

"Pooh Bear" wrote in message
...


Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.

These were tube amps.

That's only because OTL amplifiers behave entirely differently. If a solid
state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell the amplifier
what the output is doing.

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.

These were tube amps.

In which case that's relatively unusual. There can be circuitry to detect the
issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely differently. If a solid
state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell the amplifier
what the output is doing.

Not *always* by any means and even so, one that I know certainly does have this
feedback was nevertherless instantly trashed when someone accidentally
disconnected the speaker.

Graham

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.

These were tube amps.

In which case that's relatively unusual. There can be circuitry to detect
the
issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely differently. If a
solid
state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell the
amplifier
what the output is doing.

Not *always* by any means and even so, one that I know certainly does have
this
feedback was nevertherless instantly trashed when someone accidentally
disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing it.

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Not *always* by any means and even so, one that I know certainly does have
this
feedback was nevertherless instantly trashed when someone accidentally
disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing it.

Well known. The output tranny's primary magnetising inductance turns the output
stage into a 'flyback psu' and it overvolts the tubes. Nice flash followed by an
expensive smell.

Graham

wrote:
Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

TIA

Rob (UK)


Dont run the tube amp unloaded. My 1965 VOX AC10 (guitar amp) had an
open speaker once and I was treated to a very pretty fireworks show
inside my EL84 output tubes. Little hot-white sparks were flying in
all directions inside the tube like tracer bullets from a machine gun.
I found the problem pretty fast as the screw that holds the chassis in
was too long and butting against the speaker terminal block ccausing an
open. It was beautiful to watch but scary so I shut the amp off. The
amp works fine today but I would not want that to happen again.

Karl Uppiano wrote:
"Pooh Bear" wrote in message
...
Karl Uppiano wrote:

"Pooh Bear" wrote in message
...
Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.
These were tube amps.
In which case that's relatively unusual. There can be circuitry to detect
the
issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely differently. If a
solid
state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.
An unloaded transformer still has feedback windings that tell the
amplifier
what the output is doing.
Not *always* by any means and even so, one that I know certainly does have
this
feedback was nevertherless instantly trashed when someone accidentally
disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing it.



Listen, you've gotten some pretty good advice already. Either take it
or not. If you don't I think you'll get to see some pretty sparks, or
even get to watch the smoke get let out of your tube amp.

If you do, you won't. Take your choice. IMO, it's like the agnostic
who said that he didn't know if God existed, but he wasn't taking any
chances....

jak

windcrest wrote:
wrote:
Hi - I have a HT amp (Yamaha, with preamp out), a valve power amp
(Beard P100, driven from the Yamaha pre), and one set of speakers
(Dynaudio A52).

I want to be able to switch between the two amps safely - can someone
suggest a switchbox, DIY schematic, or confirm it's a non-starter.

TIA

Rob (UK)


Dont run the tube amp unloaded. My 1965 VOX AC10 (guitar amp) had an
open speaker once and I was treated to a very pretty fireworks show
inside my EL84 output tubes. Little hot-white sparks were flying in
all directions inside the tube like tracer bullets from a machine gun.
I found the problem pretty fast as the screw that holds the chassis in
was too long and butting against the speaker terminal block ccausing an
open. It was beautiful to watch but scary so I shut the amp off. The
amp works fine today but I would not want that to happen again.


Yes, thanks - that appears to be the consensus. What would *nice* is a
switch that could provide a dummy load when not connected - but I'll be
happy enough soldering up a switchboard using banana plugs/sockets.

Rob

"jakdedert" wrote in message
. ..
Karl Uppiano wrote:
"Pooh Bear" wrote in message
...
Karl Uppiano wrote:

"Pooh Bear" wrote in message
...
Karl Uppiano wrote:

I cannot recall a single failure
that was attributable to unloaded operation.
These were tube amps.
In which case that's relatively unusual. There can be circuitry to
detect the
issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely differently. If a
solid
state
amp has an output tranny it would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.
An unloaded transformer still has feedback windings that tell the
amplifier
what the output is doing.
Not *always* by any means and even so, one that I know certainly does
have this
feedback was nevertherless instantly trashed when someone accidentally
disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing
it.
Listen, you've gotten some pretty good advice already. Either take it or
not. If you don't I think you'll get to see some pretty sparks, or even
get to watch the smoke get let out of your tube amp.

If you do, you won't. Take your choice. IMO, it's like the agnostic who
said that he didn't know if God existed, but he wasn't taking any
chances....

Perhaps you've confused me for the original poster. I am an engineer trying
to understand what mysterious mechanism would cause this problem, coupled
with many examples in my real life experience of it not happening -- in
harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never exceeds
90 degrees due to impedance changes, and that the overvolt situation for the
ends of a center-tapped primary only approaches 2 x B+. True, I might be
missing something.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of any
such authoritative and scientific analyses, please point me to them.

Karl Uppiano spake thus:

"jakdedert" wrote in message
. ..

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

I cannot recall a single failure that was attributable to
unloaded operation.

These were tube amps.

In which case that's relatively unusual. There can be circuitry
to detect the issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely
differently. If a solid state amp has an output tranny it
would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell
the amplifier what the output is doing.

Not *always* by any means and even so, one that I know
certainly does have this feedback was nevertherless instantly
trashed when someone accidentally disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing
it.

Listen, you've gotten some pretty good advice already. Either take it or
not. If you don't I think you'll get to see some pretty sparks, or even
get to watch the smoke get let out of your tube amp.

If you do, you won't. Take your choice. IMO, it's like the agnostic who
said that he didn't know if God existed, but he wasn't taking any
chances....

Perhaps you've confused me for the original poster. I am an engineer trying
to understand what mysterious mechanism would cause this problem, coupled
with many examples in my real life experience of it not happening -- in
harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never exceeds
90 degrees due to impedance changes, and that the overvolt situation for the
ends of a center-tapped primary only approaches 2 x B+. True, I might be
missing something.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of any
such authoritative and scientific analyses, please point me to them.

So far, my money's on you: your real-world empirical experience trumps
the usual Usenet speculation. (Although I do wonder about that other
poster who experienced real-world arcing in the output tubes of his
guitar amp when the speaker got disconnected ...)


--
I hope that in a few years it [Wikipedia] will be so bloated that it
will simply disintegrate, because I can't stand the thought that this
thing might someday actually be used as a serious reference source.
Because in its current form, it's not to be taken seriously at all.

- Horst Prillinger (see
http://homepage.univie.ac.at/horst.p...06/000623.html)

"David Nebenzahl" wrote in message
.com...
Karl Uppiano spake thus:

"jakdedert" wrote in message
. ..

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

"Pooh Bear" wrote in message
...

Karl Uppiano wrote:

I cannot recall a single failure that was attributable to
unloaded operation.

These were tube amps.

In which case that's relatively unusual. There can be circuitry
to detect the issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely
differently. If a solid state amp has an output tranny it
would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell
the amplifier what the output is doing.

Not *always* by any means and even so, one that I know
certainly does have this feedback was nevertherless instantly
trashed when someone accidentally disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing
it.

Listen, you've gotten some pretty good advice already. Either take it or
not. If you don't I think you'll get to see some pretty sparks, or even
get to watch the smoke get let out of your tube amp.

If you do, you won't. Take your choice. IMO, it's like the agnostic who
said that he didn't know if God existed, but he wasn't taking any
chances....

Perhaps you've confused me for the original poster. I am an engineer
trying to understand what mysterious mechanism would cause this problem,
coupled with many examples in my real life experience of it not
happening -- in harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never
exceeds 90 degrees due to impedance changes, and that the overvolt
situation for the ends of a center-tapped primary only approaches 2 x B+.
True, I might be missing something.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of
any such authoritative and scientific analyses, please point me to them.

So far, my money's on you: your real-world empirical experience trumps the
usual Usenet speculation. (Although I do wonder about that other poster
who experienced real-world arcing in the output tubes of his guitar amp
when the speaker got disconnected ...)

I am sure there are situations where this can happen for some reason or
another -- a poor design, for example, with tubes rated for barely more than
B+ (either by design, or replaced with the wrong types). Or an amp that is
only marginally stable under favorable conditions and begins to oscillate
when unloaded. BTW, the "flyback" behavior of an unloaded transformer might
come into play if there is high frequency oscillation.

A well-designed, unconditionally stable amplifier with a proper feedback
design should automatically reduce its drive so that the unloaded output
voltage is practically the same as the loaded output voltage. That leaves
only the poorly designed, unstable amps to blow chunks when unloaded, IMHO.
Sort of Darwinian justice, isn't it?

Karl Uppiano wrote:

Perhaps you've confused me for the original poster. I am an engineer trying
to understand what mysterious mechanism would cause this problem,

It's not mysterious.

coupled
with many examples in my real life experience of it not happening -- in
harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never exceeds
90 degrees due to impedance changes

You know about the real and imaginary components of impedance ?

and that the overvolt situation for the
ends of a center-tapped primary only approaches 2 x B+. True, I might be
missing something.

Read up about flyback power supplies. The output stage ceases operating like a
nice linear amplifier when the secondary load is removed. Issues like phase lag
which apply to nice sine wave operation cease to apply.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of any
such authoritative and scientific analyses, please point me to them.

Why should there be ? They don't often publish any design guides.

Graham

David Nebenzahl wrote:

Karl Uppiano spake thus:

Perhaps you've confused me for the original poster. I am an engineer trying
to understand what mysterious mechanism would cause this problem, coupled
with many examples in my real life experience of it not happening -- in
harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never exceeds
90 degrees due to impedance changes, and that the overvolt situation for the
ends of a center-tapped primary only approaches 2 x B+. True, I might be
missing something.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of any
such authoritative and scientific analyses, please point me to them.

So far, my money's on you: your real-world empirical experience trumps
the usual Usenet speculation. (Although I do wonder about that other
poster who experienced real-world arcing in the output tubes of his
guitar amp when the speaker got disconnected ...)

His example was classic. Overvolted tubes.

Karl needs to stop thinking linear amplifier theory to understand what's going on.

Graham

Karl Uppiano wrote:

So far, my money's on you: your real-world empirical experience trumps the
usual Usenet speculation. (Although I do wonder about that other poster
who experienced real-world arcing in the output tubes of his guitar amp
when the speaker got disconnected ...)

I am sure there are situations where this can happen for some reason or
another -- a poor design, for example, with tubes rated for barely more than
B+ (either by design, or replaced with the wrong types). Or an amp that is
only marginally stable under favorable conditions and begins to oscillate
when unloaded. BTW, the "flyback" behavior of an unloaded transformer might
come into play if there is high frequency oscillation.

No oscillation needed ! You really don't understand flyback do you ? Why do
relay coils need reverse biased diodes across them ? It's the same mechanism. V
= -L.dI/dt

A well-designed, unconditionally stable amplifier with a proper feedback
design should automatically reduce its drive so that the unloaded output
voltage is practically the same as the loaded output voltage. That leaves
only the poorly designed, unstable amps to blow chunks when unloaded, IMHO.
Sort of Darwinian justice, isn't it?

Why do you have such trouble believing this ? Several ppl have told you it
happens. Feedback doesn't work when an amp is overdriven ( the feedback goes
open loop ) and in any event the low feedback used in tube amps can only do so
much to help and some amps ( esp guitar amps) don't have feedback round the
output stage.

Graham

"Pooh Bear" wrote in message
...


Karl Uppiano wrote:

So far, my money's on you: your real-world empirical experience trumps
the
usual Usenet speculation. (Although I do wonder about that other poster
who experienced real-world arcing in the output tubes of his guitar amp
when the speaker got disconnected ...)

I am sure there are situations where this can happen for some reason or
another -- a poor design, for example, with tubes rated for barely more
than
B+ (either by design, or replaced with the wrong types). Or an amp that
is
only marginally stable under favorable conditions and begins to oscillate
when unloaded. BTW, the "flyback" behavior of an unloaded transformer
might
come into play if there is high frequency oscillation.

No oscillation needed ! You really don't understand flyback do you ? Why
do
relay coils need reverse biased diodes across them ? It's the same
mechanism. V
= -L.dI/dt

Actually, I do understand that, but thanks for the insult. I want to know
what is causing enough signal slope to cause significant flyback. Clipping
or oscillation are the only things that come to mind.

A well-designed, unconditionally stable amplifier with a proper feedback
design should automatically reduce its drive so that the unloaded output
voltage is practically the same as the loaded output voltage. That leaves
only the poorly designed, unstable amps to blow chunks when unloaded,
IMHO.
Sort of Darwinian justice, isn't it?

Why do you have such trouble believing this ? Several ppl have told you it
happens. Feedback doesn't work when an amp is overdriven ( the feedback
goes
open loop ) and in any event the low feedback used in tube amps can only
do so
much to help and some amps ( esp guitar amps) don't have feedback round
the
output stage.

I already said "I am sure there are situations where this can happen". What
part of that says I don't believe it happens? What I am saying is, in my own
experience, I have never seen this happen even when tube amps are unloaded
as a part of normal operation, even with large signals applied. In at least
one case, I had an oscilloscope attached, and the output signal changed
imperceptibly when the speaker was removed. If the amp doesn't have feedback
around the output stage, then I guess it could be removed from the gene pool
when you disconnect the speaker.

Karl Uppiano wrote:

[failure mode of power amplifiers with output transformers]

If this is a known phenomenon,

It is. It is how cars ignition systems used to work.


Kind regards

Peter Larsen

Karl Uppiano wrote:


I already said "I am sure there are situations where this can happen". What
part of that says I don't believe it happens? What I am saying is, in my own
experience, I have never seen this happen even when tube amps are unloaded
as a part of normal operation, even with large signals applied. In at least
one case, I had an oscilloscope attached, and the output signal changed
imperceptibly when the speaker was removed. If the amp doesn't have feedback
around the output stage, then I guess it could be removed from the gene pool
when you disconnect the speaker.



The solution is quite simple... Either, telephone Marshall or Fender or
Hiwatt or any other manufacturer of high powered valve amps, and ask
them, or better still, go out and buy 'say' a 100watt Marshall amp
head, take it home, plug it in without speakers crank it up to 11 and
play a few chords. If it blows up, you can telephone the manufacturers
and tell them that their design is rubbish and demand your money back.

Report back with your findings

Ron(UK)

--
Lune Valley Audio
Public Address Systems
Hire Sales Maintenance
www.lunevalleyaudio.com

"Peter Larsen" wrote in message
...
Karl Uppiano wrote:

[failure mode of power amplifiers with output transformers]

If this is a known phenomenon,

It is. It is how cars ignition systems used to work.

Different application. Different circuit topology. Different waveforms.