[zyx]

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?

[Eeyore]

zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?

You won't do any harm.

Graham

[Patrick Turner]

zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?


That should be fine, jam away.

Its always OK to use a higher ohm value for speakers instead of a lower
value
on nearly all amps.

Patrick Turner.

[FLY135]

Patrick Turner wrote:
zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?


That should be fine, jam away.

Its always OK to use a higher ohm value for speakers instead of a lower
value
on nearly all amps.

Patrick Turner.

That's nearly all **solid state** amps. The Epi is a tube amp.

[Lord Valve]

Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

Lord Valve
Expert

VISIT MY WEBSITE: http://www.nebsnow.com/LordValve
I specialize in top quality HAND SELECTED NOS and
current-production vacuum tubes for guitar and
bass amps. Good prices, fast service.
Authorized dealer for QSC amps, Sovtek/Electro-Harmonix,
Behringer, Hammond-Suzuki, Leslie, Rolls, Weber VST, etc.!


- Partial Client List -
* Derek Trucks (Allman Brothers Band/Derek Trucks Band) *
* Meatloaf * Catherine Wheel * Yo La Tengo * Let's Go Bowling *
* Kofi Burbridge (DTB) * Tod Smallie (DTB) * Susan Tedeschi *
* Roy Pritts (former head, Audio Engineering Society) *
* Rob Hyckys (guitarist for Commander Cody) * Waky Amps *
* Jamie McLean (guitarist for the Dirty Dozen Brass Band) *
* John Pierce (bassist for Huey Lewis and the News) *
* Rob Eaton (guitarist for Dark Star Orchestra) *
* Detroit Frank DuMont (guitarist, Frank DuMont and the Drivin' Wheels) *
* Coco Montoya * Clint Black * Bill McKay * Mojo Watson * Dick Dale *
* Fleetwood Mac * Tyrin Benoit * Eugene Fodor * Dale Bruning *
* Komet Amplification * Dr. Z * Maven Peal * Blockhead Amps *
* Jim Kelley * Balls Amplification * Roccaforte Amplifiers *
* Gerhart Amplification * Aiken Amplification * Germino Amplification *
* Lots More *

NBS Electronics, 230 South Broadway, Denver, CO 80209-1510
Phone orders/tech support after 1:30 PM Denver time at 303-778-1156

- Our 25th Year -

VISA - MASTERCARD - PAYPAL - DISCOVER

"It ain't braggin', if ya can do it." - Dizzy Dean



zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?

[GregS]

In article , Lord Valve wrote:
Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

Lord Valve
Expert


Consider the rated Z is just a nominal figure, and the real Z will rapidly
rise with frequency, every amp would be at risk according to your
advice. The rated Z is at a bass frequency. if you don't play
bass, then its not a relavant issue.


greg

VISIT MY WEBSITE: http://www.nebsnow.com/LordValve
I specialize in top quality HAND SELECTED NOS and
current-production vacuum tubes for guitar and
bass amps. Good prices, fast service.
Authorized dealer for QSC amps, Sovtek/Electro-Harmonix,
Behringer, Hammond-Suzuki, Leslie, Rolls, Weber VST, etc.!


- Partial Client List -
* Derek Trucks (Allman Brothers Band/Derek Trucks Band) *
* Meatloaf * Catherine Wheel * Yo La Tengo * Let's Go Bowling *
* Kofi Burbridge (DTB) * Tod Smallie (DTB) * Susan Tedeschi *
* Roy Pritts (former head, Audio Engineering Society) *
* Rob Hyckys (guitarist for Commander Cody) * Waky Amps *
* Jamie McLean (guitarist for the Dirty Dozen Brass Band) *
* John Pierce (bassist for Huey Lewis and the News) *
* Rob Eaton (guitarist for Dark Star Orchestra) *
* Detroit Frank DuMont (guitarist, Frank DuMont and the Drivin' Wheels) *
* Coco Montoya * Clint Black * Bill McKay * Mojo Watson * Dick Dale *
* Fleetwood Mac * Tyrin Benoit * Eugene Fodor * Dale Bruning *
* Komet Amplification * Dr. Z * Maven Peal * Blockhead Amps *
* Jim Kelley * Balls Amplification * Roccaforte Amplifiers *
* Gerhart Amplification * Aiken Amplification * Germino Amplification *
* Lots More *

NBS Electronics, 230 South Broadway, Denver, CO 80209-1510
Phone orders/tech support after 1:30 PM Denver time at 303-778-1156

- Our 25th Year -

VISA - MASTERCARD - PAYPAL - DISCOVER

"It ain't braggin', if ya can do it." - Dizzy Dean



zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?

[Ian Iveson]

greg said:

Consider the rated Z is just a nominal figure, and the real Z will
rapidly
rise with frequency, every amp would be at risk according to your
advice. The rated Z is at a bass frequency. if you don't play
bass, then its not a relavant issue.

Perhaps you might, step by step, check the logic of your argument.

Assuming similar characteristics, an 8 ohm speaker will rise to a
maximum impedance of twice that of the 4 ohm speaker. An amp designed
to be reliable at the maximum impedance of the 4 ohm speaker may not
be reliable at the maximum impedance of an 8 ohm speaker.

It would be OK if you stuck to bass.

cheers Ian

[Eeyore]

Ian Iveson wrote:

greg said:

Consider the rated Z is just a nominal figure, and the real Z will
rapidly
rise with frequency, every amp would be at risk according to your
advice. The rated Z is at a bass frequency. if you don't play
bass, then its not a relavant issue.

Perhaps you might, step by step, check the logic of your argument.

Assuming similar characteristics, an 8 ohm speaker will rise to a
maximum impedance of twice that of the 4 ohm speaker. An amp designed
to be reliable at the maximum impedance of the 4 ohm speaker may not
be reliable at the maximum impedance of an 8 ohm speaker.

Do tell me more about this reliability Ian !

Especially when a speaker's impedance can rise at some frequencies to easily TEN
TIMES the 'nominal' figure.

By your argument, driving the amplifier at such frequencies will destroy it.


It would be OK if you stuck to bass.

It would be OK if you stuck to not making stuff up about which you know nought.

Graham

[Chris Hornbeck]

On Wed, 17 Jan 2007 17:53:35 +0000, Eeyore
wrote:


Especially when a speaker's impedance can rise at some frequencies to easily TEN
TIMES the 'nominal' figure.

By your argument, driving the amplifier at such frequencies will destroy it.

And furthermore a nominally stable (with nominal load) amplifier,
with nominal levels of voltage feedback, which includes conventional
guitar amplifiers, have very, very largely the same circuit voltages
irrespective of load, over a wide range.

The very real danger that has promulgated these myths is
with an unstable amplifier, for example a legacy pentode-
output guitar amplifier that oscillates when driven hard
unloaded; not unknown for lots of classic designs.

Oscillation, or even just overdrive, rapidly puts the
output valves into class C; dI/dT makes multi-kiloJolt
spikes across the output transformer when unloaded.

Any remotely reasonable load keeps dI/dT to as safe a
level as any other remotely reasonable load.

Much thanks, as always,

Chris Hornbeck
"History consists of truths which in the end turn into lies,
while myth consists of lies which finally turn into truths."
- Jean Cocteau

[Lord Valve]

GregS wrote:

In article , Lord Valve wrote:
Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

Lord Valve
Expert

Consider the rated Z is just a nominal figure, and the real Z will rapidly
rise with frequency, every amp would be at risk according to your
advice. The rated Z is at a bass frequency. if you don't play
bass, then its not a relavant issue.

Sorry, realworld experience puts the lie to your statement. No offense.

LV






greg

VISIT MY WEBSITE: http://www.nebsnow.com/LordValve
I specialize in top quality HAND SELECTED NOS and
current-production vacuum tubes for guitar and
bass amps. Good prices, fast service.
Authorized dealer for QSC amps, Sovtek/Electro-Harmonix,
Behringer, Hammond-Suzuki, Leslie, Rolls, Weber VST, etc.!


- Partial Client List -
* Derek Trucks (Allman Brothers Band/Derek Trucks Band) *
* Meatloaf * Catherine Wheel * Yo La Tengo * Let's Go Bowling *
* Kofi Burbridge (DTB) * Tod Smallie (DTB) * Susan Tedeschi *
* Roy Pritts (former head, Audio Engineering Society) *
* Rob Hyckys (guitarist for Commander Cody) * Waky Amps *
* Jamie McLean (guitarist for the Dirty Dozen Brass Band) *
* John Pierce (bassist for Huey Lewis and the News) *
* Rob Eaton (guitarist for Dark Star Orchestra) *
* Detroit Frank DuMont (guitarist, Frank DuMont and the Drivin' Wheels) *
* Coco Montoya * Clint Black * Bill McKay * Mojo Watson * Dick Dale *
* Fleetwood Mac * Tyrin Benoit * Eugene Fodor * Dale Bruning *
* Komet Amplification * Dr. Z * Maven Peal * Blockhead Amps *
* Jim Kelley * Balls Amplification * Roccaforte Amplifiers *
* Gerhart Amplification * Aiken Amplification * Germino Amplification *
* Lots More *

NBS Electronics, 230 South Broadway, Denver, CO 80209-1510
Phone orders/tech support after 1:30 PM Denver time at 303-778-1156

- Our 25th Year -

VISA - MASTERCARD - PAYPAL - DISCOVER

"It ain't braggin', if ya can do it." - Dizzy Dean



zyx wrote:

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want to try
it on a 12" speaker which is 8 ohms.

Jam away?

[Eeyore]

Lord Valve wrote:

Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

From just a 2:1 change in load impedance ?

You jest surely. Having an open on the output is another matter entirely of
course.

Graham

[Lord Valve]

Eeyore wrote:

Lord Valve wrote:

Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

From just a 2:1 change in load impedance ?

You jest surely. Having an open on the output is another matter entirely of
course.

Graham



Sorry, realworld experience puts the lie to your statement.

Hope you are *profoundly* offended, you America-hating
****tard.

Lord Valve
Expert

[Chris Hornbeck]

Despite what you may have seen from a loud-mouthed
know-it-all on this NG, don't worry.

Loudspeaker impedances are only the roughest kind
of estimate, good within a factor of ten, maybe.

Operation without load is dangerous because it can
cause (somewhat indirectly, but "cause") high
voltage arcing, often in the output transformer's
windings' insulation.

Operation with any load even remotely like a speaker
attached is safe. Never operate without a speaker
attached, but don't worry about some bogus number.

All good fortune,

Chris Hornbeck
"History consists of truths which in the end turn into lies,
while myth consists of lies which finally turn into truths."
- Jean Cocteau

[Lord Valve]

Chris Hornbeck wrote:

Despite what you may have seen from a loud-mouthed
know-it-all on this NG, don't worry.

Well, ****-fire and boy howdy, son...

You're wrong, but you can derive some small solace
from the fact that you're wrong for all the right reasons,
chief among them being ignorance regarding actual
field operating conditions for *guitar* amplifiers.

You've also never seen an Epiphone Valve Junior,
nor the schematic thereto. Right?

So - this amp runs a single EL84, giving an output
of around 4 watts RMS. One 12AX7. SS bridge
rectifier for the HV, as well as one for the heaters.
No screen resistor. The entire shebang - including
the speaker - *retails* for $139, and can often be
had for less than $100, brand new in the box. In
short, it's a cheap-ass piece of Chinese ****,
a bare-bones minimum design intended to sell
for the lowest price possible - less, in fact, than
most audiophools spend on a couple of snob-glass
preamp tubes.

Think any corners were cut?

I've had around a half-dozen of these across my
bench in the last year. All but one had the same
problem: fried socket. Cause: "I hooked it up
to my (brand XYZ) speaker cabinet cuz it wasn't
loud enough for me to play with a drummer."
Said speaker cabinet, of course, was nearly
always an 8-ohm box. One was 16.

This is a 4-watt amp commonly purchased by
the worst gear abusers on the planet: guitar
pickers. Since it's only 4 watts, it is *always*
played at full-bore output clipping, for max
distortion and "blues tone." Its (often short)
life is one of maximum stress, placed upon
the sleaziest components imaginable.

Now, son - you may know what you may
know, and it may well be at odds with what
I've said, but I have hands-on time with this
particular gizmo, and I know for certain it
eats tube sockets when operated into a
higher than spec load. If you want to show
me the math which proves that a bumblebee
can't fly, be my guest. I've seen 'em fly, so
I'll take your "knowledge" with a grain of salt,
if that's OK with you. You don't catch my ass
on this NG telling the audiophools about their
SET 300B snob designs for a fairly good
reason: I know jack **** about 'em. The
difference between me and the audio******s
is that I can admit it.

Lord Valve
Expert Guitar Amp Dude

[Chris Hornbeck]

On Thu, 18 Jan 2007 14:34:29 GMT, Lord Valve
wrote:

Despite what you may have seen from a loud-mouthed
know-it-all on this NG, don't worry.

Well, ****-fire and boy howdy, son...

First of all, let me apologize for what, last night,
seemed like a funny play on your larger-than-life
reputation, but today just reads as awful. You're
a real gentleman to have let it slide so easily.

You're wrong, but you can derive some small solace
from the fact that you're wrong for all the right reasons,
chief among them being ignorance regarding actual
field operating conditions for *guitar* amplifiers.

You've also never seen an Epiphone Valve Junior,
nor the schematic thereto. Right?

Spent more than three decades doing bench repairs, but
fortunately never met these Chinese Firecracker Champs.
That you're having to repair 'em at all makes me
wonder if the repairs are on your dime - my heart
goes out to ya either way. Lotta work for little return.

It's too late to fix their design issues, but it'd be
interesting to know how the external speakers connect.
Speaker cabs are wired several different ways. Do these
connect the external speakers in parallel with the internal
or do the jacks disconnect the internal speaker? Is
the jack make-before-break? They could even be connected
in series. God dwells in the details.


You're obviously seeing a real issue here, but I'll have
to stand by my statement that the nominal speaker impedance
is the wrong place to be looking. Uhh... I think.

Much thanks, as always,

Chris Hornbeck
"History consists of truths which in the end turn into lies,
while myth consists of lies which finally turn into truths."
- Jean Cocteau

[Patrick Turner]

Lord Valve wrote:

Chris Hornbeck wrote:

Despite what you may have seen from a loud-mouthed
know-it-all on this NG, don't worry.

Well, ****-fire and boy howdy, son...

You're wrong, but you can derive some small solace
from the fact that you're wrong for all the right reasons,
chief among them being ignorance regarding actual
field operating conditions for *guitar* amplifiers.

You've also never seen an Epiphone Valve Junior,
nor the schematic thereto. Right?

So - this amp runs a single EL84, giving an output
of around 4 watts RMS. One 12AX7. SS bridge
rectifier for the HV, as well as one for the heaters.
No screen resistor. The entire shebang - including
the speaker - *retails* for $139, and can often be
had for less than $100, brand new in the box. In
short, it's a cheap-ass piece of Chinese ****,
a bare-bones minimum design intended to sell
for the lowest price possible - less, in fact, than
most audiophools spend on a couple of snob-glass
preamp tubes.

Think any corners were cut?

I've had around a half-dozen of these across my
bench in the last year. All but one had the same
problem: fried socket. Cause: "I hooked it up
to my (brand XYZ) speaker cabinet cuz it wasn't
loud enough for me to play with a drummer."
Said speaker cabinet, of course, was nearly
always an 8-ohm box. One was 16.

This is a 4-watt amp commonly purchased by
the worst gear abusers on the planet: guitar
pickers. Since it's only 4 watts, it is *always*
played at full-bore output clipping, for max
distortion and "blues tone." Its (often short)
life is one of maximum stress, placed upon
the sleaziest components imaginable.

Now, son - you may know what you may
know, and it may well be at odds with what
I've said, but I have hands-on time with this
particular gizmo, and I know for certain it
eats tube sockets when operated into a
higher than spec load. If you want to show
me the math which proves that a bumblebee
can't fly, be my guest. I've seen 'em fly, so
I'll take your "knowledge" with a grain of salt,
if that's OK with you. You don't catch my ass
on this NG telling the audiophools about their
SET 300B snob designs for a fairly good
reason: I know jack **** about 'em. The
difference between me and the audio******s
is that I can admit it.

Lord Valve
Expert Guitar Amp Dude


Lordy Lord. I agree mostly.

But the failures when speakers are changed from 8 to 16 ohms isn't
why it fails; failures due to arcing is gonna happen anyway
since a speaker presents a high Z at each siode of its power range, and
chinese crapologists dunno how to make good ****.

And the chinese dunno what they are doing except they try to copy ****
and
finish up with ****tier **** that you can poke a stick at.....

They make 'Goochi' handbags for sheilas, 'Rolecks' watches for the
un-wary,
and any ****en thing goes as long as some poor ******* buys it.

Patrick Turner.

[Alan Rutlidge]

"Patrick Turner" wrote in message
...


Lord Valve wrote:

Chris Hornbeck wrote:

Despite what you may have seen from a loud-mouthed
know-it-all on this NG, don't worry.

Well, ****-fire and boy howdy, son...

You're wrong, but you can derive some small solace
from the fact that you're wrong for all the right reasons,
chief among them being ignorance regarding actual
field operating conditions for *guitar* amplifiers.

You've also never seen an Epiphone Valve Junior,
nor the schematic thereto. Right?

So - this amp runs a single EL84, giving an output
of around 4 watts RMS. One 12AX7. SS bridge
rectifier for the HV, as well as one for the heaters.
No screen resistor. The entire shebang - including
the speaker - *retails* for $139, and can often be
had for less than $100, brand new in the box. In
short, it's a cheap-ass piece of Chinese ****,
a bare-bones minimum design intended to sell
for the lowest price possible - less, in fact, than
most audiophools spend on a couple of snob-glass
preamp tubes.

Think any corners were cut?

I've had around a half-dozen of these across my
bench in the last year. All but one had the same
problem: fried socket. Cause: "I hooked it up
to my (brand XYZ) speaker cabinet cuz it wasn't
loud enough for me to play with a drummer."
Said speaker cabinet, of course, was nearly
always an 8-ohm box. One was 16.

This is a 4-watt amp commonly purchased by
the worst gear abusers on the planet: guitar
pickers. Since it's only 4 watts, it is *always*
played at full-bore output clipping, for max
distortion and "blues tone." Its (often short)
life is one of maximum stress, placed upon
the sleaziest components imaginable.

Now, son - you may know what you may
know, and it may well be at odds with what
I've said, but I have hands-on time with this
particular gizmo, and I know for certain it
eats tube sockets when operated into a
higher than spec load. If you want to show
me the math which proves that a bumblebee
can't fly, be my guest. I've seen 'em fly, so
I'll take your "knowledge" with a grain of salt,
if that's OK with you. You don't catch my ass
on this NG telling the audiophools about their
SET 300B snob designs for a fairly good
reason: I know jack **** about 'em. The
difference between me and the audio******s
is that I can admit it.

Lord Valve
Expert Guitar Amp Dude


Lordy Lord. I agree mostly.

But the failures when speakers are changed from 8 to 16 ohms isn't
why it fails; failures due to arcing is gonna happen anyway
since a speaker presents a high Z at each siode of its power range, and
chinese crapologists dunno how to make good ****.

And the chinese dunno what they are doing except they try to copy ****
and
finish up with ****tier **** that you can poke a stick at.....

They make 'Goochi' handbags for sheilas, 'Rolecks' watches for the
un-wary,
and any ****en thing goes as long as some poor ******* buys it.

Patrick Turner.

I have to agree with you. Unlike the Japanese, the Chinese copiers are not
very good at copying technology and getting it right.
Sure there are exceptions to the rule, but by in large most Chinese audio
gear is crap. Cosmetically it may look acceptable but under the skin there
is a lot of technically compromised "design", demonstrating a fundamental
misunderstanding of electronic engineering principles.

And yes you are correct in so much as whilst there are fools who will buy
this crap, they will continue to manufacture it. Just goes to prove that 50
years of communism didn't kill off the capitalist ideology of making a quick
buck.

Cheers,
Alan

[Patrick Turner]

Lord Valve wrote:

Despite what you may have seen from a couple of
the know-it-alls on this NG, don't do it. Audiophools
rarely stress their gear, and consequently rarely
consider what may happen to it when it is operated
beyond design parameters - as guitar amplifiers
*routinely* are. You risk a condition known as high-
tension flyback if you run a tube guitar amp into a higher
than rated impedance; this will fry your power tube
socket, along with the tube in it. Lower-powered
amps are certainly not immune to this, judging by
the number of tube sockets I've replaced in Fender
Champs and small Gibsons over the years. Get
the correct impedance speaker.

Lord Valve
Expert


Yes, we hear what you are saying and without any load on the amp
the anode to anode signal voltage can be maybe 2,000Vrms instead of a
maximum
of say 500Vrms with a load of below 20 ohms is at the output.

The majority of guitar amps are set up for maximum power into a load
which is slightly less than what SHOULD be used anyway
so that the typical operation is to have a small amount of class A%
power
in the total class AB power.

So changing from say 4 ohms to 8 ohms is never going to cause a problem
of overloading.

However, the speaker will have a resonant frequency where the Z of the
speaker is very high
at some bass F, and 50+ ohms would not be uncommon, so regardless of
what sort
of speaker is fitted there will be an impedance at bass which is way
above the rated Z
of 4 or 8 ohms which is only valid for the mid F, say at 400Hz.
The inductive nature of the voice coil will have the Z rising at
6dB/octave above a pole
at say 500Hz, so by 5kHz, the speaker Z has gone very high, and as you
know
ppl run their amps with treble and brightness turned right up, so that
the amp is overloaded by HF
before the bass F cause overloading. The resulting distortions are much
favoured by rock'n'rollers,
and when the wave forms become clipped the HF content is even greater,
and there is even more risk of arcing.

So regardless of what you say Lord V, there is always a risk of arcing
at the tube sockets
because there is so little ever done to prevent it, like say
having diodes connected from each anode to ground, to prevent the Vswing
ever moving negative with respect to 0V, and thus prevent Eamax ever
swinging more than twice
the supply voltage.
There is never ever any Zobel network connected across the output so
there is a HF load on the amp.

Guitar amp makers rarely ever do anything to prevent over voltaging the
tube sockets and OPT
insulation because they don't give a **** if an amp blows up because its
going to then need replacing,
and they like the new business, any way they can get it.

So after considering what LV has siad, and has NOT said, and what I have
brought to folk's attention,
I would maintain that changing from a 4 ohm speaker to 8 ohms is OK.
The amount of class A power is increased, although maybe the overall
power is reduced,
which won't alter percieved volume levels much.
The tubes won't be subject to such high peak currents and should perhaps
last a few gigs longer.

Patrick Turner.

[John Byrns]

In article ,
Patrick Turner wrote:

So regardless of what you say Lord V, there is always a risk of arcing
at the tube sockets
because there is so little ever done to prevent it, like say
having diodes connected from each anode to ground, to prevent the Vswing
ever moving negative with respect to 0V, and thus prevent Eamax ever
swinging more than twice
the supply voltage.

You are assuming the transformer has no leakage inductance between
primary halves, is this a valid assumption for the typical guitar amp?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

John Byrns wrote:

In article ,
Patrick Turner wrote:

So regardless of what you say Lord V, there is always a risk of arcing
at the tube sockets
because there is so little ever done to prevent it, like say
having diodes connected from each anode to ground, to prevent the Vswing
ever moving negative with respect to 0V, and thus prevent Eamax ever
swinging more than twice
the supply voltage.

You are assuming the transformer has no leakage inductance between
primary halves, is this a valid assumption for the typical guitar amp?

The leakage inductance between halves of guitar amp OPTs is usually
high, but
nevertheless the negative going voltages which are shunted are arrested
by the diodes, and the positive going
voltages at the other end of the primary are also arrested..
And BTW, really high power tube amps for say 1,000 watts+ using
anode supply voltages of say 2.5kV and high ohm anode loads used spark
gaps adjusted so excess voltage would be
harmlessly shunted in a spark. Usually OPT were mounted in a sealed box
full of oil with ceramic stand off connections.
This does not appear in Fender or Marshall.

Back emfs are a fact of life...

Patrick Turner.

Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Ian Iveson]

zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

cheers, Ian

[Eeyore]

Ian Iveson wrote:

zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

Oh puhleeze ! Is that nonsense or what ?


As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

[Patrick Turner]

Eeyore wrote:

Ian Iveson wrote:

zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

Oh puhleeze ! Is that nonsense or what ?

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

Tube amps can be load matched to allow any low value load to be
connected since there is an OPT which may have adjustable configurations
of windings.
Even many crummy old tube amps had matching for 4 or 16 ohms by
paralleling or seriesing
the two secondary OPT windings. This allowed a range of anything above 2
ohms, because the
4 ohm load may have given a match for class A, and the 2 ohm would be
tolerated
since the load is still an OK class AB load.

The typical low levels used with hi-fi permit some load mismatch, but
not in guitar amp world,
a world of plectrum demons.

SS amps have no such matching OPT or other device, and need current /
voltage limiting.
What is NEVER fitted to an SS amp is an intelligent circuit that detects
if the load
is below 2 ohms and TURNS THE AMP OFF even if it is used at low level.
This safety option would save all the amps who expire when connected to
shorted speaker cables
or to fried/faulty speakers/crossovers.

The makers of SS amps have had 40 years to fit decent active protection
but they
prefer you buy a ****ing new amp, rather than have the old one continue
for too long.

Tubes will also glow red and thermal out if the load is too low, but it
just takes longer for it to happen.
If an OPT is destroyed in the process, it can be very expensive!


Patrick Turner.

[Eeyore]

Patrick Turner wrote:

Eeyore wrote:
Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

Oh puhleeze ! Is that nonsense or what ?

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

Tube amps can be load matched to allow any low value load to be
connected since there is an OPT which may have adjustable configurations
of windings.

I had in mind the 'typical' 4, 8 and 16 ohm taps on most power amplifiers.

Have you ever seen a 2 ohm tap btw ?

Graham

[Sander deWaal]

Eeyore said:


Have you ever seen a 2 ohm tap btw ?


Yes.
My own OPTs have 2.83 ohm taps, in fact.

I use them to drive 2 paralleled Magnepans per channel.

--

- Maggies are an addiction for life. -

[Lord Valve]

The RAT's ass mumbled:

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

Tube amps can be load matched to allow any low value load to be
connected since there is an OPT which may have adjustable configurations
of windings.

I had in mind the 'typical' 4, 8 and 16 ohm taps on most power amplifiers.

Have you ever seen a 2 ohm tap btw ?

Where ya been, junior?

Two of the "Holy Grail of Tone" amps - the Fender '59
Tweed Bassman and the Fender '65 blackface Super
Reverb - have 2-ohm outputs. (So does the famous
Ampeg SVT, designed to drive one or two 4-ohm
cabinets containing eight 10" 32-ohm speakers
wired in parallel, with a switching jack for selecting
the 2-ohm tap when two cabinets are used. Try running
one of those into an upward mismatch - 8 ohm box
on the 4-ohm tap - and watch the fireworks.) Both of the
Fender amps drive four 10" 8-ohm speakers wired in parallel.
A few times a year - *every* year since I have been repairing
guitar amps, and that would be about 40 of 'em years)
I've had to fix several of these amps which had the
speaker wiring "corrected" to the much more common
series-parallel arrangement that produces an 8-ohm
load from four 8-ohm speakers. Said "correction"
usually performed by some audiophool "expert"
who "knew" that tube amps don't drive 2-ohm
loads. Of course, as soon as this "repair" was
accomplished, the poor *******'s amp started
blowing fuses. Seems the sockets were arced.

Imagine that.

Lord Valve
Expert

[Iain Churches]

"Eeyore" wrote in message
...


Patrick Turner wrote:

Eeyore wrote:
Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

Oh puhleeze ! Is that nonsense or what ?

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve
amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

Tube amps can be load matched to allow any low value load to be
connected since there is an OPT which may have adjustable configurations
of windings.

I had in mind the 'typical' 4, 8 and 16 ohm taps on most power amplifiers.

Have you ever seen a 2 ohm tap btw ?

Yep. I have an amp on my bench with a stock Sowter UK transformer.
The secondary windings can be connected in series and in parallel for
1,4,8, and 16 Ohms.

Iain


Graham

[Patrick Turner]

Eeyore wrote:

Patrick Turner wrote:

Eeyore wrote:
Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

Oh puhleeze ! Is that nonsense or what ?

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

Tolerant to a point ! I wouldn't advise it. In fact I doubt any valve amps will
drive a 1 - 2 ohms load very well, yet many SS amps do it admirably.

Graham

Tube amps can be load matched to allow any low value load to be
connected since there is an OPT which may have adjustable configurations
of windings.

I had in mind the 'typical' 4, 8 and 16 ohm taps on most power amplifiers.

Have you ever seen a 2 ohm tap btw ?

Graham

What I said in my other posts on the subject was that many old amps have
a pair of
2 ohm windings. When seriesed, you get a match for 8 ohms, because there
are twice the turns on the sec,
and 2 squared = 4, so 2 ohms becomes 8 ohms.

When paralleled, you still only get 2 ohms, since there is no difference
to the turn count.

But many old amps were meant for 4 or 16 ohms by juggling the two lousy
windings offered.
It was either 4 or 16, and having 8 ohms meant the windings had to be
divided awkwardly, and awkward extra
matches cost extra money, so it rarely happened properly.
The 4 or 16 was for for NOMINAL MATCHES of course, never literally,
and usually the nominal meant you had 10k : 16 ohms for 20 watts of
class A, or for anything over
8 ohms for class AB, or 10K : 4 ohms, for class A, or for anything
between 2 and 4 giving acceptable class AB.

To repeat, the 16 ohms configuration meant you could possibly have 5k
a-a : 8 ohms for class AB,
or if the 8 ohms was connected to the two paralleled 4 ohm windings
you'd get 20k a-a as the load,
and a "low distortion, low power class A connection, with low output
resistance".
You could connect 4 ohms to the 4 ohm config for class A but 2 ohms
worked fine since the a-a
load only dropped to 5k a-a, and still drivable by an average pair of
6L6/KT66;
;if more power into lower loads is needed use KT88/KT90.

The OPT is a like an electronic gear box which allows you to drive up
any hill no matter how steep.

Remember the fundementals.

More basic/not so basic info is at http://www.turneraudio.com.au

Patrick Turner.

[Eeyore]

Patrick Turner wrote:

What is NEVER fitted to an SS amp is an intelligent circuit that detects
if the load
is below 2 ohms and TURNS THE AMP OFF even if it is used at low level.
This safety option would save all the amps who expire when connected to
shorted speaker cables
or to fried/faulty speakers/crossovers.

Actually QSC have a very crude system that does do this.
http://www.freepatentsonline.com/4321554.html

It's an area I've been looking into myself actually as it can hugely benefit
amplifier design by eliminating 'hard' current limiting.

Graham

[Patrick Turner]

Eeyore wrote:

Patrick Turner wrote:

What is NEVER fitted to an SS amp is an intelligent circuit that detects
if the load
is below 2 ohms and TURNS THE AMP OFF even if it is used at low level.
This safety option would save all the amps who expire when connected to
shorted speaker cables
or to fried/faulty speakers/crossovers.

Actually QSC have a very crude system that does do this.
http://www.freepatentsonline.com/4321554.html

What is the URL for the schematic?

Patrick Turner.

It's an area I've been looking into myself actually as it can hugely benefit
amplifier design by eliminating 'hard' current limiting.

Graham

[Patrick Turner]

Ian Iveson wrote:

zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

SS amps have a load of NFB and there is a Zobel network which loads the
output,
and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,
so risk of arcing in SS amps is not a problem. Sensible SS guitar amps
have diodes from the output to
the rails to prevent swings on the output ever going more than the rail
voltage peak value,
since this can occur with the inductive load of a speaker, and cause
instant death to the output
bjts with reverse flow currents.

Tube guitar amps under discussion don't have any precautions to limit
anode voltage swings way above what happens with 4,8, or 16 ohms.


Low value loads on tube guitar amps means much earlier death for the
tubes,
as evidenced in amps used for heavy metal / grunge band use where the
guitar sound
is a continuous stream of square waves during gross output tube
saturation.


NEVER use a lower than recommended load on a tube amp.
If you do, send the bill for new tubes to Ian who will pay because he
advises its OK.

Patrick Turner.

cheers, Ian

[Eeyore]

Patrick Turner wrote:

Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

SS amps have a load of NFB and there is a Zobel network which loads the
output, and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,

Not to mention diodes fron output to supply rails too.

Graham

[Phil Allison]

"Eeysore the ****ing Pommy ****head "

Patrick Turner wrote:

SS amps have a load of NFB and there is a Zobel network which loads the
output, and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,

Not to mention diodes fron output to supply rails too.



** Ahhh - the diodes are to catch those pesky flyback spikes, yet again.

The ones this ASININE POMMY ****HEAD denies exist.

ROTFLMAO !!!


What an absolute hoot this Stevenson Criminal Nutter is !!




........ Phil

[Eeyore]

Phil Allison wrote:

"Eeysore the ****ing Pommy ****head "
Patrick Turner wrote:

SS amps have a load of NFB and there is a Zobel network which loads the
output, and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,

Not to mention diodes fron output to supply rails too.


** Ahhh - the diodes are to catch those pesky flyback spikes, yet again.

The ones this ASININE POMMY ****HEAD denies exist.

ROTFLMAO !!!

What an absolute hoot this Stevenson Criminal Nutter is !!

None of which has anything to do with loading a 4 ohm tap with 8 ohms.

Graham

[Patrick Turner]

Eeyore wrote:

Patrick Turner wrote:

Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

SS amps have a load of NFB and there is a Zobel network which loads the
output, and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,

Not to mention diodes fron output to supply rails too.

Graham

But I went on to say how inductive speaker loads on an SS amp could give
rise to destructive back emfs.

SS amps need all the protection they can get.

Patrick Turner.

[Eeyore]

Patrick Turner wrote:

Eeyore wrote:
Patrick Turner wrote:
Ian Iveson wrote:
zyx wrote

I have an epi valve jr combo with 8" speaker that is 4 ohms. I want
to try it on a 12" speaker which is 8 ohms.

Jam away?

A bit risky. Anode voltage may rise higher than some parts can cope
with reliably.

As a general rule, SS amps don't care too much about a higher load
impedance, and valves are tolerant of a lower one.

SS amps have a load of NFB and there is a Zobel network which loads the
output, and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,

Not to mention diodes fron output to supply rails too.

Graham

But I went on to say how inductive speaker loads on an SS amp could give
rise to destructive back emfs.

SS amps need all the protection they can get.

I don't think they're *inherently* any more fragile to be honest.

Graham

[John Byrns]

In article ,
Patrick Turner wrote:

SS amps have a load of NFB and there is a Zobel network which loads the
output,
and there is no OPT, so no high voltages with no load, and supply
voltages are lower anyway,
so risk of arcing in SS amps is not a problem. Sensible SS guitar amps
have diodes from the output to
the rails to prevent swings on the output ever going more than the rail
voltage peak value,
since this can occur with the inductive load of a speaker, and cause
instant death to the output
bjts with reverse flow currents.

I have had that happen in an amp without the diodes you mention. When
it went it made quite a show, the transistors were in TO-3 packages and
when it went one stated arcing between the emitter post inside and the
top of the package. The arc cut a hole through the top of the package
and started blasting out a jet of hot metal and flame sort of like some
sort of plasma torch, until I killed the power.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Phil Allison]

"Patrick Turner"

Sensible SS guitar amps
have diodes from the output to
the rails to prevent swings on the output ever going more than the rail
voltage peak value,
since this can occur with the inductive load of a speaker, and cause
instant death to the output
bjts with reverse flow currents.


** Plenty of reliable BJT SS guitar and hi-fi amps have no such diodes.

( MOSFETS have such reverse diodes inherent inside the device.)

The need for "flyback diodes" only arises when the particular BJT amp
incorporates VI limiting circuitry to protect output devices from over
dissipation and SOA induced failures.

See: http://sound.westhost.com/vi.htm




........ Phil

[Patrick Turner]

Phil Allison wrote:

"Patrick Turner"

Sensible SS guitar amps
have diodes from the output to
the rails to prevent swings on the output ever going more than the rail
voltage peak value,
since this can occur with the inductive load of a speaker, and cause
instant death to the output
bjts with reverse flow currents.

** Plenty of reliable BJT SS guitar and hi-fi amps have no such diodes.

( MOSFETS have such reverse diodes inherent inside the device.)

The need for "flyback diodes" only arises when the particular BJT amp
incorporates VI limiting circuitry to protect output devices from over
dissipation and SOA induced failures.

See: http://sound.westhost.com/vi.htm

....... Phil

Your article at the above address goes a long way twoards explaining the
pitfalls
of BJt and mosfet use and VI limiting.

Rod Elliot explains further.....

-------------------------------------------------------------------------------------------------------------------------------------
Some amplifier circuits show diodes connected between the output and the
+ve and -ve power rails - these are shown in grey in Figure 1. The
purpose of these
should now be obvious - they will "catch" the spikes generated under the
conditions Phil has explained. These will (hopefully) prevent the
destruction of output
transistors by shunting the excess voltage and current back into the
power supply which simply absorbs the energy. The diodes prevent the
amplifier's output
voltage from ever exceeding the supply by more than 0.65 volt or so, and
therefore prevent reverse biasing of the transistors under fault
conditions.

Needless to say, this provides zero sonic benefit, since the spikes will
still occur, but they will now be limited to a little over the
amplifier's supply voltage. The amp
will probably not self destruct, but it will sound just as horrible when
driven beyond its current limits.

High power amplifiers (i.e. anything over about 400W) nearly always use
MOSFETs rather than bipolar transistors, and this is especially true of
professional units
for sound reinforcement. There are very good reasons for this, as Phil
has explained.

The ability to drive virtually any load without the need for VI limiter
protection is one of the all time great advantages of power MOSFETs.
Naturally, the
requirement that there are sufficient output devices to handle the load
still exists, but MOSFETs are far more tolerant of fault conditions than
bipolar transistors, and
are not subject to a most undesirable problem known as "second
breakdown".

This occurs in bipolar transistors when a small section of the silicon
die becomes hotter than the rest due to an overload condition, or where
the devices are too
small for the job. This increases the gain in that small section, and
it therefore does even more of the work, causing it to get hotter
still. Thus a repeating the cycle is
started, which results in device destruction. This happens very fast
(in as little as a few tens of milliseconds), and it is virtually
impossible to protect the device once
second breakdown has started. The VI limiters are designed to prevent
the device(s) from ever exceeding the manufacturer limits where second
breakdown is
known to occur.

As Phil has shown, some attempts at achieving that goal are somewhat
less than a complete success.
-------------------------------------------------------------------------------------------------------------------------------------


My experience agrees generally with what Phil and Rod are saying.

I was taught early that having fuses in the rails and at the output
will prevent gross sudden current surges if the fuses are fast blow
types especially.

Mosfets have self limiting behaviour because they are transconductance
devices like tubes,
but will still produce huge currents, and if multiple mosfets are used
in an output stage it
it wise to have back to back external zeners of considerably less than
the 12V zeners fitted internally
to limit gate to source voltage, and hence maximum current flow via
drain to source.

I still place the diodes between output and rails anyway, even though it
may be argued that
these diodes to limit the output voltage ever swinging more than the
rails may not really be necessary.
I believe there is utterly no sonic penalty for these "clamping diodes".

Another failure mechanism in SS amps can be due to cross conduction
where
the pnp and npn devices fail to turn off quickly between each half of
the
wave form in class AB operation at F above say 10kHz.
When tested up to clipping level at say 30kHz, one sees that
inexplicably there is a huge
increase in rail supply current, even without a load.
Douglas Self alerted this problem to me when I first noticed this, but
his solution
to the phenomena problem wasn't completely as successful as I'd wish,
and I am still left wondering what could be done that would work better
than the Self method.
I have not noticed the same bother with mosfets, and not with vacuum
tubes of course.

Perhaps you may like to shed light on the phenomena of cross conduction.

Patrick Turner.

[Phil Allison]

"Patrick Turner"
Phil Allison wrote:


** Plenty of reliable BJT SS guitar and hi-fi amps have no such diodes.

( MOSFETS have such reverse diodes inherent inside the device.)

The need for "flyback diodes" only arises when the particular BJT amp
incorporates VI limiting circuitry to protect output devices from over
dissipation and SOA induced failures.

See: http://sound.westhost.com/vi.htm


Your article at the above address goes a long way twoards explaining the
pitfalls of BJt and mosfet use and VI limiting.


** Barely a beginner's primer - in fact.

Like most seemingly simple things - it soon gets all complicated when you
look right into it.

UK songwriter, Pete Townshend , said it all over 40 years ago.

In a little tune called "Substitute".

No allusions to BJTs intended.....



Rod Elliot explains further.....


** Well, he had to use his " editor's discretion " - didn't he ?.





....... Phil