[Lost'n Found]

Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven, the
speaker seems it is about to pop out, the memberane jumps all the way up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

[Andy Evans]

It sounds like you are designing a stage amp where you want a kind of
controlled distortion. Take a look at some existing designs, say by
Fender for starters - put that into Google. I'm no expert on these
circuits, but I believe the principle is to overdrive the input stage
in a controlled way, and not to distort the output stage, and where you
put your volume controls affects this. Studying existing designs might
help you see where your own design is different or flawed.

[Eeyore]

Andy Evans wrote:

It sounds like you are designing a stage amp where you want a kind of
controlled distortion. Take a look at some existing designs, say by
Fender for starters - put that into Google. I'm no expert on these
circuits, but I believe the principle is to overdrive the input stage
in a controlled way, and not to distort the output stage, and where you
put your volume controls affects this. Studying existing designs might
help you see where your own design is different or flawed.

The output stage distorts like the clappers.

That's where most of the colouration in a guitar amp comes from. You're quite
wrong as ever.

Playing a guitar amp quietly won't have 'that sound' nor will a hi-fi valve
design be particularly suited to a guitar application.

Graham

[Eeyore]

Lost'n Found wrote:

Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven, the
speaker seems it is about to pop out, the memberane jumps all the way up.

The speaker is being overdriven. The voice coil should stay in the magnetic gap
at all times.


I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

It won't be a lack of a tone control per se.

I suspect your HT voltage ( B+ ) is sagging and modulating the output with the
envelope of the music dynamics. That would explain it.

Graham

[Ian Iveson]

Eeyore wrote:

I suspect your HT voltage ( B+ ) is sagging and modulating the
output with the
envelope of the music dynamics. That would explain it.

How?

Please.

How can this young student test for this condition?

Thanks.

Ian

[Eeyore]

Ian Iveson wrote:

Eeyore wrote:

I suspect your HT voltage ( B+ ) is sagging and modulating the
output with the envelope of the music dynamics. That would explain it.

How?

Please.

As B+ reduces, the gain reduces too. Supply fluctuations also directly affect
the audio. That's one way you get hum in the output too. If the B+ fluctuations
are large, then the signal will be modulated by that affect and the effect will
indeed be low-frequency too. Also see 'motorboating' for another example of the
effect.


How can this young student test for this condition?

With a common or garden DVM.

Meaure the B+ in absence of signal and then when 'flat out'. A large droop will
be cause for concern.

Graham

[west]

"Lost'n Found" wrote in message
...
Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven,
the
speaker seems it is about to pop out, the memberane jumps all the way up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

Has anyone considered a DC offset (DC getting into the OPT). That would push
the speaker out. Should only take less than 1 minute to check.

west

[Bob H.]

Lost'n Found wrote:
Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

It does sound like low freq feedback.

How much filter capacitance are you using for each stage of the amp?

What size resistor is used between the driver tube and output tubes?

What is the power rating of the power supply transformer?

thanks,
Bob H.

[Bob H.]

What size resistor is used between the driver tube and output tubes?
I mean the dropping resistor in the power supply feed.
Bob H.

[Andy Evans]

Eeyore wrote:
Andy Evans wrote:

It sounds like you are designing a stage amp where you want a kind of
controlled distortion. Take a look at some existing designs, say by
Fender for starters - put that into Google. I'm no expert on these
circuits, but I believe the principle is to overdrive the input stage
in a controlled way, and not to distort the output stage, and where you
put your volume controls affects this. Studying existing designs might
help you see where your own design is different or flawed.

The output stage distorts like the clappers.

That's where most of the colouration in a guitar amp comes from. You're quite
wrong as ever.

Playing a guitar amp quietly won't have 'that sound' nor will a hi-fi valve
design be particularly suited to a guitar application.

Graham

What on earth are you talking about? What you want is a clean output
stage - that provides a basically clean and clear sound. The deliberate
overload that guitarists use is achieved by controlled overloading of
the input valve - that provides the 'whine' and singing tone that rock
guitarists are so fond of and which is a particular characteristic of
valves. That's why valves are uniquely suitable for this - they have
the right overload characteristics.

[PhattyMo]

west wrote:
"Lost'n Found" wrote in message
...
Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven,
the
speaker seems it is about to pop out, the memberane jumps all the way up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

Has anyone considered a DC offset (DC getting into the OPT). That would push
the speaker out. Should only take less than 1 minute to check.

west


Perhaps if this amp has neg. feedback from the OPT secondary,and it is
mis-wired?
But most guitar amps don't have any NFB..

[Lord Valve]

PhattyMo wrote:

But most guitar amps don't have any NFB..

Know a lot about those, eh?

LV

[Eeyore]

Andy Evans wrote:

Eeyore wrote:
Andy Evans wrote:

It sounds like you are designing a stage amp where you want a kind of
controlled distortion. Take a look at some existing designs, say by
Fender for starters - put that into Google. I'm no expert on these
circuits, but I believe the principle is to overdrive the input stage
in a controlled way, and not to distort the output stage, and where you
put your volume controls affects this. Studying existing designs might
help you see where your own design is different or flawed.

The output stage distorts like the clappers.

That's where most of the colouration in a guitar amp comes from. You're quite
wrong as ever.

Playing a guitar amp quietly won't have 'that sound' nor will a hi-fi valve
design be particularly suited to a guitar application.

Graham

What on earth are you talking about? What you want is a clean output
stage - that provides a basically clean and clear sound. The deliberate
overload that guitarists use is achieved by controlled overloading of
the input valve - that provides the 'whine' and singing tone that rock
guitarists are so fond of and which is a particular characteristic of
valves. That's why valves are uniquely suitable for this - they have
the right overload characteristics.

Total ********.

A guitar amp output stage distorts like the clappers.

Graham

[Andy Evans]

A guitar amp output stage distorts like the clappers

Only if you want the sound to break up completely (numetal etc). Have
you ever gigged with an intrument like the guitar or bass?

[Ian Iveson]

Eeyore wrote

I suspect your HT voltage ( B+ ) is sagging and modulating the
output with the envelope of the music dynamics. That would
explain it.

How?

Please.

As B+ reduces, the gain reduces too. Supply fluctuations also
directly affect
the audio. That's one way you get hum in the output too. If the B+
fluctuations
are large, then the signal will be modulated by that affect and the
effect will
indeed be low-frequency too. Also see 'motorboating' for another
example of the
effect.


How can this young student test for this condition?

With a common or garden DVM.

Meaure the B+ in absence of signal and then when 'flat out'. A large
droop will
be cause for concern.

Graham

I see. Thanks.

So, ignoring the extraneous point about motorboating, and the
irrelevant mistake about hum, what reading would confirm your
diagnosis?

Just to recap. The OP has stated that the amp is OK at low volumes,
but when the volume is turned up, the speakers reach the extent of
their travel, noticeable visibly at low frequencies. You say this may
be because of HT sag, which may be tested with a voltmeter.

What meter reading would confirm your diagnosis, please?

Thanks, Ian

[Ian Iveson]

Andy Evans wrote

A guitar amp output stage distorts like the clappers

Only if you want the sound to break up completely (numetal etc).
Have
you ever gigged with an intrument like the guitar or bass?

How would that help? If gigging guitarists knew how their guitars and
amps work, Lord Valve for one might be short of a job.

Many guitar amps are designed with overdrive of the output stage in
mind. If you look at the driver stage you should be able to see that,
in many cases, the source impedance seen by the output stage has been
arranged to give smooth sagging of the signal as the output valves'
grids begin to pass current.

cheers, Ian

[Eeyore]

Andy Evans wrote:

A guitar amp output stage distorts like the clappers

Only if you want the sound to break up completely (numetal etc). Have
you ever gigged with an intrument like the guitar or bass?

I've been mixing bands for 35 yrs if that helps.

Graham

[Phread]

"Andy Evans" wrote in message
oups.com...
A guitar amp output stage distorts like the clappers

Only if you want the sound to break up completely (numetal etc). Have
you ever gigged with an intrument like the guitar or bass?

Andy,

The other day a guy brought his guitar in to test a little solid state Fender I'd
fixed for him. He put on an amazing 5 minute show with the amp in severe
distortion; he had everyone in the shop standing around listening and
applauding by the time he was done. He remarked, "I can do that on my
Marshall stack, too, but you'd have to leave the room."

The distortion rock guitar players love is in the output stage. To get the 100
watt Marshall into distortion, it has to get REALLY LOUD! That's why a lot of
Marshall owners also own power plates & such, resistive loads that soak up
most of the power an amp puts out, to keep themselves and their audience
from going deaf when they put their amps into distortion in a smaller room.

Fred

[Nelson Gietz]

"Here in Ohio" wrote in message
...
On Tue, 21 Nov 2006 13:06:59 -0500, Here in Ohio
wrote:


Has anyone considered a DC offset (DC getting into the OPT). That would
push
the speaker out. Should only take less than 1 minute to check.


I guess a good question would be whether the speaker "pops out" and
stays there, or whether it "jumps in and out." The first would seem to
indicate a DC offset (which would be a good trick since the OPT won't
pass it) or whether the speaker is being overdriven.


Yeah, I can type, really. :-)

That should have been:

The first would seem to indicate a DC offset (which would be a good
trick since the OPT won't pass it) and the second might mean the
speaker is being overdriven.

Any chance the original poster is using an air suspension speaker
in an open box? Lack of air loading on the speaker would let it run
'way too freely, unless he's got one with stiffer suspension.
Cheers,
Nelson

[Andy Evans]

I think you're both right and you're both wrong. :-)

Different strokes for different folks and all that.

What I mean about overloading the first valve is the way it's done in
circuits with two volume controls - before and after the first stage -
which you see on some but not all stage amps. That gives you the choice
of where you want the overload to take place. I assumed that to get
that mellow whine sustaining like a violin, you mostly overloaded the
first stage, because you don't actually want the sound to break up, as
you do with nu-metal for instance. You understand what I mean here? You
can do it anyway you like - depends on what effect you want out of it.
Hence the two volume controls.

I'm a bass player (mostly double bass), so I've always gone for the
clean sound with a bit of growl and sustain, and when I play keyboards
it's the same. The guitar world is a whole theatre of its own, and
whatever rocks your boat........... as you say.

[PhattyMo]

Lord Valve wrote:

PhattyMo wrote:

But most guitar amps don't have any NFB..

Know a lot about those, eh?

LV

Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

[Eeyore]

Nelson Gietz wrote:

Any chance the original poster is using an air suspension speaker
in an open box?

For the excursion to be so high I suspect that must be the case.


Lack of air loading on the speaker would let it run
'way too freely, unless he's got one with stiffer suspension.

Absolutely right.

Graham

[Eeyore]

PhattyMo wrote:

Lord Valve wrote:
PhattyMo wrote:

But most guitar amps don't have any NFB..

Know a lot about those, eh?

LV

Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

Well I have !

Graham

[Patrick Turner]

Lost'n Found wrote:

Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven, the
speaker seems it is about to pop out, the memberane jumps all the way up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

The muddy splashy sound is due to distortions.
The amp is also producing very low frequency
waves at the output due to dc bias swings probably
because of too large a value of coupling cap to the output tubes
from the driver stage; 0.033uF is usually big enough
wheras in a hi-fi amp one might use 0.47uF.

The slow cone movements are due to dc voltage changes and trying to
re-set itself up after overloads.

Guitar amps only need to go down to 40Hz, -3dB,
unless its a bass amp, and 30Hz is usually plenty.

Since you say you are designing an amp project for your school,
then you'd need to be conscious of just what happens dynamically in an
amp
during low distortion use and after overload when distortion
is double figure %.

Perhaps you have a circuit error that you have overlooked....

Patrick Turner.

[PhattyMo]

Eeyore wrote:

PhattyMo wrote:

Lord Valve wrote:
PhattyMo wrote:

But most guitar amps don't have any NFB..
Know a lot about those, eh?

LV
Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

Well I have !

Graham



Guitar amps? Got any links?

[jh]

PhattyMo schrieb:

Eeyore wrote:

PhattyMo wrote:

Lord Valve wrote:
PhattyMo wrote:

But most guitar amps don't have any NFB..
Know a lot about those, eh?

LV
Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

Well I have !

Graham


Guitar amps? Got any links?
#

ampwares.com go to the fender field guid
drtube.com
duncanamps.com
......

BTW most guit amps have NFB

regards

Jochen

[Sander deWaal]

jh said:


Guitar amps? Got any links?
#

ampwares.com go to the fender field guid
drtube.com
duncanamps.com


http://www.ax84.com/
http://www.drtube.com/guitamp.htm
http://www.diyguitaramp.com/tech.html
http://www.schematicheaven.com/
http://blueguitar.org/schems.htm
http://www.kbapps.com/tubeamps.html
http://www.diyguitarist.com/AmpStuff.htm
http://www.triodeel.com/schindex.htm
http://www.thetubestore.com/schematics.html


--
- Ever seen someone with 5.1 ears? So, what does that tell you? -

[Superhet]

Anyone wonder why the poster is not participating? I think he just
wanted to get a rise, jerk your chain as it were!

Patrick Turner wrote:

Lost'n Found wrote:
Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven, the
speaker seems it is about to pop out, the memberane jumps all the way up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

The muddy splashy sound is due to distortions.
The amp is also producing very low frequency
waves at the output due to dc bias swings probably
because of too large a value of coupling cap to the output tubes
from the driver stage; 0.033uF is usually big enough
wheras in a hi-fi amp one might use 0.47uF.

The slow cone movements are due to dc voltage changes and trying to
re-set itself up after overloads.

Guitar amps only need to go down to 40Hz, -3dB,
unless its a bass amp, and 30Hz is usually plenty.

Since you say you are designing an amp project for your school,
then you'd need to be conscious of just what happens dynamically in an
amp
during low distortion use and after overload when distortion
is double figure %.

Perhaps you have a circuit error that you have overlooked....

Patrick Turner.

[Eeyore]

PhattyMo wrote:

Eeyore wrote:
PhattyMo wrote:
Lord Valve wrote:
PhattyMo wrote:

But most guitar amps don't have any NFB..
Know a lot about those, eh?

LV
Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

Well I have !

Graham

Guitar amps? Got any links?

They'll be sketched on paper if I can even find them !

Graham

[Andy Evans]

Anyone wonder why the poster is not participating? I think he just
wanted to get a rise, jerk your chain as it were!

I think OPs are too stunned at the weirdness of the responses to
continue.

[Ian Iveson]

jh wrote

ampwares.com go to the fender field guid
drtube.com
duncanamps.com
.....

BTW most guit amps have NFB

In several places.

But not global, obviously. Generally nfb from the output is returned
to an inverter/driver stage. Not much gain in the loop, and the fb is
weak.

cheers, Ian

[Ian Iveson]

Superhet wrote

Anyone wonder why the poster is not participating? I think he just
wanted to get a rise, jerk your chain as it were!

Poor old Lost. Being Found was an illusion.

cheers, Ian

[PhattyMo]

flipper wrote:
On Tue, 21 Nov 2006 17:30:31 -0800, PhattyMo
wrote:

Lord Valve wrote:
PhattyMo wrote:

But most guitar amps don't have any NFB..
Know a lot about those, eh?

LV
Not really,but in my schematic browsing,I don't think I've seen any
circuits with NFB taken from the OPT secondary..
Just an observation,really.

Don't know how you missed 'em but keep browsing because it's common as
dirt.

*bangs head on desk* I don't either..sheesh,I must be blind..



Try here for Fenders. http://www.ampwares.com/ffg/

See the Narrow Panel Champ, 5E1, for a single ended example.

See the Black Face Tremolux, AA763, for an example of classic feedback
into the long tail pair phase splitter (also serves as the tail
resistor bootstrap).

I just finished building a little single channel 20 watter of my own
design with 6CW5s on the output using a similar 12AT7 LPT with 11dB of
feedback, except I added a resistor splitting the feedback and
bootstrap to fine tune the two separately.

[jh]

Ian Iveson schrieb:
jh wrote

ampwares.com go to the fender field guid
drtube.com
duncanamps.com
.....

BTW most guit amps have NFB


In several places.

But not global, obviously. Generally nfb from the output is returned
to an inverter/driver stage. Not much gain in the loop, and the fb is
weak.

cheers, Ian



Ian

of course most times only in the power amp, with some exceptions to the
rule.
and it´s enough to *hear and feel* the difference if you take it out of
the circuit.

Jochen

[Lost'n Found]

Hello all,

Thanks for your replies. They all helped. I had a bit of flaws here and
there in the amplifier.
I think the main problem was the coupling caps to the output tube. I found 1
uf rated at 450v, so I used them assuming that if they are higher than what
I need, they should be fine.

I am rewiring the amplifier now and adding a tone contorl to the circuit.
I'll keep you all posted.

Thanks for all your post. Now I am ready for many questions to be asked :P





"Superhet" wrote in message
...
Anyone wonder why the poster is not participating? I think he just wanted
to get a rise, jerk your chain as it were!

Patrick Turner wrote:

Lost'n Found wrote:
Hello,

I am designing a tube amplifier as a project for my school.

It works now fine when the volume is low and sound is clean.

However, when I raise the volume to overdrive the amp, a muddy sound
comes
out from the speaker, like the sound is splashing.

I am using 12AX7 in the drive stage and 6550A in the output stage in AB
push-pull configuration, HT = 350v

I was looking at the speaker and noticed that when that when overdriven,
the
speaker seems it is about to pop out, the memberane jumps all the way
up.

I don't have a tone control yet in the circuit. Is it because of low
frequencies that the speaker cannot keep up with?

And if the signal is clipping in the drive stage, shouldn't it give the
regular guitar distortion sound? Or are there some tricks to this? It
sounded like there is not enough distortion.

The muddy splashy sound is due to distortions.
The amp is also producing very low frequency waves at the output due to
dc bias swings probably
because of too large a value of coupling cap to the output tubes
from the driver stage; 0.033uF is usually big enough wheras in a hi-fi
amp one might use 0.47uF.

The slow cone movements are due to dc voltage changes and trying to
re-set itself up after overloads.

Guitar amps only need to go down to 40Hz, -3dB,
unless its a bass amp, and 30Hz is usually plenty.

Since you say you are designing an amp project for your school,
then you'd need to be conscious of just what happens dynamically in an
amp during low distortion use and after overload when distortion
is double figure %.

Perhaps you have a circuit error that you have overlooked....

Patrick Turner.