Let's talk about tube amps

[sortech]

I posted a while back about rusty transformers and got lots of response.
It turns out they were rusty because the amp had been ridiculously
over heated. The amp is a mesa boogie tipple rectifier and some fool had
added a resistor to the bias supply increasing the cathode current. He
must have had some interesting sounds as the power transformer primary
shorted and B+ went up and up, it still does this but no output tubes
were in place as I tested it. lots of nasty stuff under the power trans
that is some kind of potting compound. So it needs that replaced but I
am not sure if the output trans is good or not. The primary reads 21 and
23 ohms from the center tap. This is a 160 watt amp with 6-6l6GC's.
The output trans is good and rusty so I am sure it got overheated as well.
Also there was lots of glass chips from the output tubes under the
chassis, could the tubes have exploded? Any thoughts?

[PeterD]

On Fri, 31 Jul 2009 09:20:49 -0600, sortech > wrote:

>I posted a while back about rusty transformers and got lots of response.
> It turns out they were rusty because the amp had been ridiculously
>over heated.

No the were not. heat doesn't make rust, moisture and air do.

[Ian Iveson]

PeterD wrote wrote:

>>I posted a while back about rusty transformers and got
>>lots of response.
>> It turns out they were rusty because the amp had been
>> ridiculously
>>over heated.
>
> No the were not. heat doesn't make rust, moisture and air
> do.

Moisture, oxygen, heat and a voltage or current source do.

Overheating may remove surface protection, and repeatedly
evaporating condensation from bare steel by heating it can
accelerate the rusting process dramatically, especially in
the presence of imposed electric currents.

Ian

[Engineer[_2_]]

On Jul 31, 4:38*pm, PeterD > wrote:
> On Fri, 31 Jul 2009 09:20:49 -0600, sortech > wrote:
> >I posted a while back about rusty transformers and got lots of response.
> > *It turns out they were rusty because the amp had been ridiculously
> >over heated.
>
> No the were not. heat doesn't make rust, moisture and air do.

Peter is right, of course. Your amplifier needs a lot more careful
tests than the inconsistent forensics you have reported. Take the OPT
out of circuit and test with 6.3 VAC on the secondary and measure the
ratios P-P and P to center tap. Impedance matching is ratio squared.
The power transformer needs similar testing - run 6.3 VAC into the
heater winding and measure rectifier P-P and P to centre tap volts.
High current in the 6 volt test supply would indicate shorted turns.
Then there's all the o/p tube wiring to check... I suggest you let
tech do it!
Cheers,
Roger

[Phil Allison[_3_]]

"sortech"

>I posted a while back about rusty transformers and got lots of response. It
>turns out they were rusty because the amp had been ridiculously over
>heated.

** Utter nonsense.

The amp has been allowed to get wet - most likely from condensation while
in storage.

I see that all the time.



> The amp is a mesa boogie tipple rectifier and some fool had added a
> resistor to the bias supply increasing the cathode current. He must have
> had some interesting sounds as the power transformer primary shorted and
> B+ went up and up,


** No such thing happens.

Shorted turns in the primary causes large currents to flow and makes the AC
fuse blow.


> lots of nasty stuff under the power trans that is some kind of potting
> compound. So it needs that replaced but I am not sure if the output trans
> is good or not. The primary reads 21 and 23 ohms from the center tap.

** Sounds about right.


> This is a 160 watt amp with 6-6l6GC's. The output trans is good and rusty
> so I am sure it got overheated as well.

** Drivel.

> Also there was lots of glass chips from the output tubes under the
> chassis, could the tubes have exploded?

** If output tubes get wet during use, the glass often breaks.

They break if you hit them hard enough.

They break if they bang into each other ....



...... Phil

[Ian Iveson]

sortech wrote:

>I posted a while back about rusty transformers and got lots
>of response. It turns out they were rusty because the amp
>had been ridiculously over heated.

Overheating might have played some part, but damp is the
primary culprit.

>The amp is a mesa boogie tipple rectifier and some fool had
>added a resistor to the bias supply increasing the cathode
>current. He must have had some interesting sounds as the
>power transformer primary shorted and B+ went up and up,

In what way does "up and up" differ from "up"? What do you
mean by shorted? I can understand that you want to join in
with all the fun we have here, so I don't wish to dampen
your ardour, but you need to think and write more clearly.
Try proper paragraphs.

It might be useful, for the purpose of grasping how
transformers work, to consider what happens if just some of
the primary windings become shorted. Would that lead to an
increase in secondary voltage? There is an obvious
theoretical reason to think that it might, and a rather more
complicated reason, to realise that it most probably won't.
The best assumption is that the manufacturer has used as few
turns as possible.

Your shorted primary hypothesis is not best tested by
plugging it into the mains.

But it made me think of a nice example of a "reductio ad
absurdam". If you continue to short primary turns, then
secondary voltage should keep rising because the turns ratio
increases. In that case, when you have no turns, then you
should have a very high secondary voltage indeed.

> it still does this but no output tubes were in place as I
> tested it.

Still does what, exactly? HT voltage will normally be higher
with valves removed, but by how much depends on the details
of the circuit, which few here will be familiar with.

> lots of nasty stuff under the power trans that is some
> kind of potting compound. So it needs that replaced but I
> am not sure if the output trans is good or not. The
> primary reads 21 and 23 ohms from the center tap.

Likely to be normal. The difference is because the two
halves will have the same number of turns, but can't occupy
the same space, so one will have a different average coil
diameter, and so a different length of wire.

> This is a 160 watt amp with 6-6l6GC's. The output trans is
> good and rusty so I am sure it got overheated as well.
> Also there was lots of glass chips from the output tubes
> under the chassis, could the tubes have exploded? Any
> thoughts?

Explosions are scarce, I should imagine, although you could
have fun thinking up a possible scenario. I wonder how much
of the contents would have to vaporise before sufficient
positive pressure resulted?

Implosions are more likely, because of the vacuum.

Ian

[Phil Allison[_3_]]

"sortech"


> The amp is a mesa boogie tipple rectifier ...


** The drinking man's favourite model.




...... Phil

[Phil Allison[_3_]]

"sortech"

> The amp is a mesa boogie tipple rectifier


** Here is a pic of the Mesa Boogie " Triple Rectifier " :

http://img1.iwascoding.de/1/2009/05/23/9F694DD29A474D3B90F2C438618CA3BA.jpg


No rusty trannys in this one....



..... Phil

[Fred[_8_]]

"Ian Iveson" > wrote in message ...
> sortech wrote:
>
>>I posted a while back about rusty transformers and got lots of response. It turns out they were rusty because the amp had been
>>ridiculously over heated.
>
> Overheating might have played some part, but damp is the primary culprit.
>
>>The amp is a mesa boogie tipple rectifier and some fool had added a resistor to the bias supply increasing the cathode current. He
>>must have had some interesting sounds as the power transformer primary shorted and B+ went up and up,
>
> In what way does "up and up" differ from "up"? What do you mean by shorted? I can understand that you want to join in with all the
> fun we have here, so I don't wish to dampen your ardour, but you need to think and write more clearly. Try proper paragraphs.
>
> It might be useful, for the purpose of grasping how transformers work, to consider what happens if just some of the primary
> windings become shorted. Would that lead to an increase in secondary voltage? There is an obvious theoretical reason to think that
> it might, and a rather more complicated reason, to realise that it most probably won't. The best assumption is that the
> manufacturer has used as few turns as possible.
>
> Your shorted primary hypothesis is not best tested by plugging it into the mains.
>
> But it made me think of a nice example of a "reductio ad absurdam". If you continue to short primary turns, then secondary voltage
> should keep rising because the turns ratio increases. In that case, when you have no turns, then you should have a very high
> secondary voltage indeed.

I love it, Ian, the ultimate divide by zero example! Infinite
current into the primary; infinite voltage on the secondary!
Well done!

- Fred

>> it still does this but no output tubes were in place as I tested it.
>
> Still does what, exactly? HT voltage will normally be higher with valves removed, but by how much depends on the details of the
> circuit, which few here will be familiar with.
>
>> lots of nasty stuff under the power trans that is some kind of potting compound. So it needs that replaced but I am not sure if
>> the output trans is good or not. The primary reads 21 and 23 ohms from the center tap.
>
> Likely to be normal. The difference is because the two halves will have the same number of turns, but can't occupy the same space,
> so one will have a different average coil diameter, and so a different length of wire.
>
>> This is a 160 watt amp with 6-6l6GC's. The output trans is good and rusty so I am sure it got overheated as well.
>> Also there was lots of glass chips from the output tubes under the chassis, could the tubes have exploded? Any thoughts?
>
> Explosions are scarce, I should imagine, although you could have fun thinking up a possible scenario. I wonder how much of the
> contents would have to vaporise before sufficient positive pressure resulted?
>
> Implosions are more likely, because of the vacuum.
>
> Ian

[sortech]

Ian Iveson wrote:
> sortech wrote:
>
>> I posted a while back about rusty transformers and got lots
>> of response. It turns out they were rusty because the amp
>> had been ridiculously over heated.
>
> Overheating might have played some part, but damp is the
> primary culprit.
>
>> The amp is a mesa boogie tipple rectifier and some fool had
>> added a resistor to the bias supply increasing the cathode
>> current. He must have had some interesting sounds as the
>> power transformer primary shorted and B+ went up and up,
>
> In what way does "up and up" differ from "up"? What do you
> mean by shorted? I can understand that you want to join in
> with all the fun we have here, so I don't wish to dampen
> your ardour, but you need to think and write more clearly.
> Try proper paragraphs.
>
> It might be useful, for the purpose of grasping how
> transformers work, to consider what happens if just some of
> the primary windings become shorted. Would that lead to an
> increase in secondary voltage? There is an obvious
> theoretical reason to think that it might, and a rather more
> complicated reason, to realise that it most probably won't.
> The best assumption is that the manufacturer has used as few
> turns as possible.
>
> Your shorted primary hypothesis is not best tested by
> plugging it into the mains.
>
> But it made me think of a nice example of a "reductio ad
> absurdam". If you continue to short primary turns, then
> secondary voltage should keep rising because the turns ratio
> increases. In that case, when you have no turns, then you
> should have a very high secondary voltage indeed.
>
>> it still does this but no output tubes were in place as I
>> tested it.
>
> Still does what, exactly? HT voltage will normally be higher
> with valves removed, but by how much depends on the details
> of the circuit, which few here will be familiar with.
>
>> lots of nasty stuff under the power trans that is some
>> kind of potting compound. So it needs that replaced but I
>> am not sure if the output trans is good or not. The
>> primary reads 21 and 23 ohms from the center tap.
>
> Likely to be normal. The difference is because the two
> halves will have the same number of turns, but can't occupy
> the same space, so one will have a different average coil
> diameter, and so a different length of wire.
>
>> This is a 160 watt amp with 6-6l6GC's. The output trans is
>> good and rusty so I am sure it got overheated as well.
>> Also there was lots of glass chips from the output tubes
>> under the chassis, could the tubes have exploded? Any
>> thoughts?
>
> Explosions are scarce, I should imagine, although you could
> have fun thinking up a possible scenario. I wonder how much
> of the contents would have to vaporise before sufficient
> positive pressure resulted?
>
> Implosions are more likely, because of the vacuum.
>
> Ian
>
>
I meant that when first powered up the voltage was normal for a low load
condition and over a period of a few minutes the voltage increased about
12 percent. The transformer also buzzed more as it warmed and this was
the case with a Marshal I repaired a few years ago that needed a power
transformer.

I checked the line voltage and it was constant at 117 volts. My area has
an old power system. So I was thinking only a few windings had shorted
before I discontinued the test. The leaking potting material also lead
me to think the transformer was likely to be defective.

"The drinking man's favourite model." Yes this is the most probable
explanation for the glass fragments. Perhaps a collision between power
tubes and beer bottle lead to the amps present condition.

Is it not likely that a test not using line voltage would not heat the
transformer and therefor not show a short?

I did fail to provide enough information in my first post but would be
pleased to let the blame fall on the US educational system for the
sentence structure.

Eric

[sortech]

Engineer wrote:
> On Jul 31, 4:38 pm, PeterD > wrote:
>> On Fri, 31 Jul 2009 09:20:49 -0600, sortech > wrote:
>>> I posted a while back about rusty transformers and got lots of response.
>>> It turns out they were rusty because the amp had been ridiculously
>>> over heated.
>> No the were not. heat doesn't make rust, moisture and air do.
>
> Peter is right, of course. Your amplifier needs a lot more careful
> tests than the inconsistent forensics you have reported. Take the OPT
> out of circuit and test with 6.3 VAC on the secondary and measure the
> ratios P-P and P to center tap. Impedance matching is ratio squared.
> The power transformer needs similar testing - run 6.3 VAC into the
> heater winding and measure rectifier P-P and P to centre tap volts.
> High current in the 6 volt test supply would indicate shorted turns.
> Then there's all the o/p tube wiring to check... I suggest you let
> tech do it!
> Cheers,
> Roger
Perhaps, but engineers often come to me.
Eric

[Phil Allison[_3_]]

"sortech"

> I meant that when first powered up the voltage was normal for a low load
> condition and over a period of a few minutes the voltage increased about
> 12 percent.


** Bizzare.

Been servicing valve guitar amps for 4 decades and never seen that.

Even a few shorted turns blows the AC fuse, real quick.


> The transformer also buzzed more as it warmed

** Very common for loose laminations to begin to buzz when the tranny warms
up.


> I checked the line voltage and it was constant at 117 volts. My area has
> an old power system. So I was thinking only a few windings had shorted
> before I discontinued the test. The leaking potting material also lead me
> to think the transformer was likely to be defective.


** That last bit indicates a bad tranny.

Plus an evil smell and popping /cracking noises as it heats up.


> "The drinking man's favourite model." Yes this is the most probable
> explanation for the glass fragments. Perhaps a collision between power
> tubes and beer bottle lead to the amps present condition.


** The " Triple Rectifier" is famously known as the model played by Saddam
Hussein.

Even his friends said he sounded like Sheite ....




...... Phil

[Eeyore[_3_]]

sortech wrote:

> I posted a while back about rusty transformers and got lots of response.
> It turns out they were rusty because the amp had been ridiculously
> over heated.

NO !

Heat doesn't cause rust in any way or form. Moisture does.

Graham

--
due to the hugely increased level of spam please make the obvious adjustment
to my email address

[Eeyore[_3_]]

PeterD wrote:

> On Fri, 31 Jul 2009 09:20:49 -0600, sortech > wrote:
>
> >I posted a while back about rusty transformers and got lots of response.
> > It turns out they were rusty because the amp had been ridiculously
> >over heated.
>
> No the were not. heat doesn't make rust, moisture and air do.

I suspect quite a few of these answers.

Being left in a warm damp place ( e.g. Florida ) would encourage rust too.

Graham


--
due to the hugely increased level of spam please make the obvious adjustment
to my email address

[Eeyore[_3_]]

Phil Allison wrote:

> "sortech"
>
> > The amp is a mesa boogie tipple rectifier
>
> ** Here is a pic of the Mesa Boogie " Triple Rectifier " :
>
> http://img1.iwascoding.de/1/2009/05/23/9F694DD29A474D3B90F2C438618CA3BA.jpg

WTF did they use toob rectifiers for ? To increase the price ? Hype ?

Graham

--
due to the hugely increased level of spam please make the obvious adjustment to
my email address

[Phil Allison[_3_]]

"Eeyore"
Phil Allison wrote:
> "sortech"
>>
>> > The amp is a mesa boogie tipple rectifier
>>
>> ** Here is a pic of the Mesa Boogie " Triple Rectifier " :
>>
>> http://img1.iwascoding.de/1/2009/05/23/9F694DD29A474D3B90F2C438618CA3BA.jpg
>
> WTF did they use toob rectifiers for ? To increase the price ? Hype ?



** Like any audio myth - it has a long and *very sordid* history !!!!

Fact: The earliest tube guitar amps all used " tube " rectifiers.

Fact: Lectic gittar players are obsessed with the " sound " of early
amplifiers - since they falsely associate that with the distinctive
playing of many, now legendary, early electric blues and rock guitarists.

Fact: Tubes are automatically considered way cool.

Fact: It is a given that more tubes are better than less tubes.

Fact: The use of tube rectifiers is responsible for maybe just a * tad more
noticeable * amount of amplitude compression when the amp is heavily
overdriven as compared with otherwise similar SS rectifier models.

Fact: If you are in the business of FLOGGING tube guitar amps for an
income to live on - then clutching at such tenuous straws is perfectly
kosher.

Fact: If two is better than one - then three is one better again.


PLUS :

Never forget THE number one, most fundamental tenet of the AMERICAN way of
life:

" If some's good,

then more is better and

TOO MUCH is just right !! "





...... Phil

[Ian Iveson]

Eeyore wrote:
>>
>> No the were not. heat doesn't make rust, moisture and air
>> do.
>
> I suspect quite a few of these answers.
>
> Being left in a warm damp place ( e.g. Florida ) would
> encourage rust too.

Corrosion is in general a consequence of the operation of a
cell. Whatever is required for a cell to operate is needed
for corrosion to happen.

I would expect an optimum temperature for any particular
circumstance. It's likely that at very low temperatures
nothing much will happen at all, and if it's too hot
something else will happen instead. Steel, in particular, is
a very complicated thing. Do rig operators need to replace
sacrificial anodes more often in warm seas because of the
higher temperature, or less often because of the reduced
oxygen content of the water?

Boiler flues, for example, can disappear in next to no time,
even if they're made of "stainless" steel. There are any
number of apochryphal tales of engines destroyed in short
order because the cylinder head has been used as an earthing
point...particularly old vehicles with bolted-up chasses.

Ian

[Eeyore[_3_]]

Ian Iveson wrote:

> Eeyore wrote:
> >>
> >> No the were not. heat doesn't make rust, moisture and air
> >> do.
> >
> > I suspect quite a few of these answers.
> >
> > Being left in a warm damp place ( e.g. Florida ) would
> > encourage rust too.
>
> Corrosion is in general a consequence of the operation of a
> cell. Whatever is required for a cell to operate is needed
> for corrosion to happen.
>
> I would expect an optimum temperature for any particular
> circumstance. It's likely that at very low temperatures
> nothing much will happen at all, and if it's too hot
> something else will happen instead. Steel, in particular, is
> a very complicated thing. Do rig operators need to replace
> sacrificial anodes more often in warm seas because of the
> higher temperature, or less often because of the reduced
> oxygen content of the water?
>
> Boiler flues, for example, can disappear in next to no time,
> even if they're made of "stainless" steel. There are any
> number of apochryphal tales of engines destroyed in short
> order because the cylinder head has been used as an earthing
> point...particularly old vehicles with bolted-up chasses.

One thing for sure. I doubt those transformers were vacuum varnish
impregnated.

Graham


--
due to the hugely increased level of spam please make the obvious
adjustment to my email address

[Phil Allison[_3_]]

"Eeyore knows Nothing"

>
> One thing for sure. I doubt those transformers were vacuum varnish
> impregnated.
>
> Graham


** Who gives *flying ***** what some manic, ASD ****ed, know nothing
pommy idiot like Graham Stevenson "doubts" ??

Based entirely on his MONSTROUS IGNORANCE !!!

FYI:

The AC supply and valve OP trannys in Mesa Boogie amps ARE nearly all,
totally resin impregnated types.

Prevents the re-winding of damaged ones.

Real PITA.



...... Phil

[Bret L]

>
> Fact: The earliest tube guitar amps all used " tube " rectifiers.
>
> Fact: Lectic gittar players are obsessed with the " sound " of early
> amplifiers - since they falsely associate that with the distinctive
> playing of many, now legendary, early electric blues and rock guitarists.

It isn't false, it's a big component. However, so were the strings
used in those days, which were like telephone wires as compared to
modern ones.
>
> Fact: Tubes are automatically considered way cool.
>
> Fact: It is a given that more tubes are better than less tubes.

If you use enough rectifier tubes there is no sag, defeating the
purpose.
>
> Fact: The use of tube rectifiers is responsible for maybe just a * tad more
> noticeable * amount of amplitude compression when the amp is heavily
> overdriven as compared with otherwise similar SS rectifier models.
>

Tube rectifier sag makes a BIG difference in dynamic response in most
of the vintage amps when highly overdriven.

[Bret L]

>
> The AC supply and valve OP trannys in Mesa Boogie amps ARE nearly all,
> totally resin impregnated types.
>
> Prevents the re-winding of damaged ones.

There are effective resin strippers that will leave you with clean
lams and bare copper wire. You can reuse the endbells and lams, I
guess.

[Phil Allison[_3_]]

"Bret LIAR"


> Tube rectifier sag makes a BIG difference in dynamic response in most
> of the vintage amps when highly overdriven.


** Laughably stupid ********.



...... Phil

[Ian Iveson]

Eric wrote:

>>> The amp is a mesa boogie tipple rectifier and some fool
>>> had added a resistor to the bias supply increasing the
>>> cathode current. He must have had some interesting
>>> sounds as the power transformer primary shorted and B+
>>> went up and up,
>>
>> In what way does "up and up" differ from "up"? What do
>> you mean by shorted? I can understand that you want to
>> join in with all the fun we have here, so I don't wish to
>> dampen your ardour, but you need to think and write more
>> clearly. Try proper paragraphs.
>>
>> It might be useful, for the purpose of grasping how
>> transformers work, to consider what happens if just some
>> of the primary windings become shorted. Would that lead
>> to an increase in secondary voltage? There is an obvious
>> theoretical reason to think that it might, and a rather
>> more complicated reason, to realise that it most probably
>> won't. The best assumption is that the manufacturer has
>> used as few turns as possible.
>>
>> Your shorted primary hypothesis is not best tested by
>> plugging it into the mains.
>>
>> But it made me think of a nice example of a "reductio ad
>> absurdam". If you continue to short primary turns, then
>> secondary voltage should keep rising because the turns
>> ratio increases. In that case, when you have no turns,
>> then you should have a very high secondary voltage
>> indeed.
>>
>>> it still does this but no output tubes were in place as
>>> I tested it.
>>
>> Still does what, exactly? HT voltage will normally be
>> higher with valves removed, but by how much depends on
>> the details of the circuit, which few here will be
>> familiar with.
>>
>>> lots of nasty stuff under the power trans that is some
>>> kind of potting compound. So it needs that replaced but
>>> I am not sure if the output trans is good or not. The
>>> primary reads 21 and 23 ohms from the center tap.
>>
>> Likely to be normal. The difference is because the two
>> halves will have the same number of turns, but can't
>> occupy the same space, so one will have a different
>> average coil diameter, and so a different length of wire.
>>
>>> This is a 160 watt amp with 6-6l6GC's. The output trans
>>> is good and rusty so I am sure it got overheated as
>>> well.
>>> Also there was lots of glass chips from the output tubes
>>> under the chassis, could the tubes have exploded? Any
>>> thoughts?
>>
>> Explosions are scarce, I should imagine, although you
>> could have fun thinking up a possible scenario. I wonder
>> how much of the contents would have to vaporise before
>> sufficient positive pressure resulted?
>>
>> Implosions are more likely, because of the vacuum.
>>
>> Ian
> I meant that when first powered up the voltage was normal
> for a low load condition and over a period of a few
> minutes the voltage increased about 12 percent. The
> transformer also buzzed more as it warmed and this was the
> case with a Marshal I repaired a few years ago that needed
> a power transformer.
>
> I checked the line voltage and it was constant at 117
> volts. My area has an old power system. So I was thinking
> only a few windings had shorted before I discontinued the
> test. The leaking potting material also lead me to think
> the transformer was likely to be defective.
>
> "The drinking man's favourite model." Yes this is the
> most probable explanation for the glass fragments. Perhaps
> a collision between power tubes and beer bottle lead to
> the amps present condition.
>
> Is it not likely that a test not using line voltage would
> not heat the transformer and therefor not show a short?
>
> I did fail to provide enough information in my first post
> but would be pleased to let the blame fall on the US
> educational system for the sentence structure.

Shorted primary isn't the same thing in most ppls heads as
shorted primary turns.

At bottom-left here:

http://www.ivesonaudio.pwp.blueyonder.co.uk/TransformerIllustration.GIF

is the usual and *very* useful low-frequency model of a
transformer. At high frequencies it becomes necessary to
include winding capacitance and leakage inductance, but for
your purpose the simple example shown is OK.

From left to right, there's the primary winding resistance
in series with the primary inductance. In parallel with the
primary inductance is the load, made up of the secondary
winding resistance in series with the resistance presented
by your amp, both of which should be multiplied by the
square of the turns ratio, Tprimary/Tsecondary.

Notice that the primary winding resistance carries the
current attributable to the load, *and* that which flows
through the primary inductance. If you short turns on the
primary, you reduce the turns ratio *and* the primary
inductance. Both changes result in extra primary current.
Considering also that the inductance falls by the square of
the proportion of turns lost, the increase in current is
much greater than the reduction in primary winding
resistance, so the power it dissipates rises steeply, so it
gets hotter. If it was already hot enough to cause a fault,
then hotter still is likely to lead to catastrophic failure.

If you must test using the mains, then you should at least
work out beforehand what you need to measure during the
test, and what you expect the measurements to be if your
theory is correct. You should also eliminate as much
extraneous circuitry as possible...easily done in your case
by disconnecting the transformer from the amp and using a
suitable resistor instead. Then you can do your test quickly
and effectively and just once, thus minimising the risk of
causing further damage.

If you measure the AC primary and secondary voltages with no
load, and divide the one by the other, you get the turns
ratio, hopefully in the no-fault condition.

If you measure primary current and secondary voltage under
load, then wait for the fault to occur and measure them
again, you will have all the data you need. A little algebra
will then tell you whether your theory fits the facts.
Remember that inductance is proportional to the square of
the number of turns. Once you've calculated the change in
turns ratio, you can infer the proportional reduction in
primary inductance. Then you can work out the expected
increase in current due to load and inductance, and compare
that to your measurement. If you are right you will be
seeing something that Phil has never seen before, so it's
probably very rare and an experience to be savoured.
Certainly it's not a condition that's likely to last for
very long.

Theory is likely to be of academic interest only, as it
sounds like you need a replacement transformer anyway.

Ian

[Ian Iveson]

PS...

There's a complication that's worth putting as a question
because it's a bit interesting to think about.

As primary turns are reduced, will there come a point at
which the core saturates?

If it does approach saturation in your case, then inductance
is reduced still further. I've assumed that, if you do get a
higher secondary voltage when the fault occurs, then the
core permeability is roughly constant. That may not be a
safe assumption. If permeability varies significantly, then
inductance will no longer be proportional to the square of
the number of turns, the experiment becomes invalid, and
impossible anyway because you are using a reasonably rated
fuse, of course, which will blow quicker than you can
measure.

Ian

[Patrick Turner]

Phil Allison wrote:
>
> "sortech"
>
> >I posted a while back about rusty transformers and got lots of response. It
> >turns out they were rusty because the amp had been ridiculously over
> >heated.
>
> ** Utter nonsense.
>
> The amp has been allowed to get wet - most likely from condensation while
> in storage.
>
> I see that all the time.
>
> > The amp is a mesa boogie tipple rectifier and some fool had added a
> > resistor to the bias supply increasing the cathode current. He must have
> > had some interesting sounds as the power transformer primary shorted and
> > B+ went up and up,
>
> ** No such thing happens.
>
> Shorted turns in the primary causes large currents to flow and makes the AC
> fuse blow.
>
> > lots of nasty stuff under the power trans that is some kind of potting
> > compound. So it needs that replaced but I am not sure if the output trans
> > is good or not. The primary reads 21 and 23 ohms from the center tap.
>
> ** Sounds about right.
>
> > This is a 160 watt amp with 6-6l6GC's. The output trans is good and rusty
> > so I am sure it got overheated as well.
>
> ** Drivel.
>
> > Also there was lots of glass chips from the output tubes under the
> > chassis, could the tubes have exploded?
>
> ** If output tubes get wet during use, the glass often breaks.
>
> They break if you hit them hard enough.
>
> They break if they bang into each other ....
>
> ..... Phil

I have to agree with your assessment. The OP does have some further
learning to achieve before he understands why **** happens in guitar
amps; maybe with time and experience, he may learn, but usually it takes
an average mind about a year or two working as a tech in a repair shop
to to fully understand guitar amps after fixing maybe 50 of them, and
seeing all manner of **** that's happened.

Patrick Turner.

[Patrick Turner]

Phil Allison wrote:
>
> "Eeyore knows Nothing"
>
> >
> > One thing for sure. I doubt those transformers were vacuum varnish
> > impregnated.
> >
> > Graham
>
> ** Who gives *flying ***** what some manic, ASD ****ed, know nothing
> pommy idiot like Graham Stevenson "doubts" ??
>
> Based entirely on his MONSTROUS IGNORANCE !!!
>
> FYI:
>
> The AC supply and valve OP trannys in Mesa Boogie amps ARE nearly all,
> totally resin impregnated types.
>
> Prevents the re-winding of damaged ones.
>
> Real PITA.
>
> ..... Phil

Yes, but good varnishing does make a given tranny less likely to fail
early.

Now I have had to repair many amps and have never ever bothered to try
to unwind a traumatised tranny then remove E&I lams which have been
glued together with varnish and re-wind it after trying desperately
trying to save the bobbin . The cost in time is absurd, and its always
cheaper to buy a spare and install it. They are widely available.
However, sometimes one might want to recover the core material so it can
be re-used, and then one has to place the pharqued tranny in a small
wood fire until it just glows dull read for a few minutes and this
vapourizes and burns off all the plastics and varnish. I've done barrow
loads of stuffed PT, OPT, and chokes in my open fireplace in my
loungeroom, so I have a stock of various sized cores which I use mainly
for new chokes. The core material is left to cool slowly and next day
the wire can be cut off with side cutters or angle grinder and bolts
removed and the laminations will easily all fall apart. The magnetic
properties should not be changed. Re-winding can be done, but with a new
bobbin and wire, but usually I always try to re-wind with a taller stack
of iron with more turns per volt and less losses to make a better
transformer than original.

Where you have an E&I core which has been machine welded together, and
which has no bolt holes, then it is always a throwaway item.

The commonly used GOSS lams are often now machine welded after just
butting a pile of E and I without interleaving the E and I. The
permability obtained is high enough.

Patrick Turner.

[Patrick Turner]

Phil Allison wrote:
>
> "sortech"
>
> > I meant that when first powered up the voltage was normal for a low load
> > condition and over a period of a few minutes the voltage increased about
> > 12 percent.
>
> ** Bizzare.
>
> Been servicing valve guitar amps for 4 decades and never seen that.
>
> Even a few shorted turns blows the AC fuse, real quick.


It depends where the shorted turns are. If you had a couple of shorted
turns in a low voltage heater winding, the tranny would not blow a mains
fuse immediately, but after 1/2 an hour it would be scorching hot and
maybe hum a bit, and maybe the fuse blows when other shorted turns
develop as turns near the initial sorted turn get roasted by the high
local heat.

>
> > The transformer also buzzed more as it warmed
>
> ** Very common for loose laminations to begin to buzz when the tranny warms
> up.

And when the plate current begins to be drawn, and the peak charge
currents in the rectifiers become substantial.


>
> > I checked the line voltage and it was constant at 117 volts. My area has
> > an old power system. So I was thinking only a few windings had shorted
> > before I discontinued the test. The leaking potting material also lead me
> > to think the transformer was likely to be defective.
>
> ** That last bit indicates a bad tranny.
>
> Plus an evil smell and popping /cracking noises as it heats up.

If the tranny is estimated to be a 150VA tranny, the input current from
the mains with no loads connected on the secondary should be less than
15% of the full load current. But with all tubes out of the sockets and
no load, including rectifier tubes. With no load, and Si diodes left
connected, B+ should not exceed the capacitor voltage ratings. Novices
won't know what dangers lurk by leaving all the tubes out and they don't
know what to check and look for whiloe testing things.
The transformer temperature should not rise more than 15C above room
temp with no load after 1 hour; if it gets really hot its a bad tranny.

>
> > "The drinking man's favourite model." Yes this is the most probable
> > explanation for the glass fragments. Perhaps a collision between power
> > tubes and beer bottle lead to the amps present condition.
>
> ** The " Triple Rectifier" is famously known as the model played by Saddam
> Hussein.
>
> Even his friends said he sounded like Sheite ....


Well, I really doubt they would have ever said that, because they would
have known the consequences were not a happy story.....

If they didn't like his tunes, he would give them his old worn out 6L6,
and tell them to eat them for breakfast at gunpoint.

Even on a sunni day.

Patrick Turner.
>
> ..... Phil

[Patrick Turner]

Bret L wrote:
>
> >
> > Fact: The earliest tube guitar amps all used " tube " rectifiers.
> >
> > Fact: Lectic gittar players are obsessed with the " sound " of early
> > amplifiers - since they falsely associate that with the distinctive
> > playing of many, now legendary, early electric blues and rock guitarists.
>
> It isn't false, it's a big component. However, so were the strings
> used in those days, which were like telephone wires as compared to
> modern ones.
> >
> > Fact: Tubes are automatically considered way cool.
> >
> > Fact: It is a given that more tubes are better than less tubes.
>
> If you use enough rectifier tubes there is no sag, defeating the
> purpose.
> >
> > Fact: The use of tube rectifiers is responsible for maybe just a * tad more
> > noticeable * amount of amplitude compression when the amp is heavily
> > overdriven as compared with otherwise similar SS rectifier models.
> >
>
> Tube rectifier sag makes a BIG difference in dynamic response in most
> of the vintage amps when highly overdriven.

Nope, the B+ sags more than 20% under high load, and the dynamics tend
to become compressed. And because so many guitar amps have **** all B+
filtering, and work in near class B and even in class C during highest
over drive power, then the hum voltage at the OPT CT becomes high making
the square waves produced contain a "gravelly" tone, much loved by those
who are confused about what constitutes real music.

I have noticed that Marshall and Fender have often supplied amps to
export markets say in Oz **without** filter chokes and a second B+ cap
which is seen on the schematics for the amps concerned when you read the
Groove Tube circuit book.

The exporters save in 3 ways. Less expense to make the amp, less expense
of freight, and less chance of unhappy rock stars in distant nations
turning up at the factory wanting to shove the guitar amp up the CEOs
arsole because it failed to meet their expectations.

Many guitar amps **do** have good B+ filtering just like most hi-fi
amps. But they all work in class C to make a bunch of square waves when
the input goes 20% above that needed for sine wave clipping.

When I weigh up what musicians say about the tone of their guitar amps
and about their idea of technicals I can only smile.

And if you want the sound George Benson or Mark Knoffler gets as their
signature sound, be prepared to think slightly differently.

Patrick Turner.

[boardjunkie]

On Aug 30, 4:20*am, Patrick Turner > wrote:
> Nope, the B+ sags more than 20% under high load, and the dynamics tend
> to become compressed.

Now see...thats what makes this particular amp a retarded design. 3
pairs of 6L6s....when are you going to be able to open it up enough to
hear a difference from voltage sag? Unless yer playing a 12,000 seat
arena and refuse to use the house PA, its kinda useless. Marketing
hooey. A 15-35w amp? Sure then you'd be able to wind it up to get the
desired compression. It'll still be damn loud.

I have that same model "rectumfrier" on the bench right now. Came in
with 2 filament failures....one recto tube and the first preamp tube.
He's getting the 2 good 5U4s back in a cardboard box. The guy never
uses the tube recto'd supply anyway so there's no need to even have
them in the sockets pulling filament current.

[WB]

Hi,
The tube rectifier can make a pretty big difference in the dynamics
of a guitar amp. But, another important factor is the actual design and
how the thing is biased. I build a guitar amp using a pair of cathode
biased EL34's where, at idle, the thing pulls darn near the same plate
current through the EL34's as it does at the point where it starts to
clip. I use a 5U4 rectifier and the same power transformer as found in
Dynaco's ST70 power amp to power this 32 watt push pull guitar amp.
My power supply, at clipping sags about 20 volts. My Fender Bandmaster
(1965 version) sags more if you compare the B+ during a light load and
then during light clipping. The Bandmaster has solid state rectifiers in
it's B+ power supply. Also, depending on which of the three negative
feedback settings you have the feedback switch set to on my amp, the
dynamics, and especially the tightness of the bass, change a lot.
If the phase splitter isn't adjusted correctly for AC balance, the amp
can sound rather odd as well and lack punch and power.
Your comment about strings is right on! Wow, someone else realizes
this. Thick strings were very much the norm back in the day. Also, the
pickups used back then were a lot different sounding. There are a lot of
variables when designing a guitar amp. The electronics is only one part
of the puzzle.


Patrick Turner wrote:
>
> Bret L wrote:
>>> Fact: The earliest tube guitar amps all used " tube " rectifiers.
>>>
>>> Fact: Lectic gittar players are obsessed with the " sound " of early
>>> amplifiers - since they falsely associate that with the distinctive
>>> playing of many, now legendary, early electric blues and rock guitarists.
>> It isn't false, it's a big component. However, so were the strings
>> used in those days, which were like telephone wires as compared to
>> modern ones.
>>> Fact: Tubes are automatically considered way cool.
>>>
>>> Fact: It is a given that more tubes are better than less tubes.
>> If you use enough rectifier tubes there is no sag, defeating the
>> purpose.
>>> Fact: The use of tube rectifiers is responsible for maybe just a * tad more
>>> noticeable * amount of amplitude compression when the amp is heavily
>>> overdriven as compared with otherwise similar SS rectifier models.
>>>
>> Tube rectifier sag makes a BIG difference in dynamic response in most
>> of the vintage amps when highly overdriven.