Spot the Design Error 2

[Phil Allison]

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.



...... Phil

 

[John Robertson]

On 2019/10/22 9:06 p.m., Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.



..... Phil

Shorting the output by (apparently) unplugging the speaker whilst
playing a deep, loud, note would not be good...

John :-#(#

 

[Jasen Betts]

On 2019-10-23, Phil Allison <pallison49@gmail.com> wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.

By unplugging the loudspeaker it appears that the output transformmer
is short-circuited, and the feedback path is grounded!

--
When I tried casting out nines I made a hash of it.

 

Phil Allison <pallison49@gmail.com> wrote in
news:f27ee20b-05e9-4bf4-a18a-9704e6578886@googlegroups.com:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender
"Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was
hazardous, instantly damage or destroy output tubes and/or the
output transformer.

This during normal use and involving no misconnection or missing
connections.

Special tube knowledge is not needed, just basic electrical
circuit theory.



..... Phil

Plug in a quarter inch jack with nothing connected to it.

 

[piglet]

On 23/10/2019 07:29, piglet wrote:

On 23/10/2019 07:13, Phil Allison wrote:
  piglet wrote:

--------------




Open the "standby" switch? Inductive kickback from TR2?



** Piglet is getting warm.

    Another step or two.



.....  Phil


Well I guess depending on the current flowing at the instant the switch
was opened the kickback could be high enough to arc over inside the
tubes or breakdown output transformer insulation?

piglet

And the worst time to open the switch would be when playing loud?

piglet

 

[piglet]

On 23/10/2019 05:06, Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.



..... Phil

Open the "standby" switch? Inductive kickback from TR2?

piglet

 

[Phil Allison]

Jasen Betts wrote:

---------------------

Phil Allison


Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.


----------------------------------------

By unplugging the loudspeaker it appears that the output transformmer
is short-circuited, and the feedback path is grounded!

** Unplugging the speaker at the amplifier results in shorted output - a safety feature designed by Leo Fender. The output tubes see a low impedance and may get hot if driven for a while but no other risk.

Unplugging at the speaker box end results in an open input and some risk of high voltage events.


..... Phil

 

[Phil Allison]

piglet wrote:

--------------

Open the "standby" switch? Inductive kickback from TR2?

** Piglet is getting warm.

Another step or two.



...... Phil

 

Phil Allison <pallison49@gmail.com> wrote in
news:4104aef6-67e6-4484-a0f5-27bc144de4b8@googlegroups.com:

John Robertson wrote:

----------------------

Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender
"Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was
hazardous, instantly damage or destroy output tubes and/or the
output transformer.

This during normal use and involving no misconnection or
missing connections.

Special tube knowledge is not needed, just basic electrical
circuit theory.





Shorting the output by (apparently) unplugging the speaker whilst
playing a deep, loud, note would not be good...


** No shorted or open connections involved.

This is something more insidious.




..... Phil

Well, the standby switch creates a by design open or short (because
that's what a switch does). And if that could cause a problem, it
would qualify as insidious.

 

[Phil Allison]

DecadentLinux...@decadence.org wrote:

----------------------------------------

Well, the standby switch creates a by design open or short (because
that's what a switch does). And if that could cause a problem, it
would qualify as insidious.

** Well said.

What could be a bad time to operate that switch ?



...... Phil

 

[piglet]

On 23/10/2019 07:13, Phil Allison wrote:

piglet wrote:

--------------




Open the "standby" switch? Inductive kickback from TR2?



** Piglet is getting warm.

Another step or two.



..... Phil

Well I guess depending on the current flowing at the instant the switch
was opened the kickback could be high enough to arc over inside the
tubes or breakdown output transformer insulation?

piglet

 

[Phil Allison]

John Robertson wrote:

----------------------

Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.





Shorting the output by (apparently) unplugging the speaker whilst
playing a deep, loud, note would not be good...

** No shorted or open connections involved.

This is something more insidious.




...... Phil

 

[gray_wolf]

On 10/23/2019 1:08 AM, piglet wrote:

On 23/10/2019 05:06, Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".


https://schematicheaven.net/fenderamps/fender_bassman50.pdf


Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous,
instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.



.....  Phil





Open the "standby" switch? Inductive kickback from TR2?

piglet

I've heard this for years. I had a dummy load box with a rotary switch
that went from 0, 2, 4, 8, 16 ohms. Over my 50 years on both tube and
solid state amps, including Fender I've never seen a one misbehave. I figured
the negative fb kept things sorted. It's rather interesting to switch through the
load range and see how the output voltage changes. Gives you a feel for the
DUT's output impedance. The few Crown amps I've worked on were very
impressive. They didn't hardly blink. I'm well aware of inductive kickback.
I've felt it's tingle when removing my ohm meter from a transformer's leads
while checking for continuity.

Seriously, why doesn't the pole pig outside my house explode when I open the
main breaker so it has no load at all on it's secondary winding. Am I missing
something?

I've seen ignition coils fail when a spark lead developed such high resistance it
allowed the output voltage to rise high enough to cause internal breakdown.
How many Fender outputs have been replaced over the years?

Once, I had the boss's son call me from another shop telling me that he had
replaced a Fender's output xformer and... before he finished, I told him "It
oscillates".
He then says HTF did you know. I said I read a lot and don't sit around smoking
weed all day. It was worse than that. His dad offered me a grand (in 1970) to
straighten him out. I said I thought it was too late. About 6 months after I'd left
a friend told me that the son and several of his friends were busted in an
attempted
armed robbery of a drug store.

 

[Phil Allison]

piglet wrote:

---------------

** Piglet is getting warm.

Another step or two.


Well I guess depending on the current flowing at the instant the switch
was opened the kickback could be high enough to arc over inside the
tubes or breakdown output transformer insulation?

** Once the s/b switch is opened, the DC current in TR2 changes direction and goes up.




..... Phil

 

[piglet]

On 23/10/2019 08:09, Phil Allison wrote:

piglet wrote:

---------------




** Piglet is getting warm.

Another step or two.


Well I guess depending on the current flowing at the instant the switch
was opened the kickback could be high enough to arc over inside the
tubes or breakdown output transformer insulation?



** Once the s/b switch is opened, the DC current in TR2 changes direction and goes up.




.... Phil

Oh, sorry I thought TR2 was a choke and tried to keep current flow in
the same direction.

piglet

 

[Arie de Muynck]

On 2019-10-23 06:06, Phil Allison wrote:

Hi to my many admirers,


see schem for tube instrument amplifier, the famous Fender "Bassman 50".
https://schematicheaven.net/fenderamps/fender_bassman50.pdf

Thousands made and sold, yet it has a fatal design flaw.

Owners can, in a single step that they would never imagine was hazardous, instantly damage or destroy output tubes and/or the output transformer.

This during normal use and involving no misconnection or missing connections.

Special tube knowledge is not needed, just basic electrical circuit theory.

..... Phil

Apart from owner interaction - failure of the negative bias circuit for
the output tubes will cause severe overheating of the end stage. I've
seen melted 6L6's. Fortunately they were cheap then.

Arie

 

[Phil Allison]

piglet wrote:

---------------

** Once the s/b switch is opened, the DC current in TR2
changes direction and goes up.




Oh, sorry I thought TR2 was a choke and tried to keep current flow in
the same direction.

** TR2 is a 2H choke with about 100 ohms R.

Telling you any more is like posting the answer.


...... Phil

piglet

 

[George Herold]

On Wednesday, October 23, 2019 at 2:14:04 AM UTC-4, Phil Allison wrote:

piglet wrote:

--------------




Open the "standby" switch? Inductive kickback from TR2?



** Piglet is getting warm.

Another step or two.

OK, I was confused by the 'ground' switch with a big cap to ground...
what's that about?

George H.

..... Phil

 

[Phil Allison]

George Herold wrote:

---------------------

OK, I was confused by the 'ground' switch with a big cap to ground...
what's that about?

** Potential user safety issue, when the AC outlet has no ground.

Those are know among guitarists as "death caps".



..... Phil

 

[Phil Allison]

legg wrote:

According to the schematic, unplugging the load shorts the output.

** As already posted, that is a safety feature incorporated by Leo Fender.

Tube plates many glow if audio drive in sustained.

However, the event causing damage happens in an instant.



...... Phil