Spot the Design Error 2

[legg]

On Tue, 22 Oct 2019 21:06:18 -0700 (PDT), 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.



..... Phil



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

RL

 

[Phil Allison]

Dimitrij Klingbeil wrote:

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

Let me hazard a guess:

- Start with the amp off, but with standby switch closed.
- Turn on mains and immediately switch it into standby.

(with one hand movement, actuating both switches in opposite directions
almost simultaneously - or maybe even in the same direction, depending
on how the switches are oriented on the front panel wrt. each other)

If you leave very little time between "mains on" and "stdby open",
it may be possible to catch the 20uF 525V capacitor in the middle of
charging, with TR2 conducting the maximum peak current. Interrupting
TR2 supply at this point will "discharge" TR2 into TR3 and the 6L6s
in opposite polarity. Either TR3 or one 6L6 is likely to arc over.

** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.



...... Phil

 

[Dimitrij Klingbeil]

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.

Let me hazard a guess:

- Start with the amp off, but with standby switch closed.
- Turn on mains and immediately switch it into standby.

(with one hand movement, actuating both switches in opposite directions
almost simultaneously - or maybe even in the same direction, depending
on how the switches are oriented on the front panel wrt. each other)

If you leave very little time between "mains on" and "stdby open",
it may be possible to catch the 20uF 525V capacitor in the middle of
charging, with TR2 conducting the maximum peak current. Interrupting
TR2 supply at this point will "discharge" TR2 into TR3 and the 6L6s
in opposite polarity. Either TR3 or one 6L6 is likely to arc over.

Dimitrij

 

[Phil Allison]

Dimitrij Klingbeil wrote:

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

** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.

OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.
- Panic and flip the standby switch to kill the squealing.

This would likely catch TR3 at maximum instantaneous current, which
is way higher than TR2 maximum operating current could ever get to.

** One more step.

Think about instantaneous damage.



...... Phil

 

[Phil Allison]

John Robertson wrote:

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

** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.


..... Phil



Hmm, the owner decided to turn the amp on? 'cause it won't blow up if it
remains powered off...

** Reminds me of the false argument sometimes used by speaker repairers to deny warranty. Goes like this:

Speaker tech:

Your speaker has a burnt voice coil, so the amplifier has damaged it.

Customer:

So connecting the speaker to an amplifier with less power than recommended by the maker voids the warranty ?



....... Phil

 

[Dimitrij Klingbeil]

On 2019-10-24 23:32, Phil Allison wrote:

Dimitrij Klingbeil wrote:

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


Let me hazard a guess:

- Start with the amp off, but with standby switch closed.
- Turn on mains and immediately switch it into standby.

(with one hand movement, actuating both switches in opposite directions
almost simultaneously - or maybe even in the same direction, depending
on how the switches are oriented on the front panel wrt. each other)

If you leave very little time between "mains on" and "stdby open",
it may be possible to catch the 20uF 525V capacitor in the middle of
charging, with TR2 conducting the maximum peak current. Interrupting
TR2 supply at this point will "discharge" TR2 into TR3 and the 6L6s
in opposite polarity. Either TR3 or one 6L6 is likely to arc over.


** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.

OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.
- Panic and flip the standby switch to kill the squealing.

This would likely catch TR3 at maximum instantaneous current, which
is way higher than TR2 maximum operating current could ever get to.


Alternatively:

- Get the amp to squeal and yank out the cable on the speaker side.

A user might deem that an acceptable "emergency stop" measure.

 

[John Robertson]

On 2019/10/24 2:32 p.m., Phil Allison wrote:

Dimitrij Klingbeil wrote:

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


Let me hazard a guess:

- Start with the amp off, but with standby switch closed.
- Turn on mains and immediately switch it into standby.

(with one hand movement, actuating both switches in opposite directions
almost simultaneously - or maybe even in the same direction, depending
on how the switches are oriented on the front panel wrt. each other)

If you leave very little time between "mains on" and "stdby open",
it may be possible to catch the 20uF 525V capacitor in the middle of
charging, with TR2 conducting the maximum peak current. Interrupting
TR2 supply at this point will "discharge" TR2 into TR3 and the 6L6s
in opposite polarity. Either TR3 or one 6L6 is likely to arc over.


** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.



..... Phil

Hmm, the owner decided to turn the amp on? 'cause it won't blow up if it
remains powered off...

(ducking)

The idea is based on "Unsafe At Any Speed" - remember that book?

John ;-#)#

 

[Dimitrij Klingbeil]

On 2019-10-25 00:15, Phil Allison wrote:

Dimitrij Klingbeil wrote:

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


** Most creative reply so far - and kinda in the right ball
park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.

OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.
- Panic and flip the standby switch to kill the squealing.

This would likely catch TR3 at maximum instantaneous current, which
is way higher than TR2 maximum operating current could ever get
to.


** One more step.

Think about instantaneous damage.

You've got me stumped here, not many things remaining that a normal
user could likely consider natural. It has to be a simple action...

- Turn the power switch on with the standby switch open.
- Close the standby switch while the cathodes are still cold.

TR2 and the 20uF 525V cap together form a series LC circuit.
Tubes with cold filaments pull no current and provide no damping.

The step response of a series LC circuit is an oscillatory transient
with a peak amplitude equal to twice the step amplitude.

This could in theory produce a ring-wave of 890 V peak on that cap and
therefore on the second grids of the 6L6s. In reality, the ringwave is
damped because there is one more 20uF 525V capacitor connected through
a 1k resistor. This resistor will provide damping and decrease the Q.

Not sure what the actual peak will be, this would need TR2 inductance
and the ESR values for TR2 and the caps to model accurately.

 

[John Robertson]

On 2019/10/24 3:13 p.m., Phil Allison wrote:

John Robertson wrote:

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

** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.


..... Phil



Hmm, the owner decided to turn the amp on? 'cause it won't blow up if it
remains powered off...



** Reminds me of the false argument sometimes used by speaker repairers to deny warranty. Goes like this:

Speaker tech:

Your speaker has a burnt voice coil, so the amplifier has damaged it.

Customer:

So connecting the speaker to an amplifier with less power than recommended by the maker voids the warranty ?



...... Phil

Comparing this schematic with one of my old-time Wurlitzer (model 2100)
jukebox amps shows that whereas the output tube cathodes were tied
directly to ground on this Fender amp, my jukeboxes had the cathodes
tied together - then there was a resistor in parallel to a 25ufd cap.

Also the plates had their RF suppression tied to the cathodes of the
tube instead of using the grid doing that job.

See attached png...

Transients can be deadly even to tube amps.

John :-#)#



--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[Phil Allison]

Dimitrij Klingbeil wrote:

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

** One more step.

Think about instantaneous damage.

You've got me stumped here, not many things remaining that a normal
user could likely consider natural. It has to be a simple action...

** You have the scenario correct.

Its what happens in the next few mS.



...... Phil

 

[Winfield Hill]

Phil Allison wrote...

Its what happens in the next few mS.

Please, Phil, seconds are lower-case s.
S is Siemens, the unit of conductance,
so it's ms, not mS.


--
Thanks,
- Win

 

[Phil Allison]

Winfield Hill wrote:

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


** You get some sick pleasure out of being pedantic - do you Win ?

Compensates for getting so much else wrong?

Its what happens in the next few mS.

Please, Phil, seconds are lower-case s.
S is Siemens, the unit of conductance,
so it's ms, not mS.

** Go tell Rigol and other scope makers they have wrong front panel labelling.

S, mS and uS are very commonly used in electronics.

Two of my four scopes use capital S for the time base.

Long as there is no confusion, there is no error.



..... Phil

 

[Mikko OH2HVJ]

Phil Allison <pallison49@gmail.com> writes:

Dimitrij Klingbeil wrote:
OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.
- Panic and flip the standby switch to kill the squealing.
This would likely catch TR3 at maximum instantaneous current, which
is way higher than TR2 maximum operating current could ever get to.

** One more step.

Think about instantaneous damage.

Good sniping. Does this cause a strike through to G3/anode and run full
supply current through the tube to destroy it ?

--
mikko

 

[legg]

On Thu, 24 Oct 2019 15:15:56 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

Dimitrij Klingbeil wrote:

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


** Most creative reply so far - and kinda in the right ball park.

But no cupie doll.

It's single, seemingly innocent user action - damage is instant.

OK, another scenario:

- Mess up something with acoustic feedback so that the amp squeals.
- Panic and flip the standby switch to kill the squealing.

This would likely catch TR3 at maximum instantaneous current, which
is way higher than TR2 maximum operating current could ever get to.


** One more step.

Think about instantaneous damage.



..... Phil

There's no such thing as instantaneous damage in a tube amp, or a
solid state one, either.

RL

 

[Phil Allison]

legg wrote:

----------

There's no such thing as instantaneous damage in a tube amp, or a
solid state one, either.

** You feeing all right ?

Haven't had a stroke or a blow on the head lately - have you ?

A visit to your doctorf might be advisable.



...... Phil