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Alex Pogossov Alex Pogossov is offline
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Default Overvoltage protection

If you are experimenting on a running tube amp, you might be at some stage
pulling a hot output tube from its socket while the amp is running.
Alternatively you might accidentally send a high negative pilse to a grid of
the tube while working on the circuit.

In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the load
giving you a loud crack in the speaker. A residual smaller energy stored in
the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in
a snubber of say 2200pF+5K usually connected in parallel to the OPT primary.

But what is the load is not connected? Then the huge magnetisation energy
will either cause:
- arcing in the tube;
- breakdown in the OPT winding insulation;
- breakdown of the subber capacitor (say 2200pF);
- arcing elsewhere in the wiring.or in the tube socket.

Would it be a good idea to placa a varistor rated slightly above the +B
across the primary? Or a spark gap of some sort, or gas discharge surge
arrester tube (the later have miniscule capacitance and very reliable). I
remember in vertical deflection stages of old TVs such varistors were a
must, since the abrupt cutoff of the current (during flyback) in EL84 often
used for this purpose was the mode of operation.

However I have never seen any discussions on the varistor protection issue
on this site.
What is your opinion?

Regards,
Alex


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Overvoltage protection


"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the
load giving you a loud crack in the speaker. A residual smaller energy
stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely
dissipated in a snubber of say 2200pF+5K usually connected in parallel to
the OPT primary.

But what is the load is not connected? Then the huge magnetisation energy



** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?




..... Phil




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Alex Pogossov Alex Pogossov is offline
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Posts: 95
Default Overvoltage protection


"Phil Allison" wrote in message
...

"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the
load giving you a loud crack in the speaker. A residual smaller energy
stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely
dissipated in a snubber of say 2200pF+5K usually connected in parallel to
the OPT primary.

But what is the load is not connected? Then the huge magnetisation energy



** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?


Apparently 0.5J.

It is the same as charging a 10uF to 315V or 47uF to 160V. If you are in
Europe or Australia, try charging 10uF from the grid through a diode and
then discharging it with a screwdriver... The closer you keep the cap to
your face when discharging -- the better... Then you will see if it is huge
or not... It is all subjective after all... If you are in the US, use 47uF
for a similar experiment.

Enjoy

Alex


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Posts: 500
Default Overvoltage protection


"Alex Pogossov"
"Phil Allison"
"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the
load giving you a loud crack in the speaker. A residual smaller energy
stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely
dissipated in a snubber of say 2200pF+5K usually connected in parallel
to the OPT primary.

But what is the load is not connected? Then the huge magnetisation
energy



** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?


Apparently 0.5J.



** IOW - **** all.

Even the smallest Varistor you can buy will absorb it with ease.

Like this one for $ 1.58 + gst

http://au.element14.com/epcos/b72205...9?Ntt=100+4399


..... Phil


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Alex Pogossov Alex Pogossov is offline
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Posts: 95
Default Overvoltage protection


"Phil Allison" wrote in message
...

"Alex Pogossov"
"Phil Allison"
"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a
decent amp. If a load is connected, then the energy will be dissipated
in the load giving you a loud crack in the speaker. A residual smaller
energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be
safely dissipated in a snubber of say 2200pF+5K usually connected in
parallel to the OPT primary.

But what is the load is not connected? Then the huge magnetisation
energy


** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?


Apparently 0.5J.



** IOW - **** all.

Even the smallest Varistor you can buy will absorb it with ease.

Like this one for $ 1.58 + gst

http://au.element14.com/epcos/b72205...9?Ntt=100+4399

Agree, it is no problem to clamp the surges, but the point is: why I have
not seen varistors across the OPT primaries in various schematics? Is this
sort of protection not needed? Or is the assumption that no one will be
changing tubes on a working amp? Or occasional arcing and sparking is not a
problem?




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Phil Allison[_3_] Phil Allison[_3_] is offline
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Posts: 500
Default Overvoltage protection


"Alex Pogossov"
"Phil Allison"

But what is the load is not connected? Then the huge magnetisation
energy


** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?

Apparently 0.5J.



** IOW - **** all.

Even the smallest Varistor you can buy will absorb it with ease.

Like this one for $ 1.58 + gst

http://au.element14.com/epcos/b72205...9?Ntt=100+4399

Agree, it is no problem to clamp the surges, but the point is: why I have
not seen varistors across the OPT primaries in various schematics?



** Cos the usual RC network suppresses the spike sufficiently, along with
the load.


Or is the assumption that no one will be changing tubes on a working amp?


** Well, not with no load connected.


Or occasional arcing and sparking is not a problem?



** The Aussie made 6V6GT used in my first valve amp liked to spark
internally with no load.

Never did it, the A&R OP tranny or the socket any permanent harm.

But if you wanna go belt and braces - fine with me....

BTW - how much HT have you got ??


..... Phil




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[email protected] arthrnyork@webtv.net is offline
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Posts: 81
Default Overvoltage protection

On Jul 8, 11:16*pm, "Alex Pogossov" wrote:
"Phil Allison" wrote in message

...







"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the
load giving you a loud crack in the speaker. A residual smaller energy
stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely
dissipated in a snubber of say 2200pF+5K usually connected in parallel to
the OPT primary.


But what is the load is not connected? Then the huge magnetisation energy


** Why huge ???


If Lp is say 100H and *Ia = 0.1A *then how many Joules is that ?


Apparently 0.5J.

It is the same as charging a 10uF to 315V or 47uF to 160V. If you are in
Europe or Australia, try charging 10uF from the grid through a diode and
then discharging it with a screwdriver... The closer you keep the cap to
your face when discharging -- the better... Then you will see if it is huge
or not... It is all subjective after all... If you are in the US, use 47uF
for a similar experiment.

Enjoy

Alex- Hide quoted text -

- Show quoted text -


We used to use 100 uF@ 500VDC in my highschool to play jokes in the
electronics shop. Gives a very nice wake-up jolt , especially to those
with sweaty palms . . .
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NT NT is offline
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Posts: 27
Default Overvoltage protection

On Jul 9, 3:08*am, "Alex Pogossov" wrote:
If you are experimenting on a running tube amp, you might be at some stage
pulling a hot output tube from its socket while the amp is running.
Alternatively you might accidentally send a high negative pilse to a grid of
the tube while working on the circuit.

In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent
amp. If a load is connected, then the energy will be dissipated in the load
giving you a loud crack in the speaker. A residual smaller energy stored in
the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in
a snubber of say 2200pF+5K usually connected in parallel to the OPT primary.

But what is the load is not connected? Then the huge magnetisation energy
will either cause:
- arcing in the tube;
- breakdown in the OPT winding insulation;
- breakdown of the subber capacitor (say 2200pF);
- arcing elsewhere in the wiring.or in the tube socket.

Would it be a good idea to placa a varistor rated slightly above the +B
across the primary? Or a spark gap of some sort, or gas discharge surge
arrester tube (the later have miniscule capacitance and very reliable). I
remember in vertical deflection stages of old TVs such varistors were a
must, since the abrupt cutoff of the current (during flyback) in EL84 often
used for this purpose was the mode of operation.

However I have never seen any discussions on the varistor protection issue
on this site.
What is your opinion?

Regards,
Alex


I've seen spark gaps on the primary on amps occasionally. But not
often.


NT
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