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Overvoltage protection



 
 
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  #1  
Old July 9th 11, 03:08 AM posted to rec.audio.tubes
Alex Pogossov
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Posts: 95
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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  #2  
Old July 9th 11, 03:36 AM posted to rec.audio.tubes
Phil Allison[_3_]
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Posts: 500
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




  #3  
Old July 9th 11, 04:16 AM posted to rec.audio.tubes
Alex Pogossov
external usenet poster
 
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


  #4  
Old July 9th 11, 04:32 AM posted to rec.audio.tubes
Phil Allison[_3_]
external usenet poster
 
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


  #5  
Old July 9th 11, 07:43 AM posted to rec.audio.tubes
Alex Pogossov
external usenet poster
 
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?


  #6  
Old July 9th 11, 07:55 AM posted to rec.audio.tubes
[email protected]
external usenet poster
 
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 [email protected] 500VDC in my highschool to play jokes in the
electronics shop. Gives a very nice wake-up jolt , especially to those
with sweaty palms . . .
  #7  
Old July 9th 11, 07:59 AM posted to rec.audio.tubes
Phil Allison[_3_]
external usenet poster
 
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




  #8  
Old July 9th 11, 12:25 PM posted to rec.audio.tubes
Alex Pogossov
external usenet poster
 
Posts: 95
Default Overvoltage protection


"Phil Allison" > wrote in message
...

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


If a load is connected -- correct. If no load, then as I have mentioned you
need microfarades + several kiloohms to absorb it, not a few nanofarades
usually connected to dump HF ringing of the leakage inductance.
>
>
>> Or is the assumption that no one will be changing tubes on a working amp?

>
> ** Well, not with no load connected.


What about an amp being fool proof then?

>> 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 ??


Only 250V. But I do not want to risk any possibility of insulation breakdown
in an old lousy OPT.
Probably a small varistor rated at 350...400Vdc and with less than 500pF of
capacitance will be OK.

The tread is also of general nature -- why do not we see varistors in the
tube amp circuits. Probably you are right -- no one suffered serios damage
of this particular stress mode.


  #9  
Old July 9th 11, 11:03 PM posted to rec.audio.tubes
mick
external usenet poster
 
Posts: 130
Default Overvoltage protection

On Sat, 09 Jul 2011 21:25:39 +1000, Alex Pogossov wrote:

> "Phil Allison" > wrote in message
> ...
>
>> ** Cos the usual RC network suppresses the spike sufficiently, along
>> with the load.

>
> If a load is connected -- correct. If no load, then as I have mentioned
> you need microfarades + several kiloohms to absorb it, not a few
> nanofarades usually connected to dump HF ringing of the leakage
> inductance.
>>
>>
>>> Or is the assumption that no one will be changing tubes on a working
>>> amp?

>>
>> ** Well, not with no load connected.

>
> What about an amp being fool proof then?
>
>>> 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 ??

>
> Only 250V. But I do not want to risk any possibility of insulation
> breakdown in an old lousy OPT.
> Probably a small varistor rated at 350...400Vdc and with less than 500pF
> of capacitance will be OK.
>
> The tread is also of general nature -- why do not we see varistors in
> the tube amp circuits. Probably you are right -- no one suffered serios
> damage of this particular stress mode.



It's probably traditional. :-) Small, effective varistors are a newer
invention than valves. The two would never have met when the circuits
were being developed. That, and engineers having it hammered into them
that you *never* run a valve amp without a load.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.
  #10  
Old July 9th 11, 11:17 PM posted to rec.audio.tubes
John Byrns
external usenet poster
 
Posts: 1,441
Default Overvoltage protection

In article >,
"Alex Pogossov" > wrote:

> The tread is also of general nature -- why do not we see varistors in the
> tube amp circuits. Probably you are right -- no one suffered serios damage
> of this particular stress mode.


It's interesting to note that 1950s, 1960s, and 1970s Telephones used to be full
of varistors, both for protective purposes, and for regulating and controlling
the speech level.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
 




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