Hi,
OK, valve-based circuits require extreme caution due to high voltages when dealing with the internals.
However, I never read anything about possible safety issues "outside the case".
For example, consider any preamp design whose output is directly connected to the output stage (either a common cathode, a cathode follower, an SRPP, etc). In either case, there is only a capacitor between the output terminals and the valve.
How big is the risk if the valve or some other component fails and the high voltage goes through to the output (perhaps only as a spike due to the cap)?
For example, if the anode of a common cathode is disconnected in the socket due to some contact problem, the high voltage goes directly to the output....
Or, can a cathode follower conduct due to some malfunction so much that the outuput voltage is dangerously high?
OK, valve-based circuits require extreme caution due to high voltages when dealing with the internals.
However, I never read anything about possible safety issues "outside the case".
For example, consider any preamp design whose output is directly connected to the output stage (either a common cathode, a cathode follower, an SRPP, etc). In either case, there is only a capacitor between the output terminals and the valve.
How big is the risk if the valve or some other component fails and the high voltage goes through to the output (perhaps only as a spike due to the cap)?
For example, if the anode of a common cathode is disconnected in the socket due to some contact problem, the high voltage goes directly to the output....
Or, can a cathode follower conduct due to some malfunction so much that the outuput voltage is dangerously high?
There is a risk, at least to the following equipment. Properly designed tube circuits will include a muting circuit
to preclude HV from the output terminals. Often the outputs are shorted to ground during turn-on and turn-off
for a certain period of time. Further measures are necessary in the event of component failure.
Of course, an OTL amplifier without an output capacitor is much more dangerous.
to preclude HV from the output terminals. Often the outputs are shorted to ground during turn-on and turn-off
for a certain period of time. Further measures are necessary in the event of component failure.
Of course, an OTL amplifier without an output capacitor is much more dangerous.
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All the valve radios I repaired had capacitor coupled valve stages those old capacitors would have high leakage impairing or blocking the radio from working , yes you could get a slight shock from the leakage but unless there is a dead short your body resistance stops a build up of current and the usual values of inter-stage capacitors didn't allow a large current to pass.
What I came across was valves run into the ground that developed internal short circuits , this was quite common on old CRT TV,s and my analyzer could remove the shorts by a high voltage pulse charge .
I don't hear so much about shorts in audio tubes here but the possibility remains.
I have survived 1000,s of electrical shocks only because of my high body resistance /thick skin that's dry and yes ( touch wood ) a good heart , not everybody is born with that physical plus point .
There is a well known saying "its the current that kills " and its true EHT voltages in old CRT TV,s had many 1000,s of volts connected to the tube but at only a few milliamps so it wasn't that sore compared to 250 volt mains electricity where one of my arms was practically paralyzed for several days .
350 V DC or more can be sore but it depends on your particular physiology= wet moist hands-- thin skin and a family history of heart problems ? then be very-very careful.
Under NO circumstance while working on equipment touch an earthed object with one hand and a live one with the other as that is the classic example of the current running through your heart --either stopping it or causing an irregular heartbeat.
What I came across was valves run into the ground that developed internal short circuits , this was quite common on old CRT TV,s and my analyzer could remove the shorts by a high voltage pulse charge .
I don't hear so much about shorts in audio tubes here but the possibility remains.
I have survived 1000,s of electrical shocks only because of my high body resistance /thick skin that's dry and yes ( touch wood ) a good heart , not everybody is born with that physical plus point .
There is a well known saying "its the current that kills " and its true EHT voltages in old CRT TV,s had many 1000,s of volts connected to the tube but at only a few milliamps so it wasn't that sore compared to 250 volt mains electricity where one of my arms was practically paralyzed for several days .
350 V DC or more can be sore but it depends on your particular physiology= wet moist hands-- thin skin and a family history of heart problems ? then be very-very careful.
Under NO circumstance while working on equipment touch an earthed object with one hand and a live one with the other as that is the classic example of the current running through your heart --either stopping it or causing an irregular heartbeat.
Yes the minimum that should be done but unfortunately often is omitted. Less/not necessary when coupling with tube gear but a must when using a solid state amplifier.
Protection against part failure maybe is a bit overdone (except with OTL)?!
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By the same token there is only insulation between mains voltage and you.
1) any competent designed preamp will have a very good "plastic" capacitor at its output.
Forget old time paper in oil capacitors which NOBODY serious uses any more.
I said serious, that does non include Boutique, Esoteric and similar MOJO/Faith based designs.
2) only way to get shocked is to have one hand touching chassis while the other touches only the connector tip.
Why would you do that?
Only "current" connector which allows that is a Guitar cable plug which has an exposed tip, and maybe an RCA connector if you try hard and rab just the tiny tip.
In which case I woud think you are trying hard to get damage.
In normal use, not an issue.
PS: and in any case, a capacitor would be connected to a tube plate which will be connected to +V through a highish value resistor, say 100k, about what you find inside a mains testing neon bulb screwdriver (hint: not "dangerous") or to a cathode, about 1/2 the +V present.
In most ( all ?) cases the curret is limited to a few mA, 50mA is considered potential lethal.Hi,
OK, valve-based circuits require extreme caution due to high voltages when dealing with the internals.
However, I never read anything about possible safety issues "outside the case".
For example, consider any preamp design whose output is directly connected to the output stage (either a common cathode, a cathode follower, an SRPP, etc). In either case, there is only a capacitor between the output terminals and the valve.
How big is the risk if the valve or some other component fails and the high voltage goes through to the output (perhaps only as a spike due to the cap)?
For example, if the anode of a common cathode is disconnected in the socket due to some contact problem, the high voltage goes directly to the output....
Or, can a cathode follower conduct due to some malfunction so much that the outuput voltage is dangerously high?
The limitation comes from plate resistor ( 47k and 300Volt gives 6mA )if
a common cathode stage where an output cap would short.
A cathode follower typically has less voltage ( 2k and 50Volt gives 25mA)
As current is also is series with the contact resistance of the finger or whatever comes in contact with the "live signal conductor" it's unlikely that it's lethal.
Much greater risk comes from the AC supplying the amp where the chassies in not connected to ground. This as the casing will be "live" and touching the casing might be all that is needed.
I prefer bidirectional TVS diodes.
SA12CA is the one I've used the most.
https://m.littelfuse.com/~/media/el...des/littelfuse_tvs_diode_sa_datasheet.pdf.pdf
SA12CA is the one I've used the most.
https://m.littelfuse.com/~/media/el...des/littelfuse_tvs_diode_sa_datasheet.pdf.pdf
OK, so a common cathod type of circuit is safer because of the low anode current, and this is independent of tube failure. But, a cathode follower (or other similar designs) are problematic in the case of grid failure when large anode current may take place, right?
Hi,
OK, valve-based circuits require extreme caution due to high voltages when dealing with the internals.
However, I never read anything about possible safety issues "outside the case".
For example, consider any preamp design whose output is directly connected to the output stage (either a common cathode, a cathode follower, an SRPP, etc). In either case, there is only a capacitor between the output terminals and the valve.
How big is the risk if the valve or some other component fails and the high voltage goes through to the output (perhaps only as a spike due to the cap)?
For example, if the anode of a common cathode is disconnected in the socket due to some contact problem, the high voltage goes directly to the output....
Or, can a cathode follower conduct due to some malfunction so much that the outuput voltage is dangerously high?
A long shot to solve a problem never seen is to use a thyristor triggered by
an elevated output that shorts the B+. Commonly called "crowbar"