It's possible to damage the output transformer if the audio level is cranked very high without a load on the transformer.
If the amp isn't passing any audio, it's perfectly safe.
If the amp isn't passing any audio, it's perfectly safe.
If it has heavy NFB (enough to destabilize... which may not be that much if it's a really bad design), it may oscillate without anything connected. If the speakers make a slight pop when you connect them, or you measure voltage on the output...
Tim
Tim
Hi,
You may have seen the writing on the wal by now:
While most tube amps will survive without any harm done, people tend to forget and accidents do happen...
Make it a habit to have a load connect to the secondary side of the OPT just as you would follow a turn on, turn off sequence to avoid clicks and plops in the speakers...Use a mute switch on the preamp if you have one.
Do all of the above and nine out of ten you won't ever experience burned out OPTs (they can be expensive to replace, especially as you'd have to replace the pair of them) or other silly defects you can hold only yourself responsible for.
Cheers,😉
You may have seen the writing on the wal by now:
While most tube amps will survive without any harm done, people tend to forget and accidents do happen...
Make it a habit to have a load connect to the secondary side of the OPT just as you would follow a turn on, turn off sequence to avoid clicks and plops in the speakers...Use a mute switch on the preamp if you have one.
Do all of the above and nine out of ten you won't ever experience burned out OPTs (they can be expensive to replace, especially as you'd have to replace the pair of them) or other silly defects you can hold only yourself responsible for.
Cheers,😉
i always use a 25W 8R resistor on my amps. it won't hurt to have this as a safety precaution. in addition, i have modified RCA plugs that are internally shorted which i also use (when getting voltage reading from the amp).
Spark safety gap to protect amplifier
A protection spark gap may be fabricated from two short pieces of solid #14 copper wire and wired across the primary winding of the output transformer using a standard eyelet terminal strip to launch the short #14 wires from that form the gap. Leave at least one unconnected terminal on the strip between the terminals used for the spark gap or the terminal strip will likely break down and lead to a conductive carbonized short before the spark gap fires. Depending on the power output of the amp and the plate voltages involved, and also the inductance value of the output transformer a gap spacing of from 1/16" to 3/32" should be sufficient to protect your amp in case of a voltage surge resulting from operating under no-load conditions.
If the gap fires while playing music loud there will be a very loud, very rude noise from the speaker. If this happens then the gap setting is too small and should be increased until it no longer fires during normal hi-powered operation. Best to set the gap by testing the amp into a suitable dummy load resistor on the bench after installing the safety gap.
Naturally all safety rues apply when working inside tube amplifiers like making sure the unit is unplugged from the mains and all electrolytic capacitors are discharged properly. NEVER use a screwdriver or shorting wire to discharge electrolytic power supply capacitors. A 10 ohm, 10-25 watt wirewound resistor on clip leads works well.
A protection spark gap may be fabricated from two short pieces of solid #14 copper wire and wired across the primary winding of the output transformer using a standard eyelet terminal strip to launch the short #14 wires from that form the gap. Leave at least one unconnected terminal on the strip between the terminals used for the spark gap or the terminal strip will likely break down and lead to a conductive carbonized short before the spark gap fires. Depending on the power output of the amp and the plate voltages involved, and also the inductance value of the output transformer a gap spacing of from 1/16" to 3/32" should be sufficient to protect your amp in case of a voltage surge resulting from operating under no-load conditions.
If the gap fires while playing music loud there will be a very loud, very rude noise from the speaker. If this happens then the gap setting is too small and should be increased until it no longer fires during normal hi-powered operation. Best to set the gap by testing the amp into a suitable dummy load resistor on the bench after installing the safety gap.
Naturally all safety rues apply when working inside tube amplifiers like making sure the unit is unplugged from the mains and all electrolytic capacitors are discharged properly. NEVER use a screwdriver or shorting wire to discharge electrolytic power supply capacitors. A 10 ohm, 10-25 watt wirewound resistor on clip leads works well.
Clever idea! I guess you could use a spark-plug, if you could find some convenient way of mounting it. It'd look pretty cool on top of the chassis.
I seem to remember that Mullard recommended connecting a 1k resistor across the speaker terminals in the 5-10 amplifier, to guard against damage in the event that the speaker was not connected. They omitted it in the 5-20 schematic, however.
I seem to remember that Mullard recommended connecting a 1k resistor across the speaker terminals in the 5-10 amplifier, to guard against damage in the event that the speaker was not connected. They omitted it in the 5-20 schematic, however.
Hi,
That's indeed very good practice.
Use a carbon comp. 1K/1W resistor.
By doing so your amp is still safe in case one of the speaker leads should accidentally come lose.
Cheers,😉
That's indeed very good practice.
Use a carbon comp. 1K/1W resistor.
By doing so your amp is still safe in case one of the speaker leads should accidentally come lose.
Cheers,😉
A word of caution
The protection spark-gap is a good idea, but could lead to trouble if semiconductors are used anywhere in the amp. When a discharge occurs, a large amount of energy gets dumped. This can cause currents to be induced into other circiuts, causing damage. Small signal semiconductors are especially susceptible.
The protection spark-gap is a good idea, but could lead to trouble if semiconductors are used anywhere in the amp. When a discharge occurs, a large amount of energy gets dumped. This can cause currents to be induced into other circiuts, causing damage. Small signal semiconductors are especially susceptible.
I have seen a fast or ultra-fast diode with a voltage rating of at least *double* the HT voltage wired reverse polarity from anode to cathode of each power tube. The idea is that if one anode tries to go greater than 2x HT then it's mate will have it's anode try to go below earth. The diode clamps the voltage to no lower than earth potential.
Instead of a spark gap, how about a couple of 1500W transils/transorbs in series from anode to anode? These things have *picosecond* response time. I don't think MOV's would be very suitable here.
One thing I have never seen on a tube amp is a series RC network of say 10R + 100nF across the output like in many IC chip amps. Keeps a load on the transformer secondary at high frequencies when the speaker impedance begins to rise, or is in fact open cicuit.
Instead of a spark gap, how about a couple of 1500W transils/transorbs in series from anode to anode? These things have *picosecond* response time. I don't think MOV's would be very suitable here.
One thing I have never seen on a tube amp is a series RC network of say 10R + 100nF across the output like in many IC chip amps. Keeps a load on the transformer secondary at high frequencies when the speaker impedance begins to rise, or is in fact open cicuit.
"One thing I have never seen on a tube amp is a series RC network of say 10R + 100nF across the output like in many IC chip amps. Keeps a load on the transformer secondary at high frequencies when the speaker impedance begins to rise, or is in fact open cicuit."
The HH Scott amplifiers (299C) have series RC networks on the transformer secondaries.
The HH Scott amplifiers (299C) have series RC networks on the transformer secondaries.
Circlotron said:
Precisely. Something that cannot happen in linear operation so stays out of circuit in normal circumstance. Turner Audio uses this, as well as zener diodes stacked to equal twice the power supply voltage.
Tim
Re: A word of caution
John,
You raise a valid point indeed. The only tube amp I used this technique on was a long time ago (in my teens) and it had no sand in it whatsoever. Even the B+ rectifiers were vacuum tubes (two 5u4's). This was a high power amp with a 120 watt Hammond O/P xfmer and four KT77's P-P-P.
CP Claire (the relay manufacturer) manufactures little precision spark gaps that have a small amount of radiactive material inside to promote ionization. They can be obtained in many different voltages and act very fast. I suspect this is what I would use today if I wanted to employ this idea. Simply adding a small series R would nullify the dangerous EMP issue of which you speak. Come to think of it, a series R would also make the open spark gap I suggested work better too!
dhaen said:
John,
You raise a valid point indeed. The only tube amp I used this technique on was a long time ago (in my teens) and it had no sand in it whatsoever. Even the B+ rectifiers were vacuum tubes (two 5u4's). This was a high power amp with a 120 watt Hammond O/P xfmer and four KT77's P-P-P.
CP Claire (the relay manufacturer) manufactures little precision spark gaps that have a small amount of radiactive material inside to promote ionization. They can be obtained in many different voltages and act very fast. I suspect this is what I would use today if I wanted to employ this idea. Simply adding a small series R would nullify the dangerous EMP issue of which you speak. Come to think of it, a series R would also make the open spark gap I suggested work better too!
Hi RCAVictim,
The resistor idea with precision spark-gaps sounds good 🙂
TV manufacturers used an earthing scheme where all earths join at the earthly end of the spark gaps. This, with careful wiring paths and lead dressing increased the ability to survive flashovers from just a few to almost infinity.
The resistor idea with precision spark-gaps sounds good 🙂
TV manufacturers used an earthing scheme where all earths join at the earthly end of the spark gaps. This, with careful wiring paths and lead dressing increased the ability to survive flashovers from just a few to almost infinity.
Can anyone explain more about what happens to an amp without load? Is there oscillation going on? Bias going unstable? Or?
The general consensus seems to point to an increase (buildup) of voltage on the primary side of the OPT, which will eventually cause the windings to break down, resulting in catastrophic failure. There may be "other" things going on at the same time, like arching inside the output tubes (or at the socket) due to the excessive voltage present.
Please realize I'm not an EE, hence the rather basic explanation.😉
jeff
No load means high gain in the output stage, so it easily clips. NFB can make this worse. Sudden clipping can dump a lot of energy into the OPT HF resonance and it has nowhere to go so tries to continue conduction via a spark. This damages the insulation and the likely outcome is either a shorted turn or an open circuit winding.
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