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Old 8th January 2011, 11:06 PM   #1
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Default Tube output protection

What methods and techniques are there for protecting tubes and the output transformers in the case of a short or open circuit? I am building a tube amplifier using kt88's in a push-pull configuration. I would like the amplifier to have protection incase the amplifier is ran without a load or if a load shorts.

Thanks!
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Old 8th January 2011, 11:15 PM   #2
Loren42 is offline Loren42  United States
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Fuses!

I use fuses on the mains and on the high voltage lines just after the rectifiers.
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Old 8th January 2011, 11:21 PM   #3
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Fuses! Thats a simple and cheap way, thanks. I have very little experience with tube amps and this is my first build. I am assuming fuses will protect against short circuits but what about open circuits? I have read that having no load connected to a tube amplifier can damage that output transformer. Is there anyway of protecting from an open circuit?
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Old 8th January 2011, 11:38 PM   #4
Loren42 is offline Loren42  United States
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Whatever is going to damage a transformer boils down to a single cause; current. Fuses open when the current reaches the breaking threshold for that fuse and interrupts the current.
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Old 9th January 2011, 12:12 AM   #5
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Quote:
Originally Posted by Loren42 View Post
Whatever is going to damage a transformer boils down to a single cause; current. Fuses open when the current reaches the breaking threshold for that fuse and interrupts the current.
A Tube amp with Transormer output will be damaged by VOLTAGE generated in the O/P Tx as there is no load to control it.

Fuses Will NOT protect for this condition--Until its TOO Late!

A 'Snubber' network on the Primary of summit like a 0.01 and 100 ohm can help prevent excess volts from destroying the trans. if its run no-load....
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Old 9th January 2011, 12:26 AM   #6
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Quote:
I have read that having no load connected to a tube amplifier can damage that output transformer. Is there anyway of protecting from an open circuit? ........Whatever is going to damage a transformer boils down to a single cause; current.
NOT IN THIS CASE! A tube amp, especially a pentode amp can be damaged if the load goes open during use. An open load draws NO current. In normal operation one (or more) output tube is conducting a fixed current into the OPT primary. The OPT will transform the primary current into a voltage that is applied to the load. If the load is suddenly removed the current has nowhere to go so the voltage will rise to an extremely high value (several KV is possible) causing an arc somewhere. The situation is magnified several times over if the amp is operating at the clipping level (a guitar amp) since the output tubes are now functioning like switches and the OPT works like an ignition coil.

The most likely place for an arc to start is inside the OPT itself. This will fry it quickly. Most of the common audio output tubes have the plate on pin 3 and the heater on pin 2. I have seen an arc start at the base of one of the output tube sockets from pin 3 to pin 2. If the heater winding is not grounded it is possible for this to blow the power transformer. Very high power guitar amps (the Ampeg SVT) have been known to burst into flames if the speaker goes open during use. I saw it happen back in the early 70's. Autopsy revealed blown OPT, blown power transformer, two of the 6 6550's and several small parts, all fried because a 300 watt amp was played into a 100 watt speaker cabinet at an outdoor show where everything was set on 11.

There is no way to protect against this with a fuse. The usual protection method is to place a 100 ohm 10 watt or so resistor across the speaker terminals. This will steal a couple of watts of output power, and is not a guarantee either. Gas tube spark arresters like used in telephone equipment and big zener diodes across the speaker terminals is another possibility.

NEVER FUSE THE SPEAKER LEADS IN A TUBE AMP that uses an OPT. OTL's may need fuses to protect the speakers from blown tubes.
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Old 9th January 2011, 01:37 AM   #7
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My prefered protection is:

Use a MOV and series resistor across each OT primary half winding. DC working voltage of MOV slightly greater than max DC of HT supply. Resistor = approx half PP loading impedance - used to damp the transient if MOV starts conducting. Use smallest disc MOV available to keep capacitance down to a negligible level (the 7mm disc types typically have low capacitance <100pF).

Use a fuse in each cathode, with a parallel high value cathode bias resistor (say 10k) to bias tube deep in to cutoff, but not exceed heater-cathode voltage rating. You can put a 1R current sensing resistor from 0V to fuse, for easy bias maintenance checking. Aim for fuse rating at or just above the peak tube current, to give nominal 20% above likely worstcase rms.

Ciao, Tim
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Old 9th January 2011, 06:37 AM   #8
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Originally Posted by trobbins View Post
My prefered protection is:

Use a MOV and series resistor across each OT primary half winding. DC working voltage of MOV slightly greater than max DC of HT supply.

Use a fuse in each cathode...

Ciao, Tim
In a PP amp the voltage of the anodes will swing to up to twice as high as the HT supply. An MOV will typically clamp at twice the rated working voltage, so something slightly greater than the HT for the rated working voltage should be good, but be very careful how you read the specs when you buy MOVs.

The fuse idea I'm not so fond of. For one thing, the parallel fuse/resistor idea on the cathode is a good way to burn up the other output tube if you're using a CCS on your output stage like I do - it will force the other tube to double its bias current. For another thing the fuse is highly non-linear as it heats and cools. I would just take my chances. If the fuse blows the tube is probably shot anyway. I'm not sure what you're protecting there. The amp fusing should kick in for something catastrophic.

As for a short in the output transformer, I feel safer with a large current capacity MOV on the secondary of the OT to ground. If the primary shorts internally to the secondary there's no guarantee where it will happen, and any fusing may or may not blow on the primary side depending on the location of the short.

Just my 2 cents.

PS: Why would you run your amp with no load???

Last edited by sampleaccurate; 9th January 2011 at 06:42 AM.
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Old 9th January 2011, 01:44 PM   #9
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In a PP amp the unloaded winding will nominally have a voltage equal to the turns ratio - ie. HT to a peak level determined by opposite loaded anode in saturation (a magnitude slightly less than HT). If there is blocking going on then both windings can be unloaded. If the output is unloaded, then things get uglier. The aim of the MOV is to start conducting if a substantial overvoltage peak occurs, and even then it is a damped loading applied to the overvoltage portion only. Yes the MOV specified DC operating level has to be somewhat above HT, and then there is a large tolerance on the 1mA DC voltage level, which is somewhat higher still. The intent is to alleviate overvoltage stress on OT insulation - as the OT is typically more expensive than the tubes, and sometimes there are no replacements per se.

I must admit I have not heard of OT's shorting primary to secondary, but it is not uncommon for instrument amps to have the speaker plug pulled out, or someone to change speakers during testing and to forget one terminal.

Agreed that the fuse and parallel resistor technique is for typical fixed bias PP. The intent is to provide a lower level of over-current stress to the OT (than say a HT fuse) - for same reasons as above. The fuse voltage drop, if sized for no more than 70-80% rated, is I suggest negligible (I've seen 0.11 ohm for a hot 2A 3AG) for most applications (especially instrument amps).

Ciao, Tim
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Old 9th January 2011, 06:54 PM   #10
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definelty fuses. keep resistors slightly undersized compared to the tubes max power ratings so that the resisters blow first instead of the tubes or trannies. inrush limiters on the power supply. warm up circuit approaches. also think about what could be done in the driver tube area to prevent driver tube problems from feeding into the output stages. a shorted driver tube could glow red a power tube never blowing fuses.
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