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Vacuum tube am radio

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Mdkanz

New Member
I have a vacuum tube am radio that i made a 1/4'' jack output but i was reading somewhere that it can damage a tube if there is no load. so would it solve that problem to put a 10ohm resistor across + & - with a head phone jack so when the jack is plugged in the resistor is disabled?
 

flat5

Member
Use a 100 ohm (or 10x the expected speaker impedance) and don't bother with a switch.

What I'd do is try a 47 ohm and see if I can even hear a difference when it is across the speaker or not.
 
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k7elp60

Active Member
Yes it is possible for damage to occur as the tube type radio's used a step down audio transformer. With out a reasonable load the secondary, where the normal speaker connnected the reflected impedance in the primary was very high.
A normal solution is to use a 1/4 jack that has a switch contact built in to the jack. The contacts are normally closed with out a plug in the jack, thus putting a load on the secondary. When a plug is inserted into the jack the contacts open disconnecting the load, and the load on the plug now becomes the load on the secondary.
See the attached figure.
 

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Mdkanz

New Member
Yes it is possible for damage to occur as the tube type radio's used a step down audio transformer. With out a reasonable load the secondary, where the normal speaker connnected the reflected impedance in the primary was very high.
A normal solution is to use a 1/4 jack that has a switch contact built in to the jack. The contacts are normally closed with out a plug in the jack, thus putting a load on the secondary. When a plug is inserted into the jack the contacts open disconnecting the load, and the load on the plug now becomes the load on the secondary.
See the attached figure.
Attached Thumbnails

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What size resistor should i use?
 

k7elp60

Active Member
I am guessing a resistor about 5 to 10 ohms @ about 2 watts. As I recall a lot of the speakers in those days were 3.2 ohms, and the amplifier was capable of about 2 watts output.
 

BrownOut

Banned
Yes it is possible for damage to occur as the tube type radio's used a step down audio transformer. With out a reasonable load the secondary, where the normal speaker connnected the reflected impedance in the primary was very high.
I don't get it. The reflected impeadance is high. Then what? I grew up in the age of silicon.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
I don't get it. The reflected impeadance is high. Then what? I grew up in the age of silicon.
It's quite simple:

Transistor amplifiers are destroyed by a short circuit on the output, valve amplifiers are destroyed by an open circuit on the output.

It's a bit like a protection diode across a relay, with no load the transformer can arc internally destroying it, or it can arc over inside the valves.
 

marcbarker

New Member
Tube guitar and PA amplifiers were destroyed with a spectacular puff of smoke if the speaker circuit broke if the amp was cranked up at full power. The manufacturers followed the trend set by repairmen, who arranged for the the Speaker socket to short-circuit the amplifier if no speaker plugged in. It was better to have glowing red hot tubes with a shorted output, than to have an output transformer destroyed by internal arcing. Quite nice colours in the tube, red hot anode and blue sparkling inside, as a reminder to plug the speaker back in.
 

flat5

Member
I used to repair a lot of Fender guitar amps in the 1970s.
Fender dealt with the problem by either using a neon bulb on the output transformer primary (tube side) or a one watt resistor (or two watt) (I don't remember the value but I think it was 47 ohms) across the speaker jack.
Not switched. Always in the circuit.
 
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