I have some general Q's about tubes!

[hells.saints]

I purchased my first tube amp in December, and fell in love with it. I just got another one this past weekend. I have some questions about them. Feel free to respond to all or one of these following questions:

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What is the purpose of the metal covers that go over the preamp tubes? Does it hurt anything to use the amp while these are removed?

I have heard that if you replace a power tube with another tube of the same hardness rating, the tubes don't need to be biased. Is this true?

I have a 5 watt tube combo. I have been searching and searching, and I cannot for the life of me find a bias pot. Could it be that this amp, although cheap, simple, and featuring only one power tube, does not need biasing?

If you don't connect a speaker load to a tube amp, you can damage the amp, as I am told. What if, instead of connecting a speaker, I connect a resistor of precisely the same wattage and resistance of the speaker that the amp is made to use? EX: It's a 5 watt tube combo with a built in 8 ohm speaker. I hook up a 5 watt 8 ohm resistor instead of the speaker. Theoretically, would this work? Would there be any damage?

[tunghaichuan]

Welcome to the forum. Please stop by the Fret Players section in the Lounge and introduce yourself. It's great group here, and everyone is very friendly.


I purchased my first tube amp in December, and fell in love with it. I just got another one this past weekend. I have some questions about them. Feel free to respond to all or one of these following questions:


What is the purpose of the metal covers that go over the preamp tubes? Does it hurt anything to use the amp while these are removed?


They shield the tube from noise and interference. If they fit tightly enough, they may reduce some microphonics from mechanical vibrations. It won't hurt anything to leave them off.



I have heard that if you replace a power tube with another tube of the same hardness rating, the tubes don't need to be biased. Is this true?


Maybe. This was started by Mesa Boogie, who used to build their amps without bias pots. They would buy a lot of tubes, test them and then use only the ones that fell into the acceptable range for their amps. Groove Tubes has also perpetuated this myth.

Any amp, regardless of biasing method, cathode bias or fixed bias can benefit from adjusting the bias when changing output tubes. Preamp tubes are cathode biased, also called self biased, so they don't need to be biased when they are changed.



I have a 5 watt tube combo. I have been searching and searching, and I cannot for the life of me find a bias pot. Could it be that this amp, although cheap, simple, and featuring only one power tube, does not need biasing?


If it is 5W, I would guess that it is a cathode biased amp. That means there is a resistor connected to the cathode and ground that biases the tube for you. That's why cathode biased is called self biased.



If you don't connect a speaker load to a tube amp, you can damage the amp, as I am told. What if, instead of connecting a speaker, I connect a resistor of precisely the same wattage and resistance of the speaker that the amp is made to use? EX: It's a 5 watt tube combo with a built in 8 ohm speaker. I hook up a 5 watt 8 ohm resistor instead of the speaker. Theoretically, would this work? Would there be any damage?
[/QUOTE]

It will work fine. But I would use at least double the wattage of the amp, personally I use 4X the value of the wattage. So for a 5 watt amp, I usually use a 20 watt resistor.

What are you trying to accomplish with this? If you plug a resistor into the output of the amp and don't have a speaker plugged in, you won't get any sound out.

[DeanEVO_Dude]

If I migh add a little layman's understanding to what tunghaichuan said about the sleves on the preamp tubes...
Understand first that a guitar signal is very, very low and must be amplified alot before it goes to the power amp then the speaker(s). As a result, preamp tubes are very suseptable to extraneous noises such as electrical interference and vibration. On a (what I suspect is) 5 watt, 2 tube (1 preamp, 1 power amp), small combo, it is not very noticeable, but once you get into tube amps with 2, 3, or more preamp tubes, the noise can be amplified alot. They also serve a second purpose, to hold the tube firmly into the socket. EL84 power tubes have a bracket that goes over the top of the tube (ala the Crate V5), 6V6 tubes have a couple of metal tabs that are "part" of the socket that hold onto the base of the tube to hold it into the socket (ala the Fender Champion 600 RI). Preamp tubes have a metal cylinder with a spring inside that covers the tube and help to shield it from electrical interference (i.e. computers, flouresent lights, etc.) as well as hold the tube tightly into the socket. Since preamp tube run much cooler than power tubes, heat is not much of an issue with them.

I hope this additional info helps.