Number of Tubes in an Amp? - Jemsite
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post #1 of 3 (permalink) Old 07-27-2009, 01:50 PM Thread Starter
Join Date: May 2008
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Smile Number of Tubes in an Amp?

Hi guys, I was just wondering why some tube amplifiers use less tubes than others and if it is a sign of quality? I have done some research (hoping that I would be able to own a tube amp myself someday ) and found that some amplifiers require a certain number of tubes to be able to put out a certain wattage, but other designs use fewer tubes and are still able to produce the same wattage. For instance, The Koch Twintone II requires 4 preamp tubes and two power tubes, while the Peavey ValveKing needs only 3 preamp tubes and two power tubes.

I was thinking maybe this was due to some amps being class A or class AB in operation, or design issues, or maybe something else. Please educate me
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post #2 of 3 (permalink) Old 07-27-2009, 02:50 PM
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Re: Number of Tubes in an Amp?

Well, I'll start by saying that the number of tubes is no definition of the quality of the amp. I believe, in the preamp, the more tubes usually means the more pre-amp gain available. An electrical engineer will also be able to explain how certain pin types of various tubes will allow you to produce greater amounts of gain in the circuit but I'm not very knowledgable on the subject. There is no hard and fast rule that says this many tubes in the preamp = x amount of gain on tap because there are many types of preamp tubes that yeild different results.
Powertubes are the same way, different tubes will yeild different wattages/results. 2 EL84 will not give the same output as 2 EL34 or 6v6 vs. 6L6. All tubes being equal (which they're not), the rule of thumb is that the more tubes in it, the more headroom the amp can produce. Meaning that more tubes require a higher level of power to push them into powertube distortion, you'll maintain a clean tone at higher volumes. It has to do with the number of tubes, but it's also due to the type of tubes used.
Class A and Class A/B refers to the efficiency of the power usage in the amp's circuit. My very basic understanding, and I'm sure someone else will come and tell me I'm not totally correct, is that Class A runs current the whole time and is less efficient in it's power usage while Class A/B alternates the power in a "push/pull" effect that allows the tubes to run longer in a more efficient fashion.
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post #3 of 3 (permalink) Old 07-27-2009, 03:09 PM
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Re: Number of Tubes in an Amp?

Indeed, the number of preamp tubes has no relation to quality or power (wattage).
To understand a bit of what preamp tubes do, and why some amps have three, four or even five look here:

The number and type of power tubes does affect power, as does the class of amplification.

Typical power pentodes/beam tetrodes (EL34, 6L6, KT66 etc.) in Class AB configuration are rated at about 50 Watt per pair in Guitar amp applications.

50W is about the maximum for the 6L6, 5881, 6550 or KT66, but the EL34, or KT88 can deliver up to 100W/pair. Usually in guitar amps these will be rated at 50W-60W/pair, but it may also be as low as 30W/pair. It may depend on the power supply, transformers, tube or solid state rectification, cabinets used etc. Class A designs are less efficient so will be rated lower as well.

A typical 100W amp will have two pairs of pentodes (so four power tubes in total) such as the classic Marshall Plexi (the 100W Superlead) which uses four EL34 tubes, or the Mesa Dual Rectifier, which uses four 6L6 tubes.

The Marshall Major used four KT88s and was rated at 200W! But the Fender Twin got only 80W with four 5881s.

Another pentode power tube that you may come across is the EL84, it can only deliver ~17W/pair, these amps are usually rated at 15W. The famous AC30 uses four EL84 tubes for 30W.
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