• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

power tubes..newbie question.


2008-02-14 6:22 pm
I'm currently building a guitar amplifier (push pull, 50W)!
What I would like to do is to have the possibility to use different kind of output tubes (El34, 6L6, 5881, KT88). I know I cannot change this kind of tubes without modifying my amp..But As I'm building the amp from scratch I would like to know what I have to keep in mind in order to be able to use different kind of tubes?
(I mean If I have a proper bias supply swing for all the kind of tubes (fixed bias), and anode voltage)..what I have to consider? (I.e. grid resistor (how to calculate them)) and how is the impact of the load impedance seen by the Output transformer? I know that for example a typical impedance for 2 EL34 is 4.4 KOhm in PP configuration (if I'm not wrong)..but should be different for 6L6 for example...
There are some amplifiers that works with different tubes..how can they do?
(I mean there is a kind of tube the amp is designed for? and the other tubes just works but not used at their best?)
Sorry for my english...
Hope you can help me!
Thanks .


diyAudio Moderator Emeritus
2004-01-27 8:55 am
I think it's best to design the amp based on a beam tetrode (6L6, 6550, KT88, etc.) because those tubes need a low value for the grid resistor, otherwise the bias can become unstable. A pentode happily tolerates a significantly higher grid resistance, which makes it easier to drive, but it is also equally happy with a low grid resistance.


2007-08-08 12:12 am
The impedances will match better with a specific tube type, but in a guitar amp it will work as long as the plate load is not radically out of spec. You *will* get more distortion, but it's a guitar amp. 4,000-6,000 ohms on the primary will probably work ok with all of those tubes at common currents and voltages.