Q: PP tube amp

[km-r]

i had this similar thread somewhere here but few tube people seem to find it so ima try this board.

im going to build a cathode biased 18W PP AB amp with 2x el84 [marshall]
but the tubes im going to use are of different brand and are not electronically matched.

will this do something very dangerous?
anything i can do to make use of these tubes in PP?

oh yeah... got these from a local antique electronics store...



sorry the pic might be tooooo biiigg...

[brett]

Hi
there's no strict answer to this.  Firstly, no two tubes are perfectly matched.  Even the "matched" tubes don't match right across their charcateristic curves - they are just close (within 5 or 10%) at one point.

So, it is important to know how mismatched your tubes are.  Do you have any information about their Gm s? If they are different by more than 20% you will have difficulty getting a p-p amp working with them.  Small er differences can be partially ofset by biasing the tubes individually.  Because you are cathode biasing the tubes, this is fairly easy.  IF, AND ONLY IF YOU ARE COMPETANT AND SAFE AT DEALING WITH LETHAL VOLTAGES, use the cathode resistor or OT shunt method (my prefered method, see geofex.com for details) to bias the tubes to the same idle current.  That prevents any significant DC voltage from flowing through the OT at idle (which is quite a bad thing).

EL84s can be biased quite "hot".  If I recall correctly, I used to run my 18watter at about 30mA (or 10W at idle).  25mA would give longer tube life.

With mismatched tubes you'll get a little extra distortion.  Probably not enough that you'll notice.

But I'm wondering why you want to use mismatched tubes?  EL84s are cheap to buy.  And they'll only last a year or two anyway.

cheers