Tube biasing question

[daveg777]

I have a cathode biased tweed clone head that came with 6L6's and a 5U4 rectifier. If I swap to EL34's, does the bias have to be reset and must I change to a 5AR4 type rectifier?

Dave

[WaltC]

Almost certainly yes to the first question and no to the second (unless you want to use the 5AR4/GZ34 of course<G>).

[daveg777]

Would it cause damage to my amp if I made the swap and didn't re-set bias? Sorry for my ignorance


Dave

[Old Tele man]

...if the amp's "cathode-biased" then the EL34's will work OK, but probably not at exactly 100% perfect biasing!

...remember, 6L6 and EL34 tubes have different transconductance (ie: gm) values, so their respective idle currents will be different to the common cathode-bias votlage generated.

[WaltC]

Yeah! That's what I was trying to say <G>

Walt

[kt77]

check yer PT temp if you pop those '34s in there...they draw a LOT more heater current

[Old Tele man]

+1,000,000 on kt77's point! Power tubes are NOT simple "plug-&-try" subsitutions like preamp tubes are! Other considerations (which can affect "smoke" content and release there-of), in semi-importance order:

1) Filament current (affects PT)
2) Control-grid "bias" voltage (or cathode-bias resistor!)
3) Max. plate dissipation rating (affects tube & PT)
4) Max. plate voltage level (affects tubes, then PT)
5) Reflected speaker impedance (affects OT & tubes)
6) Idle "heat" into chassis (affects ALL components in chassis "oven")