flat || threaded

	Should speaker impedance be halved when using 6L6s in 5E Darren Burt ( ) Fri Oct 1 02:57:33 2004 I think I read this somewhere....   I'm using 6L6 tubes in my Mission 5E3; do they want to "see" half the plate impedance that 6V6 tubes do?     Cheers,   Daz

---	Carl Gigun ( ) Fri Oct 1 04:27:14 2004 Yeah it will make more power with a heavier load, but that doesn't mean it it won't work or sound good with lighter loads. Does your output transformer have taps for different speaker impedances? If it does you can mismatch the speaker load to reflect a lower impedance to the tubes to see if you like the sound better.     -Carl

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Re: Should speaker impedance be halved when using 6L6s i Darren Burt ( ) Fri Oct 1 04:35:59 2004 The OT is not multi-tap.   The reason I'm wondering is that I've been running 6L6's in my 5E3 clone with an 8 ohm speaker for a while now with no ill effects. I originally built it with the 6V6s and the (correct) 8 ohm speaker. Now, I've ordered two 10" speakers for it (each at 8ohms), which I'll run in parallell for 4 ohms.     I just figured that an 8 ohm speaker with the 6L6 tubes was an impedance mismatch and that the 2x10 @ 4 ohms would be "correct".

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Dave B. ( ) Fri Oct 1 07:21:32 2004 Mismatch is not always a "problem" Fender OT's can handle 50% mismatch with no problem. Some amp builders use the impedance switch for the speakers loads as a "tone" effect. Early JTM45 with KT66's (a 6L6 variant) are reported to sound best with the 8 ohm tap of the RS spares OT. Certainly not the "standard" operating parameters.     Neil Young has used 6L6's for years in his tweed deluxe. Never heard about any problems with mismatch.     If it sounds good don't stress too much about correct impedence, especially if it's a bit on the higher side. I'd be more concerned with going with a lower than recommended impedance.     DB

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Carl Gigun ( ) Fri Oct 1 12:32:18 2004 Luckily there's no one and only "Correct" impedance for a pair of tubes. If you wanted to get technical about it you could select the impedance to give maximum output power at your operating point, or select a different impedance for lowest distortion. Apparently those two ideals are never the same point for any known tube (O'Connor). You can also use the impedance that sounds good to you, that's what's so fun about tube guitar amps. As I saw somebody say once "Ratings are for transistors, Tubes have guidelines."     -Carl

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Mick ( ) Fri Oct 1 15:34:09 2004 Hey all,     What Carl said is correct. There is no absolute output impedance for vacuum tubes at least for what we are after as guitar players. The recommended plate impedance that a manufacturer suggests is a "best case" trade-off between output power and distortion. Varying the load impedance that the tube sees within a reasonable amount will affect distortion content but may not hurt output power too much. You need to looks at the load vs. distortion charts for a particular tube. The RCA Radiotron Designer's Handbook has some good info on this.       Mick