screamingdaisy
Well-known member
Has anyone ever figured out the output power of a Dual Recto when it's in spongy/diode, full/tube, or spongy/tube?
94Tremoverb said:Yes, that's right.
If you really want low power, in the Spongy/Tube mode you can also use 6V6s, which roughly halve the power again, so you could actually run it at only about 15-20W with just a pair of them. The only problem would be that in order to get a correct impedance match you would need a 16-ohm cabinet, connected to the 4-ohm jack - but it would probably be OK with an 8-ohm cab.
(If you pull two tubes, or use 6V6s, or EL84s with converters, you need to double the speaker impedance relative to the amp's output, because all these have about twice the impedance of four 6L6s/EL34s. So if you use only *two* 6V6s or EL84s, you need to multiply by four.)
94Tremoverb said:It's not the transformers that are overstressed if you don't correctly reset the impedance, it's the tubes. The transformer is at no risk whatever, since the total power output is only about half (or less) anyway.
If you don't reset the impedance when you pull two tubes, the remaining tubes are then operating into half their matching load, which is effectively the same as running a 4-ohm load from the 8-ohm output in normal operation. Tubes will tolerate this quite well (unlike transistors!) but it isn't good for their life expectancy and usually doesn't sound as good - it can get quite flubby and squashed-sounding.
I don't know how Mesa get around this problem on the amps that switch output tube configuration - maybe they just ignore it, or maybe the transformer ratios are set to be either a compromise between the two (so both are somewhat mismatched, but not badly) or maybe the ratio is designed to be correct for the two-tube setting and a high mismatch with four - this would actually make sense because impedance matching is only really critical when the amp is being pushed into distortion, for clean sounds it's much less important.
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