Recto Output Power

[screamingdaisy]

Has anyone ever figured out the output power of a Dual Recto when it's in spongy/diode, full/tube, or spongy/tube?

 

[94Tremoverb]

Yes, but I forgot again .

Something like 100W Bold/Diode, 80W Bold/Tube, 70W Spongy/Diode and 60W Spongy/Tube, but don't rely on that as dead on until I've checked again. Some power tube types can change it by 10% or more either way, too.

 

[lhrocker]

If you pull out 2 tubes, does all of the above cut in half again? I would think that it would still be safe to use the spongy/bold and diode/tube options as long as you pull the middle 2 tubes and a rectifier tube, right? That way you can get down to 35 watts...

 

[94Tremoverb]

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.)

 

[blazer986]

Interesting, I didn't know about the impedance difference between 6v6s and 6l6s. Very good to know.

 

[screamingdaisy]

I didn't know you could stick 6V6s in an 6L6/EL34 amp. I knew they used the same sockets, but I didn't think they were interchangeable.

 

[94Tremoverb]

They are, but only if the voltages are low enough (under 450V plate and preferably under 400V screen), which is why you must use the Spongy (and preferably Tube rectifier) settings. The bias voltage requirement is about the same as for EL34s (~ -30V), so you need to set the amp to that - no harm would occur if set to 6L6, it just wouldn't work very well. Since there aren't too many EL34 or 6L6 amps that do have such low voltages, you can't normally do it without using a Variac to get the same sort of voltage drop as the Spongy mode (or Tweed on the Mark series).

 

[YellowJacket]

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.)

EL-84s with converters at 4 times the impedance . . . Are you referring to using something like a yellowjacket that has resistors to suck power, or just a straight up socket converter? I always run EL-84s with yellowjackets set to tube and spongy so that there isn't so the plate voltage is low on the amp. Now I am curious to buy some 6V6s as well as 6550s (for my bass)! I think I'm going to need to expand my tube carrying case!!

 

[lhrocker]

I thought in the Mesa manual, it said that speaker impedance wasn't too important as the transformers are really tough. So with pulling a couple of tubes, EL or 6L6's, you don't need to really connect the 8 ohm jack to 16 ohm speakers and such. Also, on amps like Roadster and the new multi-watt recto's, there's no way to actually change speaker impedance if you have a 100 watt channel 1 with a 50 channel 2 and 3.

 

[94Tremoverb]

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.

 

[screamingdaisy]

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.

If it's any hint, with the LSS you're supposed to plug the 8ohm speaker into the 4ohm tap. It produces 5w, 15w and 30w this way (according to Mesa).

They advertise that 8ohm into 4ohm is a little darker sounding on 30w, whereas moving it to the 8ohm tap provides a brighter tone with more headroom... which leads me to believe that the amp is operating at a mismatch when in 4ohm->8ohm/30w mode.

With the 8ohm speaker plugged into the 8ohm jack it produces 35w. It has 'Dyna-Watt', which as I understand it is that they use a stiffening cap to provide extra power when attacking low notes.

 

[94Tremoverb]

Yes, that makes sense perfectly.