Higher Impedance Jack when using 1/2 power???

[Monsta-Tone]

Had the Heartbreaker on 2 yesterday at practice. It was still louder than the PA or the drummer!
I've always hated the 60/100 watt power switch on the back. It's never really sounded that good in the 60 watt setting.
Then.....something occurred to me this morning while I was trying to wake up......

If you half the power tubes, the transformer is not seeing the correct load. Very similar to the way that using 6V6's means that you need to plug the speaker into the next higher output jack. For instance, 8 ohm speaker goes in the 16 ohm jack.

Has anyone tried using their amp in the 60 watt setting (on any 100 watt Mesa) but using the higher ohm speaker jack?
I don't know why I never thought about this before. Guess that's just how my tiny brain works, day late....dollar short!
Maybe the newer manuals actually say to do this. The Heartbreaker manual has a bunch of typos in the power tube and power amp section.

 

[YellowJacket]

I thought the 8 ohm speaker went into the 4 ohm jack to correct an impedance mismatch of that sort??

 

[Monsta-Tone]

:lol: It's early here! :lol:
Should have read a few more manuals.
The Mark IV and Lonestar manuals say to plug the 8 ohm speaker into the 4 ohm jack.

But....if you read transformer manufacturer information it says to do the opposite.
Here is a section from a Heyboer transformer that is a direct replacement for a Deluxe Reverb (2x6V6) or 2 x EL-84 amp:

made in the USA by Heyboer this unit features a special high performance core steel for maximum output and clean bass. 7,000 ohm primary to 8 or 16 ohms. Also works great with a 2x6L6 or 2xEL34 amp (with 4 or 8 ohm speaker load) where the intention is to limit the output with core saturation for natural compression and sustain.

If you look at replacement transformer information for Fender amps (I know we are talking about Mesa amps, but the theory is the same), these are general primary impedance ratings:
2 x 6V6 or EL-84 = 6,600 - 8,000 ohms
4 x 6V6 or EL-84 = 4,000 ohms
2 x 6L6 or EL-34 = 4,000 - 4,200 ohms
4 x 6L6 or EL-34 = 2,000 ohms

So, the basic rule of thumb is that 2 power tubes will need roughly twice the primary impedance as 4 power tubes do. Wonder why Mesa says to do the opposite in their manuals?
Most amp manufacturers will use the same transformer for a 50 6L6 amp as they do for a 40 watt 6V6 or EL-84 amp.

So.....anyone try this in 60 watt mode with either 6L6's or EL-34's?
:shock: All this circular thinking is making me wanna try it and see which tap sounds the best! :shock:

 

[Monsta-Tone]

Can't remember where I got this info from, probably Dr. Tube or Lord Valve. I found this years ago.

Boy, we've been dragging this one out a lot, as of late: JJ 6V6's will handle that plate voltage quite
easily. This is one tough, modern 6V6.

The impedance figure will double, i.e.: an 8 ohm amp will be 16 ohms, a 4 ohm amp will be 8 ohms. And
of course check the bias.

I use JJ 6V6s in a 6L6 amp with plate voltages of 420 and 1K screen resistors. I have had no problems
and its been gigged many times.

FWIW, my 1967 tube caddy shows the 6V6 as a replacement for the 6L6.

One concern is the mismatched load (6V6s want to see 2X the output load for 6L6s) but they'll probably
still last 1/2 as long (or longer) than usual.

 

[94Tremoverb]

Mesa are correct. You're doing the math backwards .

Running a 4-tube amp with only 2 tubes means that the primary impedance has doubled, so you need to double the secondary impedance. This means that you need to *halve* the setting on the amp relative to the speaker, e.g. for an 8-ohm speaker you use the *4* ohm jack.

If you use the 16-ohm tap with an 8-ohm speaker and only two tubes you're effectively running them into 1/4 of their matching impedance, which is probably not a good thing.


Similarly, if you fit 6L6s or EL34s to a 6V6 amp, the amp now wants to see *half* the impedance, so you would want a 4-ohm speaker instead of 8-ohm.

The Fender Deluxe Reverb is actually a special case because the stock transformer ratio is incorrect; the matching impedance isn't 8 ohms but closer to 16 - this is one of the things that gives the Deluxe the characteristic flubby sound when it's pushed hard. If you want to tighten it up, fit a 16-ohm speaker. This also means that running 6L6s in one does *not* mean you need a 4-ohm speaker - the 8-ohm speaker is now a better match.

 

[Monsta-Tone]

Ahhh! I stand corrected. Pretty confusing when transformer manufacturers say the opposite in some cases!


So....anybody use the 4 ohm tap for an 8 ohm speaker when in 50 or 60 watt mode? :lol:

 

[igfraso]

Mike Bendinelli always says that running a combo (with an 8 ohm speaker) in 100 watt, with the speaker in the 8 ohm tap, is similar to running it in 60 watt with the same speaker in the 4 ohm tap.

If you want to have a darker and smoother sound, run the amp in 100 watt with the 8ohm speaker in the 4 ohm tap. According to Mike, this is the way to mimic a Simul-Class power section when using a 100/60 watt amp.