Running a Twin Reverb on two output tubes

[Randall]

A Twin Reverb Re-issue came in today for a cleaning and a check over. It only has two 6L6s in it. I'd like to get a consensus on this as to what to say to the customer, a local church. It would seem that whomever did this was knowledgeable to the point of removing one tube from each side. But is this really an OK thing to do?

[Leo_Gnardo]

Won't hurt a thing, especially if they're using it a low to medium volumes.

Ideally you'd want to double the load impedance but I'm sure there's no budget for that.

Some of my crustomers like to run their Twins, 100W Marshalls and other 4 output tube amps on 2 tubes whether removing the tubes or installing a switch to break the ground connections to 2 cathodes. Can't see anything's been damaged as a result, OTOH the power doesn't obviously go to half, it seems only a little less powerful. That's the diff between measurement and perception, ain't it always the case?

[Justin Thomas]

In a Twin, you could just disconnect a speaker to double the load impedance... I'd remove it entirely - any "lightening" would be welcomed, I'm sure... though, honestly - how big is the church? Betcha a Twin Reverb Reissue a Princeton would be great!

Justin

[The Dude]

In a Twin, you could just disconnect a speaker to double the load impedance... I'd remove it entirely - any "lightening" would be welcomed, I'm sure... though, honestly - how big is the church? Betcha a Twin Reverb Reissue a Princeton would be great!

Justin

+1 to that. It will make a more noticeable difference in volume and make the amp less "midrangy" because of the mid coupling of speakers in close proximity, which is often perceived as "quieter". A lot of the old guys would disconnect a speaker in smaller clubs. It works well.

[Leo_Gnardo]

In a Twin, you could just disconnect a speaker to double the load impedance... I'd remove it entirely - any "lightening" would be welcomed, I'm sure... though, honestly - how big is the church? Betcha a Twin Reverb Reissue a Princeton would be great!

Justin

Simply disconnecting one speaker, the non powered speaker's cone will move in opposition to the powered speaker, almost a "speakers out of polarity" effect.

Remove the non powered speaker, now you have a huge hole in the baffle, still some of that "out of phase" effect with the energy from the working speaker finding its way out that hole.

Cover the hole, now we're getting somewhere. Bet nobody will exercise the energy to do it.

Just run it with the two tubes. Keep calm & carry on. Pass the Lord and praise the ammunition.

How big is the church? Don't matter, some of those church bands get rockin' LOUD. Some don't. Some churches wish they could dial down the band's volume, just like many a bar/club venue. Might find out what kind of volume they really need so proper advice can be given.

[The Dude]

Leo,

In an open back cabinet, the "passive radiator effect" is very minimal. Sure it will move, but not enough to make a big difference. Different story in a sealed or ported box. Like I said, I used to see lots of guys do that in the 70's with good results.

[Dave H]

In an open back cabinet, the "passive radiator effect" is very minimal. Sure it will move, but not enough to make a big difference.

Would it help to short the + and - terminals together on the disconnected speaker to damp any cone movement?

[Mike Sulzer]

Won't hurt a thing, especially if they're using it a low to medium volumes.

Ideally you'd want to double the load impedance but I'm sure there's no budget for that.

Some of my crustomers like to run their Twins, 100W Marshalls and other 4 output tube amps on 2 tubes whether removing the tubes or installing a switch to break the ground connections to 2 cathodes. Can't see anything's been damaged as a result, OTOH the power doesn't obviously go to half, it seems only a little less powerful. That's the diff between measurement and perception, ain't it always the case?

But since the two remaining tubes are operating into the wrong impedance, you lose more than half the power, but not enough to sound "half as loud". I think that requires more like a 10 db loss.

[Mick Bailey]

Yet another factor is the B+ goes up due to the decreased loading of the output tubes.

[Chuck H]

I think the mechanisms at work and the changes is operating conditions have been considered with the advice given so far. It's not uncommon for players to use a four tube amp with only two tubes installed. It's good to be aware of the drawbacks though and I believe they were considered.

So, how about a switch that lifts the cathodes on two tubes (installed on the two that are missing now, presumably) and also adds a load resistor (say, 6R) in series with the existing load? THAT would make for a useful volume decrease, somewhat correct the operating impedance and the amp could be used stock by just flipping the switch (once it has four tubes in it of course).

[Leo_Gnardo]

But since the two remaining tubes are operating into the wrong impedance, you lose more than half the power, but not enough to sound "half as loud". I think that requires more like a 10 db loss.

Indeed so. How many times, as a monitor mixer, I've been asked "double the volume" or "half the volume". If I move it 3 dB, the musician notices . . . nothing. And starts barking. Yes, when they say "half" or "double" a 10 dB move is what they really mean, better be really noticeable.

FWIW those amps with triode/normal switches, the difference really is noticeable, it's more than a simple volume change.

As for wiring a short across the disconnected speaker, yes that's a sort of good idea. Until some smart person comes along and decides to reconnect the speaker, doesn't notice the short. Churches are notorious for having lots of "experts" with their hands on the PA and band gear. "My uncle's best friend was a 'lectrican, therefore I know 'zackly what to do!"

Best case was a church that had a Blues Junior, plenty enough amp for the purpose. "Doesn't make a sound" was the complaint. Yes, because there were NO tubes in it. Maybe yanked out by unattended kids? Awww, the li'l angels...

[Mike Sulzer]

Indeed so. How many times, as a monitor mixer, I've been asked "double the volume" or "half the volume". If I move it 3 dB, the musician notices . . . nothing. And starts barking. Yes, when they say "half" or "double" a 10 dB move is what they really mean, better be really noticeable.

FWIW those amps with triode/normal switches, the difference really is noticeable, it's more than a simple volume change.

As for wiring a short across the disconnected speaker, yes that's a sort of good idea. Until some smart person comes along and decides to reconnect the speaker, doesn't notice the short. Churches are notorious for having lots of "experts" with their hands on the PA and band gear. "My uncle's best friend was a 'lectrican, therefore I know 'zackly what to do!"

Best case was a church that had a Blues Junior, plenty enough amp for the purpose. "Doesn't make a sound" was the complaint. Yes, because there were NO tubes in it. Maybe yanked out by unattended kids? Awww, the li'l angels...

Yep. If you want a push pull parallel amp for living room volume levels, you have to go all the way down to the EF86. Too bad they are such expensive little things!