4 Power Tubes in a 3 channel triple Rec?

[mexirican]

So I went to practice yesterday to a different sounding amp. Looked in the back and noticed that one of the tubes was out. (3rd one from the left) I killed the amp, jiggled the tube, tube turns back on. Then it went out after 3 min of playing. I won’t have the money to replace the tube until I get paid next week. Here are my questions.
1 - I recently changed all the power tubes to JJs 6L6s. Could I use the original tubes that I know are good in the middle two sockets until I get the new tubes?
2 - Can I run the amp with out the middle 2 tubes?

Thanks for the help.

 

[94Tremoverb]

1 - yes, but if they were a matched set you'd probably be better using *all* of the old tubes because they are unlikely to match the new ones.

2 - yes. You may be better to run the cabinet from the impedance below it, ie 4 ohm jack for an 8-ohm cab. It's not strictly a 2:1 mismatch but it's closer than running at the 'correct' (which now isn't) impedance. It doesn't really matter unless you're cranking it though.

And -

- the problem with the tube is probably a bad solder connection in the tube pin, on either pin 2 or 7 which are the filament pins. Have a careful look at these pins and you may see a crack around the central wire which is inside the pin itself. I've come across this on quite a few occasions, not only on JJs but a good number of them were. You can fix it by re-soldering the tube pin by heating the tip and adding a small amount of fresh solder - just be careful not to make the pin tip larger or it will stretch the socket contacts. If you do accidentally make it larger, file or scrape away the excess.

Yes, you can (sometimes) repair tubes!

 

[mexirican]

So just take out the 2 middle tubes run it @ 4ohms?

 

[domct203]

So just take out the 2 middle tubes run it @ 4ohms?

Yes, and remove one (1) Rectifier tube (5U4GB). Each pair of powertubes require one Rectifier tube to supply the correct voltages. It won't hurt the amp if you leave all three, it just won't sag as much.

If all you use is diode, don't even worry about the 5U4GB.

Dom

 

[mexirican]

I am currently running it @ 4ohms because in playing through a full stack. Still OK? Im asking so many questions because I don't want to hurt anything.

Thanks again.

 

[YellowJacket]

It is probably better to run a halfstack until you get those tubes replaced. Losing one 4 x 12 shouldn't affect your stage presence all that much =-o

(I do agree that fullstacks sound amazing)

 

[94Tremoverb]

I would just run it with the two tubes (and possibly one rectifier) pulled into the two cabs. Yes, it's a mismatch but Mesa amps are pretty tolerant of that, you're developing less power overall so nothing (other than the tubes) is working harder than it would be normally, and - unless you're playing at completely insane volume, with those cabs! - you won't have it fully cranked anyway. Impedance matching only becomes fairly critical when you're pushing the power stage really hard, and at lower volumes - which on that amp will still be pretty high - it makes very little difference.

Do check the dead tube though, you could save yourself a fair amount of money for two minutes' work if you're lucky, as well as not having to worry about mismatching. An intermittent filament connection that comes and goes when you move the tube is very likely to be this problem.

 

[stratele52]

NO NOT 4 OHMS , but 16 ohms

Removing tubes give higher impedance ( ohms) on the primary of your transformer so you have to plug secondary to higher ohms speakers

 

[94Tremoverb]

Sorry, that's wrong. When removing tubes you must use a *lower* impedance tap on the amp. You're correct that the impedance on the primary is raised, so you need a higher impedance on the secondary too. The way you do this is to connect the *same* speakers to a *lower* impedance tap, not higher - ie the 4 ohm jack for an 8-ohm cab.

Please check this elsewhere if you don't believe it, I know for 100% certain that it's right.

 

[domct203]

Trem is correct.

Why not just use your old set of powertubes until you have your new tube issues straightened out?

Dom

 

[mexirican]

I only have 4 of the original 6 power tubes. one was broken and I dropped the other when I was changine them. :roll:

 

[MesaENGR412]

I only have 4 of the original 6 power tubes. one was broken and I dropped the other when I was changine them. :roll:

I have done the same exact thing.....

-AJH

 

[stratele52]

I hope this will help:
Removing tubes / output impedance

Tube Guitar Amplifiers Essential, by Gerald Weber, page 213

......When you remove two of the output tubes, the primary must be doubled to accommodate de two remaining output tubes. An easy way to do this is to use a speaker load that is twice of what you would normally use when running all four output tubes.

For instance, let's say you have a Twin Reverb amp, wich normaly runs at 4 ohms, and you remove two of the tubes. You could disconnect one of the Twin's speakers to change the secondary to 8 ohms........

In case of 100 watts Marshall that two output tubes were remove, you would set the impedance selector at 8 ohms if you were using a 16 ohms cabinet "

 

[94Tremoverb]

That's correct and is the opposite of what you said earlier. "NO NOT 4 OHMS , but 16 ohms"

You then said "Removing tubes give higher impedance ( ohms) on the primary of your transformer so you have to plug secondary to higher ohms speakers" which is correct if you do have higher impedance speakers, but if you're using the *same* speakers (as most people will be) you must then use a *lower* impedance tap to give the same result, ie *4* ohms with an 8-ohm cab, *not* 16 ohms.

If you meant this, your first sentence was extremely confusing and reads that you need to reset the impedance to 16 ohms.

Not being pedantic, this really matters. Using the 16-ohm output on the amp with an 8-ohm cab while running with tubes pulled is then a *very* low mismatch and could blow the tubes.

 

[MesaENGR412]

I did not think you could run a Triple Rectifier while missing power tubes like you could a dual rectifier, since the impedance could not be cut by 1/3s? I know you can with the new multi-watt heads, but that is with the flip of a switch. Can it safely be done with the older 3 channel and two channel Triples?

-AJH

 

[94Tremoverb]

It's true that you can't create an exact match, but it's not that critical - just a question of getting a bit closer. Most (not all, true) tube amps will handle a 2:1 mismatch in either direction fairly safely, and Mesas certainly will, so a 1.5:1 mismatch is really nothing to worry about. The new models with the switch can only be doing the same thing really, unless they have many more taps on the OT, which I think is unlikely... it would make the internal switching really complicated.

The older Mesa models with half-power switches don't even reset the impedance, although they do recommend in the manual that you swap to the lower tap if you're running it like that - I know many people don't though. On the Blue Angel, they gave you three different combinations of power tubes (2, 4 or 6) and only the standard 4/8/16-ohm taps, none of which can be right at all of the settings.

 

[MesaENGR412]

that makes sense..thanks for the great explanation man.

-AJH

 

[stratele52]

Mark IV you are RIGHT.

I'm sorry, my excuse is I'm french speaking and some time I make mistake for translating english in french and to understand what people write. And this subject " ..plug to 4 ohms jack a 8 ohms speaker" is not easy .

Very sorry because you look to know a lot on amps

 

[94Tremoverb]

No problem! I would not be able to write on a forum in French at all, I can only barely speak a little (and very out of practice at that) so you're already doing better than me .