duplicating the load on the B channel of my 2:90

[ibanezgtr]

I am running out of the A channel of the simul class 2:90. I talked to Marcus from Mesa Boogie and he said I need to plug the B channel to an extra speaker or cut off the end of a guitar cable and solder the black to white to make it parrallel and plug that in the B channel speaker jack to duplicate the load and not cause the tubes to osciallate. Is there any other alternatives like using a guitar cable or a footswitch with an unshielded cable? or a quarter inch adaptor for headphones? Don't have an extra cabinet or a soldering iron.

I need a shorting jack, can you buy one of these?

 

[hanson2713]

Unless i am not unserstanding your question, why cant you just turn the volume off on channel B?

 

[ibanezgtr]

I guess turning the volume down should be fine, but Marcus from mesa boogie said it could still do damage. I have been running it this way for 5 years and everything has been fine, so I guess it is ok.

 

[hanson2713]

Yeah, I have a 2:50 and mesa told me that I should just turn one side off, Ive never had any problems.

 

[spikenet]

hey man, you should never run a tube amp without a load ie: speaker or attenuator. If you do there's a good chance you will fry the output tranny and be up for big $$ repair.

I run my 20/20 mono and just plug the unused channel into an appropriate block of resistors I made. still run it set a 0 volume and max presence but the tranny is happy!

cheers

 

[hanson2713]

its is not an issue if the volume on that channel is down.

 

[AdmiralB]

Why wouldn't you just pull the tubes from that side? If you're not using it, you're (slowly) wearing them for no good reason.

 

[hanson2713]

Im not sure of the reason but you cannot pull the tubes out of one side. You will damage the amp. You can however just alternate channels every few months, like rotating your tires.

 

[AdmiralB]

I can see absolutely no reason for that to be the case.

The only affect on the channel in use will be slightly increased voltages, since the load on the power supply will be reduced. Not nearly enough to affect operation.

 

[hanson2713]

From the Boggie manual. NOTE: If you opt to use only one channel of the 2:50 with speakers connected to that channel’s output only, you must turn the unused channel’s level control to zero! Otherwise the channel will be operating without a load, and damage to the tubes
and/or the transformer will result.
Doesnt mention pulling tubes though I've read somewhere that it not good.

 

[AdmiralB]

From the Boggie manual. NOTE: If you opt to use only one channel of the 2:50 with speakers connected to that channel’s output only, you must turn the unused channel’s level control to zero! Otherwise the channel will be operating without a load, and damage to the tubes and/or the transformer will result.

This is true.

Doesnt mention pulling tubes though I've read somewhere that it not good.

Doesn't mean that what you read was correct, though. If the schematic for the 2:90 is accurate, there is no reason you cannot pull the tubes from one side.

In fact, the only situation I can think of where pulling tubes could ever be harmful would be in cathode-biased amps. I've never seen a cathode-biased tube power amp, and even if there were some, you wouldn't share cathode resistors across channels.

 

[again with this]

To the OP. Go with Marcus told you. There is some bad info in this thread, namely pulling the tubes. You can get away with that on some heads, to run at half power. But, in general, that is a terrible idea.

Take a trip to Radio Shack and pony up $20 bucks for a soldering iron (you can buy one ther in a kit with solder and everything you'd need for $15) and a mono 1/4" plug for $5.

Take a piece of wire and thread it through the holes in the conductors and solder.

Heck... for $5, just buy the plug and wrap a wire very tightly around the conductor leads. No solder needed.

I did the EXACT same thing to make a "modern" plug for my 2:100.

 

[AdmiralB]

To the OP. Go with Marcus told you. There is some bad info in this thread, namely pulling the tubes. You can get away with that on some heads, to run at half power. But, in general, that is a terrible idea.

OK Einstein, tell my WHY it's bad info.

I said pull ALL the tubes from one channel. These are essentially two discrete amps that share a power supply (as opposed to the Marshalls, which really are two discrete amps).

 

[18&Life]

MANUALS !!!!!!!!!!!!!

2.90 manual = volume 0 presence maximum.

That's it nothing more.
If mesa do 2.90's manual wrong,well it's the end of the world :lol:

 

[spikenet]

I definitely disagree with the manual! my 20/20 says the same thing

My amp tech gave me some advice about this same issue when I first got my 20/20 and from my own experience you should never run a tube amp without a load. PERIOD

buy yourself a speaker load (or make it yourself for a few $), plug that into the unused channel and save yourself the heartache down the track.

 

[msi]

What I have done is pulled the fuse for the unused channel. That way the tubes are still in the 2:90. They are pulling current for their heaters, but because the fuse is on the cathode there is no current through the output transformer. I still keep the volume down like in the manual. The best method is to have some load on the unused channel, even if it's just a static load like a resistor. Make sure that the resistor is rated for the proper load.

 

[AdmiralB]

If there's no AC signal on the unused side, the tubes don't need a load. So the manual is correct.

Pulling the HT fuse is a bad idea, because then you'll have the tubes' heaters going with no current flowing - this results in cathode poisoning, and it'll (slowly) kill the tubes).

 

[msi]

Yes, fair enough. You are correct and I am corrected. I have old tubes on channel b anyways so that was never a concern of mine.

 

[ibanezgtr]

Thanks guys for all the feedback. Marcus said not to pull the tubes too. I may just put some old tubes in it's place or alternate channels every few months just to play it safe. Also when I can, I will keep it plug to another 4x12 when I am only playing out of one cab. I have kept my volume at 0 and presence all the way up for 5 years and it has been good. So we'll see. Thanks