2:50 RUNNING ONLY 2 TUBES CAUSE DAMAGE?

[mribanezmetal]

HI all! I have the newer red light version 2:50. I only use one channel and have recently installed EL34's in CHL A and flicked the bias switch. Seeing Im a tight arse I didnt want to fork out the dough on another set of tubes for CHL B that I never use - if the volume for CHL B is down is this OK?

I know the manual states this when using one chl if all 4 tubes are in but I wasnt sure if the unused chl 'needed' a closed circuit voltage flow?

Noobish question but I just want to make sure its similar to pulling tubes out of your favourite old Marshall banger

 

[mr_fender]

No it will not cause any damage. In fact it is safer to pull the unused tubes. I never liked the whole leaving the unused channel's volume down thing. If the preamp stage or inverter for that channel were to malfuction and/or go microphonic, there could be a possibly large signal going to the output section even with the volume turned all the way down. With no speaker plugged into that channel, the result for the output transformer would be bad. With the unused channel's tubes pulled, there is absolutely no current flowing through the output transformer for that channel. This will have some effect on the power supply's load, since there are two less heaters present, but I doubt it would make an audible difference in this amp.

 

[Old BF Shred]

+1 on that! You are perfectly safe with just the 2 tubes.

 

[mribanezmetal]

Thank you guys for the advice - much appreciated!!

 

[mr_mojo]

does this work for the 20/20 too?

thanks!

 

[haha]

And the older 50/50?

 

[elvis]

Yes and yes.

 

[haha]

Yes and yes.

Any idea why it's not mentioned in the manual?

 

[elvis]

There's a LOT that's not mentioned in Mesa's manuals. Too busy building amps, I guess. Their saving grace is excellent customer support. This is exactly the sort of thing I would call them about.

 

[crane]

Just be aware that in all these amp only one power tranny do the job. So if you pull out 2 tubes, other voltages will raise a bit, and some tubes may not like that.
For example the 20/20 is set to 390V on the EL84 plates. I know Mesa's tubes are selected for these but a voltage increase may kill some other tubes (EL84 are rated for 300V max...).

 

[elvis]

That is a fair point. Could affect the supply bypass capacitors as well. The supply should sag a bit under load, though, so it would tend to behave more like it would if one channel is merely turned down. I'm not sure how much DC current the power tubes are biased for.

On the 50/50 the B+ is 436 V v. a nominal rectified supply of 460V. The loss is in the choke. Pulling 2 tubes might raise B+ by 10V or so. The caps on B+ are 600V rated. (Based on the schematic, which is a bit blurry on the numbers)

 

[guitarman52]

I run either my 20/ 20 or my 50/50 (old style with power switch and blue light) Have never removed tubes.... I believe according to Mesa they "Suggest" alternating between Chanels if you are using the Amps in MONO Mode...... and on the 20/20 they tell you to leave the Unused Channels Volume OFF but to turn the Presence Control UP..... On my 50/50 I leave both Volume and Presence OFF on the UNUSED channel..... been running my racks in this fashion for over 15 years and haven't had any problems......

My fear of continuely playing with the tubes in and out.....would be destroying the small brass inserts where the tubes sit...... so I like putting them in place....... and leaving well enough alone..... (jmo here..)

George..

 

[crane]

Did you change channel sometimes ? I used to run my 50/50 mono too, but the unused channel is hissing now. Seem to e a problem on stereo amp played mono. OT dislike it, whatever the manual says.
It's quite easy to build a dummy load ;-)

 

[guitarman52]

Yes I switch channels every couple of months if I am using it alot....
and doing it this way so far has worked for me..... haven't experienced anything wierd with either of my power amps......