Mesa Boogie Bass 400+ tubes question

Don't do it, there's no practical benefit and plenty of potential problems. The internal voltages are in part based on having all tubes in and operating. Yes, there will be a significant effect on the impedance matching.

You are right, you don't want to blow your amp because repairs on it are expensive.

 

This is a thing... to remove parts in hope of better performance?

 

That's not at all true, another bit of incorrect information posted on the internet.

When the cathode is lifted, the tube is effectively shut down. There is no plate current, there is no screen current, there is no cathode current, and therefore there is no contribution from the tube to the source impedance driving the primary of the output transformer. Therefore the more tubes that are shut down, the greater the impedance mismatch.

The secondary problem can be that unless the amp was designed for this feature, the operating voltages can climb above the safe design values with the reduction of quiescent current in the output stage. This amp already has a pretty high B+ supply, you really don't want to run into an overvoltage flashover event that could damage tubes and the output transformer.

 

Cost of maintenance and upkeep is important to factor into the equation when purchasing gear.

If the 400+ needs retubed, and you can’t afford it, it might make sense to sell or trade it for something that doesn’t have high maintenance cost in the long run, even if it’s at a bit of a loss.

A working amp you can afford to keep working long term, is a far better amp than one you can’t afford to keep operational.

 

What's wrong with the existing tubes?

 

Pulling tubes to reduce power is actually not as uncommon as you might think. Some guitarists pull two tubes from their 100W heads. There are also some bass amps that essentially deactivate tubes to reduce power. The Mesa Prodigy and Strategy call it Multi-Watt. My Trace Elliot V8 has a switch labeled half power.

Basically if you can remove half of the tubes from each side of the output transformer, the expected load doubles. So for example a 4 ohm output becomes and 8 ohm output. The Mesa Strategy drops from 8 to 4 tubes for half power. Then it drops from 4 to 2 tubes for 1/4 power.

It might be possible to run the 400+ with 6 tubes, but you can't really go any lower and resolve the impedance mismatch. The Ampeg SVT has the same problem. Since there are three tubes on each side of the transformer, you can't pull half of them.

Also as Agedhorse has mentioned, there are other consequences you need to consider. As you pull tubes from the amp, the power supply voltages will increase. This will cause the remaining tubes to conduct harder, which means the bias should be checked to ensure tube current is within range. The Mesa 400+ is not factory equipped with variable bias. Also, the 400+ runs 480V for the plate and screen supply. If you check the limits on tube data sheets, most brands of 6L6GCs list 500V plate and 450V screen as the limits. If the power supply voltage rises to far, tube reliability will suffer because the voltages are too far above the limits.

 

Tubes aren't really that expensive considering how infrequently you should need to retube a large power amp like that. Because either you're not pushing it that hard, in which case, just turn down the volume (and if you want lots of overdrive at low volume, you should just get an appropriate wattage head and/or use a different cab setup), or you are driving the amp hard all or most of the time, in which case pulling tubes doesn't make sense anyway.

Either way, it's not really the right solution.

What you might want to do is just reserve the 400 for special occasions and put some money towards a lower wattage tube amp for more "every day" usage.

 

Keep it lit up with the 12 6l6's... Retube when you have the Cash. I have the 400+ and the 400 with the 400 having an adjustable bias mod and KT88's installed .. Both kickass!

 

I think one issue with the 400\+ with tubes is that there is no adjustable bias in the circuit, so unless the tubes were purchased from Mesa who would have matched them reliably to each other and to the operating parameters of the amp (or in some rare cases another tube source that actually reliably tests and matches tubes and knows the amp) then it’s quite possible it was retubed with a set that didn’t work well with the amp. When I bought my old 400+ ages ago this was clearly the case- it sounded congested and weak and blew a fuse not long after I got it (it had non-Mesa Sovteks in it). I took it to a tech that knew the amp, checked it for other issues, and retubed it with a set of JJs that sounded phenomenal for the next 5-6 years until I stupidly sold it. Reliable techs that know these amps well and also test their own tubes in house are becoming a rarer breed these days though- I would definitely suggest having one install an adjustable bias circuit to open up your retubing options. Either way I would say buying a set of tubes from internet dealers can be a potentially expensive crapshoot, even if you may save money on the front-end.

 

That's a good reason for replacement, not the 30 years but the coming apart bit.

 

the glue loosening part reminds me of this sweet 70s aluminum-frame Italian racing bicycle I had that was completely held together with aircraft adhesive (no welds). Great stuff that allowed for a slick lug-free bike, but obviously has a finite shelf life which happened to run out on mine mid-ride one time as I felt my top tube separate from my head tube at 35mph (fortunately not faster). Somehow I came out unscathed!

 

That's the equivalent of having a rein break on your horse...hopefully not at a bad time either.

 

Bench the thing for the moment, and buy what you can afford when you can, store them somewhere safe until you have everything you need imo. $360-$400 hurts a lot less when you break it down over a period of weeks/months.

 

In a car, maybe. To make it lighter.

Can't think of how this would be successful in music gear.

 

Ya... This is more like pulling three spark plugs out of your six cylinder engine to make it more fuel efficient.
Or in this case, put in the three plugs to get around for a while until getting the other three on payday.

Yes, I am very aware that active cylinder deactivation is a thing these days.
Just having some fun conversation since this was solved on the second post.