• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

400+ Tube removal.

#10
You can pull any numbered pair per your diagram. i.e. 1,1 ... 2,2 ...3,3 etc. This will not change you impedance. I pulled a pair last week during sound check. Sounded funky, a back row tube was burning up so i pulled it and it's pair. That is the purpose of the design, to keep you playing in a problem. But if I felt i needed less output with an equal amount of drive, which seems to be you intention, I'd invest in a marshall power brake, or a hot plate. Tube circuits, especially an array of this size should probably be left to function by design.

BTW, i love my 400+
It's the 3rd coolest thing i've ever owned.
 
#11
Tualatin said:
You can pull any numbered pair per your diagram. i.e. 1,1 ... 2,2 ...3,3 etc. This will not change you impedance. I pulled a pair last week during sound check. Sounded funky, a back row tube was burning up so i pulled it and it's pair. That is the purpose of the design, to keep you playing in a problem. But if I felt i needed less output with an equal amount of drive, which seems to be you intention, I'd invest in a marshall power brake, or a hot plate. Tube circuits, especially an array of this size should probably be left to function by design.

BTW, i love my 400+
It's the 3rd coolest thing i've ever owned.
Click to expand...

I've thought about the hot plate but they are getting harder to find.

I actually wanted a little more drive honestly. It was just too clean too loud. I'm jumping between a tour possibly with average 700-1000 person venues and after 200 person venues


so.

Little less power would be nice.
 
#12
Also, I might recommend using a JJ gold pin 12au7 in the far left/furthest from the inputs preamp section and cranking your input volume while using less master to achieve what you want. They cost $20 a piece and run much cleaner, giving you less preamp over drive and more flexibility and tube compression at lower master volumes, while retaining all of your power tubes.
 
#15
While you can remove tube pairs to drop the available output power it is inadvisable. As B-string posts you will have a mismatch to your cabinets. This can lead to damage of your amp. If you need less power get a smaller amp or just turn the amp you have down. If you're trying to get dirt add a VT pedal.

Paul
 
#19
Read This, it explains it very well.


The most common “internet information” is that you can take two of the tubes out and cut the power in half - right, but which tubes?

Again, the amp is a push pull amp, with the two tubes on the right being one side of the push pull circuit, and the two tubes on the left as the other side of the push pull circuit.

Taking out the wrong tubes will, again, send the amp and the output transformer into crossover distortion - an ugly tone that no one can use.

So which tubes do you take out? (Or disconnect.) Take a look at the drawing for clarity. Either the two tubes in the middle, or the two tubes on the outside. Not both right hand or both left hand tubes.

Cut the power in half? Again, misunderstandings. This does not drop the volume to half, or anywhere close.

All other things being equal, it takes 10 times as much power to make a sound appear to be twice as loud to our ears. So, the opposite is true, it takes 1/10th as much power to sound half as loud.

Cutting from 100 to 50 watts is not 1/10th, it is only half. 10 watts would be half as loud (with everything else equal.)

But cutting the power will allow the amp to distort or break up earlier, at a more reasonable volume.

There are other concerns with removing or disconnecting tubes. The output transformer was made for the four tubes - there is what is called a “primary impedance” - a load that the amp and tubes expect at the input of the output transformer.

For example, a 4 x 6L6 transformer primary may expect an impedance of 1,900 ohms from the tubes. Taking two of them out doubles that impedance to 3,800. You did not change the transformer, so now there is a considerable mis match and the transformer has no way to “fix” it other than putting out a different secondary impedance (to the speakers.) Depending on the “winds ratio” of the transformer, now the speaker impedance is wrong, doubled, or close to doubled.

The amp will still work, but not as well.

Why all this? Simple, a 100 watt amp with two tubes removed doesn’t sound as good as the same model amp really made as a 50 watt amp with the correct transformer.

The “10 times” audio factor is also the reason audio taper pots are used in guitars and amps to control powered audio signals. The taper is adjusted so that 1/10th as much power appears across the pot at 5 as at 10, otherwise, we don’t hear very much of a taper at all, most of an “on-off” condition.
Click to expand...
 
#20
Well my amp tech is a certified boogie repair guy as well as doing custom work that boogie does not certify. He works on amps for major touring bands that you have heard of if you listen to heavy music as well as some blues guys that I have no clue if they are well known are not.
He says it is a super bad idea to pull tubes in a bass 400/400+. If you are playing clean through the amp and pull a pair off tubes that is okay for one or two shows. If you are driving the power section hard regularly and pull some tubes than you are asking for a repair bill.
As stated above the power tubes have a total impedance that is seen by the transformer. The amps are designed around this impedance which of course will vary a bit with different tubes. pulling out tubes changes this a lot.

If you want the same tone at a lower volume scale back your cabinets. a 1x15 will be about half as loud as a 2x15 and so on. Heck I used to know a guy that ran his 400+ into a 1x15 for just that reason.
 

Latest posts

Top Bottom
Back