valve question

[tom skool]

I have just come into owning a marshall vba 400 which i absolutely love.
What i dont love about it though is the thought of having to replace all 8 of the 6550 power valves when they go as its gonna cost a bit.
I have heard of people (mainly guitarists wanting to drive there amp more)removing a couple of poweramp valves and i was wondering i f could do the same. I dont need any where near 400watts (i have'nt had it past 2 on the volume!) and then i would have some matched spare valves for when the time came.

Is this a good idea or not? Would it do the amp any harm running it this way?

[alanbass1]

It's done safely in guitar amps by removing the two outer valves in a four valve set up. Not sure of the pairing in an 8 valve set up and whether the concept is the same. What you will find is that those valves remaining will be working much harder and liable to wear out quicker so you might not get what you are looking for, which is overall longer valve life. Guitarist do this to get the compression/overdrive earlier and valve life is part of the thinking.

[Ancient Mariner]

Removing half the valves in a guitar amp is done to halve the power output and drop clean headroom. The 2 remaining valves work just as hard as they did before at half the power level. However if the amp was previously set to produce 50 watts with all 4 valves and is again set to produce 50 watts on only 2 they yes, they will be working harder than before.

Removing one pair of valves will also change the double the impedance that the output needs to see. If previously you used a 4ohm load then you will need to change it to 8 ohms. Failure to do that will also wear the valves out more quickly. Valve amps do appear to cope with quite a wide range of impedances, but it is not good for them to be run at the wrong impedance, and it will likely shorten the life of the output transformer as well as the power valves.

[umph]

wouldn't recomend pulling valves for less power it doesn't work like people hope it will.

If you want less power buy a smaller amp.

[tom skool]

I'm not wanting to do it solely fo less power, i find the volume knob fulfills that job just fine. I just wondered if i could save myseilf from having to replace all 8 valves when just one has blown. But maybe with the amount of work they do perhps that day is along way off.
I dont like the thought of changing the impediance either, is there any way of checking what the output needs to see if i did take a couple out, ie with a meter reader?

I know the sensible option would be to just get a smaller amp. i Previously had a 50watt valve head. Ive gone from too small to too big, its ridiculous!

[TankJon]

Seriously, I wouldn't worry about it. Valves will go for ages before dying. Check the colour of them from time to time, when they have a weak, blue-ish glow it means they are on the way out. Having said that though even then they will still go on for a while yet. My mate had a Mesa Triple Rec for 9 years without changing the valves and the amp spent a lot of its life on the road touring. My Mesa F30 combo hasn't had a valve change in 3-4 years and still sounds as sweet as a nut and I gig the balls off that!

Although I would say one thing and that is try not to drop the thing or you will be changing valves

Edited May 6, 2010 by TankJon