Pulling tubes

[Pearse]

hey i know pulling two power decreases the headroom alot and i tend to like it at time but i was told that you also have to set the independance to half the value of the cab. is this true, cheers.
Pearse

 

[steelhorse]

Why not get a complete high gain tube kit and forget pulling tubes, it's better and safer for your amp.

 

[lerxst88]

well if you rebias the amp for fewer tubes you should be ok. as far as speaker impedances it doesnt sound right to me

 

[MajorWatt]

hey i know pulling two power decreases the headroom alot and i tend to like it at time but i was told that you also have to set the independance to half the value of the cab. is this true, cheers.
Pearse

Yes. If you're running a 16 ohm cab, place selector to 8 ohms when you pull two power tubes. Never had to re-bias myself.
Remember, pull either the two outer or the two inner tubes.

 

[MKB]

Overall, what you have to do depends on the amp. Some newer Marshalls do not have chokes, but a power resistor for the screens filter supply. If you remove two tubes, the screen voltage on the remaining tubes will go up, which means effectively you just biased the tube a little hotter (i.e. more current). Also your plate voltage may go up as well, leading to the same effect. Might be best to check your bias just in case. The good thing is that if you bias the amp with two tubes, when you install two more the overall bias is a bit colder, if it sounds OK it is the best solution.

The designer of the Marshall Vintage Modern, Steve D, does not approve using a 100W VM with only two power tubes, as the voltages in the amp will increase where the voltages on the power tubes will be too high and might lead to failure. This is the first time I've heard this warning, but it makes sense. To be totally safe, have a technician check out the amp with two tubes, and make sure the tubes aren't being stressed by overvoltage or too hot a bias.

 

[Pearse]

Overall, what you have to do depends on the amp. Some newer Marshalls do not have chokes, but a power resistor for the screens filter supply. If you remove two tubes, the screen voltage on the remaining tubes will go up, which means effectively you just biased the tube a little hotter (i.e. more current).

does the bias only go up doing this with a resistor or does it happen with both the choke or the resistor. Bdw mine has a choke and how much does it increase the current by roughly ie does it double it or does it only add a little more current.cheers.
Pearse