"Sound" of master volumes

[philster]

Hi Folks
Since I got my Bassman up and running I've been been having a lot of fun cranking the thing up and just experimenting with the sound. She's darn loud for home use and I don't get a proper band rehearsal until Friday so fitted a type 3 trainwreck master volume to let me crank the input at home a little.
Obviously I'm not expecting too much from this but I was surprised to find that there was much more distortion for a given setting with the master volume turned down than with the master turned fully up (volume excepted). Is this normal or is it that this type of MV is not best suited to this amp? And I guess I should build an Air Brake if I want a more realistic sound at lower volume?
Cheers
Phil

[statorvane]

Is this normal or is it that this type of MV is not best suited to this amp?

Yep. that's pretty much normal. The idea is to get some power tube breakup/crunch at lower overall volume. If you'd rather deal with preamp distortion you can use a pre-PI MV like the one in a JCM800.

[Darkbluemurder]

Hi Folks
Is this normal or is it that this type of MV is not best suited to this amp? And I guess I should build an Air Brake if I want a more realistic sound at lower volume?

Phil,

Yes, this is normal. The reason for it is that htis master is after the phase inverter (PI) and the amp has a feedback loop. Once you turn down the master you also limit the signal which goes into the feedback loop. Once a certain threshold is reached the feedback loop collapses which causes more distortion - what the feedback loop tried to correct.

Cheers Stephan

[philster]

Thanks Gents!
Stephan - that makes total sense. I was getting way more distortion than expected - I've cranked the amp up and it is nowhere near that overdriven at volume. Bloody loud, though
Going to fit a Lar/Mar PPIMV tonight and see how I like that one.
Cheers
Phil