Blues master pi

[Andyjoe]

What do you gain and what do you loose by putting a bm pi in a non hrm circuit?

Andy

[Dr d]

You will get an earlier break up in the output and an allround "bluesier" tone. Consequently if you want particularly"clean" sound, you will lose out.

[JD0x0]

without hijacking too much... How about going in the opposite direction. Has anyone tried a Hiwatt/Hifi PI in their ODS for maximum clean headroom

[llemtt]

max clean headroom can't go much higher than the standard ODS 6L6 power amp configuration

cheers
teo

[JD0x0]

max clean headroom can't go much higher than the standard ODS 6L6 power amp configuration

cheers
teo

Im not so sure about that man. My OTS built by Nik has very low clean headroom and breaks up before "3" on the master. This is not the blues master either it is the modded S&M otherwise known as OTS.

I can definatly hear the PI distortion as it gets gritty and nasty. At about "7" on the master it gets into powertube OD

My 80's hiwatt you can put the master on "10" and get very little grit which is all coming from the EL34's unless you OD the preamp too

[llemtt]

My OTS built by Nik has very low clean headroom and breaks up before "3" on the master

there must be something wrong with your OTS, none of my amps behaves like that

my power tubes (6L6) get into breakup long way before PI and there is lot of clean headroom till max power of 45Wrms (measured with power tubes starting to grid clip), raising the master further takes a while to get PI into overload and, besides seeing it on the scope, I can hear the sound turning from pleasant to nasty as you say

can you check your poweramp voltages as it gets into overdrive?

anyway to increase PI headroom basically you can:
- increase idle current by lowering the cathode resistor
- increase plate swing with higher value plate resistor
- decrease tail resistor
- increase supply voltage

solution 1 & 2 also increase the PI gain
solution 3 increases the AC imbalance

BM PI does 1 & 3, and by increasing gain it lets you overdrive powertubes earlier on your master settings too

cheers
teo