Bias Change arrangement

[Vaughan]

Hi Everyone - this is my first post.
I have on my bench a nameless 30watt amp using a very standard 6L6 P/P o/p stage. At some time, someone has removed the fixed bias arrangement, and has changed it to cathode bias. The fixed bias voltage was originally fed to the centre of the normal pair of 220K resistors. I always assumed that this centre connection should be grounded in this arrangement , but on this amp the centre has been left floating!!
The amplifier seems to work OK (came in for unrelated problem)
Could someone with a bit more theory than me, please enlighten me as to whether this would affect the output and/or the sound
I would appreciate any help on this one

[Enzo]

And when you ground that point like it shuold be, how does the amp sound?

[Vaughan]

Hi Enzo
Haven't tried as yet - I was a little worried as to whether any other mods had been done that I had not come across. Will try later when I return to the workshop. The amp actually sounds quite good, but overdrive comes in on about 2 on the gain control and there doesn't seem to be much clean headroom.
I'll post again when I've tried.
Vaughan - Banbury UK

[Vaughan]

Hi - It's me again
Had another look at this amp tonight. It seems the removal of the fixed bias was only partial - The supply had been removed , but the adjustment components were left in situe on the PCB. The centre of the 220K resistor pair was finding a path through a 56K
resistor and the adjustment pot. This was affecting the grids of both O/P tubes. I haven't looked at the actual grid voltage levels but I suspect that it maybe causing incorrect biasing.
Then I removed all the remaining bias adjust components and connected the centre point direct to ground. Hey Presto!!! Sound Quality improved and a much better clean output. Now it starts to breakup at about 4-5. just about right (for my customer anyway).