Reducing gain by way of the PI

[Decko]

Hi all,

I want to increase the "clean headroom" on my express in order to take advantage of the beautiful clean tones of this circuit.

I assume that the Phase Inverter acts like the throttle valve to overdrive the power tubes.


Considerations:

1. Guitar volume pot with more gradual taper
2. Amp volume pot - same as 1
3. Split load resistors on the plates at the PI
4. Increase resistance on the Cathodes at pins 3 and 8? 470R is the standard on the Express schematic.
5. Use lower gain tubes
6. ?

I went with item 3 and installed split load plate resistors. I noticed some difference. However I desire more of a gradual increase of "clean to mean" with the sweep of the volume pot on the guitar.

I tried item 4 and increase the cathode resistance from 470R. This made a noticeable difference. However I am concerned that this will mess with the linearity of the amp. Or am I completely off track?

What would be your recommendation?

Regards,
Decko

[jjman]

A true Express circuit distorts in the OP tubes 1st. So you have to increase the clean volume potential at some point after there. In other words, via more or more effecient speakers.

This assumes you are looking for more audible volume headroom, and not volume-knob-numerals headroom.

[statorvane]

I found this out in a Marshall 50 watt clone head - try a 12AY7 in the PI. This really opened up that amp's headroom. Probably different in an express, but If you've got some lower gain novals give 'em a shot. You have nothing to lose, except maybe 5 minutes.

[Decko]

Thanks for the feedback gents. I will Definitely try the 12ay7...

regards Decko

[jjman]

I found this out in a Marshall 50 watt clone head - try a 12AY7 in the PI. This really opened up that amp's headroom. Probably different in an express, but If you've got some lower gain novals give 'em a shot. You have nothing to lose, except maybe 5 minutes.

That will increase volume-knob-numerals headroom, but not audible headroom.

[M Fowler]

What affect does increasing NFB have on keeping the signal cleaner?

[ChrisM]

What affect does increasing NFB have on keeping the signal cleaner?

More signal is tapped off at the output when you increase the amount of NFB. When you increase the amount of signal tapped off the output and fed back to the PI you are decreasing the gain more. More signal being attenuated/cancelled out.

[Decko]

What affect does increasing NFB have on keeping the signal cleaner?

The further clockwise I turn the NFB the more distorted the signal gets...

Decko

[rock_mumbles]

more NFB needs a smaller feedback resistor ... using a larger feedback resistor gives less NFB.