Laney aor100 halfpower switch

[gsk]

Hey guys,

A friend of mine has this amp and he asked me if I could build in something like a half power switch. He now runs it with only two powertubes instead of four, but he would like to run four, and just using a switch to make it less loud.

This is the schematic:
http://www.schematicheaven.com/newam...00_series2.pdf
What do you guys think would be best to do.

Thanks in advance everyone, it would be great if you could help

[Chuck H]

IIRC there was a discussion here (long ago) about the MO for cutting out two tubes in a 4 tube power amp. The old standard was to simply lift the cathodes on two of the tubes (at opposite ends of the OT obviously). But I think it was concluded that lifting the signal input and grounding the grids of those two tubes was better because it maintains an impedance match in the OT. Not sure about the validity of that but i'm sure you can find it in the archives under something like "1/2 power switch".

If you decide to lift the the signal and ground the grids with a switch, a shorting switch will make less noise so it could be switched on the fly. Whereas a non shorting switch will leave the power tube grids without a ground ref for a moment and so the amp would be best placed in standby before switching. The resistors to target are R69 and R70 or R71 and R72. Be sure the switch grounds the power tube grid and not the input signal.

The 1/2 power switch that Marshall used for the 900 amps just lifts the screen grid resistors from thier power supply node and connects them to the plates. Kind of a psuedo "triode/pentode" thing. I've never heard it in use. But it does keep all four power tubes in the loop so tube wear will remain more consistent. On the Laney it would be R73, R74, R75 and R76. Lift the B+ node end of each resistor and connect each one to it's own tubes plate.

HTH

Chuck

[gsk]

Thanks for the answer, I think I'll do it the marshall JCM900 way. Does anyone know which option makes the biggest volume difference.
By the way which kind of switch should I use. I never worked with switches switching two things at once.

[Chuck H]

I've used regular rotory switches for triode/pentode switches before. But IIRC the voltage and current rating on the standard switches is below the task. I've never had one blow up, but if I were putting it in someone elses amp I might buy the better switch. They're pretty spendy though. I think on the order of about $35.00 US. You'll need a 4P2T (or 4PDT depending on how they list it) Mouser has them.

Chuck