Technical question for Steve; VM half power (triode) mode?

[MKB]

Hi Steve, have you tried or heard of anyone that configured either a 2266 or 2466 for the half power triode mode used in Jubilee and JCM900 Marshalls? For those not familiar, this is where the screen grids of the power tubes are disconnected from the screen supply and connected directly to the plates of each tube. This cuts the power roughly in half without removing any power tubes. It would also permit a 50W power amp with two tubes to be set to a 25W mode.

I'm curious if you've tried it, how it sounded, and if this would cause any issues with the 2466. It would probably change the tone somewhat; the preamp voltages may go up a bit due to the screen load being removed, and in my experience triode mode tends to make an amp sound much darker than the normal mode. But it may not cause KT66 damage perhaps as all 4 power tubes are still there.

I don't plan to try this, just curious.

[SteveD]

I haven't actually tried it on the Vintage Modern and it would change the tone for sure. It would require some PCB surgery too.

[StephenRutledge]

Hi Steve, have you tried or heard of anyone that configured either a 2266 or 2466 for the half power triode mode used in Jubilee and JCM900 Marshalls? For those not familiar, this is where the screen grids of the power tubes are disconnected from the screen supply and connected directly to the plates of each tube. This cuts the power roughly in half without removing any power tubes. It would also permit a 50W power amp with two tubes to be set to a 25W mode.

I'm curious if you've tried it, how it sounded, and if this would cause any issues with the 2466. It would probably change the tone somewhat; the preamp voltages may go up a bit due to the screen load being removed, and in my experience triode mode tends to make an amp sound much darker than the normal mode. But it may not cause KT66 damage perhaps as all 4 power tubes are still there.

I don't plan to try this, just curious.

cool idea for the 2466

[DeanM]

MKB, I've thought of that before too. a member asked in a thread on here one time and i said to look up the jubilee schematic and implement that. but never really felt the urge to try it! it would be interesting tho!

As far as tone goes, i would imagine it would change alright. its common with lower powered amps to have pentode/triode switches and with those the tone usually goes darker with less bite so it would be interesting to see how it would effect a 100watt kt66 amp!! but as i said i'v never went for it cos my VM is prob the only amp i will be buying till im finished college and have a decent job! so if it went wrong or i didnt like the tone afterwards then i prob couldnt afford a new one!

[KevinOConnor]

Hi Guys

Wiring a triode-pentode switch is very simple.

You need a DPDT that has proper safety approvals. The latter suggests that the switch is sturdily built - enough that the manufacturer paid for the approvals. Becuase of the high voltages on this switch, this should be a "large" body switch, not a subminiature type.

You have to slash two traces on the PCB to isolate the screen resistors for the 'push' side from those on the 'pull' side.

#22 wire is sufficient for all connections. Add some heatshrink tubing around the wire connections to the switch. This will help to keep any arcing from occuring over the switch body.

The switch wipers go to the now free ends of the screen resistor pairs. The same end of both poles ties to the trace that fed voltage to the screen resistors. The opposite end of both sides of the switch go to the respective plate connection (end of the OT for the tubes tied to the related screen resistors).

It does not matter which side of the switch you use for which side of the output stage. Just be careful to get the related plate and screen connections onto the same side of the switch.

Triode sound is cleaner than pentode, but it will sound "darker" due to pentode's harsher distortion.

Have fun
Kevin O'Connor

[okgb]

I think it's a good option for anything
Anyone know how it sounds with el84's ?