100/50 watt switch curcuit

[bigdrums925]

Hi everyone. I have been asked to install a switch to cut the output power of a 4 x 6L6GT Jet City guitar amp down to 50 watts. Thinking about this a little I wonder if it could be as simple as turning off the plate voltage to 2 of the power tubes? I don't think it would be that simple but would love to hear your input. I have an old Acoustic Control Model 160 guitar amp that has this feature, but finding a schematic that is readable for it is difficult. So lets discuss it!

[Leo_Gnardo]

Typically this sort of thing is done by disconnecting 2 of the cathodes not plates. Mesa & Marshall have done this way for instance. Keep in mind, half the # of output tubes, doubles the impedance the amp wants to drive, so your 4 ohm tap becomes 8, 8 ohm tap becomes 16. In spite of this last "fine point" the object is to reduce wattage so most folks don't bother with the impedance change.

Also, the amp's just a little bit less loud when "half power" is done this way. It's not a huge change, won't turn a lion into a kitten.

[ThermionicScott]

The Jim Kelley FACS did it that way, too.

If it were my amp, I'd probably switch in some high-value cathode resistors rather than leaving them completely disconnected in order to stave off cathode poisoning of those tubes if the amp will be used in half-power mode for long periods of time. They'd still allow a small trickle of current to flow, but wouldn't contribute meaningfully to the sound.

[Chuck H]

The Jim Kelley FACS did it that way, too.

If it were my amp, I'd probably switch in some high-value cathode resistors rather than leaving them completely disconnected in order to stave off cathode poisoning of those tubes if the amp will be used in half-power mode for long periods of time. They'd still allow a small trickle of current to flow, but wouldn't contribute meaningfully to the sound.

Definitely a better idea. People have been simply switch lifting the cathodes for ever, but any time you can avoid creating a hard switch in an audio power circuit it's better. I think a high value resistor, like 10k, should hold the tubes in cutoff under all operational conditions.

Another method would be to short two of the output tube grids to the opposite side of the bias resistors. You need to do this in a way that doesn't short the other two output tubes. I haven't seen the schematic, but this should effectively short any AC through the bias circuit filter cap. This silences two tubes without taking them out of the impedance equation. There's been some debate about that here though. As I remember, a couple of the higher minds thought it was a good idea.

Overall I think this is a moot mod. The actual volume difference between 100 and 50 watts through the same OT and speakers is really small gains. We're talking "occupancy 5000" vs. "occupancy 2500" as far as venue size difference You're not going to pump that sucker up with two tubes out of service at a 250 occupancy club without doing some serious character damage and pissing off a lot of people.

I once did a switch that cut out two tubes and put the other two in triode operation. Much more satisfying volume drop. Much more tonal degradation . Still an option for Marshall owners. Not sure about a Jet City. Is it Jet City or Sound City amps that commonly have ridiculous voltages? At any rate... You probably don't want to run triode with really high voltage because it'll stress the screen grids.

Just thinking out loud and covering some options.

[drewl]

I did it with my AC30 reissue but being cathode biased it switched in a different cathode resistor also.

Sounded really nice on that.

[Leo_Gnardo]

I think a high value resistor, like 10k, should hold the tubes in cutoff under all operational conditions.

I've used 1K 5W wirewound "ant coffin" resistors for this function. Works fine, doesn't let the cathodes drift up to who knows where, effectively takes the switched-out tubes out of the picture. 1 resistor per tube.

And yes it's much more effective to switch the output tubes into triode mode.

For a quick n dirty test of "howzit gonna sound" with just 2 output tubes, remove 2, just take 'em out. In a set of 4 I typically remove # 2 & 3 simply so there's more room around 1 & 4 for cooling.

[mhuss]

Another way I've seen this done is to insert a largish value resistor in the screen circuit. This provides a lower-voltage screen that effectively shields the higher potential plate from the cathode, and the higher impedance prevents excessive screen current. I suspect this changes the output impedance significantly as well, but I've never tested this theory.

FWIW, I've never liked the way this mod nor the triode connection sounds when overdriven.

[Chuck H]

mhuss just reminded me of another idea I've had, but was discouraged from trying... I'll share it again now

Why couldn't you branch the screen supply and use a four tiered totem arrangement for the filter. If you used carefully valued and burly enough components, could you then tap from divisions at 1/2 voltage and 1/4 voltage for the screens? I'm vaguely remembering a 90's Marshall model that may have done this.

[pdf64]

For a quick n dirty test of "howzit gonna sound" with just 2 output tubes, remove 2, just take 'em out

That should be fine for fixed bias, but if cathode biased with a shared cathode resistor, eg AC30, then the remaining 2 tubes would be horribly under biased, possibly to the point of failure in short order, bearing in mind that they may already be dissipating above the limit at idle, even in the stock configuration.

[Mick Bailey]

FWIW I've never thought that cutting the power in half creates any meaningful difference. People who ask for this usually want a noticeable reduction in volume and they're disappointed with the results just by (effectively) removing a pair of tubes - especially if it's a 6L6/EL34 amp. The underlying actual requirement is more often a drastic reduction - sometimes from 100W to maybe 10W or so to go from 'stage' to 'practice' mode.

Before making any changes it's probably worthwhile asking the customer what they actually want or expect from the lower power setting.

[J M Fahey]

FWIW some 80's tube Carvins offered power reduction by lowering screen voltages through switchable voltage divider resistors, they found proper values to offer 100W (full) / 50W / 25W
All reviewers mentioned the sound "getting farty"