1/2 power switch anyone?

[Heater]

Howdy all,
I have a D Clone that I recently bought from someone partially completed. It's mostly a Hybrid A design. It has 4 6l6's in the output section though. It was intended to have a 1/2 power switch. Does anyone have a schematic showing the proper implementation of the 1/2 power switch? Specifically, I'm wondering about driving the LED indicator.

Thanks in adavance,
Chris

[Funkalicousgroove]

Just use it to lift 2 of the power tubes' cathodes from ground (One on each side of course) I tap the LED lamp off the Heater supply and use a DPDT. One side does the LED one side does the cathodes. The correct way was posted by AYAN some time ago, you'll have to do a search.

[swt]

the tut series books suggest to lift the cathode ground with a big enough resistor, 22k or higher up to 100k. that way it's like it's isolated, but it's still referenced to ground somehow, but turned almost off. i guess it's to protect the tubes. just use a switch with a resistor across two poles, and make it shorted when you want full power.
the other way is to have triode pentode switching, but in this way you have a noticeable change in sound. i like this in some type of amps...specially marshall derived...since it has a high treble roll off, it sounds like a really cranked amp at lower volume, and really rotten. don't think it's the right way to do in a dumble though...

[Zippy]

Is there a difference between these two methods in what the OT perceives as primary impedance? Should one still adjust the output selector if using the tut method to best match the load to the speakers?

[Luthierwnc]

Yes. When you cut out two of four tubes, you double your primary impedance. You need to double the secondary as well.

Skip

[Zippy]

I understand that point.

What I was asking was... grrr... it's right there. How may I make it more simple?

Is there a difference between the two methods of halving the output power: Lifting the cathodes entirely vs limiting current to the point that the tubes are not participating?

[Luthierwnc]

The easiest way is to pull the cathodes. It is just a SPST (heavy-duty) switch. I have also used a DPDT on-off-on switch for dual use as a standby (in the center position) on one side and 1/2 power on the other.

A couple of gotchas; ground the cathodes at the same place, even if you have to run some extra wire to and from the switch. Also, you want a fairly stout power transformer for this. If the PT can barely supply the draw to four tubes, switching to two can really raise your preamp voltages. These "D" clones are very fussy about preamp plate volts so make sure you don't lose the sweet spot. If you do, take that same DPDT switch and use one side to put a little more resistance between the choke and the rest of the rail.

sh

[Luthierwnc]

Just to be sure... if you aren't letting current flow through the tubes, for any reason, you are increasing the impedance. On is on, off is off.

sh

[Zippy]

Thanks.

On other builds that incorporate this type of half power implementation, I like to use a Matchless-style power ladder with one pair of the output tubes in parallel to the rest of the chain. This way, the current through (and hence the voltage drop across) the choke or resistor is fairly constant regardless of whether both pairs of output tubes are driving or not - voltage to V1 is consistent.

If, as you mention, the transformer headroom is marginal and the supply voltage is a function of loading, all bets are off. I wouldn't use that trannie for a number of reasons.

[908ssp]

Just to throw a little fuel on the fire. I think a half power switch on most amps is a waste of time and effort especially so on a Dumble clone. I can see it on a 100 watt Marshall so you can safely run a 100 watt attenuator but other then that they don't do any thing worth having.