I Have 4 Output Tubes and Want to Cut 2 Off With Switches

[The New Steve H]

More weird questions.

I am still fiddling with my "Wingman" amp, which is like a Bassman hooked up to the 6BM8 output of a Little Wing. It has 4 output tubes, though, not 2.

The amp was originally a lunatic design with two OT's. The PT got hot all the time, and I felt like it pooped out at high volume. I took one OT and two output tubes out, and it worked fine. Now I am putting a cabinet together for it.

I have been looking it over, and there is no reason I can't connect four 6BM8 tubes to the remaining OT, which is rated for 15 watts. Thing is, I really like the volume and sound I get with two tubes, and I am thinking that if I double the wattage, the PPIMV will have to be turned down more at practice volume, and the amp won't sound as good.

Here is the obvious question: Is there any reason why I can't put a switch on it so two output tubes are disconnected from the OT when I play at low volumes? I was thinking I would just disconnect the + plates.

I know the difference between 7 and 15 watts isn't a lot, but I figured I would ask. I really love this amp.

[The New Steve H]

I think I got the answer. I paralleled the output tube plates for the two sets of output tubes, and the amp works fine on two or four. I may put a switch in there for fun.

I have read that I may have to make some sort of impedance adjustment, but nothing has blown up so far.

[pdf64]

Connecting power tube plates to a switch seems a bad idea to me, eg massive voltages, increased parasitic coupling from non-ideal lead layouts.
Better either to lift the cathodes (add a ground reference resistor for them) or break the signal path to the grids.

See discussion on the topic http://ampgarage.com/forum/viewtopic.php?t=28977

[The New Steve H]

Thanks.

Also wondering about the possibility of just shooting the current from two tubes through a big fat resistor, so the amp thinks the impedance hasn't changed.

[pdf64]

Power tubes have a very high (plate) impedance, same reason as a pre-amp pentode.
You may be getting the dc operating point confused with the dynamic ac conditions.
Replacing tubes with resistors that draw the same idle current would act to damp the speaker's resonance.

[The New Steve H]

I'm not talking about replacing tubes with resistors. I'm talking about running the output from tubes through a resistor instead of a speaker.

[Phil_S]

I'm pretty sure a dummy load, or in this case a partial dummy load, does not behave like a speaker.

[pdf64]

I'm talking about running the output from tubes through a resistor instead of a speaker.

Sorry, I don't get what you mean with this - could you sketch something up, or point at a schematic which has the arrangement in question?

[The New Steve H]

I'm too lazy to try to draw a schematic after wrestling with upholstery all day.

Ordinarily, you have a connection from the plate to the OT, and then you go to the speaker. Some people have put resistors after the OT so the tube output goes into the resistor silently. I can't do that because I have 4 tubes, and they go into two wires, so I would not be able to separate two tubes. I would have to put a resistor after the plates of two tubes.

I think it would be simple. Just divert the plate output wires to the ends of the resistor. Maybe I'm missing something. I have not spent a lot of time on it.

I don't know what I'm going to do right now. I feel like I'm trying to do everything at once with this amp, when I should really focus on getting the cabinet finished. I plan to get that over with before I fiddle with the output.

[Phil_S]

I don't know the answer, but I'm pretty sure it doesn't work. Think about it. If it was that simple to modify the primary impedance of an OT, why would anyone bother winding them for secondary multiple taps when you could put the primary on a sort of decade box? Umm...I think you can't get there from here.

[The New Steve H]

I'm not trying to modify the impedance. I'm just trying to bleed the power into something that doesn't make any sound.

This is what Garnet Amps did with the Herzog, supposedly, but they did it after the OT because there was only one output tube.

[Charlie Wilson]

Take a trip over to the Dumble discussion section and look at a half power switch on an Overdrive Special.
CW

[Jered]

Isnt the Herzog single ended?

[sluckey]

I'm not trying to modify the impedance. I'm just trying to bleed the power into something that doesn't make any sound.

This is what Garnet Amps did with the Herzog, supposedly, but they did it after the OT because there was only one output tube.

Sounds like you just want a dummy load connected to the secondary of the OT. But this will silence the entire amp. It will not work to simply kill the sound from two tubes in a quad output section.

The Herzog is a special case. It's not being used as an amplifier to produce sound, therefore, it uses a dummy load to kill all sound. The Herzog is a small SE amp (think champ) that is being used just like a distortion stomp box. Plug guitar into Herzog input, plug output of Herzog into the input of your gigging amp. This produces a super distorted/overdriven sound.

You can use any amp as a Herzog by simply using a dummy load, volume pot, and true bypass switch. These few components can even be assembled into a small footswitch box.

[The New Steve H]

I think it's important to read all the details.