1/2 Power Mod

[devbro]

Hi everyone,
First post here. How difficult is it to add a 1/2 power switch to a Fuchs TDS-100? It uses 4 6L6's and is simply too loud for most gigs.

thanks...........

[Enzo]

It would be a matter of disabling two of the tubes and adjusting the output impedance connection. A switch to open the cathode connection would be one way.

You realize that cutting the power in half drops the loudness only 3db.

[devbro]

Thanks Enzo.
I think I'll try pulling 2 power tubes and see if there's any noticable difference before I do any modifications. I'm afraid 3db isn't going to achieve the result I'm after.

[trevorus]

Pentode/triode may be the way to go about this... I tried it with a Fender Deville, and the result was cool. I didn't end up keeping it becaue the amp itself wasn't me. (Got an AC30 now.)

[Regis]

Just some thoughts...

Make sure you pull the right two power tubes if you do that. You might want to check with Andy about this.

Pentode\cathode switch would dramatically drop the volume and dramatically change the tone of the amp. You might want to check with Andy about this.

Dunno how Andy would feel about using a power attenuator with one of his amps, but that would probably solve your problem. A couple of clicks to about -6 db down on just about any power soak will get the volume to manageble levels.

Enzo, would you go into more detail about disabling two of the tubes and adjusting the output impedance connection? Would the switch disabling the cathode achieve both those results?

Thanks and good luck,

regis

[hasserl]

I'm not Enzo, but run the cathodes of one tube from each side of the OT to a toggle switch, then from the toggle switch to ground. When the switch is closed the cathode circuit is complete and the tubes are in operation, when the switch is open the circuit is broken and the tubes are inoperable. The effect is the same as if the tubes were physically removed from the amp, which means the primary impedance should be doubled for the remaining two tubes. If the amp has an impedance selection switch, set the impedance switch to half of what the actual load is; i.e. with an 8 ohm load set the impedance switch to 4.

[devbro]

Thanks guys,
I spoke with Andy about this and he indicated that removing the 2 outside tubes is the same as the 1/2 power switch he offers as a mod. It also will only result in a small reduction in volume (about 3db). He said as long as I plug my 4 ohm cabinet into the 8 ohm tap after removal of the tubes it would be ok. I'm going to try it today.

I'm intrigued about the cathode/pentode approach especially if it will result in a greater volume reduction. How difficult is this mod?

[Bruce / Mission Amps]

Thanks Enzo.
I think I'll try pulling 2 power tubes and see if there's any noticable difference before I do any modifications. I'm afraid 3db isn't going to achieve the result I'm after.

-3dB is noticeable but in order to sound half as loud (or twice as loud) it takes a 10dB drop or increase.
I think the triode mode with two only two power tubes running might get you where you want though.
Personally, I don't really like the sound of a faux triode, modded set of power tubes.
They always seems to have a smeared and duller tone to me.
Maybe I have not heard one truly set up correctly though.

[hasserl]

Thanks guys,
I spoke with Andy about this and he indicated that removing the 2 outside tubes is the same as the 1/2 power switch he offers as a mod. It also will only result in a small reduction in volume (about 3db). He said as long as I plug my 4 ohm cabinet into the 8 ohm tap after removal of the tubes it would be ok. I'm going to try it today.

I'm intrigued about the cathode/pentode approach especially if it will result in a greater volume reduction. How difficult is this mod?

Are you sure he didn't say to plug your 8 ohm cab into the 4 ohm tap? The way you put it is backwards, that would be halving the primary impedance, not what you want to do. You want to double the primary impedance if you remove two tubes. If you plug the 4 ohm cab into the 8 ohm tap you'll only have half the primary impedance that you should have for FOUR tubes, and only one fourth of what it should be for two tubes.

[trevorus]

Why not grab yourself an attenuator?

https://taweber.powweb.com/weber/masslite.htm

[dai h.]

how about selling the amp and buying something the 20-30 watt range? If you get an attenuator, that is one more thing to carry around, which might be a factor.

[pdf64]

Too loud is a combination of speaker efficiency as well as amp power - what speakers are you using?
Re amp power, the use of yellow jacket type adaptors for el84s is another option to consider - Peter