Half power mod.

[midnightlaundry]

I was looking at some Boogie schematics, and I seen a switch that lifts the inner pairs cathodes to ground. Is that all it takes for a fixed bias amp?

 

[phsyconoodler]

Yes.It works on most 4-tube fixed bias amps.

 

[midnightlaundry]

Cool. What about a cathode biased amp?

 

[pdf64]

Just my opinion, but this and the triode mode type half power switches are marketing led features. Amps generally sound best when configured to do what they are supposed to do; the half power options sound naff (due to mismatched loads and super stiff supplies) and are imperceptably less loud than full power. They certainly won't enable a great, quiet cranked tone.
Everyone I know that's got one of these switches just leaves them on full power.
To implement it on a 4 tube cathode biased amp, the shared cathode resistor / bypass cap would have to be changed to individual per tube, or 2 shared between push-pull pairs.
Pete

 

[phsyconoodler]

Not too many 4-tube cathode biased amps out there,but yes,you would need to have a cathode resistor for each tube.
Cathode biased amps are easier to have lo power switches but they don't lift the cathode resistor off ground,they lift the bypass caps instead.
A pentode/triode switch is better sounding in a 4-tube fixed bias amp IMHO.
That being said,a twin reverb with a half-power switch would sound best with one speaker disconnected so the load is correct,same with other high-powered twin-speaker amps that are wired in paralell

 

[wilto]

Don't know why more amp builders haven't followed up on the Music Man power switching , that cuts the voltage at the output transformer, simple idea.

 

[phsyconoodler]

Lots of Fender amps do that.The Evil Twin,The new Fender Twin Amp,Fender 75,some older Fender Super amps,etc,etc....

They all sound kind of cheap in those modes,and the circuitry involved is rather complex.
In reality the swiss-army amp is not great at any of it's multiple modes.If it were they would be selling like hotcakes.
No voltage adjustment can sound like a 5 watt Supro on 10 or a Champ or even a Tweed Deluxe.
They just don't cut it like the real deal does.
Keep the Twin Reverbs loud and clean and get a smaller amp.
Or design in power scaling or voltage regulation that's adjustable and tracks the bias.Hey wait! I already do that on some amps I build.Lots of others are too,it's just not that great a sound.The platform is wrong to start with when you use a 100 watt amp as a baseline.

 

[pdf64]

Exactly, my view is that a key aspect of a great cranked amp tone is that the power supply sags / gets modulated by the signal; drop from four to two power tubes and the current draw will halve, and that modulation will also halve (at least, probably more so if there's an impedance mismatch too).
A (dual track) post phase splitter master volume control is almost as simple to implement, would sound better, and provide an infinitely variable range of control.
And as psychonoodler points out, VVR (rather more expensive and complex) will provide a much better result.
Pete
Pete

 

[teemuk]

Don't know why more amp builders haven't followed up on the Music Man power switching , that cuts the voltage at the output transformer, simple idea.

You mean at power transformer? Lot's of amps do that. Already a few decades ago that idea even evolved into introducing continuously variable B+ voltage controls.

 

[teemuk]

Exactly, my view is that a key aspect of a great cranked amp tone is that the power supply sags / gets modulated by the signal; drop from four to two power tubes and the current draw will halve, and that modulation will also halve (at least, probably more so if there's an impedance mismatch too).

A (dual track) post phase splitter master volume control is almost as simple to implement, would sound better, and provide an infinitely variable range of control.
And as psychonoodler points out, VVR (rather more expensive and complex) will provide a much better result.

Unfortunately, both of those also pretty much eliminate all sag:
- PPIMV will surpress the power tube drive and the amp will produce less output power, draw less current, sag less
- VVR is often a plain voltage regulator that produces a very "stiff" unchanging B+ output, otherwise it would be pointless to call it a regulator. One can go to extremes "Maven Peal style" and simulate the power supply sag but so far I'm aware of only two amps that do this: Maven Peal and one VHT preamp. It's not a feature found from any usual VVR schemes.

 

[zenas]

I did the half power mod on a silver twin reverb way back when. Pull outside 6L6s disconnect one speaker and cut the volume by almost nothing.

 

[teemuk]

^ That. Power decreasing by half is only -3 dB. It's basically just "slightly noticeable" decrease in loudness.

 

[danielperez808]

Do you guys think pulling the outside tubes and maybe changing speakers to two 30W speakers on each side will be ok?

Everyone seems to pull tubes, but what about changing the speakers to lower wattage as well? Anyone try this?

 

[smolder]

Do you guys think pulling the outside tubes and maybe changing speakers to two 30W speakers on each side will be ok?

Everyone seems to pull tubes, but what about changing the speakers to lower wattage as well? Anyone try this?

I have a twin reverb head I run with 2 power tubes (= 8 ohm load) and put it on a 1x15 16 ohm cab. The downside is that because its a jbl, in the end the volume is not that much less.

A lower efficiency speaker would help with the overall volume. The thing is... a twin sounds pretty similar in tone from about 2.5 to 8 on volume. It's one of the few amps where the volume knob really does primarily change the volume.

 

[pdf64]

what about changing the speakers to lower wattage as well? Anyone try this

The power rating of a speaker is different and unrelated to its sensitivity, ie how loud it will be for a given input power.
Pete

 

[JerryP]

Half power switches can be dangerous. When you remove 2 tubes and leave the mains fuse the original value the amp is now overfused and you leave yourself wide open for a major meltdown. I had a local customer that pulled 2 tubes on his amp and it smoked bigtime, almost caught fire.

If you're going to use one make sure you adjust the impedance to correct for the mismatch and cut the mains fuse value in half.
Jerry

 

[psychodave]

While it is not a "true" 1/2 power option, I had Mark Cameron add a pentode/triode switch on my CCV. He told me he did it in a specific way to not stress the power tubes.... I never looked how he did it, but I love being able to switch between pentode and triode for different sounds or late at night playing.

 

[woad_yurt]

When you remove 2 tubes and leave the mains fuse the original value the amp is now overfused and you leave yourself wide open for a major meltdown.

I am currently running a 135 watt Twin with a pair pulled. Should that fuse value be halved? I don't want to melt my sweetheart.