Half Power Switch for (4) 6L6 Clone

[lymo]

Hi all,
my first posting, have been building amps for about 7 years
I'm looking for guidance on adding a half power switch in a friends Dumble clone. I used the 1977 schematic, it came out nice, except it was too bassy at higher volumes, so I lowered the coupling caps after the clean and dirty stages, and it seemed to be the solution.

I've been searching for example schematics of half power switches without much luck, should I just lift the ground off of a tube on each side of the OT? Is there another detail to be concerned about? I realize the bias will be different, as the plate voltage will increase without the two tubes. The switch is a dpdt slide type on the back of the chassis, not too much room in there for any more stuff, relays etc... I'd like to keep it simple, the amp has relays for switching and fx loop, I don't want another one in there.

I've enjoyed what I've seen in this forum, looks like some very intelligent advice has been issued, and quite politely for the most part.
thanks,
Todd (LYMO on this site)
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MARY JANE

[Fischerman]

You can just lift the cathodes of two power tubes but then the voltages do change as you mentioned. There may be other issues.

Another way would be break the connection from the PI to the grid of two tubes while still retaining the bias voltage to the power tube grids. This way the tubes are still idling but they just don't get any signal.

But FWIW/IMO/etc., these half power switches barely reduce volume and castrate the amp. It isn't that much less volume but it loses all of it's cajones. Everytime I try one it stays on 'Full Power'.

[Bob-I]

But FWIW/IMO/etc., these half power switches barely reduce volume and castrate the amp. It isn't that much less volume but it loses all of it's cajones. Everytime I try one it stays on 'Full Power'.

I agree. In fact I just tried this on a SLO clone, same result. Apparently there's nothing I can do to quiet this amp down.

On the plus side, I can use it to kill small rodents.

[LeftyStrat]

Every half power amp I've ever played was the same. The only really noticeable difference between full and half, is the bass is more authoritative in the full power mode. Or, for example, a 50 watt Marshall has less headroom and compresses a little more than the 100, but can compete volume-wise on stage, ask Dickey Betts.

Unfortunately half power isn't half volume. To halve the perceived volume of a 100 watt amp, you need to drop down to about 10 watts. Of course this is theoretical, other factors weigh in like speaker compression, etc.

[heisthl]

I posted this once before - here's an alternate method used by Carvin https://tubeamparchive.com/download/file.php?id=2010
If you try this make sure it's not changing the voltages of V1 and V2.

[lymo]

Guys, thanks for all the lucid and rapid responses, I'll have my friend read them and see if he's talked out of it. I used the switch position to go cathode biased on my clone (well on a couple I've done it). It makes a difference that's for sure, I think it's more useful on the clean side, a bit too spongy with all the gain stages on the dirty side I think, to be too useful, but someone probably would disagree. I'm glad this forum is here, it's not easy to find tube amp builders to talk to as one wanders...

thanks again,

Todd
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No2 Vaporizer

[heisthl]

I've done the cathode bias switch thing too, here's one way that works well for 2 6L6 tubes using both sides of a DPDT switch.

[Bob-I]

....Or, for example, a 50 watt Marshall has less headroom and compresses a little more than the 100, but can compete volume-wise on stage, ask Dickey Betts.

Not a valid argument Dickie Betts used a 100 Watt Marshall lead and Duane Allman used 2 - 50 watt Marshall Bass heads.

[LeftyStrat]

....Or, for example, a 50 watt Marshall has less headroom and compresses a little more than the 100, but can compete volume-wise on stage, ask Dickey Betts.

Not a valid argument Dickie Betts used a 100 Watt Marshall lead and Duane Allman used 2 - 50 watt Marshall Bass heads.

Oops had it backwards AND I left out an amp. The Gods of Southern Rock are frowning on me

I have played in a band where one guitarist was playing a strat into a 100 watt Marshall, and the other playing a Hamer w/ hot buckers into a fifty, and there were no problems with the volume (well, as long as you didn't ask the sound man

Half the wattage is a 3 db difference.

I tend to think of wattage as being more about headroom and bass reproduction than volume. Sometimes a 15 watt amp will keep up with a loud drummer if you like playing distorted all the time. If you want clean with a loud drummer, you may need 50 or 100 watts.

Of course, most of my experience comes from the days when a PA was something used by singers and whimpy drummers.

[Bob-I]

Oops had it backwards AND I left out an amp. The Gods of Southern Rock are frowning on me

There's so many variables in this it's easy to get mixed up.

I've read that Dickie used JBL's, I believe it, first off because I saw the silver domes at the Filmore, and second because of the cleanness of the tone. That would give him an additional 3Db over Duane who used Cerwin Vega speakers (I've read)

Add that to Dickie's clean tone vs. Duane's hard driven tone and you get close to a balance. At the theatre though Duane was WAY louder than Dickie... WAY louder!!!!

Half the wattage is a 3 db difference.

Which is almost impossible to hear.

I tend to think of wattage as being more about headroom and bass reproduction than volume. Sometimes a 15 watt amp will keep up with a loud drummer if you like playing distorted all the time. If you want clean with a loud drummer, you may need 50 or 100 watts.

Of course, most of my experience comes from the days when a PA was something used by singers and whimpy drummers.

Mine too. But we started miking amps around 1977, to get them over the top and lower stage volume. Today I play with 2 bands, one uses straight stage volume, the other, 100% PA. I'm trying to convince the first band to change.

[rutledj]

This may not be what you are looking for but I had a Carvin with a 25,50,100% switch. Looking at the schematic they were changing the voltage on the PI plates. Seemed to work ok.

Rut