Half power switching

[M Fowler]

What are the best ways to implement 25w/50w power switch in 50w Marshall type amp running EL34's?

Mark

Two-EL34-Half-Power.jpg

[sluckey]

That's a pentode/triode switch. It will certainly reduce the power in triode mode. Don't know if it would be a 50% reduction though. Give it a try and make some measurements.

[M Fowler]

Steve is there a better 1/2 power switching circuit to use?

Mark

[martin manning]

If you are using a FWB rectifier with a center tapped HT and a stacked reservoir, you could do a half voltage switch like on some Fenders, although that would be a (oops) 25% power switch.

[Reeltarded]

Do you have to do it?

Let me guess; someone asked for it and they imagine it does something. There's your problem.

[pdf64]

If the customer insists on having a 'sounds worse' switch that reduces power output, then there plenty of ways that could be implemented, it seems pointless to agonise over the exact % of power reduction.

Though...

If you are using a FWB rectifier with a center tapped HT and a stacked reservoir, you could do a half voltage switch like on some Fenders, although that would be a 70% power switch.

Won't halving the HT roughly quarter the power output?

[martin manning]

You are right Pete, -6dB or 25% power.

Mark, I had a quick look for a transformer that would work for a half voltage scheme and didn't see anything readily available. Maybe you already have the power supply built anyway. Maybe a VVR for this?

[Reeltarded]

plop two amps down that show the difference between output volume.. then if he still asks for it, give him a switch that cuts highs a little bit...

the customer is ALWAYS wrong

wasn't always so..

[norburybrook]

IMHO the half power thing only works when you're going from 4 to 2 valves, usually 100 to 50w. On my last second gen dumble ODS I had a hole and a switch so I thought I'd do the pentode/triode thing with my 50w 2x6L6's.

It works but just sounds worse on the 25w. So instead of 50-25w it should say 'better-worse'

my 100w BM sounds great at either setting however the difference in actual noise level is very little, the 100w just sounds fuller but no louder.


Just my 2 cents


M

[martin manning]

Based on everything I hear, that’s £2 worth.

[norburybrook]

Based on everything I hear, that’s £2 worth.

indeed...….BREXIT...…

about 10 years ago I was working in Japan with a big star and we had an international band so we were all paid in dollars at the time,that meant I was halving my pay as the exchange rate was $2 to the pound.....I was also paying $1000/week withholding tax!!!!


bloody governments...

I've just bought a Les Paul and had a glass of wine so ignore me...………


M

[Reeltarded]

Get paid up front in cash and spend it all on booze from now on..

lol

[M Fowler]

Sounds like I will just tell the potential customer that 50w amp only no custom switching.

I have not started on his amp nor has he paid down payment so I can say forget it, my choice.

I fixed a lot of his Marshalls and other amps but he heard my 50w hot rod plexi and his marbles started turning in his head.

Mark

[nworbetan]

I'm just going to play Devil's Advocate for a second and point out that a pentode/triode switch can situationally add some versatility to an amp (even though most of the time it really is a "worse" switch). If you're doing say a JCM 800ish build, a "hi-fi" sounding triode output topology combined with the low gain input that doesn't have any cathode bypass capacitors in the signal chain (and maybe some minor tweaks to the low freq roll off points) can allow your JCM 800 (plugged into a sealed 15" cab) to be used as a bass amp in the studio, or with a small low volume group. Or maybe you can use it for a ultra clean pedal platform in a wet/dry rig. Yeah, we all know a quad of EL34s sound best as pentodes with an overabundance of drive voltage, but using them as triodes can also work well enough in some niche situations.