Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pinkphiloyd on May 20, 2015, 01:56:41 pm

Title: Re: Hi/Low power.
Post by: pinkphiloyd on May 20, 2015, 01:56:41 pm

I've been thinking about this, out of boredom as much as anything else, really.

I've seen the hi/low power option on tube amps approached from a couple of different directions.  It usually comes back to lifting (or switching  a large resistance in series with) the cathodes of 2 power tubes.  Opinions on the sonic results of this method seem to vary considerably.  So (everybody stand back!) I got to thinking about all of this...

If your PT had multiple primaries, say 120 and 220 V for the sake of conversation, and you're living in a country with ~120V line voltage, what would the results be if you utilized a switch that switched your line voltage input between the two primaries for hi and low power modes, respectively.  I know this would effect the heater voltages... any other problems?

Title: Re: Hi/Low power.
Post by: sluckey on May 20, 2015, 02:41:04 pm

Probably work similar to an on/off switch.  :icon_biggrin:

If you feed 120v into a pt that expects 240v, then all the secondary voltages will be 1/2 the expected voltage. Some tubes will probably choke.

If you have a variac you can simulate this for yourself. Just dial it down to 60v. Does the amp like it?

Title: Re: Hi/Low power.
Post by: kagliostro on May 20, 2015, 02:49:54 pm

The big problem is the heater voltage

to half B+ will give the effect of an early compression like using a VVR

but if at the same time you half the heater voltage, easily the amp will not work


and assuming it do, it will happen in a bad way

Franco

Title: Re: Hi/Low power.
Post by: AZJimC on May 20, 2015, 04:23:47 pm

Having tinkered with voltage reduction methods for lower power settings, I can say I much prefer to drop voltage only to the power tubes, while allowing the preamp full voltage. My current goto amp has a switch that places a large resistor in screen circuit, which allows easier power tube overdrive more so than dropping power, not much volume reduction. Then it switches in a large resistor on plate power to offer even more distortion, still not a lot of volume drop. Lowest power setting would be about 1/3 power, which still won't get it into bedroom volume, so a LARMAR type master vol helps tame it even more.

Allowing the heaters to drop to half would likely render the tubes too cold to do much at all.

Title: Re: Hi/Low power.
Post by: pinkphiloyd on May 21, 2015, 10:11:09 am

Well, that's why I asked.   :icon_biggrin: