B+ reduction strategies

[rsi]

I thought of something last night while reading one of the Audio Classroom articles from Glass Audio Magazine. Situations pop up constantly where a lower B+ is needed, so there's invevitably discussion about using zeners or a mosfet circuit or even a variac to do this.

Has any one reduced their voltage by changing from a capacitor input to a choke input supply? I don't recall seeing any guitar amps that use a choke input setup. I was just curious how this works out, if anyone has actually tried this.

[Alexo]

I recently did this on my office stereo to adapt it to a cheaper tube than the EL34's I was burning through.

Works just as the electronic theory books say - went from 375 VDC to something more like 250 or 225, but you need to have a choke rated for a lot of current, and you will get a very stiffly regulated power supply, which may not be desirable in a guitar amp.

One unexpected consequence is that the choke (a big, robust Hammond) produces a very audible hum/buzz - not through the circuit, mind you, but a mechanical hum that comes from the amp itself, as that choke is now soaking up a huge amount of AC ripple.

[selloutrr]

I can't think of any production guitar amps that use the Choke input supply. It's a better regulated supply, however it lowers the voltage considerably compared to the Capacitor and requires a very large physical sized choke capable of handling 250mA+. Where as a capacitor filter supply only needs a small ~5-10mA choke to help even out the left over supply.

You would need roughly a 1000Volts unloaded using a choke input supply to end up in the 420volt ball park. The choke would be roughly the size of a 50watt transformer.

[Cliff Schecht]

Choke input filters have a problem with excessive ringing too when you flip the standby switch. Caps prevent the current from changing instantaneously, chokes prevent the current from changing instantaneously but the voltage will do what it wants (i.e. rise waay too high). Even if this is only momentarily, it stresses the standby switch and the components in the circuit.

[husky]

I thought of something last night while reading one of the Audio Classroom articles from Glass Audio Magazine. Situations pop up constantly where a lower B+ is needed, so there's invevitably discussion about using zeners or a mosfet circuit or even a variac to do this.

Has any one reduced their voltage by changing from a capacitor input to a choke input supply? I don't recall seeing any guitar amps that use a choke input setup. I was just curious how this works out, if anyone has actually tried this.

Slap power scaling in there or you can use a secondary small transformer wired out of phase with the secondary. I have done large chassis mounted Zeners but they can get hot, Also lowering the screens can be an alternate

[Gaz]

I think the new Slash Marshall just scales the screens actually. Never tried it myself...

[selloutrr]

I think the new Slash Marshall just scales the screens actually. Never tried it myself...

Isnt it just a reissue of the Jubilee with Slashes logo and cheaper parts? Which was a knock of a Risson which was a mod'd marshall circuit.

[Gaz]

I dunno, it's called the AFD or something and has power scaling of some sort.