A friend asked me if I knew how to attenuate his guitar amplifier.I asked him the reason, his reply was so that you would still get the same effect at a lower level as you would at the higher. He beat me to the punch because my next question was going to be "What about the volume Control"
This is what I read it does

"In conjunction with an electric guitar amplifier, a power attenuator is used to divert and dissipate some or all of the amplifier's excess or unneeded power in order to reduce the volume of sound produced by the speaker"

He has a Peavey Classic 30. it uses 4 x EL34 - 3 x 12AX7

My question is, would lowering the B+ have the same effect, it uses a bridge rectifier ?

Hi Rudolf, the idea is to present the amplifier with a resistance close to the rated value so the amp works normally, but, insert a series resistance to the speaker set to reduce the power delivered, yes its a "volume control".
Exactly what I do in the workshop if I have a high powered amp say 500 watt, where I want to run at full power but not rattle next doors windows out. The resistance dummy load diverts current and a low powered speaker then monitors what is going on.

Audio places used to sell these as "speaker volume controls" with multiple wound resistive elements maintaining a constant load back to the amp while driving the speaker with a variable resistor to get the volume control as you turn the knob.
Jaycar used to sell a 4 ohm unit rated around 100 watt.

I would be very careful using any thing that may accidently disconnect the speaker load from a high powered valve amp! if you open circuit a speaker load on high drive you can get transient voltages that can be high enough to arc over inside the valves and sockets, been there , done that………...pooof!.


Don't know about lowering the entire HT voltage, too many issues to look out for. Certainly would lower the clipping point on the output valves.
Some resistance put in series with the plate/screen supply to the EL34's may do the trick, to "weaken off" the valve currents just like a weak rectifier valve does. Its all experiment.

What the guy wants to do is run the amp into valve distortion but make less noise, sort of like junior heavy metal!

Just some thoughts, if you are familiar with valves and guitar amps go ahead, if not stay away.

Fred.

I would concur with Fred's comment on what happens if the VC, or secondary go open.

The transformer normally rings, much like the "ringing choke" often used to generate a higher voltage. I have mentioned several times that that will cause EHT to be generated & I have actually seen it.

Marc

Good advice, I'll let him buy a commercial one.

What he wants is the sound of over-driven output valves the solution is to over drive the pre output valves rather than heating some resistor in parallel with the speaker but to sound like jimmy micro phonics are important and hence the interplay between sound vibration and the head is paramount

Search eBay for "Power Soak", there are lots of brands available, even NOS ones from 1980. A power soak is what they were called. Not a new idea; Peter Townshend used them in the Who in the 60's. The idea is to drive the amp flat out and lower the overall volume at the speaker, but get all of that lovely gnarly valve distortion. There's was a hilarious clip on youtube of the who performing "MyGeneration" and Pete doing a crazy solo where he turns around to adjust the power soak I assume to get more volume, but not getting it he tries again, and again then a bright flash from the back of the amplifier followed by black smoke and he hurls his guitar at the amp in disgust!