Plate voltage and tone?

[slegros]

What effect doe plate voltage have on tone? For example, 2 similar amps, one running 450v on the plates biased at 70%, and another running 350v on the plates biased at 70%. Given that both amps are biased to the same 70%(the current req'd for 70% bias in each case will obviously be different due to the voltage difference), what will the effect be on tone?

Thanks!

[Frostbite Slim]

Hmm..

1.) What amp?
2.) Type of Pwr Tubes?
3.) Fixed Bias or Cathode?

[SatelliteAmps]

In two exact amps, if you had a way to limit the B+ only going to the power tubes, and you had biased them both at exactly 70%, you probably wouldn't hear that much of a difference (it would be different, but just slightly so). You would get a bit more wattage, which some amps will show with more clean headroom. Some won't.

The B+ also supplies the preamp, so when you drop it across an amp, you end up affecting everything, and can't say just what the effect is on the power amp part for tonal qualities.

[GearHeadFred]

I just did this experiment in my home-build AB763 (BFSR) by swapping the recto tube from a GZ34 to a 5U4. B+ went from about 460V to about 380V. I bias at around 65% in both modes. As Adam says, this also drops the other voltages to the other amp sections.. but...

The tone and basic character of the amp is not altered dramatically. But, it brings the "sweet spot" of the amp down in volume quite a bit. It went from break up starting at about 5 on the volume knob, to about 3.. I normally play the amp in the 2-3 range, so this was desirable for me. I was using a Hotplate to attenuate the output, but I think this is going to be a better solution for me -- I'll find out tonight at the first "low voltage" gig!

It's my understanding that this is the same fundamental logic used in "power scaling", but I am no expert in that.

[SatelliteAmps]

I'll find out tonight at the first "low voltage" gig!

It's my understanding that this is the same fundamental logic used in "power scaling", but I am no expert in that.

Cool. Where's the gig?

Dropping it across the entire amp make it very difficult to say where the tone change came from. The preamp voltage drop would make a considerable difference, as would the sag and compression from using a 5U4. It's changing so many minor things all at the same time.

It is exactly the same logic as used in power scaling. Using an amp with power scaling involved, and dropping just the B+ to the power tubes will demonstrate the idea. Most people don't notice a large tonal difference in dropping 100v off the power tube B+. The scaling works much more when the voltage drops more than that, and the main difference is overall volume, not tonal change.

[GearHeadFred]

Cool. Where's the gig?

The Kraken up in Cardiff.. If you come by, please say hello - I'll be the guy with playing the 335 and the homebrew blackface.

[slegros]

I just did this experiment in my home-build AB763 (BFSR) by swapping the recto tube from a GZ34 to a 5U4. B+ went from about 460V to about 380V. I bias at around 65% in both modes. As Adam says, this also drops the other voltages to the other amp sections.. but...

The tone and basic character of the amp is not altered dramatically. But, it brings the "sweet spot" of the amp down in volume quite a bit. It went from break up starting at about 5 on the volume knob, to about 3.. I normally play the amp in the 2-3 range, so this was desirable for me. I was using a Hotplate to attenuate the output, but I think this is going to be a better solution for me -- I'll find out tonight at the first "low voltage" gig!

It's my understanding that this is the same fundamental logic used in "power scaling", but I am no expert in that.


LOL! I did the same thing too!! But I did it by plugging my amp into a step down transformer which had outlets for 120, 115, 100 VAC. In my case B+ went from 420 down to 350. My observations were exactly in line with yours!

[sharpshooter]

I've been intrested in this too. Where I live the wall voltage is 124V. Should I get a Variac, to reduce the voltage to a more normal level? I have wondered about component life, at higher voltages.

[SatelliteAmps]

The Kraken up in Cardiff.. If you come by, please say hello - I'll be the guy with playing the 335 and the homebrew blackface.

Sorry I missed it. Maybe another time.

I've been intrested in this too. Where I live the wall voltage is 124V. Should I get a Variac, to reduce the voltage to a more normal level? I have wondered about component life, at higher voltages.

If your wall voltage is 124v, then yes. You should consider either a bucking transformer (that will drop a specific voltage all the time), or a variac.

[Champ]

Cool. Where's the gig?

Dropping it across the entire amp make it very difficult to say where the tone change came from. The preamp voltage drop would make a considerable difference, as would the sag and compression from using a 5U4. It's changing so many minor things all at the same time.

It is exactly the same logic as used in power scaling. Using an amp with power scaling involved, and dropping just the B+ to the power tubes will demonstrate the idea. Most people don't notice a large tonal difference in dropping 100v off the power tube B+. The scaling works much more when the voltage drops more than that, and the main difference is overall volume, not tonal change.

Is going from a GZ34 to a 5U4 in my Deluxe reverb clone a safe thing to do? Seems my plate voltage is high at 480v and would like to try it at a lower setting. Thanks, GT

[GearHeadFred]

The 5U4 draws 3A filament current - more than the GZ34 (or 5AR4) . You need to make sure the PT can supply it.. Otherwise, it should be perfectly safe.. Of course the bias will need to be adjusted.

The gig was good.. The 5U4 made the amp a little "mushier".. I'm not sure if I like it more or not.... Nothing is simple!

[Champ]

The 5U4 draws 3A filament current - more than the GZ34 (or 5AR4) . You need to make sure the PT can supply it.. Otherwise, it should be perfectly safe.. Of course the bias will need to be adjusted.

The gig was good.. The 5U4 made the amp a little "mushier".. I'm not sure if I like it more or not.... Nothing is simple!

It has a Mercury Magnetics PT in it but could not tell you which one. I understand they are plenty powerful. Is there a measurement I could take to make sure? Thanks, GT

[SatelliteAmps]

It will work in a Deluxe Reverb with a Deluxe clone MM PT.

[mooreamps]

I've been intrested in this too. Where I live the wall voltage is 124V. Should I get a Variac, to reduce the voltage to a more normal level? I have wondered about component life, at higher voltages.

I think having a variac, with built in voltage meter, is a great idea. You may not be so worried about small variations in plate voltages, but a more concern would be to keep the filament voltage level in check. Certainly when playing in a club, when they turn on all the stage lighting and such, and just to see how low the stage power drops. A variac can bring up the amp voltage in such a case.


-g

[SatelliteAmps]

I think having a variac, with built in voltage meter, is a great idea. You may not be so worried about small variations in plate voltages, but a more concern would be to keep the filament voltage level in check. Certainly when playing in a club, when they turn on all the stage lighting and such, and just to see how low the stage power drops. A variac can bring up the amp voltage in such a case.


-g

If the power is that bad, then using a variac to boost an amp's voltage up could be very bad in a surge situation. All it would take is one moment when they drop the lights to black, and the electrical surge to the amp could be enough to cause some damage.

If the stage voltage drops with the lights on to say 110v, and then someone uses a variac to bump the voltage up to 120v to get the filaments to an exact 6.3v, then the lighting guy pulls the fader, the voltage in the wall spikes to 125v (which is not an uncommon wall voltage), then the variac is now going to be suppling almost 145v to the mains of the amp. Probably not a great idea. There would be less risk of damage to the amp by just leaving it alone.

Plus, very few people are going to open their amps to monitor filament voltage while at a gig.

If someone really wants to setup something so their electricity is always constant, then they should invest in some kind of power conditioner with a voltage limiter in in. Those will usually take any voltage from about 80 to 150v and always put out a clean, constant 120v.