Understanding rectifier tubes

[FWAxeIbanez]

Ok, I have asked before in a round about way, but now I have to ask outright... what does tube rectification do to your sound? I keep hearing things like it adds sag and lets the amp "breathe" and I'd like a little better of an understanding... I guess I've always had an idea of what it sounds like, and have always thought I've known, untill I realized I have never owned or played an amp with tube rectifiers, so I have no clue how I would know...

 

I guess the idea I have in my head is something having to do with your pick attack... albeit with no personal experience, I've always thought it was kind of like how a singer will move the mic away from thier mouth as they sing louder to keep levels the same, but maintain the tone of singing loudly... does this make sense? and am I far off? Its like you dig in and the amp will compress naturally and provide the tone of a hardstruck chord or note but without a drastic change in actual volume, how far off am I? and more importantly, what would you guys think of adding a recto tube to my 67 bandmaster? positive or negative impact? Its 40 watts if that makes a difference

[Lord Valve]

 

Originally posted by FWAxeIbanez Ok, I have asked before in a round about way, but now I have to ask outright... what does tube rectification do to your sound? I keep hearing things like it adds sag and lets the amp "breathe" and I'd like a little better of an understanding... I guess I've always had an idea of what it sounds like, and have always thought I've known, untill I realized I have never owned or played an amp with tube rectifiers, so I have no clue how I would know... I guess the idea I have in my head is something having to do with your pick attack... albeit with no personal experience, I've always thought it was kind of like how a singer will move the mic away from thier mouth as they sing louder to keep levels the same, but maintain the tone of singing loudly... does this make sense? and am I far off? Its like you dig in and the amp will compress naturally and provide the tone of a hardstruck chord or note but without a drastic change in actual volume, how far off am I? and more importantly, what would you guys think of adding a recto tube to my 67 bandmaster? positive or negative impact? Its 40 watts if that makes a difference

 

 

All amps have rectifiers of one sort or another. Your amp currently has a fullwave rectifier made from solid state diodes. The "sag" you hear people talk about is analogous to using a compressor, although it's a bit more involved than just that.

 

Tube amps with SS diode rectifiers also sag when they are played loudly enough for the power tubes to try to use more current than the transformer is capable of delivering. A tube rectifier is rated in several ways - how much current it is capable of, how much voltage it can withstand, and the forward voltage drop at rated current, to name a few. It's this last spec that causes "sag." A 5U4, for instance, is rated for a forward drop of 50 volts when delivering its rated current of 275 mA. This means that the plate voltage, whatever it is - and that's determined by the AC voltage supplied to the rectifier by the power transformer - will go down by 50 volts when the tube is supplying full current. This drop varies with the amount of current pulled through the tube. Any time the plate voltage goes down, the amp's power output is reduced. If you hit a loud chord or note, the plate voltage will sag initially, and as the note decays, the plate voltage will rise as the current demand on the rectifier tube lessens. This means that as the note fades, the amp has more voltage to amplify with, which gives the effect of increasing the time the note takes to fade out. It's sort of like hitting a note with your volume control on 2 and then gradually turning the control up as the signal strength from the guitar decreases. Percussive attacks - like pick noises - are softened a bit, because the tube sags the plate voltage downward, sort of in a reverse image of the input signal. This effect is lessened by the size of the filter capacitors in the high voltage supply; they actually store energy for the amp to use, with the output of the rectifier tube - a string of positive-going bumps, what a tech would call "pulsed DC" - being used to "top off" the energy in those capacitors as the amp uses it to amplify your guitar. Increasing the capacitance in the HV supply will lessen the "sag" effect of the rectifier tube, but too much capacitance will *kill* the rectifier tube due to an effect known as "peak repetitive charging current." Too little capacitance will give you "ghost notes" and other unwanted artifacts.

 

There are solid-state devices which emulate rectifier tube sag - like the Weber Copper Caps, for instance - and some of them do this fairly well. I used Weber WZ-68 doghouse-style saggy rectifiers in the two custom Super Sixes I built for Derek Trucks to use with the Allman Brothers Band, providing him with a switch to select between the "saggy" Copper Cap and the normal Fender-style fullwave SS diode string. Derek prefers the sag, as it's more like his BFSRs.

 

As for your Bandmaster, you'll need to use a Super Reverb power transformer instead of the stock one if you want to install a rectifier tube. Either that, or an add-on 5V filament transformer, which you probably won't find room for on that small chassis.

 

Lord Valve

"You have been banned from TGP for the following reason: none.

Date the ban will be lifted: never."

[FWAxeIbanez]

you sir, are the {censored}... thank you

 

have you had much experience installing tube rectifiers on bandmasters? Do you think it would be a desirable change? I am wanting to tweak it into something unique...

 

also, if the output is dropping, would that effect power tube distortion during the "sag"?

[Bloozcat]

 

Originally posted by FWAxeIbanez you sir, are the {censored}... thank you have you had much experience installing tube rectifiers on bandmasters? Do you think it would be a desirable change? I am wanting to tweak it into something unique... also, if the output is dropping, would that effect power tube distortion during the "sag"?

 

 

You should call him at the number listed at the bottom of his post. Lord Valve is an amp technician extrodinnaire, and he's also one of the top tube meisters in the biz.