What line voltage do you run amps at / bias the tubes for?

bluesholyman

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I run my tube amps on a variac, so I know exactly what voltage is going into them. I've seen line voltages at home fluctuate anywhere between 115v-125v, sometimes slightly more o_O

So I've gotten in the habit, more or less, of biasing the amp at 117v and around 70% dissipation on the tubes. Just wondering what others do, if you are paying attention to the line voltages when biasing or not.

Maybe its better to bias the amp at a higher line voltage, just to be sure one does not cook the tubes if the amp is plugged in somewhere there is pretty toasty voltages going on.

Just curious how folks, who do their own biasing approach this. I'm not sure what amp techs do, but I'd imagine 120v is a more common standard.
 

AntonyB

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Depends on the amp.
I usually try to reduce the wall voltage so that the power tube plates voltage match the schematic numbers (+/- 20% as Fender says :))
The 3 amps that I use with this method are actually cathode biased so I don't need to worry about bias.

But it's well known that Fender Silverface are running a bit hot to get more power and that usually they sound better with lower power plate voltages.
70s Champ, I run at 112V
Early 60s Reverberocket, I run at 107V
Late 60s B-15N, I run at 103V

I also do use my ears... there is an audible difference, less "harsh" if I can say?

If I had to bias an amp I would still use the same method first then bias.
 

MuddyWolf

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Whatever comes out of the wall. Sometimes I get 15 minutes to hit the stage and be playing. Sometimes I get 5 minutes. I ain't got time to set up my own regulated power. Besides I play live blues and rock in a dive. Nobody cares about perfect tone. They just care whether or not the band knows sweet home alabama (brings good tips).
 

schmee

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I haven't checked a lot but find line voltage to be fairly consistent here in the PNW. I bias for what ever it is. I do find bias creeps for 10 minutes or so, so I leave the amp on for a bit before finalizing.
 

rdjones

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125V

I've been fighting this since I moved to TN.
Line voltage has always been high here, usually 125-128V, occasionally as high as 131V.
I have never seen it as low as 117V, right now it's on the low end of normal @123V.
I routinely use a Variac (actually a Sencore PR57 Powerite) or a homemade brownbox.

You don't get the same penalty for biasing slightly cool as you get with a supposedly 'correct' bias run at overvoltage mains. Heat, wasted electric power, stressed components, short(er) tube life, etc.

Disclaimer - my personal tone doesn't rely on tubes on the verge of meltdown. ;)
 

Wally

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I run my tube amps on a variac, so I know exactly what voltage is going into them. I've seen line voltages at home fluctuate anywhere between 115v-125v, sometimes slightly more o_O

So I've gotten in the habit, more or less, of biasing the amp at 117v and around 70% dissipation on the tubes. Just wondering what others do, if you are paying attention to the line voltages when biasing or not.

Maybe its better to bias the amp at a higher line voltage, just to be sure one does not cook the tubes if the amp is plugged in somewhere there is pretty toasty voltages going on.

Just curious how folks, who do their own biasing approach this. I'm not sure what amp techs do, but I'd imagine 120v is a more common standard.

I find the point where the amp works for me at the ‘chosen’ controlled voltage…say 117VAC. I then check things out at the highest voltage I have seen at the wall…..125VAC in your case. I want to know that the amp is safe there in case one gets caught having to run that higher voltage, right? More than likely, things will be safe. It takes a lot of plate dissipation to harm a power tube….75-85% for a gig is not going to do any damage. Yes, running above that 80% point does wear the tubes a bit more quickly…but it is not a sudden death. I set up a DR for a fellow who ran 117VAC. I set the amp where it sounded really good….about 62-65% of max plate dissipation. Checking at 125VAC, the plate dissipation was safe.
it is also interesting while going through this process of checking and setting to rest the bias at the higher voltage. Set it to the same plate dissipation as one had set for the 117VAC. It is educational to hear the difference. The amp was much warmer and more musical at the lower voltage, ime…..same power tube plate dissipation. we have to remember that changing the B+ voltage rebiases all of the preamp and the phase 8nverter, too…..they are all cathode bias.
 

dan40

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For longest tube life, set the input voltage at whatever voltage gives you 6.3vac on the tube's filaments. The tubes used in most guitar amps were designed to run at 6.3vac so this is the optimum voltage if tube life is your main concern. As Wally mentioned, the input voltage will affect the amp's sound and feel also, so if optimum tone is your main goal, you can adjust up or down a bit to find the sweet spot.
 

bluesholyman

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Interesting the moderator moved this to a tech forum (from the glowing tube forum,) presumably considering it a 'technical' issue. I find tube bias to be a musical issue, the main reasons why I pay so much attention to it and have a variac. 125v does not sound as good as 117v or less.

I also do use my ears... there is an audible difference, less "harsh" if I can say?

Yes, this is the observation that led me to getting a variac.

Nobody cares about perfect tone.

Valid point. I guess in this scenario, I would be concerned about driving the tubes too hard if the line voltage is aggressive.

I've been fighting this since I moved to TN.
The TVA over-delivers.
 

Euphonica

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I asked this before and the answers I got were all along the lines of “you’d have to build that.”

My wall voltage is between 115 - 123 volts, and it changes constantly. It is never static, always moving a volt or two within a minute or two. Always moving. If I wanted a magic box that would only output 120 volts, as far as I have learned there are no products available. I’d love to get 117 all day. So-called regulators do not work for this, I honestly don’t know what they do. Filter the power? The voltage still swings when it’s going through a regulator.

I’m just posting in case some product has emerged that will do this, or if anyone found another solution.
 

bluesholyman

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I’m just posting in case some product has emerged that will do this, or if anyone found another solution.

I saw a musician using something like this in a rig rundown, but I don't remember who. It basically locked the amp voltage at a set value. I remember it looking it up and it was not cheap.

I have seen some, but they have a window they allow the voltage to wander in before they start clamping it down - it was a pretty broad window though, so not great for a tube amp.
 

Wally

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Furman builds some high dollar units…and some that are much higher dollar units.
 

Euphonica

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Furman builds some high dollar units…and some that are much higher dollar units.
Where? The ones I saw were only 120 volts. And, here’s what someone said about the 1800 model ($1500):

“The product does regulate voltage within a +/-5 Volt output range. It does NOT produce rocksteady 120 V output.”
 

bluesholyman

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I’m just posting in case some product has emerged that will do this, or if anyone found another solution.

The rig rundown was Joe Bonamassa 2018 - if you blink, you miss it, but he references a Kikusui PCR2000M that he uses - does exactly what you are looking for..... $6K... found it here for slightly less.


He even throws some shade at Furman for not doing what this does....
 
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Euphonica

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The rig rundown was Joe Bonamassa 2018 - if you blink, you miss it, but he references a Kikusui PCR2000M that he uses - does exactly what you are looking for..... $6K... found it here for slightly less:
Holy SPIT! Thanks for this info. Whoa. I had an image of a box with two buttons, up or down. Maybe a single knob like a variac. This is… insane. I knew it was complicated (because y’all said so) but I never imagined something like this.

This seems like something a lab would use with super sensitive equipment that responds to changes in voltage, which throws off the performance.

If someone in guitarworld could ever make a device say,70-80% cheaper, I feel lots of tube folk would use it.

I guess I’ll be at the whim of the wall for the foreseeable future.
 
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bluesholyman

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Well, I guess if your running 2 High Power twins, a dumble or two and some marshalls, you might need some juice.
 

Euphonica

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Okay well now, what about this? It says 20 amps, 2kVA 5-500Hz 1-280 VA. This it only $265.


My goodness I am so far out of my knowledge here. I don’t even understand how that VA rating translates into regular volts and amps.
 

Milspec

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I apparently live in a great area for this issue because my line voltage is very steady and never gets above 119 v. I keep a voltage monitor on it all day and it will stay 117-119v so that is what I work with.

It seems that the newer the housing development, the higher the line voltage and all bets are off with commercial so I keep a bucking transformer handy for those situations.
 

Paul G.

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Whatever comes out of the wall. My amps get used at gigs, so they'll be running whatever the voltage is in the joint. My friends and clients will be running their amps someplace other than my house, I have no knowledge or control over that.

Fender and Marshall class AB fixed bias amps sound quite good with the bias set to 55-63% MPD. This also allows some cushion for stupid high voltages. I'll do cathode bias to 85-90%. You'll be fine if it goes higher now and then.
 




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