Set Variac to 110vac or 115vac ?
- Thread starter JMP/HBE
- Start date
If I'm playing one of my Rectifiers while others are sleeping it usually gets switched to spongy which drops the volume and the voltage down to around 100v. (17% drop iirc)
Really I do it more to lose some punch and feel a little more lively down low. The manual does say it reduces tube wear a considerable amount, but of course Rectifiers are biased very cold anyways, or at least they are in the 6L6 setting with the Mesa tubes. They are biased much warmer using EL34, but it depends on the modes and specific model.
Really I do it more to lose some punch and feel a little more lively down low. The manual does say it reduces tube wear a considerable amount, but of course Rectifiers are biased very cold anyways, or at least they are in the 6L6 setting with the Mesa tubes. They are biased much warmer using EL34, but it depends on the modes and specific model.
beaubrummels
Well-known member
There are a number interviews with Mike Soldano on the topic online. He more talks about how it really changes the sound of the amp, but doesn't mention any affect on tube life.
One article says it can burn up tubes, though that sounds counter-intuitive to me.
https://projectevh.com/2015/09/23/soldano-slo-100/
One article says it can burn up tubes, though that sounds counter-intuitive to me.
https://projectevh.com/2015/09/23/soldano-slo-100/
beaubrummels
Well-known member
In this interview, he says he figured out the 'brown sound' was 88v on the Variac, and that he lowered the B+ voltage on his Hot Rod 25 to simulate that (~90v Variac simulation), which put 310v on the plates (instead of 450v which is normal.). He didn't discuss the heater voltage, but if all things are equal, you might be able to calculate the heater voltage from the plate differential cited.
Story starts at 8:33. He mentions the plate voltages at 11:44.
devastone
Well-known member
I would guess the biggest thing would be to make sure the heaters are still getting proper voltage. The amps that have those modes built in probably separate them from the heater circuits so they remain constant.
FWIW, lowering from 120 to 110 probably won't make much difference and the heaters will work fine, the outlet voltage in different areas/buildings probably varies that much.
FWIW, lowering from 120 to 110 probably won't make much difference and the heaters will work fine, the outlet voltage in different areas/buildings probably varies that much.
JMP/HBE
Well-known member
Well it got here & i dropped the voltage & rebiased to 90 volts adjusting every 5 volts until getting to 90v.
The 6.3 heater issue bugged me from the start.
When i went to fire up the amp i had issues.
Apparently a NOS preamp tube shot crapps and the relay on the Metro Loop failed to fire.
I was s**ting a gold brick.
Fortunately i have an Engineer on speed dial and was able to trouble shoot.
My Engineer buddy has a Golden rule # If it works dont F with it.
He said if i use the variac again he's gonna charge me double to fix it. lol
I retube the preamp and re-biased all is well. "whew"
The heater voltage is definitely important. Too high and it burns up the tube, too low and it has another sort of issue.
It has to do with how a tube operates. The orange glowing rod you see in the tube is the cathode. That cathode is heated by the heater. The heater and the cathode are only separated by a very thin insulation. The cathode has a very high current flowing through it and it can only boil off so many electrons no matter how hot you get it. Too hot and the electron emission comes to a saturation point ( it won't boil off any more electrons ) and the insulation between it and the heaters will break down causing a short. If the heater voltage is too low and the cathode is too cold it reduces the ability of the cathode to boil off electrons, which actually increases its life, BUTTTTTTT it is sort of a ratio. The heater voltage and the plate voltage have to scale together if you plan on going with a lower heater voltage. If you lower the heater voltage too much and the plate voltage is too high, it has a similar effect as having too high of a heater voltage. The cathode will build up more electrons than it can boil off and it can arc/short to the heater and or break down the cathode's coating. Either way, the tube's life can be shortened considerably.
Since using a variac will draw down the entire amp, the lowered heater voltage is not generally of issue unless you have a power tube that is sensitive to heater temperature. The power tubes are the tubes generally most sensitive to heater voltage. Which makes sense, as they are the ones with several hundreds of volts flowing through them. I think in 90% of cases, using a variac set to reasonable levels of voltage reduction is of little issue. I think 90 volts is probably the lowest practical voltage you can set one though. I think with most 100 watt amps, setting it to about 90 volts reduces the amp's output to about 30 watts. I would say that for everyday use, keeping it closer to 100 volts is probably a safer bet. Most consumer products are rated for somewhere between 110v - 120v, many landing at 115v. A 10-20 volt drop is of little worry. Beyond that, you start getting into murky waters.
It has to do with how a tube operates. The orange glowing rod you see in the tube is the cathode. That cathode is heated by the heater. The heater and the cathode are only separated by a very thin insulation. The cathode has a very high current flowing through it and it can only boil off so many electrons no matter how hot you get it. Too hot and the electron emission comes to a saturation point ( it won't boil off any more electrons ) and the insulation between it and the heaters will break down causing a short. If the heater voltage is too low and the cathode is too cold it reduces the ability of the cathode to boil off electrons, which actually increases its life, BUTTTTTTT it is sort of a ratio. The heater voltage and the plate voltage have to scale together if you plan on going with a lower heater voltage. If you lower the heater voltage too much and the plate voltage is too high, it has a similar effect as having too high of a heater voltage. The cathode will build up more electrons than it can boil off and it can arc/short to the heater and or break down the cathode's coating. Either way, the tube's life can be shortened considerably.
Since using a variac will draw down the entire amp, the lowered heater voltage is not generally of issue unless you have a power tube that is sensitive to heater temperature. The power tubes are the tubes generally most sensitive to heater voltage. Which makes sense, as they are the ones with several hundreds of volts flowing through them. I think in 90% of cases, using a variac set to reasonable levels of voltage reduction is of little issue. I think 90 volts is probably the lowest practical voltage you can set one though. I think with most 100 watt amps, setting it to about 90 volts reduces the amp's output to about 30 watts. I would say that for everyday use, keeping it closer to 100 volts is probably a safer bet. Most consumer products are rated for somewhere between 110v - 120v, many landing at 115v. A 10-20 volt drop is of little worry. Beyond that, you start getting into murky waters.
LLL
New member
Jesus Mary & Joseph - 
I used to run my modded 50 watt with a variac for a handful of years. I would settle for 100VAC, as any lower and the tone would get too mushy.
Had to swap out a biasing resistor so I could bias to the general 70% dissipation range after dropping voltage.
(The trick is to drop the voltage, then bias it)
I eventually swapped back in the old biasing resistor and back to normal, because while there is a slight perceptible change in the tone, it wasn't cool enough for me to keep it.
No problems whatsoever when variac'ing, and I'm still using the same tubes that were in there.
(But then again, I know what the F I'm doing, and even if something happens, I know how to F'n fix it and don't need to call an "engineer buddy" who wets their pants at the sight of a variac.)
That being said, it would stand to reason (using common sense) that any sort of FX loop utilizing the PI power supply would suffer with a drop in voltage.
I used to run my modded 50 watt with a variac for a handful of years. I would settle for 100VAC, as any lower and the tone would get too mushy.
Had to swap out a biasing resistor so I could bias to the general 70% dissipation range after dropping voltage.
(The trick is to drop the voltage, then bias it)
I eventually swapped back in the old biasing resistor and back to normal, because while there is a slight perceptible change in the tone, it wasn't cool enough for me to keep it.
No problems whatsoever when variac'ing, and I'm still using the same tubes that were in there.
(But then again, I know what the F I'm doing, and even if something happens, I know how to F'n fix it and don't need to call an "engineer buddy" who wets their pants at the sight of a variac.)
That being said, it would stand to reason (using common sense) that any sort of FX loop utilizing the PI power supply would suffer with a drop in voltage.
LLL
New member
Well said.
LLL
New member
Yes, so they don't blow their amps.
I always thought the drop in wattage was basically linear to the drop in volts, so that a voltage drop of 20% would mean a wattage drop of 20%.
Can't remember where I read that, nor can I confirm, but it was likely on grailtone or boogie board forum since I was researching the mesa spongy setting at the time.
Can't remember where I read that, nor can I confirm, but it was likely on grailtone or boogie board forum since I was researching the mesa spongy setting at the time.
The voltage in vs. wattage out is tied directly together but does not necessarily scale together. Dropping 20 volts does not mean you will lose 20%, or 20 watts, or any general factor; lowering the power supply voltage simply means that you will reduce wattage output potential. The only way to know for sure is to measure it.
The heaters in the tube do affect the emission capability of a tube. Lower heater voltage means lower electron emission potential. I.E. less output. The voltage seen at the plate of a tube is converted into a measurable heat exchange that we know of as wattage. Dropping 5 volts at the power transformer's input does not mean you lose 5 volts at the output tube's plate. It can be 20, 30, or maybe 150 volts of reduction? The turns ratio of the transformers dictates the voltage in vs voltage out. And most amp designs have different turns ratios, meaning that there is no way to easily guess the plate voltage based on static wall voltage numbers.
As with most things, too much of a good thing is still too much, right? Well if you reduce the voltage to the power transformer too much, it will cause some unwanted outcomes, just as providing too much voltage would. It is all about reasonable amounts.
Spongy sound from an amp is part and parcel of how hard it is being run. If you reduce power supply voltage and then dime the amp, it will produce less total output than with full voltage, thus being quieter and sounding like the amp is fully saturated. That is the idea behind reducing or scaling the output wattage back/down. If you could stand to be near the amp while dimed at full potential, it would still get spongy and sound different, but it would also be unreasonably loud and less enjoyable to be around. Any scaling back of the power makes this spongy-sounding attribute more attainable at more reasonable levels. That is how the cookie crumbles.
Most amps do not sound all that great when dimed. They sound spongy, fizzy in some cases, and depending on how much distortion the speaker is adding, very crunchy and less defined. A tight-sounding amp that is also very loud has less bass in the signal path, which is why they sound thin and anemic at low volume levels. Amps like a JTM-45, or a Fender 5E3 and other full range amps tend to sound splatty, fizzy, and or very crunchy and spongy at higher volumes. Often the fix to the farty, splatty sound is to reduce bass, which free's up headroom and cleans up the signal.
The heaters in the tube do affect the emission capability of a tube. Lower heater voltage means lower electron emission potential. I.E. less output. The voltage seen at the plate of a tube is converted into a measurable heat exchange that we know of as wattage. Dropping 5 volts at the power transformer's input does not mean you lose 5 volts at the output tube's plate. It can be 20, 30, or maybe 150 volts of reduction? The turns ratio of the transformers dictates the voltage in vs voltage out. And most amp designs have different turns ratios, meaning that there is no way to easily guess the plate voltage based on static wall voltage numbers.
As with most things, too much of a good thing is still too much, right? Well if you reduce the voltage to the power transformer too much, it will cause some unwanted outcomes, just as providing too much voltage would. It is all about reasonable amounts.
Spongy sound from an amp is part and parcel of how hard it is being run. If you reduce power supply voltage and then dime the amp, it will produce less total output than with full voltage, thus being quieter and sounding like the amp is fully saturated. That is the idea behind reducing or scaling the output wattage back/down. If you could stand to be near the amp while dimed at full potential, it would still get spongy and sound different, but it would also be unreasonably loud and less enjoyable to be around. Any scaling back of the power makes this spongy-sounding attribute more attainable at more reasonable levels. That is how the cookie crumbles.
Most amps do not sound all that great when dimed. They sound spongy, fizzy in some cases, and depending on how much distortion the speaker is adding, very crunchy and less defined. A tight-sounding amp that is also very loud has less bass in the signal path, which is why they sound thin and anemic at low volume levels. Amps like a JTM-45, or a Fender 5E3 and other full range amps tend to sound splatty, fizzy, and or very crunchy and spongy at higher volumes. Often the fix to the farty, splatty sound is to reduce bass, which free's up headroom and cleans up the signal.
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