AC over-voltage knock-down at/after outlet.

[Morganfield]

I'm wondering— the thing I'm struggling with with pretty much all my "projects" is generally over-voltage. I'm wondering if it is feasible to knock 5 volts of AC before any of my equipment. AC lines here are standard at 126vac. measured with several meters which all concur with each other. I was thinking of making a power-strip/block that plugs in to the AC outlet with a huge resistor just to lower the AC a few volts. I've done this in the past with DC but don't know if putting a resistor in AC would cause problems. Even my heaters are high in every amp/tube and could easily stand being fed a volt or two less.

Any ideas or advice? I don't want to lug a Variac around, and couldn't afford one anyway.

-MM

[sluckey]

Here's one idea...

[bigsbybender]

I've built a couple of these.  I can recommend it.   In many places I find myself even having to "buck" the full 12.6v. I'm seeing more and more places running close to 130v off peak hours. Radio shack sells the tranformer...and they also sell a decent plastic/metal enclosure that you could put it in though I used a plastic electrical switch/outlet box with one plug in it  then had 2 blanks on the rest of the cover plate where I drilled for the voltage selector switch. The transformer bolted neatly inside this box.


j.

[PRR]

> a huge resistor just to lower the AC a few volts.

Do the math.

Assume 6V, just to be fun.

Your tube-pedal draws 30 watts, or about 0.25 Amps at about 120V. So the resistor should be 6V at 0.25A, or 6V/0.25A, or 24 ohms.

Your Sunn Mega-Monster 600 idles at, say, 240 Watts, or 2 Amps. 6V/2A needs a 3 ohm resistor.

That same Sunn Mega-Monster 600 may pull 430 Watts or 3.6 Amps at FULL ROAR. 6V/3.6A= 1.67 ohms.

There's no single value which will work for all amplifiers.

Steady-drain amps, such as any cathode-bias amp, can use a resistor, calculated for the situation.

Fixed-bias amps are usually fixed-bias so-that they can idle at modest power and pull MORE power when you wack them hard. A resistor will make them sag. The Sunn Mega-Monster 600 with 3 ohm resistor would get 120V at idle but 115V at full roar. This may be acceptable.

Large transistor amps may idle at 20W and roar 200 Watts drain. Any resistor big enough to do any "drop" at idle will be WAY too much drop when you need the most, and vice versa.

Richard's 100 year old idea WORKS. At least for any "sane" amplifier. The load current should not exceed 3A for very long. So it would support 360 Watts into the box. At typical tube-amp efficiency, that's up to 150 watts out of the amplifier... so pretty big stuff. And the 12V 3A transformer 273-1511 is just over $11.

> my heaters are high in every amp/tube and could easily stand being fed a volt or two less.

A volt or two? You are getting 7V and 8V?? That's a 15%-30% over-volt, not the 5%-10% you expect from 126V into 120V or 115V nominal primary.

BTW: if you are only worried about the heaters, and your wall voltage is somewhat stable, series resistors are practical.

[Morganfield]

Thank you chaps. All good feedback. I'm going to make the bucking-TX enclosure. I'll do it as a project with my youngest son. He's six and loves simple projects like this.

Thank you PRR, I've measured 6.9v on some of my heaters, What should I not let them fall below? And what happens if they do go too low?

-MM

[PRR]

> What should I not let them fall below?

Aim for 6.3V, but don't fuss. You don't go around your house checking that your "120V" lamps are really getting 120.00V, do you? Tubes are less critical.

[Morganfield]

Thanks all, for your help. I bought the bucking TX for this device today.

Just the other day I re-tubed my Jet J-20 and burned in the tubes for almost 4 hours of idle time. I had a B+ of 430vdc on the plates of Sovtek 6V6GT and eventually idled it at 21.5 and 23mA. It was HOT after the burn in and bias had crept up to about 29mA which I took back to 21.5/23mA. I have installed a nice, old, lockable/sealed Alan Bradley pot (150k) for adjusting bias. My question is: If I settle the bias with the excess AC that I was seeing, when I drop the line AC to my preferred level (120vac), will the bias move proportionally and stay safe? I'm trying not to open this amp up too often, unnecessarily.

I was thinking/wondering (dangerous) that setting the bias with the AC over-voltage present, would be a worst case scenario, and that things would improve (cool) from there as I reduce line AC? I'm not being lazy, I just have little- to-no time between day-jobs, gigs, and family stuff.

-MM

[sluckey]

I was thinking/wondering (dangerous) that setting the bias with the AC over-voltage present, would be a worst case scenario, and that things would improve (cool) from there as I reduce line AC?

That's true if you're talking about a cathode biased tube. But if you're talking about a fixed bias amp, it's not as clear.

For fixed bias amps, as you decrease the line voltage, your plate voltage will decrease, thus decreasing plate dissipation (run cooler). But, at the same time, you also decrease the fixed bias voltage, which will make the tube draw more cathode current (run hotter). Is this a net zero situation? I don't know. It's worth thinking/wondering about, but certainly not worth worring about. You could always measure/calculate plate dissipation with raw line voltage and then re-measure/calculate plate dissipation with reduced line voltage and compare. Please let us know. My gut tells me it will be cooler, but I don't know for certain.

[jjasilli]

certainly not worth worring about Agreed.  I have seen this issue come up only for hi-end hi-fi amps that use regulated fixed bias supplies.  The fact that ordinary fixed bias voltage varies in concert with plate voltage, as "wall voltage" varies, is one of life's few hidden blessings.

[Morganfield]

Okay so I made my TX enclosure. I used two switches because I didn't have a rotary switch. So I have:

Switch number one = Bypass / Operate
Switch number two = 7.2 vac drop / 14.6 vac drop

Did the math this morning during a drawdown at breakfast, I read 121.4 vac raw, 114.2 vac and 106.8 vac.
These numbers show me I'm getting a 5.93081 percent drop in position one and a 12.02636 percent drop in position two.

When I apply these percentages to my bias current that was set at off-peak overvoltage, I get:

Raw = 23mA
5.9% drop = 21.6mA
12.025% drop = 20.2mA

I have not measured idle current yet with new AC levels, I'm inclined to trust these numbers, but I'm waiting for someone to say... "Ah yes, but wait..."

Even if by some odd unseen law/factor/freak of science, the idle current increased by these percentages we're talking about a max increase of 2.8mA but with the 12.02 percent lower plate voltage of 376.5 vdc, during AC over-voltage where raw plate is 428 vdc. Right?

Considering Heaters ideal at 6.3 vac  the bigger hit/drop of 12.02 percent shows only 5.54 vac. With the lesser hit/drop of 5.93 percent shows 5.926 vac. So it appears the the 12.02 percent drop should only be used in extreme 130 vac overvoltage periods.


-MM

 

[mackie2]

Morganfield--

Mackie2 here--In the old days, I used to put a dale  high watt Res in each PT leg -- selecting IR drop to give me a lower  Filament Voltage to the tubes, ie: 5.8v-6v.  There's something about Starving the filament voltage!  I have a 20A  variac on the work bench,
whereby I lower the Voltage to see what  the Fil V  is. Most of the time Fil's are running  High!  Sometimes I  play my Jazz station through one of my amps, with the variac set for Filaments of 3.5v.  Level  drops, but she still sings low and sexy!!! Who would've thought THAT.

Mackie2

[Morganfield]

That's cool mackie2. This has proven to be a very useful tool. I did eventually go back in and measure real voltages and found that the heaters will drop to 5.6vac with the -12vac setting selected. It still sounds fine and sounds great. The idle current all lowered proportionally too as expected. I'm still wondering if there are any ill effects of running heaters down that low. If anyone has a case for not doing it I'd like to hear it we probably all would.

Also mackie2, I just installed a sag resistor in B+ similar to what you were talking about. It's a 5E3 (give or take), I also dropped the preamp voltages with an 8k2 instead of the 5k. When I max out the head I can get some laughable fuzz and compression. We laughed until we felt the temperature of that 180 ohm 10 watt resistor, which guessing, I think would last only minutes at full sail, sounds nice turned down to stun though.

-MM