Plate Voltage Varies Considerably from Day to Day.

I discovered something interesting today. Yesterday, I was measuring plate voltages on the 5881's in my Bandmaster Reverb, as part of dialing in the Bias. On one tube the plate voltage was 466. Today, when I resumed testing, without me changing anything, the plate voltage on the same tube was 448. I thought about it for a while, and considered what is different today? It's about 30 degrees cooler this morning and I have a little ceramic space heater on in my office, on the same circuit the amp is plugged into. Sure enough when I turned off the heater, the plate voltage went to 466, on half power on the heater, 458 volts, and at full power, 448 volts. Yesterday I was measuring idle plate dissipation on that tube of 80%, Today it is 67% (The voltage drop and plate current are both considerably lower). I guess I had never considered that other devices on the same circuit can significantly affect the amp biasing process.

The wall voltage on that circuit varied from 122.3 with the ceramic heater off to 117.7 with the heater on full. I also have my computer and a halogen desk lamp on that circuit.

I though this might be interesting information for anyone who messes around with tube amps.

All the folks who are obsessed with getting their builds down to the voltages specified on the schematics need only add a heater to the companion circuit!
When I rewired the wall of the office where my wife has her computer, I tapped an old, unused welder breaker to a sub panel for 2 dedicated circuits. Probably overkill now, but in the 90's we paid $10K for a computer system, (on time!), and I was worried about the power.

LOL Bill. I guess if I want to run my amp at 117 Volts out of the wall like the schematic says, the space heater acts as a Variac, although I don't know whether that's a good solution during the summer when it's in the upper 90's F.

I used to work for the power company, (mechanic), and some of the stories you would hear, would sure make you wonder about folks. The best were from people living downtown area where the original area was electrified, most with knob and tube wiring. The original service was called "A" base, and was all 110V, no 220V. The college kids renting the "apartments" above the old storefronts would sometimes add an A/C window unit, and start blowing fuses, (yeah, the screw in type!) In the 80's the services were all upgraded to 220V, and the owners were pissed that the whole place had to be brought to code to connect!

I do remember the screw in type fuses. Thankfully my house has modern wiring and a pair of 200 amp circuit breaker boxes.

I still have a couple in a box in one of the old bays, one feeds a heater, and the other lights, never had a problem!

I use a Variac with a Kill-A-Watt meter set to show voltage when I work on amps. My voltage will vary between 116 and 120 as the dehumidifier, the compressor, my son's welder, etc... turn off and on. I constantly check the Kill-A-Watt and adjust the Variac so it reads 120vac.

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Originally Posted by Don

I use a Variac with a Kill-A-Watt meter set to show voltage when I work on amps. My voltage will vary between 116 and 120 as the dehumidifier, the compressor, my son's welder, etc... turn off and on. I constantly check the Kill-A-Watt and adjust the Variac so it reads 120vac.

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I was going to suggest a Variac since wall voltages can vary so much, even within the same building. My home voltage fluctuates between 119 VAC and 124 VAC where as most vintage Fender amps specify 117 VAC. And of course high current-draw devices such as deep-freeze units, HVAC systems, MIG/TIG welders, space heaters, etc only complicate the situation.

My latest electronics project was installing a grounded outlet on this old Powerstat variable transformer. I only had to add 4 notches to the round hole to get it to fit. I removed the steel friction tabs from the rear of the outlet and reshaped them to hold the outlet firmly and permanently in place.
I had installed a grounded power cord years ago along with a permanently installed and grounded 3 prong adapter (soldered in) but was never happy with that. The only issue is that the switch is in the way of my Kill-A-Watt. I still need to use an extension.
This variable transformer was given to me many years ago.

https://live.staticflickr.com/65535/...a5cd92c6_b.jpg

Don, my "variac" was meant for a cabinet mounting, and has no enclosure! I've used it several times, just no ground connections, my nephew has a metal brake, and promised to build an enclosure, but someone needs to give him dimensions!

I'm going to piggy back on this thread with an extension cord question. I have one good outlet in my basement, I'm running a 100' power cord over to where my music stuff is. Does that much wire add any resistance that can drop the voltage a tad?

Can you get by with a shorter extension cord?

Don back in the day we played a lot of outdoor parties on generator power. I still have the 100’ 12/3 extension cord we used for the whole band. I’m sure we dropped voltage as the lights would dim on Joes power chords.
Later I came up with a 10/3 with ground that I could run directly to the breaker panel on some of our favorite venues. If you can, run a companion extension to another outlet on a different breaker on the same leg, (helps keep noise down).

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Originally Posted by DanTheBluesMan

I'm going to piggy back on this thread with an extension cord question. I have one good outlet in my basement, I'm running a 100' power cord over to where my music stuff is. Does that much wire add any resistance that can drop the voltage a tad?

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Dan, why don't you measure the voltage with a multi-meter set to A/C where the extension cord is plugged into the wall, and then measure the voltage at the end of the extension cord. The heavier the extension cord is, the better. You might even buy some 12 or 10 gauge 3 conductor cable and either fit a grounded plug onto it, or better yet tap right into the existing wall outlet and run the cable over to where you need the power and connect it to an outlet box like the one shown below. Of course, do this only if you are comfortable working with electricity, and can shut the power off to the circuit you are working on. If you do get an outlet box, make certain the black (hot) wire is connected to the smaller slot on the outlet, and the white wire is connected to the larger slot.

https://i.imgur.com/C9JWxCq.jpg

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Originally Posted by Michael Smith

You might even buy some 12 or 10 gauge 3 conductor cable and either fit a grounded plug onto it, or better yet tap right into the existing wall outlet and run the cable over to where you need the power and connect it to an outlet box like the one shown below.

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Make that cable run as short as possible.

Michael, electrical fixtures are color coded, silver is neutral, brass is black, and of course green is ground.

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Originally Posted by Bill Moore

Michael, electrical fixtures are color coded, silver is neutral, brass is black, and of course green is ground.

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+1

And come to think of it, it might be better to use commercial Romex rather than stranded, 3-conductor cable.

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Originally Posted by phantomman

+1

And come to think of it, it might be better to use commercial Romex rather than stranded, 3-conductor cable.

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Ya, I was suggesting Romex, like houses are wired with. I just didn't remember the name. It can be sucured to the wall or ceiling of the basement, using special "staples"

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Originally Posted by Michael Smith

Ya, I was suggesting Romex, like houses are wired with. I just didn't remember the name. It can be sucured to the wall or ceiling of the basement, using special "staples"

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Building code may require steel conduit. DIY will still keep the cost low.

When I added an outlet in my garage (by adding a 15AMP breaker into an unused slot in the breaker box), I covered the Romex with some kind of a plastic "channel strip", more for appearance than anything. When I added lights and an outlet in my 3rd floor attic, I just used Romex and the insulated staples. I'm not sure what the code around here requires, but I do know there is no conduit in the walls themselves.

Oddly when we moved into this house, which was built in the 1980's, the only outlets in the garage were in the 10 foot ceiling for the garage door opener, and one in the water heater closet for a recirculating pump. Luckily my breaker boxes are in the garage right near my workbench, so it was a pretty easy job to add a 4 outlet receptacle and an overhead fluorescent lamp.

Same deal here, Michael. When we bought a deep-freeze for placement in the garage, I had to have a dedicated 15-amp line installed for it. Normal current draw is about 1.2 amps but start-up draw (ie: when the compressor is cycling) is close to 12 amps. That's something the sales staff at a big-box store doesn't tell their customers unless they specifically ask.

When I wired my garage, (I did it under a homeowners permit), I knew one corner was dedicated to be our rehearsal space. 4 outlets, all on the same 20A breaker, lighting on a different circuit. I used 12 ga wire, and the dual 15/20A receptacles, no ground loops, and plenty of power for everything! No GFI on that circuit!
Like Phantomman, a dedicated circuit for a freezer!

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Originally Posted by Bill Moore

When I wired my garage, (I did it under a homeowners permit), I knew one corner was dedicated to be our rehearsal space. 4 outlets, all on the same 20A breaker, lighting on a different circuit. I used 12 ga wire, and the dual 15/20A receptacles, no ground loops, and plenty of power for everything! No GFI on that circuit!
Like Phantomman, a dedicated circuit for a freezer!

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Bill, is a GFI on a circuit a bad thing for musical gear?

I don't think it matters Michael, I know only music equipment will be plugged into that area. I put GFI breakers on the shop side and exterior walls to follow code, but with the rug in the "music room" I didn't spend the money. (No GFI on the freezer either!)

A surge protector could probably be used in lieu of a GFI outlet.

I run a power conditioner in my rack. Highly suggest them. Cheap insurance and helps quiet stuff down. It also indicates you have a proper ground, and you get the lights too.