Two Old Variacs

[MoltenVoltage]

I just inherited two old variacs and am very excited to try them out on my 50 watt amp head to get the "brown sound".

The issue is that the variacs use two prong plugs and my amp head is 3 prongs.

I'm wondering what the safest way is to connect them.

Am I making a mistake using one of these?


Thanks!

[arawn]

I would not use one of those. You lose your earth connetion, as the variac doesn't have an earth prong on it's plug either. No earth and hundreds of volts of dc=bad idea 

[defaced]

Solder a wire to the green tab of the 3 to 2 prong adapter and connect it to earth ground (that's what the green tab is for, connecting to the center screw in a duplex outlet which is earthed).  Probably also a good idea to add an earth ground connection to the variac casing too. 

[Johan]

please note that a vari-ac isn't a transformer in the normal sence, but rather a transformer implementation of what would look like a potentiometer. this means there is no galvanic isolation between primary and secondary side. things can get ugly, frustrating and dangerous pretty fast if you're not carefull...
J

[PRR]

Don't mess around with wall-power.

REPLACE THE CORD with proper modern 3-wire cord and plug, replace the outlet.

The (new) green wire goes to U-pin/hole, of course, and also to Variac CASE.

Don't solder safety-grounds if at all possible. Home Supply sells a pack of green ground screws for a buck; find the right drill and it may be self-threading. Or use a large-head sheet-metal screw.

> there is no galvanic isolation

Correct. I ass-ume he is using this with a fully-built Guitar Amplifier, which already has the required isolation. However you can not turn-down an amp far unless you split the plate-supply from the heater supply (two PTs). I can only hope he knows what he is doing.

[Joe Hart]

Don't mess around with wall-power.

I can only hope he knows what he is doing.

+1 on both of these. MoltenVoltage, have you read up on this? A Variac (and I'm using this name in the generic sense) can be used to get some different tones out of a tube amp, but in the wrong hands (or even the right hands -- accidents do happen) a Variac *can* be a super great way to eat up your tubes or send your amp to the repair shop or send the user to the morgue. I just want to make sure that you realize you can do some serious damage to your gear and/or to your person. As the hip youngsters are all texting: "just sayin"!
-Joe Hart

[theundeadelvis]

Personally, I'd recommend the BSIAB. Much less likely to kill you. Be careful! 

[iccaros]

Correct. I ass-ume he is using this with a fully-built Guitar Amplifier, which already has the required isolation. However you can not turn-down an amp far unless you split the plate-supply from the heater supply (two PTs). I can only hope he knows what he is doing.

^^^^^^^^^^^^^^
Please Read and understand PRR's warning here.
You have to split the B+ from heater voltage to make this work the way you want. High Voltage exposed... Plus you will need another transformer as you drop voltage the heaters would drop and very quickly the tubes would stop conducting. Before that happens you will start to limit emissions of the tube.

I am all for experimentation, but reading you will find that Eddi had a amp rated for 110v and he ran it @ 90 volts.. Most of his tone was created by using a dummy load and re-amping, this allowed him to control effect by placing them after the guitar amp. So in effect, he was using the Guitar head as a preamp
http://mr5150.vhvault.com/evh-brown-sound.html

running at 90v on a amp made for 110 will get you around 6.12 on the heaters, low but workable
but on a 120v transformer you get about 4.7 volts. Testing my 12ax7, I get them to light up at that, but it takes a long time...

[MoltenVoltage]

Thank you for all the feedback.

The power cords are going to be swapped anyway since they insulation is cracked on both so I'll ground them.

The amp is an Ampeg VL-502 amp head (nothing home made).
http://www.amparchives.com/album/Ampeg/Schematics/Ampeg%20VL-502,%20VL-1001%20&%20VL-1002/index.html

It sounds like it's OK to turn it down to 90 volts but any lower and I'm risking components.

The other problem is that the plug jacks on the variacs (probably from the 1930's) are two prong as well.

[merlinb]

It sounds like it's OK to turn it down to 90 volts but any lower and I'm risking components.

You can turn the voltage down as far as you want. It might not sound great, but nothing will come to harm. The heaters will just get so cool that the sound probably cuts out almost completely at some limit.

[Paul Marossy]

It sounds like it's OK to turn it down to 90 volts but any lower and I'm risking components.

You can turn the voltage down as far as you want. It might not sound great, but nothing will come to harm. The heaters will just get so cool that the sound probably cuts out almost completely at some limit.

It's an overvoltage condition that you need to be really concerned about. As long as the tube filaments can get hot enough so thermionic emission can occur, it will still operate. But there will be some lower threshold where it won't work anymore, like merlinb points out.

[Mark Hammer]

I saw the subject "Two Old Variacs", and I thought it was about a John Lennon / Yoko Ono album!

[PRR]

> 90 volts but any lower and I'm risking components.

What Merlin said. As heater voltage falls, it will fade-out quite fast, get sick and weak sounding.

There "could" be some critical voltage where electrons are ripped from a cool cathode, tubes damaged; but I don't think you'd run it like that for long.

For "weakening" an amp, taking it down from the high-strung hot-rod sound, the trick is valid. It won't turn your 50W amp into a bedroom whisperer.

Alternately you can break the heater wires, add a (hefty) heater transformer, so the heaters get constant 6.3V. Now, on most amps, you can turn the B+ way-way down. On most amps the bias will "track" the B+ so the tube current stays semi-proportional to voltage (actually a 3/2-power law). However a few (IMHO mis-designed) amps have regulated bias supplies and will go "dark" quite quick. That gives "a sound", but not a generally useful sound.

Note that you can drop B+ alone with a pot and MOSFET. Kevin O'Conner and others sell kits, DIY plans exist. Much more compact and lighter than a Variac. (Yes, I sure appreciate the "cool factor" of a hefty Variac; I just get tired of carrying them.)

> the plug jacks on the variacs (probably from the 1930's) are two prong as well.

Replace them too. If you can't find the same mount in 3-pin form (find the brand and scour their website), mount a proper outlet box and modern outlets, or good bushing and cord-end outlet.

Remember that a fault here in line wiring can start fires inside walls. Yes, the cellar fusebox is supposed to prevent that, but try to never trust "one safety".

I'd flame-test it with a "Lamp Limiter" so that any major screw-up will only light the lamp, not try to burn the fuse or house.

Weber VST sells pre-made Variacs made to reasonable safety standards. Not as cheap as "free", but you are now in for major cord and plug/socket work and a lamp-limiter.

[wavley]

ote that you can drop B+ alone with a pot and MOSFET. Kevin O'Conner and others sell kits, DIY plans exist. Much more compact and lighter than a Variac. (Yes, I sure appreciate the "cool factor" of a hefty Variac; I just get tired of carrying them.)

A few of the Traynor guys have done the power scaling thing with mixed reviews, it's pretty reversible, it's been on the to-do list for my guinea pig amp for a while.

[MoltenVoltage]

I've put up a web page with dissembly photos and an explanation of how I assume the Variac works:

http://www.moltenvoltage.com/Two_Old_Variacs.html

I'll be updating the page after I perform the surgery.

Any further suggestions would be greatly appreciated!

[Paul Marossy]

I've put up a web page with dissembly photos and an explanation of how I assume the Variac works:

http://www.moltenvoltage.com/Two_Old_Variacs.html

I'll be updating the page after I perform the surgery.

Any further suggestions would be greatly appreciated!

Cool "vintage" stuff! And kinda neat that your grandfather was an inventor, too. 

[boogietone]

Cool pics!

I have a couple of similar Variacs in the garage that I got from a guy who had retired from one of the original incarnations of Bell Labs in Jersey. Hefty beasts. I plan to do something similar at some point and will need to perform the safety mods as well.

[PRR]

The General Radio brand is high-class.

The Variac guts are the oldest I have seen in this size.

NOTE that the shaft is HOT. They insulated, but if you mess while the knob is off you could touch voltage. Note also that the whole winding is electric hot, varnished on the sides but bare where the wiper rubs.

> I assume they made another model that used 220 volts.

It's a bad way to do 200V-100V conversion-- cheaper to use plain transformer for the big step then the auto-T for fine control.

Note that it is specified for 115V input and marked for 0-130V output. This one will turn-UP. The "in" tap is 88% of the entire winding. The wiper can turn past the "in" tap to get step-up.

This is obviously useful in my lab, where wall was 112V and I might want to test 120V operation.

If you ONLY want turn-DOWN, as in "browning" an amplifier, this 13% over-voltage is pointless and sorta dangerous. By moving "in" to the FULL winding, the wiper only goes 0-100% of line voltage.

The outlet housing is a modified standard surface-mount outlet from the 1930s. (Probably Leviton.) Such castings fell out of favor with WireMold metal surface mount products. Making this kid-proof with modern connectors won't be easy.

[MoltenVoltage]

The General Radio brand is high-class.

The Variac guts are the oldest I have seen in this size.

NOTE that the shaft is HOT. They insulated, but if you mess while the knob is off you could touch voltage. Note also that the whole winding is electric hot, varnished on the sides but bare where the wiper rubs.

Thanks for the tip on the shaft being live!

Note that it is specified for 115V input and marked for 0-130V output. This one will turn-UP. The "in" tap is 88% of the entire winding. The wiper can turn past the "in" tap to get step-up.

I thought maybe it could step up, but if you look at the photo that shows the hot input soldered to one of the windings, you will see the other end of the coil that is connected to the bottom right terminal in the switch enclosure that is not connected to anything else.  It seems like it shouldn't have any effect on the voltage it you turn it past the hot input tap.  How is it still possible to step up?

The outlet housing is a modified standard surface-mount outlet from the 1930s. (Probably Leviton.) Such castings fell out of favor with WireMold metal surface mount products. Making this kid-proof with modern connectors won't be easy.

Are you saying that I shouldn't connect the third prong of a modern plug to the metal enclosure?

[PRR]

> Are you saying that I shouldn't connect the third prong of a modern plug to the metal enclosure?

No; I mean that it could be difficult to close/cover that long side slot with another box or cover in a way that is 101% finger-proof.

If your grandkid knows better than to poke at home-brew electrics, just bring wires out to a new outlet box.

All touchable metal should be bonded to your 3rd-pin ground.

> How is it still possible to step up?

Depends on how it is connected, and I can _not_ be sure from your photos. (I tend to like to turn it over and over in my hands...)

Interestingly, Staco 371 also has near-end taps on _both_ ends:

http://stephenhobley.com/blog/wp-content/uploads/2009/03/variac1.jpg

Nice picture. Can you match your taps to Staco's numbering scheme?

Here is a Variac without near-end taps. Feed the whole winding. The voltage from one end to the wiper is 0%-100%.



Below I have made a near-top tap (and checked that flux etc is not overloaded by using part of the winding). Now the output is 0%-110%.

> How is it still possible to step up?

Or are you asking how a transformer (variable or otherwise) can step-up? I'd assume you know that 2-winding transformers step up or down as designed. 1-winding transformers can also (but lack isolation). Only on AC, of course. Despite the rounded shape, this is not a pot, but an iron-core transformer, just bent round and with a hundred+ taps.

Here's a newer version of your lump. Looks like they ran out of the molded bakelite socket plates and switched to metal box with standard utility outlet.

http://www.radiomuseum.org/r/gener_raco_variac_0_140v_5a.html