Eico 1078 Variac

[Jim Coash]

Greetings:  A friend gave me an Eico 1078 variac and suggested I use it to power up my tube amp projects.  It is a real classic from the 1960s but it seems to be all there.  I would like to find a manual and schematic for it before use.  Anyone out there have one?  He was a TV tech who retired and I have done him some favors so he passed it on.  He warned me that it isn't a true isolation device.  The A/C recep on the front is not a grounded one.  He said he also used it to re-form filter caps by just slowly increasing the power over several minutes.  Jim

[sluckey]

Good friend! And a great addition to a service bench. Does it look like this?

http://www.ebay.com/itm/EICO-1078-120V-Switchable-2-1-2A-and-7-1-2A-Metered-Variac-Variable-transformer-/171584810911?pt=LH_DefaultDomain_0&hash=item27f3406f9f

I can't imagine needing a manual or schematic. They are very easy to operate. Should be easy to find a three prong outlet for it. Does it have a three prong power cord? While you're changing the outlet just draw up a schematic.

I would not be concerned about it not being an isolation device. That's not it's purpose. A big ole isolation transformer was a necessity back in the 60s especially on a tv service bench. Most of the test equipment was line powered and many of the tv sets had a hot chassis. You would plug the tv into the isolation transformer to insure the chassis was no longer connected to the line. This prevented damage to the test equipment.

I have a plain jane 10A variac made by Superior (Powerstat). One toggle switch and a voltage knob. Just last week I used it to slowly bring up a couple Hammond AO43s and reform filter caps. Works pretty good for that. I also use it to set the line voltage to exactly 120VAC so I have a good repeatable baseline for voltage readings on stuff I build. Most of the time it just sits in a storage cabinet. Invaluable when you need it though.

[HotBluePlates]

A friend gave me an Eico 1078 variac and suggested I use it to power up my tube amp projects.  ...  He said he also used it to re-form filter caps by just slowly increasing the power over several minutes.

For first-time power up of a new build, I like a lamp-limiter/light bulb limiter better, though both can protect the device under test (DUT) from excessive current draw. The light bulb limiter gives an obvious indicator of a fault condition (the light glows brightly), where the variac will require you to do some voltage measurements at low supply voltage and know what "right looks like" under those conditions. Do a forum search or Google search for "light bulb limiter" and you'll find all kinds of hits to show how to build and use one.

The variac can be handy for re-forming caps in equipment that has sat a very long time. At the end of the day, the reduced line voltage is limiting the current draw of the amp until the filter caps re-form their oxide layers. This can be done without a variac, but the variac is very handy for this purpose.

I have some test equipment which use regulated power supplies. A variac is part of the testing procedure for these units... Line voltage is adjusted above and below the rated input to evaluate the regulators' ability to maintain regulated output voltage under these conditions. For such a test, a variac is the only sensible tool (you could use transformers with fixed taps providing high/low line voltage to the DUT, but the variac is much more convenient).

You could also evaluate the effect of high/low line voltage on a new design (with unregulated supplies, as in a typical guitar amp) to determine if parts used have sufficiently-high ratings (for high line voltage conditions), or if the amp performs adequately under low line voltage conditions (a rare case; however, this could also simulate operation with old, weak tubes).

[Jim Coash]

Greetings:  I just pulled it apart.  It is simple.  The A/C cord was cracked and brittle so I installed a new 3 prong unit and grounded the green wire to the chassis.  It is very similar to the eBay unit you provided the link to, but older with a large black Bakelite knob and an ungrounded recep.  I would like to fix that but would need to find a replacement that could fit with a little modification to the existing cut out.  For now I just plugged in a three prong to two prong adapter and screwed its green ground wire to the nearest chassis screw.  With the cobwebs cleaned out and a little clean and lube on the switches it seems to work OK with just a shoplite plugged in.  Now for the big moment; do I try the Sovtek Mig-50 first or the Fender Princeton Reverb?  Why am I starting to sweat?  Jim

[octal]

I'm pretty sure that's the same model Eico which once gave me a nasty shock. As I recall, the power switch was a double pole one and switched the neutral as well.  What happened is that the "hot" side contacts  of the switch got welded together, but the neutral side was still switching OK. So, the neon power indicator light went off when I shut the power switch off, so I never noticed there was a fault with the variac. I thought I was safe working on the amp I had plugged in to the variac (as the power switch and the neon indicator were off) and  WHAMMO! I caught some AC between the now unswitched hot and ground. 


The last time I related this story on another forum, someone more or less accused me of plagarizing  it from Gerald Weber! Evidently he has a similar story in one of his books. I've never read his books and I've got the Eico sitting on my workbench at work! I replaced the switch, wired it so the hot was switched only, and wired the indicator light directly across the output receptacle.


Nathan

[Jim Coash]

I will be sure to take a look at that.  I don't need anymore stinking shocks!  Thank you.  Jim

[TubeGeek]

Is it safe to use a variac to adjust line voltage lower to what it would have been in the old days on a guitar amp?



I've seen or read some comments that suggest it is not safe.  Looking for clarification.

[sluckey]

It's safe unless you have one that does not have a properly wired three prong AC plug and socket.

[jjasilli]

Is it safe to use a variac to adjust line voltage lower to what it would have been in the old days on a guitar amp?

I've seen or read some comments that suggest it is not safe.  Looking for clarification.

Two issues: grounding & filament voltage.  Filament voltage is not a safety issue per se.  Each tube type will want a minimum filament voltage to avoid tube damage.  Lower filament voltage may also affect the tube's output.  E.g., 6V heater tubes want at least 5V.  So make sure not to use a variac in such a way as to drop heater voltage too low, below the tube's operating threshold. 


Grounding is another issue.  Some old EICO VTVM's such as the venerable 221 purposely use 2 prong power cords & isolate the unit from chassis-earth ground.  Otherwise, use of the ground (common or black) test lead will make a short circuit (as with, say the Heathkit IM-18).  I agree the EICO 221 with its metal body is dangerous in the event of a chassis ground fault.


Anyway, using the old equipment : KNOW YOUR GROUNDING SCHEME.


BTW:  I've collected all the manuals for my vintage test equipment.  They contain a lot of info & are very informative; they are repositories of old knowledge.  Especially good 'cause I missed-out on this info in its day.

EDIT:  re grounding: a variac will not make a guitar amp unsafe.  The PT offers isolation of the chassis from earth ground.  If the chassis is "live" (with relation to earth ground), due to an internal fault OR in the old "Westinghouse" design w/o a PT, then the amp is unsafe to begin with.  The variac will not make it more unsafe.  The problem is the mistaken belief that a variac provides isolation, which it does not.  A variac is an "autotransformer" -- it has only one coil -- not a primary which is physically isolated from the secondary like the typical trannie. 

[TubeGeek]

Would it be recommended to use an iso-transformer from the wall voltage and then to the variac and finally to the guitar amp?


I think I need to do a little reading up   I purchased an iso-transformer and I am trying to figure out exactly when I should use it.

[jjasilli]

IMHO an isolation transformer is a good idea.  (Sometimes an isolation transformer & variac come together in one package).


Why?  Because, without you're realizing it, a metal chassis might be live with respect to earth or house ground. 


How could this be?  i) some old equipment, like lower power amps and radios might not use a PT; the chassis might be live;  ii) the presence of a EDIT:  OPT should eliminate this problem.  But if there is a ground fault, like a short to ground, the chassis might be live anyway, despite the PT.  If you touch a live chassis you may get a shock, maybe a bad one.  If you're also in contact with earth or house ground, the consequences can be much worse.


If you don't already have it you can google & download the "Electric Guitar Amplifier Handbook" by Jack Darr.  p. 47 shows a method to always test the amp chassis.

[PRR]

> the presence of a OPT should eliminate this problem.

This should say "PT", Power Transformer. I bet that is what you typed; blame the internet for the garble.

Modern gear "should" be well isolated already. But modern gear should just work, so why are you messing with it? Usually because something is wrong. A line-chassis short can be part of that wrongness.

1960s gear with line capacitors are leaky at best, and by now many of those caps are shorted. I would assume the chassis is nasty until proven otherwise (in ALL positions of any grounding switch).

The super-cheap gear around 1960 has NO Power Transformer and has some or a lot of wall-leakage. Working with these needs special care. Ultimately they MUST be re-built with built-in power transformer, but they are hardly worth the expense, and you need a jury-rig for testing to see if their crap "tone" is worthy of the investment.

[Lectroid]

If you don't already have it you can google & download the "Electric Guitar Amplifier Handbook" by Jack Darr.  p. 47 shows a method to always test the amp chassis.

Any body reading this for current limiter/power supply tips, this test works.  Note, in the 4th edition of Darr's book, it's on pg. 52.