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Old 6th May 2013, 01:45 PM   #1
Barclay is offline Barclay  United States
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Default Can I use a variable voltage cieling fan switch as a variac?

I have an old tube amp that I am just starting up for the first time today.
Can I use a variable cieling fan switch for a variac?
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Old 6th May 2013, 02:29 PM   #2
TheGimp is offline TheGimp  United States
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Make sure the grounds are properly connected, line and neutral are correct, and the control has sufficient rating (most likely 600W or better).

Stick a 100W incandescent light bulb in series with it.

The TRIAC control may generate a lot of noise due to it's switching on at various phases of the line frequency, however for initial testing this shouldn't be a problem.
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Old 6th May 2013, 03:38 PM   #3
Barclay is offline Barclay  United States
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Can I use a 3-pole dimmer switch and a rough in light fixture?
The amp is not grounded. I know in the old amps where to tube voltage = line voltage trying to simply ground the chasis will blow it up. But this one is not like that.
Here is a sketch:
Click the image to open in full size.
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Old 6th May 2013, 04:11 PM   #4
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Skip the dimmer - the light bulb will limit the current to less than 1A. Start with tubes pulled - bulb shouldn't light. Check for expected voltages. If there's a solid state rectifier, you can check B+ and /or bias. Rectifier tube next, if used. Depending on the size of the amp, you may need a larger size than 100W to run it with all tubes installed.
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Old 6th May 2013, 04:21 PM   #5
Barclay is offline Barclay  United States
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I know this sounds like i'm being lazy, but I'm really just pressed for time.
It would take me time to figure out what pins to test, plus I would need time to run to harbor frieght for a multimeter.
Would I just be testing that voltage is going to each tube?
The one I have is an automtive and I really don't know how to use it anyway.
One person suggested wiring in a light socket before the amp (no dimmer)and starting with a 10 watt bulb for a few minutes, then a 25 watt bulb then a 50 watt then 75 then 100 then 150 up to 300, ( I don't know where I would get a 300 watt bulb)
This is an AMI DD jukebox amp.
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Old 6th May 2013, 09:31 PM   #6
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Quote:
Originally Posted by Barclay View Post
I know this sounds like i'm being lazy, but I'm really just pressed for time.
It would take me time to figure out what pins to test, plus I would need time to run to harbor frieght for a multimeter.
Would I just be testing that voltage is going to each tube?
The one I have is an automtive and I really don't know how to use it anyway.
One person suggested wiring in a light socket before the amp (no dimmer)and starting with a 10 watt bulb for a few minutes, then a 25 watt bulb then a 50 watt then 75 then 100 then 150 up to 300, ( I don't know where I would get a 300 watt bulb)
This is an AMI DD jukebox amp.
The lightbulb in series with the AC mains (I use a 25W bulb for this test) is only a protection scheme. It's not a substitute for measuring the voltages.

The multimeter is your friend! Learning to properly use one is not difficult, and is essential to checking out any piece of electronics.

Recommended action is to go to Harbor Freight and get at least 3 of their cheap red DVMs. Also pick up some clip-leads so you can safely clip the DVM into the circuit-under-test (you don't really want to be balancing multiple sets of probes inside a live amp, IMHO).

At each step, hook up the DVM(s) to the points you want to test, step back from the amp, and switch power on through a power strip. After the first couple of tests, you really don't need the light-bulb - if it glows brightly when you first power-on (with no tubes installed) there's a short somewhere that's drawing power. The light bulb should not light - that is the purpose of that particular test.

BTW - if you're lucky enough to have a Harbor Freight nearby, take advantage of it. I had to order my stuff from them and they send it FedEx, but the "cheap" way. I watched my packages bounce around the country (via the tracking number) for a week and a half - if they'd sent it USPS Priority for the same $$, I would have had it in a couple of days...

Privatization is not what it's cracked up to be...

Best luck with your project

Sam
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Last edited by rfengineer2013; 6th May 2013 at 09:35 PM.
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Old 6th May 2013, 09:47 PM   #7
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Quote:
Originally Posted by Barclay View Post
I have an old tube amp that I am just starting up for the first time today.
Can I use a variable cieling fan switch for a variac?
No. Definately not.
A variac does not switch it controls the AC voltage.
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Old 6th May 2013, 09:52 PM   #8
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Quote:
Originally Posted by Barclay View Post
Here is a sketch:
Click the image to open in full size.
If I'm not mistaken your sketch is wrong in that you have both live and neutral wires going into the bulb. It should only have the live feed which goes through the bulb. the way you show it it is always going to light up.
I found this sketch which shows how it should be done. It is from Powering Up Your Radio Safely With a Dim-bulb Tester and I hope the owner doesn't mind me borrowing it for this purpose. If so I will remove the link, I think the writer does also use this forum so apologies if I am out of order here.

Click the image to open in full size.
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Old 6th May 2013, 11:19 PM   #9
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Don't use a lamp dimmer for a transformer powered , radio or amp . Th is is the third forum you have asked this on
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Old 7th May 2013, 12:06 AM   #10
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Quote:
Originally Posted by battradio View Post
Don't use a lamp dimmer for a transformer powered , radio or amp
I would second that motion.

A light dimmer is a solid-state switch (usually a triac) that turns off the AC waveform on a cycle-by-cycle basis at a voltage set by the slider-control. It chops the AC off in mid-cycle, doing pretty horrible things to the smooth AC sine wave that your transformer is expecting to see.

The dimmer is designed to work into a resistive load (a bulb), not a reactive load (a transformer).

A variac, on the other hand, is a variable transformer that one uses to adjust the AC voltage level. The variac only affects the amplitude of the AC, it doesn't affect the shape of the AC waveform itself. A variac is the proper tool for this job if you have one handy.

Having said that, lots of guys here bring up new amplifiers all the time without benefit of a variac. It's all in following some simple rules regarding testing - they're outlined several places here and on the interwebz (sorry, don't have the links handy).

With patience and willingness, you can learn to do this safely.

EDIT - BTW, being pressed for time while checking out tube amps is not a good idea. The dangers of tube-amp innards are quite real - voltages and currents that can be lethal. Testing tube circuits requires, ABSOLUTELY requires, Patience and Time.

Once you've mastered some of the basics, you can become efficient but NEVER become "in a hurry" around these things.
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Last edited by rfengineer2013; 7th May 2013 at 12:12 AM.
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