• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Smells like a mistake

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I just finished building my first amplifier (that's not from a kit). It is a EL-84, 5Y3-GT, 6SN7. I fired it up and everything seemed fine. Dead quiet with the volume all the way down. I turned up the volume a bit and as the tubes lit up music started to come out (sounded good)and then, after about 30 seconds, smoke came out too. It appears that the smoke is coming from the power transformer. I suspect my B+ voltage is too high, as the output tubes seem too hot (REALLY GLOWING), could this cause the transformer to be over-taxed and smoke? When building the amp power supply I was worried that the B+ might be too high. It seems like it can't be anything too major as the amp works (amplifies music). I know everyone will be wanting measurements. I was just wondering if the B+ theory sounded likely off the top of my head. Thanks.
Remove all the tubes first!!!!!!!!! Then, power it up and check all the AC voltages are correct. Put the rectifier tube and check the B+ and the -ve grid bias voltage of the power tube. If the -ve grid bias volt set too low, surely the power tube would take very high current which would damage both the tube and power transformer.

Hope you could salvage your amp.

Start up proceedure

You mean you fired it up without surrounding it with all your test gear?

Well, I had my little voltmeter out. But I don't think it is going to be doing any "surrounding". I could cover one end of the amplifier in a sort of half-pincer movement I guess. Obviously we have access to different levels of money and equipment. I will remove the tubes and follow kmtang's suggestions.

Question: I should have a load (speakers) hooked up? I thought I read that you have to have a load.

Also: I searched the forum for recommended proceedures for starting up an amplifier for the first time...I obviously missed some information that is out there. If someone can recommend a source for info about this please do so. Thanks for the comments and help.
Joined 2003
Yes, I now have lots of kit. But once upon a time, I was an impecunious student, so I was really cautious. The first test is to apply power without the rectifier in place to make sure that all the valve heaters light up. Obviously, you've done that. The next is to fire the amplifier up upside down with the rectifier in and with the meter connected across the HT to check that it's what you expected. If it is, you move it around and check every other voltage you can think of. If it isn't, you switch off hurriedly, go away, and sulk. Then you come back and think hard about what might have caused the problem. Having lots of test gear simply allows you to take all the measurements simultaneously.

Oh, and it's a good idea to have a loudspeaker connected so that you can listen for hum etc.


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> I was just wondering if the B+ theory sounded likely off the top of my head.

You have presented no facts to support this top-of-head notion. WHat made you think the B+ might be wrong?

And if you had doubts, you "should" have got some clip-leads (do NOT hand-hold probes in amplifiers) and watched the worrisome voltage as the amp came up the first time. And in tube-amps, what kmtang says is generally right: start without tubes and check that the AC voltages are about-right. (They will be 5%-20% high because of no load, but they should not be wildly wrong.)

Yes, I started with just a $20 meter (back when they were $5), but invested in a 10-pack of clip-leads after nearly electrocuting myself a few times.

> smoke came out

This is a serious problem. Smoked parts are sick parts. A smoked resistor may still "work" but be far off from its nominal value. A smoked power transformer is dangerous: the insulation has been cooked. The smoke-soot may insulate well enough that it hasn't shorted-out yet, but sooner or later it will. Hopefully so dramatically it pops a fuse, but possibly in a way that it runs but shocks anyone who touches the system.

After many smoke-tests, you learn what flavor smell means what type part. And then they change the varnish formulas and all the smells change. But if you saw smoke, and can not identify a cooked resistor or capacitor, treat the transformers with extreme distrust.

If I know an amp is stable and has zero signal into it, I test without "speaker" load. Any good audio amp, even tube, can stand no-load just fine, except when driven at large signal levels which kick-back the plate winding. But a new-build always gets a load, because Murphy says it WILL oscillate at maximum level.

Over-voltage alone will not smoke a transformer. That sounds, as SY says, like over-current. If the tubes were properly biased (large cathode resistor should be fine; fixed-bias can be tested before you put tubes in) then it was oscillating (yes, an amp can oscillate wildly and still pass signal).

I once had a similar glow. 8417 is a big tube and will take lots of heat in the plate. But one of mine turned orange in seconds. Static bias was fine, on this as well as the not-hot tube. But when powered-up, the cathode current quickly rose far-far above the design current. To get a better picture, I moved a wire to get my scope on the cathode resistor.... now it was cool. I moved the wire back: quick glow. It was the plate wire laying close to a wire-wound cathode resistor (none of that thin-film power resistor stuff those days). This formed a 1:10 transformer with correct phase to feedback in the radio band. I had made a self-driven radio transmitter, but without the usual self-adjusting bias or any load so it was putting 100 watts in a 35 watt plate.
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Better still use a variac and bring the voltage up gradually while checking that everything looks ok with a voltmeter. Used variacs can be had on eBay and elsewhere like hamfests for very short money. (frequently less than $10)

I also recommend the use of a ballast lamp in series with the primary of the power transformer. Typically this would be anywhere from a 25 W bulb to a 200W bulb depending on what you are powering.. The nice aspect of doing this is that if there is a dead short somewhere the current flowing through the primary is limited to that of the fully heated bulb, and damage to the unit under test will be quite limited at worst and none at all at best. The cold bulb has low resistance, as it heats up the resistance increases thereby limiting the current. Note that the bulb will limit the voltage to a normally working device and you can tell roughly by how much by the relative brightness of the bulb. The brighter the bulb the less voltage is dropping across the device being tested. Keep a range of bulbs with 60W/100W/200W typically being the most useful.

Go to your local home center, buy a lamp socket and an extension cord, wire it in series with the hot side of the cord and you are done.

Better still get a pair of double gang metal work boxes, a ceramic lamp socket such as used in a basement or attic, a duplex outlet, and a switch, some 16-3 cord, and a plug.. Connect hot side of plug (copper color screw) via cord's black wire to switch, wire switch to lamp and other side of lamp to hot side (copper color screw) of outlet in series, connect neutral of plug via white wire in cord to neutral of outlet (silver screw), ground of plug (green screw) via green (or green/yellow wire) to ground of outlet. (green screw)

Frankly I don't even recommend direct connection the first time around without the tubes installed. Use the lamp!

Note that you can plug the lamp into the output of the variac for additional protection.

If you think the transformer was the source of smoke you need to replace it immediately! The only thing that burns in a power transformer is the varnish insulation on the wire followed by the paper between windings. It will eventually short out anyway.

:att'n: I guess you did not have a fuse in the primary wiring of the power transformer.

ALWAYS FUSE the primary! If you have a 100VA -150VA power transformer, typically a 2A - 3A medium blow type will provide enough protection without nuisance blowing due to inrush currents.

Smaller transformers require smaller fuses obviously, some transformer manufacturers will provide guidelines for their transformers.

Note that all being the same toroids generally need either larger fuses or slow blow types as compared to a conventional EI type of similar capacity due to their larger inrush currents.

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I always use a variac and turn the AC voltage about 90~100 VAC (for 117V supply) to check the high voltage level without the other tubes. Note that sometimes you can still overvoltage a cap in a series dropping string since some designs do not provide a bleeder resistor at the end of the chain.

Of course, the proper primary fuse will save a power trnasformer from drastic over current. Did the smoke have a heavy sweet smell [transformer] or more acrid smell [resistor likely]?

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