• 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.

Very old tube-amp!

My neighbour has got a seemingly old tube -amp, that has not been in use for 30 years or so.
Is it possible that it will spring to life after just standing around for 30 years? Or doesn't the tubes last that long?

I haven't got a picture but it appears to be complete, nothing broken. It reads Quad-Amp on the side of it.

Best Regards,
Christian V
 
Christian:
There should be no problems with the tubes as they can last a very long time.
But You should be careful when powering it up the first time again after such a long time. The problem are not the tubes itself but more likely the electrolytic capacitors in the power supply which may be dried out.
I strongly recommend using a VARIAC (variable transformer) for very slowly raising the line voltage to the nominated value while closely watching for smoke or strange smell.
Even better at all, if You can check the capacitors individually before power up.
I´m not a tube expert but I guess that a vintage Quad tube amp in original shape is worth quite some money and as it appears that You (he) are not so familiar with this kind of stuff, before powering it up by Yourself, it might be better to consult a person who really knows what he is doing.
Also one of the tube gurus around here could give You more detailled instructions of what to do.

Christoph
 

Brett

Member
2002-01-07 6:02 pm
It should be fine. I have a friend who just bought some NOS tubes made in the 30's and tested to spec. I have some NOS JAN 807's with a date on them of 1941, and they tested new.

Christoph has give good advice about bringing it up slowly on a variac. It'll stop the electro caps exploding if thet've dried out too much. However depending on the exact model, some Quads used oil filled caps which should be OK. But still use the variac.

Here's a Quad enthusiast page with some info on the older amps. Might help you identify it.

Old Quad amps in original or restored condition can bring quite high prices especially in Asia. The II and 22 are highly regarded.

http://www.geocities.com/ResearchTriangle/Lab/6722/info.html
 
An old amp does not necessarily blow up on power-up. But it can and while electrolytics may have dried out to 90%. the remaining moisture still can cause them to explode and to spill their content over you, your workbench, scope, whatever. A real mess. Been there ...

Methinks Christoph has given very good advice ... so do i even with every device i am not completey sure it works (because it worked just yesterday) .. use variac on 1st power-up
 

MRehorst

Member
2002-05-17 8:48 pm
Electrolytics are the biggest problem, followed by paper/wax caps (if it is THAT old).

Don't bother with a variac- it doesn't work on tube equipment. The rectifier tubes don't conduct until the voltage gets quite high anyway. Then if the electrolytics are shorted, they'll explode just like there was no variac on the line.

Before powering anything up, use your eyes and nose to look for burned transformers/components. If anything has a burned smell, it will have to be replaced before you apply any power.

Assuming there are no burned-up parts, what you should do is replace ALL electrolytic capacitors before you even consider powering up the amp. Also look for paper insulated caps (usually coated with beeswax) and replace them with modern polypropylene or polyester film caps.

Once all that is done, get some electronic contact cleaner and spray pots and switches (and rotate them back and forth a few times while doing so).

Now power it up and it will probably work like new or even better because of the improved capacitors.

The small signal tubes will probably be OK. Output tubes should probably be tested and replaced if they are out of range by much (how much? That's up to you to decide.)

If you replace or even scramble to output tubes you should readjust the bias. That will require at least a voltmeter, but usually requires an oscilloscope and distortion analzer for optimal setting. Don't worry about it too much though. Every minute that the amp operates drains away some of the performance of the tubes, so no matter how precisely you adjust it, in a few weeks you'll be back to where you started. There is a reason why they abandoned tubes years ago in favor of solid state devices ;)

MR
 
A variac is a must with tube equipment. There is a method to it and done properly takes 2 days. Tube equipment uses very high voltages and if you do not reform the capacitors it can cause a hum that ruins them even if they were not completely trashed before. As far as replacing anything in a Quad I recommend you don't touch it. They can be worth quite a lot of money and one not even tested at all is worth more than one modified. One that has been gone through by a qualified tech and is in good working order is worth even more. There is a reason quality tube equipment that is even 50 years or older is still sought after. It sounds good. It was made to last a lifetime,which it has. Parts will fail but the good stuff fixed is still good. No reason to worry about bias or anything like that untill you learn a bit first, but learn on something other than a Quad. My Dad still talks about hearing a Quad system for his first time in 1962.
 
I also agree with use of a VARIAC, it is a gold mine piece of equipment to have aroud in this situation!! Also, I wouldn't replace any caps yet since the amp would be worth a hefty sum if it's in working condition with orginal parts!

What thatch_ear means by the 2 day thing is to leave the amp on the variac for 2 days, sometimes more, and very slowly each day, gradually turn up the variac output into the amp.. This really helps to reform caps and can make the difference between an atomic cap explosion(this does happen) and a working amp!

note: When I was ten years old, I got a Dynaco MKIII 60watt amp with original KT88s from genalex(beautiful tubes!!), but, I was unfortunately very inexperienced about audio electronics. So, I believed I could just plug it in and expect it to work after sitting out in a shed for 30 years. BIG MISTAKE, the power cap blew up and left oil splattered all over my room/speakers/tv/carpet/myself, and broken tubes and schrapnel
stuck into everything that was within 5 feet of the amp!:eek:

I do however still have the amp, trust me, I never gave up. But now i got it working and it's a wonderful little monoblock/spaceheater:)

So, take my word and really that of Thatch, cause he knows what he's talkin about if ya' know what I mean;) and I hope the best of luck with that Quad, it's a nice amp, whatever model or condition. Much nicer than my Dynaco mkIII!~


And MRehorst: No offence, but you might want to study up on your tubes:cool:
 
Not everyone has a variac in their back pocket to gradually wake a slumbering vacuum tube beauty. Not even me now since my little 1 amp job bit the dust when a circlotron type bench prototype amp went somewhat stroppy and pulled way too much current and drew an arc where the carbon brush meets the winding. :( Hexfets survived just fine though.

Anyway.... if you are short of a variac then I'm sure you could pull out all the tubes except the rectifier and put a high value resistor in line with the rectifier output before it gets to anything else so the current will be limited to just 1 or 2 milliamps. Then just turn it on and let the thing sit and simmer for maybe a week? A digital voltmeter that has a 10 megohm input resistance (most are) connected across the main HT line would show the dc voltage coming up as the capacitor leakage current gradually comes down to normal. 200k to 300k worth of resistor should be about right. Two 1 watt resistors in series would be enough. I have never actually done this, so has anyone had any experience doing it like this?

Also, when large electric motors have been left lying idle for long periods of time it is not uncommon for their insulation resistance to fall to a low level, sometimes to the point that the motor will fail at or shortly after switchon. The usual thing to do before it is powered up is to connect a dc power supply across the windings and shove enough current through it that the windings gradually heat up and drive out any moisture that may have accumulated.

For an absolutely reliable startup of your amplifier after a very long time doing nothing collecting dust and moisture it *might* be a good idea to measure the power tranny primary winding resistance and calculate the dc voltage required to dissipate 10 to 20 watts inside the tranny or whatever is needed to gradually heat it up to about as warm as you can touch for an hour or so. Same goes for the output tranny, although fully encapsulated ones that are potted in resin wouldn't really need this.

GP.