Are these tubes cooked?

[Mike L.]

Hello all,

A few years ago, I obtained two amplifiers from Magnavox console record players circa 1958. They each use a single 12AX7 and four 6V6GT's. When I received the units, the tubes were packed in separate boxes so I never examined them.

Well, I finally finished replacing the resistors and capacitors in one of the units and plugged in the tubes. Very curious tubes. Four of the 6V6's are Tung-Sol with clear glass so that you can easily examine the internal structure. The other four are "Magnavox" brand and look to be coated with soot on the inside of the tube. Much like a glass door on a wood stove. The getter is still silvery and it looks like there are some parallel scratches on the internal coating. Is this soot a sign of failure, or was it done during manufacture to obscure the true source? Or was there some performance enhancement associated with this practice?

Thanks.
Mike L.

[rcavictim]

That grey sooty looking coting inside the glass is put there delberately to absorb and not re-emit secondary stray electrons that manage to escape the anode and hit the glass. These electrons are the cause of the blue fluorescent glow on the inside of the glass sometimes seen in power tubes. The result of preventing the stray electrons from coming back and hitting the anode is a superior performance tube.

I prefer the aesthetics however of an attrative showcase clear glass bottle than the greyish carbon coated envelope.

[Mike L.]

Thanks very much for the explanation. I fired the complete amp up this evening. No smoke just music.

Mike L.

[rcavictim]

Mike,

Great to hear you have a nice vintage pair of amps to enjoy! Is each amp a monoblock using four 6V6's per channel, or do you have two identical stereo amps with two 6V6's per channel? That would be especially sweet if you have a pair of identical four ouput tube monoblocks.

It is best to ramp up the mains voltage applied to vintage tube electronics over a period of hours to days when power is applied after many years in storage. This is done to allow the electrolytic capacitors to re-form their insulating layer inside which results from an electrochemical process. Sometimes you get lucky and the electrolytic capacitors manage to survive the shock of full power revival. Check the temperature of your can electrolytic(s). If they get warm you may have a problem brewing. Good electrolytics will run cold except for heat that may heat them coming from nearby tubes. Hum coming from the speaker, especially it it appears to get worse as you use the amp will be a clue that your capacitors got toasted and will need to be replaced.

The device of choice for a soft start and ramp up is called a variac, or variable autotransformer. One can create a useable substitute by wiring a standard incandescent lamp socket in series with the power cord and operating with light bulbs (the old kind with a tungsten filament, not the new spiral fluorescents) installed in this lamp socket. Start with a 40 watt lamp and then advance to a 60 watt, then 100 watt allowing a period of time running with each lamp in place. If you have the time to spend a full 12 -24 hours per lamp step will really give the electrolytics a chance to 'reform'.

Even if you have run your amps at full line voltage for a few hours without reforming the caps it is not necessarily too late to do this recovery step provoded the cap is not making hum or getting very warm. The trouble taken has a good chance of increasing the performance level and reliability of your vintage amps. In this case I'd probably begin with a 60 watt lamp for a day and then go with the 100 watt for a couple of days before resorting to full line voltage after that. You can listen to the amps at low volume during this forming operation.

I recently brought a pair of vintage 6L6 amps back to life and had trouble with the electrolytic cap in one heating abnormally. The previous owner had unwittingly powered them up at full mains without a soft start procedure. I was able to save the stressed electrolytic caps by running a regieme of reduced input voltage. I started with 50 volts and increased the mains voltage by 10 volts per step over a five day period until I reached 120 volts. Less than 50 volts was not enough for the 5U4 rectifier to get enough cathode heat (emission) to make some useful B+. I used a variac. After this forming operation my electrolytic can caps both now run stone cold and the amps run very well, and run nice and cool.

Please forgive me if you are aware of all this, perhaps a newbie will read this and be helped by the info presented.

[kevinkr]

Originally posted by rcavictim
That grey sooty looking coting inside the glass is put there delberately to absorb and not re-emit secondary stray electrons that manage to escape the anode and hit the glass. These electrons are the cause of the blue fluorescent glow on the inside of the glass sometimes seen in power tubes. The result of preventing the stray electrons from coming back and hitting the anode is a superior performance tube.

I prefer the aesthetics however of an attrative showcase clear glass bottle than the greyish carbon coated envelope.

Many also regard these "sooty" (both gray and black) tubes as being more desirable than their uncoated brethren. Look at what coated 6SN7 and 6V6 go for versus the uncoated types go for on eBay, there can be quite a premium.. In the case of the 6SN7 this treatment tended to be limited to earlier vintage versions, and they can command astronomical prices as a result.

[Mike L.]

Bob,

I have already replaced all the resistors and capacitors in the amps. I figured that after fifty years the electrolytics were probably toast and a friend of mine at work who was an electronics tech in the army 40+ years ago recommended the resistor change out. I retained the power transformer, output transformer, choke, sockets and chassis. They are monophonic amplifiers with four output tubes paralleled.

As far as the slow power-up goes, I don't have a Variac so it's usually a smoke test. I like the idea with the lightbulbs. You could even rig up a stepped attenuator using several sets of bulbs and a multi-position switch. That would be a good article for AudioXpress........

Thanks for the tip.

Mike L.

[Geek]

Hi Mike,

Ebay the Tung-Sol's and buy two more Magnavox. Then treat yourself to a steak dinner with the leftover money

Hehehehe

Cheers!

[kevinkr]

Originally posted by Mike L.
Bob,
<snip>
I like the idea with the lightbulbs. You could even rig up a stepped attenuator using several sets of bulbs and a multi-position switch. That would be a good article for AudioXpress........

Thanks for the tip.

Mike L.

Already been published in AudioXpress as of October this year.. I've been using ballast lamps for over 20yrs now and there are many instances where they provide far more protection than a variac. Unlike a variac there is a well defined upper limit to the fault current that can flow and this has saved me more than one power transformer over that time, as well as countless smoke shows I could really do without..

[rcavictim]

Originally posted by kevinkr


Already been published in AudioXpress as of October this year.. I've been using ballast lamps for over 20yrs now and there are many instances where they provide far more protection than a variac. Unlike a variac there is a well defined upper limit to the fault current that can flow and this has saved me more than one power transformer over that time, as well as countless smoke shows I could really do without..

Good point about the upper current limit provided by the light bulb in series! I guess a variac AND a selectable light bulb bank in series with the device under test would be a good idea. That would have saved the pwr xfmer in a vintage TV that I had a variac on. Talk about smoke and smell.

See, it's a floor polish AND a desert topping!