Powering up some very old high powered tube amplifiers

I have three very old high powered tube amplifiers that were originally designed to be top of the line amplifiers for theaters. They are all identical, and were rack mounted, but I build cases fro them. Back in my younger days, in the late 1960's, I used three of these amps for my stereo. Left channel, right channel, and delayed center channel using a spring reverb. I built some huge speaker towers, and ended up with a stereo capable of blowing the windows out of a house, which I proved back in my wilder days. Either way, this system had better sound than any commercial stereo.

Anyhow, I have had these speakers and the amps in storage since the late 70's. I decided it was time to bring this system back to life and brought it all home. Other than some dust and mildew, and a mouse nest in the cabinet of one of the amps, (which I removed), everything appears to be ok.

I want to power up these amps, one at a time. However, I'm a little concerned about the filter capacitors. I did replace all of them in the early 70's. They were still available back then, and they were very unusual caps because they had an Octal plug in base. I'm almost sure these can no longer be bought.

Fianlly, I want to power them up without a chance of a bad filter cap doing any damage. Is there any way to power them up without applying full voltage at first? Or should I just add a few fuses ahead of the filter caps? I should note that these amps have a main power line fuse and each of the four output tubes also has a fuse and a neon light to determine output tube faults.

I know someone will ask what brand they are. I do not know..... There are no numbers of brand names on them. However, they have about

40 lbs of transformers on each amp, and that is why they always had an awesome sound spectrum.
Reply to
jw
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This type of message belongs on news:sci.electronics.repair, or a vintage audio group.

The first thing you need to do is replace any paper coupling capacitors. They didn't age well, and develop leakage which will affect the tube bias. If you don't, you can expect to do lot of repairs, or need new power transformers from excessive current caused by leaky capacitors. If you intend to keep using the amps, just replace the electrolytics.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

Some folks reform electrolytics by operating the equipment at reduced line voltage. The cool way to do this is with a variac, but you can also power it up in series with a light bulb or three to drop the voltage and provide some current limiting.

There's a body of knowledge on this out on the web: try searching on "restoring tube amplifier" and/or "restoring tube radios".

My favorite source for stuff like your capacitors is Antique Radio Supply

-- it has been a couple of years since I've dealt with them, but they were straight-up folks when I did.

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--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim

The answer for electrolytic caps is to raise the applied voltage very slowly from 0 to full over an hour or so. If the rectifier is solid state this can be done with a variac. Either way it can be done with a variable dc psu.

If it does have paper coupling caps, chances are theyre beyond repair. Having said that I've not tried cooking them at above 100C - but with good reason. Many paper caps were impregnated with a toxic oil that should not be handled, so if you restuff old cap cases beware.

NT

Reply to
NT

The rectifiers are 5U4 tubes. I do like the idea of using light bulbs in series with the line cord. At the same time. since these 'lytic caps are plugin types, I may just apply a voltage to them using another external power supply, and see if they hold up.

Awhile back I got an old table top tube radio, plugged it in, and immediately heard 60hz hum from the speaker. I left the radio on and after awhile the hum almost vanished. I may be wrong, but it seems to me that they "heal" after some use, (assuming they are not shorted). Of course that radio is low powered in comparison (and should get new 'lytic caps anyhow).

As far as the paper coupling caps in the amps, they would be easy to replace, since they are available. The 'lytics wont be easy to get. Question on the paper caps though. What kind of caps are used as repalcements by those who rebuild these old amps or other electronic gear? Obviously they have to be the correct capacity and voltage, but there are so many choices. I know the ceramic types are stable, but maybe they wont work properly. So what is recommended? The "Orange drop" ones are similar, and are wound with some sort of plastic, not paper?

I never knew there was anything toxic in the old paper caps? They were just coated with wax, and I used to get that stuff all over my hands when I was younger and would solder them in. On the other hand, I recall some of the old stuff I worked on when I was in my teens contained 'lytics with oil inside, and several times I had them blow and shoot hot oil all over the place. Now I find out that was PCB oil, (dangerous shit). Of course these days we will all die from stress because of all the "deadly" things we are scared about due to health fanatics and medical people who want our money.......

Considering all the old electronic gear I played with when I was young, I should have died 30 years ago...... and all that lead solder..... make that 40 years ago.....

Thanks

Reply to
jw

You should run the rectifier filaments with the nominal voltage. Undervoltage with load on can lead to point emission and filament pitting with associated risk of burning it out.

If you replace the rectifier tubes with silicon diodes, please remember to add some series resistance to limit the capacitor inrush current.

I'd (temporarily) replace the 5U4's with silicon diodes and series resistors, pick out the other tubes and carefully feed the thing with a Variac, as the other posters have suggested.

You were lucky - the caps could as well throw up the electrolyte and the other insides.

I have once seen this happen. An old Telequipment scope quit working, as the HV rectifier tube slowly died. We replaced it with some 1N4007's, and got a beautiful sharp picture. There was also a pffffft sound, which quickly turned into a loud boom, and the room was filled with confetti.

Unburned PCB oil is not very toxic. Its main problem is that it is chemically very robust and dissolves readily into oils and fats. If you succeed in burning it, then it is a different story, as it will lead to dioxins, which are toxic. For more information, Google for 'Polychlorinated biphenyl'.

So do I, but I'm still alive after over 50 years of tinkering with electronics.

--

Tauno Voipio
Reply to
Tauno Voipio

better to fix the lytics by reforming. A series bulb only reduces the damage risks in the set.

it could kill them. Wind the v up very slowly, or apply full V via a high value R so leakage is small.

yes, sometimes

any type works, bar electrolytic or tant.

the outside was wax, inside was pcb oil, soaked in the paper to reduce damp susceptibility and increase capacitance.

NT

Reply to
NT
[...]

That is the way to go. But you need some resistance up front. Depending on the voltage maybe a few 120V incandescent light bulbs in series. And increase the voltage slooooowly.

If one of them blows and you really want to keep this whole amp at least authentic from its looks you may be able to open the cans and then mount a new cap inside. New ones are more modern in technology and thus much smaller for a given capacitance/voltage. Did that once on a cap that had a nice logo, was from an Italian motorcycle brand. Of course the stuff from inside needs to be disposed of properly and safely.

Sometimes they heal themselves, other times ... *PHOOMP*

Hardcore audio fans use film caps, never ceramics.

Surprisingly often they'll still be fine. But if you replace them just make sure to dispose of the old ones at the electronic waste collection. Ours is really easy, it's behind the next supermarket. The guys never minded when I came with my bag of parts that I had unsoldered over the last few months.

Most of us will die from very unrelated causes. Too many cheeseburgers, for example.

Nah. All the exercise from getting the old discarded TV sets back home over five miles on my bicycle was probably enough to offset that :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Society's do-gooders have panic attacks when there are trace levels of "contaminants" that only a few decades ago, were below the threshold of detection. (i.e., a ppm used to be safe - now it's ppb or ppt!)

May the Great Roulette Wheel in the Sky save us from the hypersensitive!

Hope This Helps! Rich

Reply to
Rich Grise

Damn, that brings back memories. I used to do the same thing. Tie a coaster wagon to my bike and haul home old electronic gear. Not to mention used lumber and stuff to build shelves and a bench for my workshop, and of course speaker cabinets. Those old console tv set cabinets made great cabinets for part storage too.

Reply to
jw

The caps probably reformed OK, but it's better to do it gently.

If you don't have a DC supply handy, you can put a resistor (or light bulbs in series to make 120V/bulb) between the rectifier tubes and the filter caps, and let that gently bring things up to voltage. The whole thing about reforming caps is that you want the applied voltage to go up without the current ever getting high enough to damage anything.

Pulling the tubes will probably make the circuit pull a lot less current, but you'd have to review the schematic to be sure.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim

Caution about pulling the tubes... some may be shunt regulator types. My famous incident involved an audio amp kept in a (dark) closet... turned it off when I left on vacation.

Flipped it on when I returned... kablooie! Multiple electrolytics lost because the shunt tubes didn't strike.

Removing tubes may have the same effect, so watch the applied voltages. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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Actually most of the PCBs were in transformer oils. Infrequent in capacitors under 1 kV, and not in electrolytics (wrong properties). Most of what you are likely to have come around would be in fluorescent ballasts (if you tore them apart).

Reply to
josephkk

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Reply to
josephkk

With old 'lytics, it's not a question of them holding up. Reforming doesn't make a working cap hold up longer, but it may change a cap that would otherwise fail when power is applied, to a cap that will work when normal power is applied to the equipment.

Your plan to pull the caps out of the equipment to do the reforming is the right approach. With vacuum tube rectifiers, it is impractical to reduce the voltage to the equipment to do the reforming.

Ed

Reply to
ehsjr

'Pulling the tubes' will raise the voltage across some of the electrolytics, and may cause damage. Electrolytics are reformed at low current. Usually through a high value resistor from a 20% or so higher than rated voltage. Then you monitor the current to see that it reforms properly. The current will drop as the capacitor reforms.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

The caps that exploded and spewed oil were on very old tube systems. They were metal can electrolytics that had a tiny hole in the center of the top of them. (probably a vent). More than once those things blew on me, spewing hot oil out of the hole, and at least once blowing the whole capacitor apart. After having this happen a few times, I was always nervous when I powered up any 'lytics with the small holes on top. I learned to put some sort of cover over them just in case.

I was working on this stuff in the late 60's early 70's and I bet these devices were 30 years old at that time......

I have never learned what these caps were called (other than being electrolytics) but a specific oil filled type, whose name I still do not know.

All I know is that they blew up much more often than any other type of capacitors.

Reply to
jw

Hmmmmm, now I'm a bit confused. First you (amd others) say to apply the voltage slowly (reduce voltage at first). Now you are saying "With vacuum tube rectifiers, it is impractical to reduce the voltage to the equipment". Why is it different for tubes than it would be for solid state rectifiers? I have heard more than once about rectifier tubes burning out from bad caps.....

While I'm here, I have been thinking. It's been years since I worked on these amplifiers, but I recall there was a pretty hefty voltage coming off the rectifiers (around 500v I believe). To reform the caps, do I need to go that high with the voltage to reform the caps, or would a 300v supply (for example) do the trick? I only ask this because I have several old (tube type) tv transformers sitting aroumd, to make up something, but they rarely went above 350v or so.

Reply to
jw

Its trivial to apply reduced dc V and wind it up veyr slowly.

With ss rects, psu V out tracks mains V, so youc an use a variac to reform the machine. With a valve rect, it doesnt conduct at all until mains v is quite high, so a variac wont work, you need to apply variable dc to the dc hv bit of the cct.

Limited current is required. There have been recommendations to use a series bulb, but this provides way too much current.

A shorting lytic means excess current which kills the rect tube.

yes

no

Use a voltage doubler. Or just 2 diodes to give +- 350v, but then you get a live chassis.

NT

Reply to
NT

It's the filaments. Generally, in those days, they had BMF power transformers with a 6.3V winding for most of the tubes, and a 5V winding for the rectifier tubes. If you turn this down, the tube filaments won't reach operating temperature, which will affect their emission, and could be hard on the tube itself. If you can run the filaments at their rated voltages, then you can put the other voltages wherever you want them. (except over the max, of course.)

Hope This Helps! Rich

Reply to
Rich Grise

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