Powering up some very old high powered tube amplifiers

[[email protected]]

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.

 

[[email protected]]

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

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

 

[Tauno Voipio]

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.

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.

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).

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.

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.......

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'.

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.....

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

 

[Joerg]

[email protected] wrote:

[...]

... 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.

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.

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).

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

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?

Hardcore audio fans use film caps, never ceramics.

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. ...

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.

... 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.......

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

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.....

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

 

[MrTallyman]

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

Metallic form lead is NOT harmful, IDIOT!

It is COMPOUNDS of lead that can cause damage.

No solder joint or solder strand you EVER touched gleaned ANY lead into
your body. You fucking FUDTard!

 

[Rich Grise]

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

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

 

[[email protected]]

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

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.

 

[josephkk]

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.......

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).

 

[josephkk]

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.

Oil yes, pcb pretty much only in caps rated 1 kV and up, NOT a full
guarantee. How many of those caps do you unwind? The paper was a physical
separator and the oil was the dielectric, the fancy properties of pcb were
not usually warranted cost wise below 1kV.

 

[ehsjr]

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.

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

 

[[email protected]]

Oil yes, pcb pretty much only in caps rated 1 kV and up, NOT a full
guarantee. How many of those caps do you unwind? The paper was a physical
separator and the oil was the dielectric, the fancy properties of pcb were
not usually warranted cost wise below 1kV.

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.

 

[[email protected]]

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

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.

 

[Rich Grise]

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.....

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

 

[Rich Grise]

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.

Please don't feel like I'm being a smartass, but they were called "oil-
filled" caps.

Cheers!
Rich

 

[[email protected]]

Please don't feel like I'm being a smartass, but they were called "oil-
filled" caps.

Cheers!
Rich

Actually that's what I suspected they were called and I have found
references to that term online. But using google (images), most
appeared to be larger caps. Apparently these were the very early
'lytics used on the very old radios and other stuff.

They looked almost identical to the aluminum can 'lytics, except the
tops had slight indented circles (like a target) and that tiny hole in
the center.

 

[[email protected]]

Anyone have a schematic for a simple variable DC power supply I can
make from salvaged tv parts, to be used for reforming old 'lytic
capacitors from tube equipment? I have plenty of parts including
transformers from old tube type tv sets. Solid state Rectifiers
preferred.

Thanks for all help.

 

[josephkk]

Please don't feel like I'm being a smartass, but they were called "oil-
filled" caps.

Cheers!
Rich

Please remember that the electrolyte in electrolytic capacitors is rather
conductive, not an oil at all.

?-)

 

[ehsjr]

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?

The filaments, as Rich said. Tubes like your 5U4 have (and need)
them, solid state rectifiers don't. The full, or close to full
filament voltage must be applied to the tube for it to rectify.
If you reduce the filament voltage a little, the tube will operate
poorly. If you reduce voltage to the equipment far enough so
that the reforming voltage would be low enough, the tube won't
operate at all.

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.

Reforming to less than full cap voltage rating is a gamble. You
might get away with it, you might not. A 350v secondary would be
good for 450VDC caps. Rectified, it will provide peak DC of about
500 volts, so you'll be able to vary from low up to the cap's rating.

Have you figured out how you're going to control the voltage?

Ed

 

[josephkk]

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.

This is where it may get tricky. If you reformed only to 70% of rated
they might still go bang if roughly subjected to rated voltage. I suggest
reforming to 125% of rated, slowly. After all, they were formed to above
rated so that they would be reliable at rated.

?-)

 

[[email protected]]

Get an old power transformer that can give you about 400V after
rectification and a low power variable transformer (Variac). Add some
rectifiers to the HV secondary. Wire the Variac between the AC line and the
transformer primary. Add an On-Off switch and a fuse between the AC line
and the Variac.

Please view in a fixed-width font such as Courier.

Pwr
Variac Xfmr FWB
Fuse +---+ +---+ +---+
/ +---+ | | | | | |
---/ ----| |--| | -----| |--------|~ +|---------(+) output
+---+ | | | | | | |
| | | | | | |
| | | | |300V | |
AC Line | |<-| | |HV | |
| | | |Sec | |
| | Pri| | | |
| | | | | |
| | | | | |
----------------| |-------| |--------|~ -|---------(-) output
| | | | | |
+---| +---+ +---+

1Amp and 3Amp variacs are easily available, and should handle your circuit.

Your drawing came thru well. Thanks

Where might I get a variac?
I'm still trying to figure out what FWB means? I know that's the
rectifier, just dont understand those initials.

One other thing, to use this as a reformer, are filter caps put after
the rectifier on this device, or do I just feed the TEST cap directly?

Aside from the variac, I should have all the parts.