Hi Folks,

After years of wondering what happened to that cute little amp
of my youth, I found it in with my Lionel train stuff while cleaning
the garage. The newspaper it was wrapped in is from 1993, so I
know when I saw it last. Anyway, its a little PP 6V6 amp with a
12AX7 and rectifier tube (didn't pull to get number). The filter
cap is a Magnavox 450V, 30uf, 15uf, and couple of more sections.

My question is this: to reform the cap, should I pull out all
but the rectifier tube? I have a 10A variac I can use. Assuming
it has a fuse (don't think it does), and it holds, and no smoking,
how fast should I bring up the voltage? Given that the tube
won't conduct until the heater gets pretty warm, can I start
1/2 way to 120Vac input? Would it make sense to sub in SS diodes
during the process and bring it up more slowly?

Beyond that, the rest of the caps look like they could use
replacing. What decisions are to be made about the value of
restoring this thing?

BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
find any reference for it.

Thx, Chris

--
Chris Richmond | I don't speak for Intel & vise versa

: My question is this: to reform the cap, should I pull out all
: but the rectifier tube? I have a 10A variac I can use. Assuming
: it has a fuse (don't think it does), and it holds, and no smoking,
: how fast should I bring up the voltage? Given that the tube
: won't conduct until the heater gets pretty warm, can I start
: 1/2 way to 120Vac input? Would it make sense to sub in SS diodes
: during the process and bring it up more slowly?

It would be a better idea to very slowly bring up a dc voltage on the
filter cap, with everything just like it is in the circuit, just
un-plugged from the wall. I reform caps in circuit all the time. You want
the cap to draw no more than 15 to 25 milli amps while it's reforming.

In stubborn caps, I've taken days up to a week to reform one cap.

Then don't trust the cap to stay up. You would then want to slowly ramp it
back up after any extended off time. Chances are, the caps preformance
will not be great.

: Beyond that, the rest of the caps look like they could use
: replacing. What decisions are to be made about the value of
: restoring this thing?

If you can get the cap to reform and stay stable... you can leave it in
and possible add a newer - smaller same value cap in parallel with it.
Don't depend on the old cap. Sometimes I don't pull them out when people
are fussy about being all original.

: BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
: find any reference for it.
: Thx, Chris

A picture of it on the web would allow many of us to tell you who made the
amp.

cheers
skipp

www.radiowrench.com/sonic

"Skipp is here" wrote in message
...
: My question is this: to reform the cap, should I pull out all
: but the rectifier tube? I have a 10A variac I can use. Assuming
: it has a fuse (don't think it does), and it holds, and no smoking,
: how fast should I bring up the voltage? Given that the tube
: won't conduct until the heater gets pretty warm, can I start
: 1/2 way to 120Vac input? Would it make sense to sub in SS diodes
: during the process and bring it up more slowly?

It would be a better idea to very slowly bring up a dc voltage on the
filter cap, with everything just like it is in the circuit, just
un-plugged from the wall. I reform caps in circuit all the time. You want
the cap to draw no more than 15 to 25 milli amps while it's reforming.

In stubborn caps, I've taken days up to a week to reform one cap.

Then don't trust the cap to stay up. You would then want to slowly ramp it
back up after any extended off time. Chances are, the caps preformance
will not be great.

: Beyond that, the rest of the caps look like they could use
: replacing. What decisions are to be made about the value of
: restoring this thing?

If you can get the cap to reform and stay stable... you can leave it in
and possible add a newer - smaller same value cap in parallel with it.
Don't depend on the old cap. Sometimes I don't pull them out when people
are fussy about being all original.

: BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
: find any reference for it.
: Thx, Chris

A picture of it on the web would allow many of us to tell you who made the
amp.

cheers
skipp

www.radiowrench.com/sonic



cant use a Variac with a Tube rectifier. you will drop the filament voltage
too.
so the rectifier wont conduct for quite a while.
you could put in some silicone for a while 1N4007 are fine
then bring up the voltage slowly
or if you have a bench power supply you could inject the voltage after the
rectifier.
bring it up slowly like that.
you could also put in a light bulb in your AC input line.
there are several websites that show how to do this.
the light bulb would limit current and keep things under control a little.

Doug

BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
find any reference for it.


Magnavox. Sams Photofacts 288-6 (AMP135) (1955) or 389-2 (AMP135BB)
(1958)

A current meter inline with the cap is very helpful. Increase the
voltage and watch the current meter. It will jump 10-20 ma or so.
Stop the voltage increase, and the current meter should drop back to
zero. Repeat slowly untill you reach rated voltage plus about 5
percent. As you back the voltage down, the current meter should move
sharply negative, indicating that the cap is discharging. If the cap
doesn't discharge current smartly, replace it.
clipping a 1k (give or take 10k ohm ; ) ) resistor across the
terminals is good for discharging the cap after testing.

I usually disconnect all the sections, and wire them in parallel to
save time. They are usually all the same voltage rating. If not,
you'll have to test them one at a time. If one section is bad, it's
best to replace the entire cap.

If the cap has a low voltage section which is used as a cathode bypass
cap, I'd suggest disconnecting it and inserting a modern bypass cap,
observing polarity.

And of course, keep your free hand away from any surfaces while doing
this. Caps can pack a lethal charge, and will discharge instantly to
a well grounded source, such as the path across your heart.

Hope this helps
Bob Hedberg



(Chris Richmond - MD6-FDC ~) wrote:

Hi Folks,

After years of wondering what happened to that cute little amp
of my youth, I found it in with my Lionel train stuff while cleaning
the garage. The newspaper it was wrapped in is from 1993, so I
know when I saw it last. Anyway, its a little PP 6V6 amp with a
12AX7 and rectifier tube (didn't pull to get number). The filter
cap is a Magnavox 450V, 30uf, 15uf, and couple of more sections.

My question is this: to reform the cap, should I pull out all
but the rectifier tube? I have a 10A variac I can use. Assuming
it has a fuse (don't think it does), and it holds, and no smoking,
how fast should I bring up the voltage? Given that the tube
won't conduct until the heater gets pretty warm, can I start
1/2 way to 120Vac input? Would it make sense to sub in SS diodes
during the process and bring it up more slowly?

Beyond that, the rest of the caps look like they could use
replacing. What decisions are to be made about the value of
restoring this thing?

BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
find any reference for it.

Thx, Chris

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

I just wanted to mention that this method is out of circuit, using ac
or dc voltage.

I'm not sure if ac voltage is rms or peak to peak. I use DC myself.
I believe the ac voltage across the terminals is max peak, not
average or rms.

Thanks
Bob

Bob Hedberg wrote:

A current meter inline with the cap is very helpful. Increase the
voltage and watch the current meter. It will jump 10-20 ma or so.
Stop the voltage increase, and the current meter should drop back to
zero. Repeat slowly untill you reach rated voltage plus about 5
percent. As you back the voltage down, the current meter should move
sharply negative, indicating that the cap is discharging. If the cap
doesn't discharge current smartly, replace it.
clipping a 1k (give or take 10k ohm ; ) ) resistor across the
terminals is good for discharging the cap after testing.

I usually disconnect all the sections, and wire them in parallel to
save time. They are usually all the same voltage rating. If not,
you'll have to test them one at a time. If one section is bad, it's
best to replace the entire cap.

If the cap has a low voltage section which is used as a cathode bypass
cap, I'd suggest disconnecting it and inserting a modern bypass cap,
observing polarity.

And of course, keep your free hand away from any surfaces while doing
this. Caps can pack a lethal charge, and will discharge instantly to
a well grounded source, such as the path across your heart.

Hope this helps
Bob Hedberg



(Chris Richmond - MD6-FDC ~) wrote:

Hi Folks,

After years of wondering what happened to that cute little amp
of my youth, I found it in with my Lionel train stuff while cleaning
the garage. The newspaper it was wrapped in is from 1993, so I
know when I saw it last. Anyway, its a little PP 6V6 amp with a
12AX7 and rectifier tube (didn't pull to get number). The filter
cap is a Magnavox 450V, 30uf, 15uf, and couple of more sections.

My question is this: to reform the cap, should I pull out all
but the rectifier tube? I have a 10A variac I can use. Assuming
it has a fuse (don't think it does), and it holds, and no smoking,
how fast should I bring up the voltage? Given that the tube
won't conduct until the heater gets pretty warm, can I start
1/2 way to 120Vac input? Would it make sense to sub in SS diodes
during the process and bring it up more slowly?

Beyond that, the rest of the caps look like they could use
replacing. What decisions are to be made about the value of
restoring this thing?

BTW, its marked as an "AMP135AA". Neither Yahoo nor Google
find any reference for it.

Thx, Chris

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

Thanks for the data and suggestions, guys!

Chris
--
Chris Richmond | I don't speak for Intel & vise versa