[combover]

Hi,
I have just dusted of a Heathkit AA-121 that I remember sounded pretty
good when I last used it.
I used a variac over several hours to get it warmed up and used the
rear bas-bal to balance socket to balance the output tubes.
One channel was rock solid and stayed at zero volts, the other channel
would slowly rise to .1 volt for a second ot two and then slowly return
to zero and repeat.
I removed the VOM, hoping that it would steady itself over time, no
such luck. There was a muted "pop" and when I inserted the VOM the
balance was way off zero volts and could not be adjusted back to zero,
also, AC voltage was present across the bas-bal socket which was not
measured on the good channel. I did an output tube change from one
channel to the other and the results was the same.
Any hint at what could be the problem would be appreciated.
Thanks, Combover

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Hi RATs!

Prolly a leaking coupling cap.

Buy a pair and replace both at the same time.

Happy Ears!
Al

[Ian Iveson]

combover wrote

I have just dusted of a Heathkit AA-121 that I remember sounded
pretty
good when I last used it.
I used a variac over several hours to get it warmed up and used the
rear bas-bal to balance socket to balance the output tubes.
One channel was rock solid and stayed at zero volts, the other
channel
would slowly rise to .1 volt for a second ot two and then slowly
return
to zero and repeat.
I removed the VOM, hoping that it would steady itself over time, no
such luck. There was a muted "pop" and when I inserted the VOM the
balance was way off zero volts and could not be adjusted back to
zero,
also, AC voltage was present across the bas-bal socket which was not
measured on the good channel. I did an output tube change from one
channel to the other and the results was the same.
Any hint at what could be the problem would be appreciated.

Blown selenium rectifier, perhaps. Replace with 1N4007. Check
associated electrolytic caps too.

I looked briefly for a circuit diagram but couldn't find one.

cheers, Ian

[combover]

Thanks Guys,
I fired up the amp after replacing 2 x .047mf coupling capacitors, I
must have changed them before and had 2 spares. The gentle pop I heard
the first time, happened again and it was was a flash within one of the
output tubes which blew the fuse immediately. I found another fuse, 3
amp rather than 4amp SB and it blew again
I'm starting to remember that I was having trouble with the amp years
ago, the bias voltage was difficult to adjust, the VM would do nothing
then just go off the range and one tube would glow like Rudolph's nose.
Thanks, Combover
wrote:
Hi RATs!

Prolly a leaking coupling cap.

Buy a pair and replace both at the same time.

Happy Ears!
Al

[Fred]

I would rather suspect that a shorted tube element is causing this. I
had the same thing happen with my Dynaco ST70. New tubes cured it.

[Fred]

Oops, forgot to mention that those old Heathkit printed-circuit board
amps were infamous for burning the board traces and components around
the output tubes, so check those carefully, too. An AA-121 is a mighty
nice sounding amp when it is working right.

[Ian Iveson]

Sometimes in threads like this, when the OP goes AWOL, I wonder if he
has electrocuted himself. Or become demented from selenium
poisoning...

"*******s...you killed my son"

Combover, if your bias supply uses a selenium rectifier, please change
it for a 1N4007. It is wise to do this whether it has failed or not.
If it fails to a short, it may make a soft pop as it releases a puff
of poisonous fumes. Then, if you escape dementia, you may measure AC
on the bias and watch the following electrolytic cap explode with a
rather louder pop, as your
valves glow and your output transformer bursts into flames, etc.

As you brought the amp up to voltage, you really should have monitored
the bias at the grids, or the current through the valves. It helps a
lot if you use a metered variac, or an ammeter in series with an
unmetered one. Preferably analogue.

cheers, Ian

[Sander deWaal]

"Ian Iveson" said:


Combover, if your bias supply uses a selenium rectifier, please change
it for a 1N4007. It is wise to do this whether it has failed or not.


In addition to this good advice, observe the polarity of this diode
very careful, the resulting voltage has to be *negative* with respect
to ground!

Also, use a 47 ohm/1 watt resistor in series with the 1N4007 to
compensate for the losses in the former selenium rectifier.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[combover]

Ian, thanks.
I blame my dementia on the coating they put on the inside of beer cans!
The Selenium diode looks to have been replaced and it looks nothing
like the one in the construction book. The replacement is a pea sized,
round ceramic looking thing.
I purchased a pair of Yorks about 6 years ago and have only listened
to them through a SS Intergrated amp even though I been meaning to try
them out with some tube stuff.
I also have tucked away 2 Heathkit W-5Ms and a Quad II outfit so
perhaps I should start looking at these?

Combover

Ian Iveson wrote:
Sometimes in threads like this, when the OP goes AWOL, I wonder if he
has electrocuted himself. Or become demented from selenium
poisoning...

"*******s...you killed my son"

Combover, if your bias supply uses a selenium rectifier, please change
it for a 1N4007. It is wise to do this whether it has failed or not.
If it fails to a short, it may make a soft pop as it releases a puff
of poisonous fumes. Then, if you escape dementia, you may measure AC
on the bias and watch the following electrolytic cap explode with a
rather louder pop, as your
valves glow and your output transformer bursts into flames, etc.

As you brought the amp up to voltage, you really should have monitored
the bias at the grids, or the current through the valves. It helps a
lot if you use a metered variac, or an ammeter in series with an
unmetered one. Preferably analogue.

cheers, Ian