hello everybody,

My dear Leak Point One is giving me trouble. I have this great tube amp
for about 20 years now and always was a pleasure to listen to. I only
did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went. I thougt it was the cold or the age of the
tube. But now I put in another GZ34 and it also blew so there's my
question;
Has anyone had this before? All suggestions are most welcome. I haven't
got the schematics so if anyone can send me a copy that would be even
better. Thanks in advance, Derk-Jan

On 11 Lug, 11:13, dadaderk wrote:
hello everybody,

My dear Leak Point One is giving me trouble. I have this great tube amp
for about 20 years now and always was a pleasure to listen to. I only
did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went. I thougt it was the cold or the age of the
tube. But now I put in another GZ34 and it also blew so there's my
question;
Has anyone had this before? All suggestions are most welcome. I haven't
got the schematics so if anyone can send me a copy that would be even
better. Thanks in advance, Derk-Jan


Indeed without the schematic is hard to give you any suggestion. First
of all, what you mean with "blew"? Does the filament on the GZ34 burn
open?
In this case I'd first determine if the heater voltage on the GZ34
socket is 5V or not. Then check the AC voltages on the two anodes pin
on the socket, it should be no more than 450V (rms) from each anode
pin to common ground, probably less.
If you don't feel confident with high voltages, bring the amp to
someone who knows how to deal with it, it could be very dangerous.

Francesco

On Wed, 11 Jul 2007 11:13:18 +0200, dadaderk
wrote:

hello everybody,

My dear Leak Point One is giving me trouble. I have this great tube
amp
for about 20 years now and always was a pleasure to listen to. I only
did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it
was
very cold and the GZ34 went. I thougt it was the cold or the age of
the
tube. But now I put in another GZ34 and it also blew so there's my
question;
Has anyone had this before? All suggestions are most welcome. I
haven't
got the schematics so if anyone can send me a copy that would be even
better. Thanks in advance, Derk-Jan


A schematic of the Leak Point One can be found here
http://www.werple.net.au/~kiewavly/Leak/tl12_cct.gif It may or may not
be the same as your amp since there were a few iterations.

Some references
http://www.werple.net.au/~kiewavly/PointOne1.html
http://www.bassboy.com.au/getreel/si...k/tl12/amp.htm
http://www.turneraudio.com.au/leakampmods.html

On Jul 11, 5:13 am, dadaderk wrote:

I only did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went.

I hope I am not understanding you correctly... "I bridged and replaced
some capacitors)... One should _NEVER_ bridge a cap if the goal is to
replace a cap that has failed. The old cap *MUST* be removed from the
circuit and the new cap put in its place.

If the goal is to add additional capacitance, it is much better
practice to remove the 'small' cap and replace it with a new 'larger'
cap. First, after 20 years the existing electrolytics may be tired,
and second, modern caps are typically much smaller than their older
cousins such that space is not an issue. Parallel caps _of the same
type and value_ *are* a legitimate and often used practice, however.

If you are replacing non-electrolytics, the same general logic
obtains.

In some circuits, excessive capacitance can cause excessive B+, so be
careful there.

It is poor and risky practice to run a system with the rectifier in
place and not the load... that is, remove all the tubes but the
rectifier.

Lastly (and long): The 5AR4/GZ34 is a very rugged tube unless you are
using some of the Chinese versions. Those tend to fail, often
spectacularly and often also taking much else with them. You have not
supplied and apparently do not have a schematic. Therefore you
probably do not know the correct voltages that should be coming off
the transformer and also what the correct B+ should be. Verify that
there is no more than 5V at the filament pins on the 5AR4. After that,
check all your work _VERY_ carefully. Polarity of the caps, solder-
blobs, poor connections, and so forth. Apply power through an ammeter,
preferably through a variac. Typically a tube rectifier will not pass
DC until it reaches ~70%+ of filament voltage, and the 5AR4 is more
like 75-80%. But when the needle on the ammeter starts to rise, you
will be able to learn much.

With the absolute final caveat that you are working in the dark and
without a schematic. So everything you are learning is more-or-less
guesswork.

Peter Wieck
Wyncote, PA

wrote in message
oups.com...
On Jul 11, 5:13 am, dadaderk wrote:

I only did some minor modifications in the beginning (I bridged and
replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went.

I hope I am not understanding you correctly... "I bridged and replaced
some capacitors)... One should _NEVER_ bridge a cap if the goal is to
replace a cap that has failed. The old cap *MUST* be removed from the
circuit and the new cap put in its place.

If the goal is to add additional capacitance, it is much better
practice to remove the 'small' cap and replace it with a new 'larger'
cap. First, after 20 years the existing electrolytics may be tired,
and second, modern caps are typically much smaller than their older
cousins such that space is not an issue. Parallel caps _of the same
type and value_ *are* a legitimate and often used practice, however.


There is usually a maximum allowable reservoir capacitance stated in
rectifier tube data sheets, exceeding this value can strip the cathode as
heavy charging current is drawn before a full space-charge has developed at
switch on from cold.

In article , dadaderk
wrote:

hello everybody,

My dear Leak Point One is giving me trouble. I have this great tube amp
for about 20 years now and always was a pleasure to listen to. I only
did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went. I thougt it was the cold or the age of the
tube. But now I put in another GZ34 and it also blew so there's my
question;
Has anyone had this before? All suggestions are most welcome. I haven't
got the schematics so if anyone can send me a copy that would be even
better. Thanks in advance, Derk-Jan


Thanks a lot already for your reply but I forgot to say it's a Stereo
20 with the 2 EL84 and ECC83 and a varislope stereo pre-amp. I did
bridge some capacitors on the lf side to make it sound better (although
the original cap's sound already really good!) and I had a hum in of
the channels so I replaced one of the rectifier cap's and then it
dissapeared (worked for 20 years). Btb I'm a television/audio repairman
but not familiar with tubes. Oh and the first the GZ34 went white and
the second time I saw a spark when starting up and then nothing worked
anymore. Doesn't seem to have done more damage than that. I'll check
the voltages and will be back. Are there also schematics for the stereo
20? Would be great, thanks. Derk-Jan

"dadaderk" wrote in message
...
In article , dadaderk
wrote:

hello everybody,

My dear Leak Point One is giving me trouble. I have this great tube amp
for about 20 years now and always was a pleasure to listen to. I only
did some minor modifications in the beginning (I bridged and replaced
some capacitors). First time I put the power on in a room where it was
very cold and the GZ34 went. I thougt it was the cold or the age of the
tube. But now I put in another GZ34 and it also blew so there's my
question;
Has anyone had this before? All suggestions are most welcome. I haven't
got the schematics so if anyone can send me a copy that would be even
better. Thanks in advance, Derk-Jan


Thanks a lot already for your reply but I forgot to say it's a Stereo
20 with the 2 EL84 and ECC83 and a varislope stereo pre-amp. I did
bridge some capacitors on the lf side to make it sound better (although
the original cap's sound already really good!) and I had a hum in of
the channels so I replaced one of the rectifier cap's and then it
dissapeared (worked for 20 years). Btb I'm a television/audio repairman
but not familiar with tubes. Oh and the first the GZ34 went white and

Going white suggests that the getter deposit has been exposed to air
(cracked glass!) other than mechanical damage before or during fitting (bent
pins etc) - this would suggest a very severe thermal shock, such as might be
caused by a dead short on the HT rail.

On Jul 11, 10:37 am, "ian field" wrote:

Going white suggests that the getter deposit has been exposed to air
(cracked glass!) other than mechanical damage before or during fitting (bent
pins etc) - this would suggest a very severe thermal shock, such as might be
caused by a dead short on the HT rail.- Hide quoted text -

Certainly the tube lost its vacuum. I hate to harp, but Chinese 5AR4/
GZ34s tend to fail either that way, or through internal melt-down.

Again, unless the OP has a schematic or finds one, much of the
troubleshooting at this point will depend on some basic instruments
and some experience in what the readings mean. And here goes, climbing
swiftly out on a limb:

4 x EL84/6BQ5 = ~ 3.5A comes to ~22 watts
2 x 12AX7/ECC83 (run at 6.3V) = ~0.6A comes to ~38 watts
1 x 5AR4/GZ34 = ~2A comes to 10 watts

Altogether, comes to ~ 70 watts, then losses, figure your quiescent
power use will be something around 75 - 90 watts. I would expect about
80 watts, but that is a blind SWAG guess.

With a good current meter and a variac you should be able to run the
current up to the point where the rectifier *just* begins to pass DC,
and see what happens on the current meter. If it starts to climb
anything above 90 watts, or so, then you have a problem that requires
further attention before you go any further. And at 90 watts, be
exceedingly careful and look for hot-spots, transformer leakage and so
forth.

Note: when a strange piece of equipment crosses my bench for the first
time untested, usually I make the calculation above and then see the
actual current draw (vs. expected draw) as my very first pass at
diagnosis. It can be very helpful.

And, looking at the schematic that Ross offered, I see a couple of
places where a shorted small-value cap could create the precise dead-
short situation that Ian mentions. I know it is not the same as yours,
but it does appear pretty typical of the species. Check them all.

Note also that _VERY_ high voltages exist inside tube equipment. Do
not become a part of them. Be careful and treat the amp with great
respect at all times.

Peter Wieck
Wyncote, PA