Need some expert advice.

I've got an Audcom AP900a (Chinese) Tube Integrated Amp. Yesterday I
turned it on and noticed one of the EL34 output tubes was completely
dead. The one physically next to it was brighter (hotter) than normal.
Needless to say I immediately shut it down.

After an hour or so, I took the cage off and gently pulled out the
"dead" tube and replaced it with a spare EL34. Well, I turned it on,
everything looked fine (and sounded fine). No tubes were brighter nor
"dead". I listed to some music for 3 or 4 hours and everything appeared
fine.

I replaced the cage and placed the amp back in its normal position.

When I reinspected the "dead" tube today, I noticed one of the pins is
missing. Not Broken per se, just completely missing from the tube.
After further research online, it appears to be Pin #6.

Of course my first reaction is to freak out! I shut down the amp and
now I am in the process of letting it cool down.

This "missing" pin is not on the ground or underneath the amp. When I
looked at the socket yesterday I didn't see anything strange or burnt or
any hint of a pin sticking out of the socket.

Could this "missing" pin actually be lodged inside the amp??!!?? It
seems to be that the whole thing would have gone up in flames if it were
rolling around in there...



Some background info on the amp:
It was purchased new about 1.5 years ago online. No warranty or service
agreement.

I ran it with the OEM chinese tubes for about a month or two, then
replaced them with a quad of matched JJ EL34L tubes. I meant to re-bias
the amp but since this required taking the bottom cover and feet off, I
never got around to messing with it.


Please give me some common sense advice as to how to proceed. My idea is
to let the thing alone until tomorrow, then remove the feet and bottom
plate to see if I can inspect for damage or the "missing" pin.

If I find nothing wrong, I intend to lower the bias so that the tubes
run a little cooler.

Does this sound reasonable? If everything runs OK, I'd like to
eventually replace the complement of tubes with some Electro Harmonix
6CA7 tubes...

On Tue, 25 May 2004 13:36:22 +0000, O. Alvarez wrote:

Need some expert advice.

I've got an Audcom AP900a (Chinese) Tube Integrated Amp. Yesterday I
turned it on and noticed one of the EL34 output tubes was completely dead.
The one physically next to it was brighter (hotter) than normal.
Needless to say I immediately shut it down.

After an hour or so, I took the cage off and gently pulled out the "dead"
tube and replaced it with a spare EL34. Well, I turned it on, everything
looked fine (and sounded fine). No tubes were brighter nor "dead". I
listed to some music for 3 or 4 hours and everything appeared fine.

I replaced the cage and placed the amp back in its normal position.

When I reinspected the "dead" tube today, I noticed one of the pins is
missing. Not Broken per se, just completely missing from the tube. After
further research online, it appears to be Pin #6.

Pin 6 isn't used on EL34, so most EL34's don't have a pin 6, and
if they do, it's simply a dummy and not connected to anything.

--
Ned Carlson Triode Electronics Chicago,IL USA
www.triodeelectronics.com

Ned Carlson wrote:
On Tue, 25 May 2004 13:36:22 +0000, O. Alvarez wrote:


Need some expert advice.

I've got an Audcom AP900a (Chinese) Tube Integrated Amp. Yesterday I
turned it on and noticed one of the EL34 output tubes was completely dead.
The one physically next to it was brighter (hotter) than normal.
Needless to say I immediately shut it down.

After an hour or so, I took the cage off and gently pulled out the "dead"
tube and replaced it with a spare EL34. Well, I turned it on, everything
looked fine (and sounded fine). No tubes were brighter nor "dead". I
listed to some music for 3 or 4 hours and everything appeared fine.

I replaced the cage and placed the amp back in its normal position.

When I reinspected the "dead" tube today, I noticed one of the pins is
missing. Not Broken per se, just completely missing from the tube. After
further research online, it appears to be Pin #6.


Pin 6 isn't used on EL34, so most EL34's don't have a pin 6, and
if they do, it's simply a dummy and not connected to anything.


Thanks for the info.

I feel better now that I know pin 6 is a dummy pin.

I managed to take a look underneath the amp with a flashlight without
taking it apart. Since it has a grill underneath there were no tiny
parts or "pins" rolling around in there.

Also, I found bias pots VR3 and VR6 in the middle of the circuit board.
I was told by somebody with the same amp that these were the the
power output bias pots, so I was easily able to adjust them
counterclockwise all the way.

The EL34s are now running a little less hot and I didn't notice any
perceptible change in audio quality.

Thanks again for the info.

"O. Alvarez" schreef in bericht
...
Need some expert advice.

I've got an Audcom AP900a (Chinese) Tube Integrated Amp. Yesterday I
turned it on and noticed one of the EL34 output tubes was completely
dead. The one physically next to it was brighter (hotter) than normal.
Needless to say I immediately shut it down.

If one tube fails in a circuit with one common cathode bias resistor
(common to both output tubes), the other tube will run hotter.


After an hour or so, I took the cage off and gently pulled out the
"dead" tube and replaced it with a spare EL34. Well, I turned it on,
everything looked fine (and sounded fine). No tubes were brighter nor
"dead". I listed to some music for 3 or 4 hours and everything appeared
fine.

I replaced the cage and placed the amp back in its normal position.

When I reinspected the "dead" tube today, I noticed one of the pins is
missing. Not Broken per se, just completely missing from the tube.
After further research online, it appears to be Pin #6.

Pin six is like your appendix, it's just a pin without a function.
I guess your EL34 has been hospitalised for pinsixcitis :-)
Check the other tube, probably it has a missing pin six too.
For EL34 pin-out have a look at:
http://frank.pocnet.net/sheets/010/e/EL34.pdf


Of course my first reaction is to freak out! I shut down the amp and
now I am in the process of letting it cool down.

Oh my, oh my, mental disorder as a side effect of pinsixcitis.


This "missing" pin is not on the ground or underneath the amp. When I
looked at the socket yesterday I didn't see anything strange or burnt or
any hint of a pin sticking out of the socket.

Could this "missing" pin actually be lodged inside the amp??!!?? It
seems to be that the whole thing would have gone up in flames if it were
rolling around in there...

Probably it's still in china....


Some background info on the amp:
It was purchased new about 1.5 years ago online. No warranty or service
agreement.

I ran it with the OEM chinese tubes for about a month or two, then
replaced them with a quad of matched JJ EL34L tubes. I meant to re-bias
the amp but since this required taking the bottom cover and feet off, I
never got around to messing with it.

Please give me some common sense advice as to how to proceed. My idea is
to let the thing alone until tomorrow, then remove the feet and bottom
plate to see if I can inspect for damage or the "missing" pin.

If I find nothing wrong, I intend to lower the bias so that the tubes
run a little cooler.

Check the plate dissipation, 25W max. or a bit less should be fine.


Does this sound reasonable? If everything runs OK, I'd like to
eventually replace the complement of tubes with some Electro Harmonix
6CA7 tubes...

I guess it's OK to run it as is for a while, but best to replace
both tubes of the channel that failed with a matched pair.

Regards,
Jan.

http://tdsl.duncanamps.com/show.php?des=EL34

Don't worry about pin 6 on EL34. It sometimes is physically there but often
omitted as it has no internal connection.


"O. Alvarez" wrote in message
...
Need some expert advice.

I've got an Audcom AP900a (Chinese) Tube Integrated Amp. Yesterday I
turned it on and noticed one of the EL34 output tubes was completely
dead. The one physically next to it was brighter (hotter) than normal.
Needless to say I immediately shut it down.

After an hour or so, I took the cage off and gently pulled out the
"dead" tube and replaced it with a spare EL34. Well, I turned it on,
everything looked fine (and sounded fine). No tubes were brighter nor
"dead". I listed to some music for 3 or 4 hours and everything appeared
fine.

I replaced the cage and placed the amp back in its normal position.

When I reinspected the "dead" tube today, I noticed one of the pins is
missing. Not Broken per se, just completely missing from the tube.
After further research online, it appears to be Pin #6.

Of course my first reaction is to freak out! I shut down the amp and
now I am in the process of letting it cool down.

This "missing" pin is not on the ground or underneath the amp. When I
looked at the socket yesterday I didn't see anything strange or burnt or
any hint of a pin sticking out of the socket.

Could this "missing" pin actually be lodged inside the amp??!!?? It
seems to be that the whole thing would have gone up in flames if it were
rolling around in there...



Some background info on the amp:
It was purchased new about 1.5 years ago online. No warranty or service
agreement.

I ran it with the OEM chinese tubes for about a month or two, then
replaced them with a quad of matched JJ EL34L tubes. I meant to re-bias
the amp but since this required taking the bottom cover and feet off, I
never got around to messing with it.


Please give me some common sense advice as to how to proceed. My idea is
to let the thing alone until tomorrow, then remove the feet and bottom
plate to see if I can inspect for damage or the "missing" pin.

If I find nothing wrong, I intend to lower the bias so that the tubes
run a little cooler.

Does this sound reasonable? If everything runs OK, I'd like to
eventually replace the complement of tubes with some Electro Harmonix
6CA7 tubes...

JKoning wrote:


Check the plate dissipation, 25W max. or a bit less should be fine.



Does this sound reasonable? If everything runs OK, I'd like to
eventually replace the complement of tubes with some Electro Harmonix
6CA7 tubes...


I guess it's OK to run it as is for a while, but best to replace
both tubes of the channel that failed with a matched pair.

Regards,
Jan.




Jan,
Thanks for poking fun at me! Actually, thanks for the response.

The OEM chinese tubes it came with have 8 pins, while apparently the JJ
Tesla tubes don't.

I'd rather ask dumb questions and learn something new than pretend I
know what's going on and make a major blunder...

Since I have your ear, I was also wondering if I two matched pairs will
perform as well as a matched quad?

Also, is it worth trying NOS tubes rather than new Electro Harmonix tubes?

Regards.
Malomarski

"O. Alvarez" schreef in bericht
...
Jan,
Thanks for poking fun at me! Actually, thanks for the response.

Your welcome for both.


The OEM chinese tubes it came with have 8 pins, while apparently the JJ
Tesla tubes don't.

My fault, cancel the China trip and re-book for Slovakia to cure
pinsixcitis.


I'd rather ask dumb questions and learn something new than pretend I
know what's going on and make a major blunder...

Since I have your ear, I was also wondering if I two matched pairs will
perform as well as a matched quad?

I'd say it depends on the number of operational hours on the tubes
and how hot they ran. You're in the position to judge that, not me.
Ran hot for more than ~5000h., replace the quad for peace of mind.
Ran "cold" for less than ~5000h., renew the failing channel tubes.
In the end it's your wallet that dictates mode of renewal.
BTW, I hate you having my ear, no good for my perception of stereo.


Also, is it worth trying NOS tubes rather than new Electro Harmonix tubes?

Does insertion of the Chinese squad make an audible difference to your
ears? If no, leave it that way. If yes, stamp them "unsuitable for audio".
NOS or EH? I can't answer that question due to lack of experience
with EH, but I can say I ran NOS Philips for years to full satisfaction.

Regards,
Jan.



Regards.
Malomarski