[Amps] Amp Supply LK-500ZBx 3-500z tube & cooling question

KB8NTY kb8nty at wowway.com
Tue Jul 8 15:09:20 EDT 2014

Asking for more help please.

My early Amp Supply LK-500ZB model has the letter X at the end, reading 
LK-500ZBx, The x is also included at the end of the three digit serial 
number 500x.

Does anyone have any idea as to what the "X" suffix in my model number may 
I have read up on some past history of the Amp Supply line and am aware of 
the changes within the same models and at times not documented.
Possibly as a long shot someone who may have been associated with the 
company may have any insight to what the "x" in the LK-500ZB model may refer 

In my researching of the LK-500ZB pictures found on line I see changes on 
the raised sub-chassis that the two 3-500z tubes mount on. Some have a 
series of holes along the top exterior side (closest to cabinet), some 
sub-chassis are open on that end. In my early model there are no holes at 
all on the sub-chassis, and the sub-chassis is complete, in other words the 
end of the raised sub-chassis on the exterior side is all one piece down to 
the bottom of amp chassis (closed, not open) like in other versions I have 

Upon reading of the overheating of the tube pins I guess Amp Supply had made 
changes along the way by drilling holes on the top of the sub-chassis and 
leaving the side open, all for better cooling ventilation.
My fan is mounted low in that a 1/4" or so is placed to blow down below the 
sub-chassis, but I guess in this earlier version, they didn't leave the 
other end open and had no ventilation holes to help the air exit.

I hate to do so but when I replace my power supply board upgrade, I am going 
to carefully take the 500z tubes out and drill out some holes into the 
sub-chassis to allow for the air from fan to flow through and exit.
I have also obtained a new ebmpapst 4600z fan, thinking I may secure it to 
the outside, side of the amp along with a speed control (as this fan really 
spins fast) to help exhaust the air flow.

Speaking of the tubes....just by chance I had the top off and turned the 
lights off in shack to get a thrill of seeing the 500z tubes glow...Sadly in 
one tube upon key down I observed a slight purple haze up near the top of 
tube right at the anode location. I mean slight...I would say 1/4 to 1/2 
inch at most. This presented itself upon key down.

Upon my reading I have read of leaks, and sometimes gas from unused tubes. I 
cannot say if the tubes were or were not used much as I only recently 
obtained the amp.

My question; I am asking for advise from the groups knowledge as to what's 
the deal?
Is my tube going to blow-up?
If so is there a chance if it does will it affect or take out any other 
Will it cause my other tube to blow?
If I am to replace, can I still use it until it gets worse or should it be 
replaced ASAP?
If I have to replace can I just replace that tube or should I be required to 
purchase both say as in a matched set?
These are Eimac's with date code of 8438

Again this purple haze is only slight, only seen in the dark, is small in 
size 1/4 to 1/2" at most and is located right at the top of the tube.
Power output don't seem to be affected at this time as I can obtain close 
to1400 watts at best with amp.

Prior to replacing the P/S board upgrade I am going to snap photos and post.

Sorry for all the questions. Trying to learn here and appreciate your valued 

Kind Regards...Ross KB8NTY

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