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Re: [Amps] Amp Supply LK-500ZB Upgrades?

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Subject: Re: [Amps] Amp Supply LK-500ZB Upgrades?
From: "Dick Green WC1M" <>
Date: Sat, 19 Feb 2011 16:47:06 -0500
List-post: <">>
I have an LK-550ZC, and while I think the amps are similar, I'm not familiar
with the specific differences. That said, I can offer the following:

1. The problem with the fan system is that it doesn't run at a high enough
speed in the Lo position. This causes the amp to flip into the Hi position
frequently, with attendant screaming freight train noise. The fans in my amp
used to flip into Hi every few minutes, even when sitting idle. There's no
need for that. It's understandable for the fans to go into Hi mode during a
long key down sequence, but not while idling. By boosting the speed a little
in the Lo position, you trade slightly more noise (not objectionable in my
opinion) for better cooling and much less frequent cycling into the Hi
position. Then you can slow the fans down in the Hi position as much as you
dare to attenuate the rest of the noise. I used a digital temperature probe
to monitor the tube temperature under operating conditions and was able to
lower the fan speed in the Hi position while still maintaining a large
margin of safety for tube temperature. And that's with three 3-500Zs! I did
this mod ages ago and I don't remember exactly how I did it. I might have
some notes and/or emails on the subject if you can't figure out how to do

4. I wouldn't bother with the AG6K kit. Many people I respect consider it to
be snake oil -- i.e., the whole VHF-parasitic oscillation thing, balls of
molten gold in the tubes, etc., is a bunch of hogwash. It's highly likely
that the "bang" people sometimes get when tuning an amp come from not
knowing how to properly tune the amp, and not from VHF-parasitic
oscillations. [Note to the reflector: I do not wish to drag this subject up
again for the one millionth debate, and will ignore any messages related to
it. Dead issue.]

The only things I would recommend that are not on your list are:

a. Put a glitch resistor in line with the HV supply to avoid damage to the
plate choke and other components in the event of a high-voltage event like a
tube arc.

b. Check the plate choke for melting or damage. If the amp doesn't have a
glitch resistor, the plate choke may already be damaged.

Hope this helps.

73, Dick WC1M

-----Original Message-----
From: Rob Santello [] 
Sent: Friday, February 18, 2011 6:57 PM
Subject: [Amps] Amp Supply LK-500ZB Upgrades?

Hello List,

   I just acquired an Amp Supply LK-500ZB which looks to be in pretty 
good condition. I took some pictures and put them on a web page at:

I have several questions regarding modifications and potential improvements.

1. The fan works at both speeds but is pretty loud in the Hi speed mode. 
Other than trying to isolate it from the chassis with some rubber 
grommets (or is there a better way to shock mount the fan), is there a 
more modern replacement fan that runs quieter and has the same or 
greater air flow? Are there any other cooling modifications that would 
be desirable to perform?

2. Even though it might not be necessary, I want to beef up the power 
supply board. I plan on replacing the caps with 470uf / 450v 
replacements and replacing the diodes with 6A versions. Are the 
currently installed 1 meg ohm resister that are in parallel with the 
diodes necessary? Can or should they be removed. I'm also replacing the 
cap bleeder resistors with Metal Oxide Film type resistors.

3. It appears the 10 meter mod is only half way installed. The input 
circuit appears to be there but there is no coil tap on the output tank 
coil. According to the manual on the BAMA web site, the tap should be on 
loop #6 as counted from the inductor end of the coil. That would put it 
2 turns from the existing 15 meter tap and 3 loops from the end of the 
coil. Is this the correct location? If you look at the input trimmer 
caps on the input board for the 10 meter position in my pictures, it 
looks kind of funky, Is this a normal install or does this need to be 

4. Are the stock RF suppressors ok or would installing the  "Richard 
Measures, AG6K - Low VHF-Q parasitic suppressor retrofit-kit" be a worth 
while investment for this amp?

I have access to some parts from a LK-550 amp and have already installed 
the step-start circuit. Also considering installing the QSK board from 
that amp as well.

Any other things that I should address or modify?

Thanks for any suggestions!

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