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[Amps] Three questions (all related to Amp Supply LK-xxx series 3-500Z a

Subject: [Amps] Three questions (all related to Amp Supply LK-xxx series 3-500Z amplifiers)
From: wb0gaz via Amps <>
Date: Sat, 21 Oct 2023 18:50:13 +0000 (UTC)
List-post: <>
 Working on a LK550 for a friend - amp has various control/metering/low voltage 
circuit problems (ALO/Meter board, QSK board, etc.) including evidence of 
probably two generations of prior issues in this area.

Three questions (arising from poor documentation) -

1. (possibly specific to LK550). The LK550's ALO/Meter board appears to accept 
unregulated 12V (I guesstimate around 13-14VDC half wave rectified and 
filtered) from the onboard low voltage/filament (two secondary winding) 
transformer (in the LK550, the HV xfmr is external and is disconnected.) The 
ALO/Meter board shows some signs of damage from a prior life (a few cooked 
traces), but not yet that far along. Presently trying to understand the use of 
D7, which is a 1N4742 12V zener diode which sits as shunt across the 
unregulated (somewhat over 12) input from the low voltage part of the power 
supply PCB. I've never seen this configuration, where a DC power supply is 
driving a shunt zener directly, without a dropping resistor. The only way I can 
see that the Zener's current is being limited is by reliance on the intrinsic 
resistance of the corresponding secondary winding on the onboard transformer. 
By the time I encountered the amp (my friend bought it in this condition), D7 
had been re

2. (likely common across the LK450-500-550-800 range) The two meter movements 
(one for plate current, one for grid current or high voltage) appear that they 
may be voltmeters in the millivolt range. They are shunted by bidirectional 
silicon diodes and a small (20-ohm-ish) resistor in series entering the diodes 
(presumably to limit damaging current into the back-to-back diodes), however, 
it is not clear from the schematic (which in the LK550 manual was evidently 
copied from a version of the amplifier without the ALO/Meter board, so 
close-but-not-exact) what the meter characteristics are (full scale volts or 
microamps, either is need to know.) I can work this out experimentally, but if 
there's anyone with a known spec, that would be really helpful.

3. (Probably common to grounded grid direct heated cathode amplifiers in 
general.) I'm working with the amp with the HV transformer removed, so the only 
power in the RF deck is filament voltage and the 12/24V low voltage DC circuit. 
The tubes are still present. When powered on, a small residual cathode current 
is seen on the plate current meter (and a small standing voltage at the grid 
current resistor-to-ground connect point.) Is this possibly due to the tubes 
rectifying a potential between the cathodes (hot with AC) and the grounded 
grids? After puzzling over this today, I think on next go I'll proceed with the 
tubes removed, but as this wasn't expected behavior, comment would be 

As the issues the amp has are limited to low voltage DC/switching type 
circuits, the HV transformer is fully unplugged to eliminate that source of 
live circuit risk.

Thank you for comments on above or related.

73 Dave WB0GAZ
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