On 2013-02-19, at 1:32 PM, GALE STEWARD wrote: > I have suspected for a number of years that the plate meter in my L4 was > reading a bit low. Today I had the amp opened up to make a minor repair and I > decided to check. Using a variable power supply, a series resistor, and a DMM > in series, I've determined that the L4 meter does indeed read a bit low. As > an example, when I adjust the power to give me 400ma on the DMM, the drake > plate meter reads ~360ma. The schematic shows the meter to be a 1.0 amp > movement and does not appear to have any external shunt resistors. BTW, the > meter was disconnected form the amplifier circuitry when I took the current > readings. > > There obviously does not seem to be any way to tweak this but I thought I > would ask. This slightly lower reading is not a problem for me but I thought > I'd check with the Drake brain trust before buttoning the the amp.
Hi Stew, My advice to you would be to either get the meter "rectified" to read properly, or use it strictly as a "guide", & not a precision instrument---at your own peril! Why do I say this...? Back in the early 90's I was finishing-up construction of a homebrew 2x813 linear amplifier that remains in daily use to this day. I wanted four physically-matching panel meters to monitor its different functions, but I was missing a 0-1000 ma. DC meter (plate current) & a 0-250 ma. DC meter (grid current)---however, I DID have a matching 0-1 ma. DC meter, as well as a 0-25 ma. DC meter... ...So I wound external meter shunts out of narrow-gauge enamelled wire, placing them in parallel with each meter to increase their respective scales, and merrily installed them. BUT I DID NOT VERIFY THE NEW SCALES TO KNOWN ACCURATE ONES BEFORE INSTALLATION! Fast-forward to the present time: I'm happily chasing DX on 160-meters, and receive sporadic complaints of key clicks and distortion---not ALL the time, but enough to make me switch exciters, experiment with grounds, install a transmatch between the linear & my transceiver, etc. Well, guess what...? I FINALLY took to examining the shunted meters by way of a series of observations: first-off, I needed 70-watts to drive the amplifier to its text-book 400 ma. of plate current, at which time, after peaking the tuning & loading caps in the amp, I would advance loading until power output went down some 10%, per a G-G tune-up procedure that I'd seen in various write-ups. This is how I would operate the amp, & key click reports were hit and miss... One day---with a friend monitoring my signal on the band---I completely discounted the amp meter readings, and simply tuned it up watching nothing by an RF power output meter. I peaked the plate tune & load caps for maximum output, then adjusted drive until I achieved a prescribed output of 600-watts. And what happened...? My plate current reading was just 300 ma.! That would translate---with an output of 600-watts, & an input of 750-watts (B+ is 2,500-VDC)---to an efficiency of 80%. Uh-uh---that is NOT gonna happen! My drive was just 50 watts at this point, & the output was a text-book perfect 600-watts, inspite of what the plate current meter showed... CONCLUSION: I had been previously targeting 400 ma. on my "suspect" meter, and was probably---in reality---actually driving the amp to some 500 ma. of plate current, and OVER-DRIVING it by 20-watts! And over-driven tubes in an amplifier can lead to all sorts of nasty things besides shortened tube life---things like key clicks... So take it from me, Stew, either fix that meter up, or use it strictly for relative readings only---CAVEAT EMPTOR! ~73~ de Eddy VE3CUI - VE3XZ
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