Measuring Pentodes With The Mutualconductance Meter

THE Type 443 Mutual-Conductance * Meter, developed several years ago for testing triodes, can he used equally well with four-element tubes. After a minor modification (which every unit in our stock now includes) both heater-type and filament type pentodes can be measured.

One of the modified mutual-conductance meters with the batteries connected for measuring a filament-type pentode is shown in Figure 1 and the corresponding circuit in Figure 2. The connecting links to the new bind ing posts, engraved "-f-" and "— marked Pentode Screen, have been installed in the left side of the panel. The cathode circuit has been broken and its two ends connected to the new posts. The screen grid of a filainent-lype pentode is connected to the prong corresponding to the cathode of heater-type tvibes. Tests in our laboratory have shown the necessity for a source of voltage for the screen separate from that for the plate in the filament-type pentode such as the 247. The connections for the screen grid are, therefore,

Figure 1. The Type 143 Mutual-Conductance Meter modified for checking filarnent-typc pentodes. Connections for heater-type pentodes are the same as for screen-grid tubes; the pentode-screen terminals are closed by the link

EVEREADY 73-4 BIAS

PENTODE SCREEN

GRID

Rai o

LOW CURRENT

HIGH CURRENT

BREAK ) FOR V-CHANGE)

PLATE . BATTERY

MAX.

PHONES -Q

Figure 2. The dotted lines in this diagram show the change in wiring necessary to make the Type 443 Mutual Conductance Meter suitable for measuring pentodes made to the two new binding posts.

For the measurement of the heater-type pentode tubes the connecting link is left in place and battery connections are made as if a screen-grid tube were being measured. Detailed instructions concerning this are found in the instruction book accompanying the instrument or in the General Radio Experimenter for July-August, 1930.

The charge for the modification is $3.20 net, and it would require about a week before return shipment could be made. If a mutual conductance meter is to be returned for this change, we suggest that i t be packed very well in a wooden case to prevent damage in transit and shipment made by express prepaid.

There are still available in our stock a few of the Typk 443 Mutual Conductance Meters that have been modified for the measurement of pentode tubes. The price is $27.00 each and immediate shipment can be made upon the receipt of an order.

— H. H. Dawes rwiHE GENERAL RADIO COMPANY mails the Experimenter, without charge, each month to engineers, scientists, and others interested in communication-frequency measurement and control problems. Please send requests for subscriptions and address-change notices to the

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