Globe King 500A Speech Amp...how to modify
Sept 2004
GLOBE KING 500 A SPEECH AMP.
This design will produce very nice audio and is set up for feed back. A EQ ,if used,will sound very good as this amp will give amplification of the full audio frequency range with little distortion.
Some of the original tubes are retained in the circuit a 6SN7phase splitter and push - pull 6B4G drivers are added, the 6B4G is the 6V filament version of a 2A3, 6B4G's are still sold new at a reasonable price. The 6B4G's should be run in AB1 not class A. The 750 ohm resistor listed may need to be higher value to put the tubes in AB1. Plate voltage of about 350 V is about right to drive the 811A's adequately.
A new filament transformer was added for the P-P drivers. These tubes have no cathode so self bias is generated in the filament center tap.
The 6B4G's drive a pair of 811A's, the original driver transformer is used. It is a P-P to P-P transformer, in the original configuration the transformer has one lead cut off on the driver side so it will be necessary to lengthen the lead and determine the center tap lead. The Transformer Inductance is 24 Hy per each primary winding and 6.2 Hy each secondary winding.
The modulation ( original) transformer is 7 Hy total primary to 5.355 Hy total secondary @ 1000hz.
Note the input impedance for the mic is low and set for a JFET device used to change impedance from a high imp crystal mic to a low impedance out put. If you use a D-104 with out the JFET increase the input resistor to 3 to 5 megs.
Determine 1C by experimenting...value may be as high as .1 Mfd to cut high freq enough to give correct operation with feed back.
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