This 1965 Gibson Skylark came into the shop because it had a LOUD hum. Evidently it didn't know the words.
Like the non-tremolo GA-5 Skylark, this amp went through many cosmetic changes. This edition is the third and definitive version that was used from roughly 1965 to 1967. It is referred to as the “White Panel” Skylark. It is difficult to find the rated output of this amp – Gibson often still referred to it as 5 Watts, which was the rating of the original single-ended Les Paul Junior, the progenitor of the Skylark. I would guess that Gibson would rate the amp at 15 – 18 Watts.
The loud hum on start-up was due to the failure of all 4 filter capacitors. These were replaced with a single Can Cap of 40/20/20/20 µF/500V; the can cap was installed with plumber’s tape and rivets, as original. The new filter caps are higher voltage rating than the original capacitors and more suited to modern wall voltages. The first filter cap was changed from 20 µF to 40 µF; this change will reduce ripple and hum, it will not affect the tone of the amplifier. The 1965 GA-5 T used diode rectification, so the 40 µF first filter cap is ok here. If this were the tube rectifier version (earlier than 1965) then the original 20 µF capacitor would be more appropriate.
The V1A triode cathode bypass capacitor was in good condition but was high in value ( spec. = 5µF ). I installed a 2µF Tantalum capacitor in parallel with the cap, which brought the value into spec at 5µF. The “fast” Tantalum cap also acts to reduce Bass “flabbiness”, common with the 10” speaker with these amps; it also acts to improve Treble response somewhat.
The cabinet was dirty and missing the two back chassis bolts. These were replaced, although not with the original style bolts. Amp and chassis were cleaned. Volume pot was very scratchy. Two applications of De-Oxit were needed to get the pot to have silent operation again.
The amp’s tone is clean and bright – very “chimy”, like early British amps using EL84s. The transformer phase inverter contributes to the very clean tone of the amp. No issues were discovered with amp operation – it sounds good and there are no odd behavior or noises.
The operating voltages were within reasonable limits. The fixed bias of the 6BQ5 output tubes, when operating at 317 VDC, results in a near 100% dissipation of the plates and a total output of the amp of 23 Watts. Although this is a very high bias setting (~37 mA) the tubes are not red-plating and there is no noticeable related distortion. This high dissipation is common with original fixed bias circuits operating at modern wall voltages. The wall power during testing was 122 VAC RMS. The circuit design calls for 117 VAC. If the customer wants to bring the bias down to a more common 70% dissipation, I can do this by changing the Cathode Resistor; its current value is as designed, 130 Ω / 5W. However – calculating the circuit design voltages, I suspect that the amp was designed to operate at or near these levels.
DELTA WHISKEY HOTEL