Mike recently acquired an Oahu guitar amplifier. The tube lineup (from right to left) was 5Y3, 6N6, 6C5, and a 'mystery tube". I traced out the wiring and came up with the following schematic: Original Oahu preamp

I was able to narrow down the possible choices to a tube with a 6 volt heater at pins 2 and 7, cathode pin 8, grids at pins 4 and 6, and plates at pins 3 and 6.

The only match I could come up with was a 6N7, which is an output dual triode - not suitable for preamp use. I tried a 6N7 metal tube anyway. It functioned, but it had low gain and a lot of hum. The only other choices for a dual triode with a common cathode also had a grid cap. No grid lead on this amp.

Mike was also unable to locate a tube chart, so I rewired it (ala Tim Taylor).

I selected the 6SK7 because of it's gain, the same filament connections, and the fact that it's available in metal, so it wouldn't require a shield. I used one of the resistance-coupled amplifier charts in my RCA tube handbook, and now it runs fine. Here's the new schematic:

Revised Oahu preamp
Next on this page will be some info about VCR tuners. The older analog types are very easy to extract from a VCR, and they make handy sources for audio and video. Hook one up to a camcorder viewfinder, and make a tiny TV set. Use one as a replacement TV tuner. There have been some articles on building spectrum analyzers with TV tuners.
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