• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6cu6 12cu6 @ 2a3

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There was a popular "urban legend" circulating about 20 years ago that pops back up every few years. Back then it was 6AV5GA = 6B4.

This started when a batch of military surplus tubes from the last days of the vacuum tube era hit the surplus market. Sylvania ECG and others needed to fulfill some government contracts as tube production lines were being shut down or consolidated. They often stuffed the glass of a tube that they had to ship with whatever guts that they could find that worked in the equipment used by the particular contract. These tubes were never sold to the general public, and not intended as a general substitute in all applications.

The most popular "stuffed glass" were some 6BG6GA's with 7027A guts in them, and the 6B4GA (a DHT) glass with 6AV5GA guts (a TV sweep pentode). I have seen the actual tubes and can confirm that they did exist.

So, can you wire a 6AV5 into a 2A3 socket. Well, first you will need to find 6.3 volt heater power, and make up some socket adapters, but it appears that it does work. Includes are pictures where I wired a pair of 6AV5's into my TSE which was set up for 2A3's. The higher rated 6FW5 works too.

The 6CU6 is the same thing as a 6BQ6GA, and some tubes carry the same number. The 6CU5 / 6BQ6GA is similar to the 6AV5 except it has a plate cap.

The 6B4GA is an octal version of the 6A3, which is a 6.3 volt version of the 2A3, so you can see how that story evolved.

Note that the 6AV5 or 6CU6 may not work in all applications, and does not sound exactly like the 2A3, but it can be made to work, and sounds pretty decent.
 

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