Assemble a HiQ guitar amplifier - circuit diagram – using 6L6 and 12AX7 vacuum tubes

RCA’s first version of this tube was an early octal base tube. Like most with this base it had a metal, rather than glass, envelope. Later versions, including the 6L6G, 6L6GA, 6L6GB, 5881, 5932, 7027, and the final version 6L6GC had glass envelopes, which made radiation cooling of the anode easier.  5881 is next to the 6L6GB. 

The 5881 should be ok with the plate voltages found in most guitar amplifiers designed for the 6L6GC but this is not guaranteed so you do risk blowing your 5881s if you put them in an amp designed for the 6L6GC.

The valve 5581 can be replaced with 6L6 and the substitution for 12AX7 is ECC83

Valves don’t usually ‘fail’; they just get weaker and weaker. The actual age of the tube is not important. It’s how much it has been played. If you play your amp regularly and it’s been a year or more since you had a valve change, it’s almost certainly time to treat yourself to a new set of valves. 

Don’t forget to buy  “matched output pair” valves. This is really important. The matched valves have been carefully selected to be a close match both in gain and bias current. If you’re going to change amp valves always choose ‘matched quads’ if you have four output tubes or ‘matched pairs’ if you only have two.

If your guitar valve amplifier is not sounding quite so good as the day you bought it, change amp valves to restore that beautiful sweet tone.  If it’s sounding a bit flat, thin and lacking in oomph, change amp valves on a regular basis if you want to keep your amp sounding its very best.

If you notice a loss in bottom-end and power, it’s most likely your power valves need replacing.  If you’re getting strange sounds coming from your amplifier, first turn your gain section up, and slowly turn the master volume down. 

If the noises continue after the master volume has been turned down, it’s most likely a power valve. If you turn the master volume down, and the noises disappear, it’s most likely a pre-amp valve.

 If you want to change amp valves always recommend changing the whole set while you are at. It only costs a little more and you know that everything is as it should be.

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