Old Tubes.....testing?

[swbo101]

My mom recently asked me to get rid of the old steros we had in my home growing up. They are both RCA's from the 1950's. I remember them as a child in the late 60's and through the 1970's that they ran on tubes. When I go to move them I am going to pull the tubes to see what I may find. I am guessing they will be pre-amp tubes, but won't know exactly until I go. If they are anything of importance should I test them before putting them in my amp? Also how should I test? Any insight would be helpful. Thanks

 

[94Tremoverb]

If they're preamp tubes, you can test them very simply... put them in your amp .

You will hear straight away if they work, have enough gain, too much noise, or excessive microphonics. There's really nothing that a proper tube tester will tell you that you need to know, that you won't find out this way. With preamp tubes, there is usually no risk of damage to the amp if one is bad - it will just sound bad, not work, or at worst squeal, hum or make other bad noises.

You can try any of the 12A*7 series tubes in the V1 position in most amps - some (eg 12AU7) will have very little gain, but it will still show whether they work properly or not. Be careful if your amp has a cathode follower position (V2 in vintage Marshalls, V3 and V4 in Dual Rectifiers for example ) - don't try 12AT or AY7s there, they don't have the right voltage ratings. (Also bear in mind that some amps don't use both halves of V1 in signal-path gain stages, so you might not get a complete result - the Blue Angel and Tremoverb are both examples - and others like the Caliber series only use it in the Lead mode.)

Power tubes are more tricky. If you're unlucky a bad power tube can damage the amp - usually by blowing the screen resistor, but occasionally worse - and even a proper tube tester won't really tell whether they will be reliable in a guitar amp, since the voltages and currents are far higher than in the tester, and often outside the original design specs. A tube can easily 'pass OK' on a tester and fail in a real amp. I have a tester I built out of a scrap tube PA amp, for this reason - so I can run the tubes under real-use stress. Also, if the tubes are from the 50s, be wary of using apparently equivalent types in modern amps - eg a 6L6GA or GB in an amp that uses 6L6GCs - the voltage ratings are lower and you'll probably blow the tubes.

Rectifiers are somewhere in between - a bad rectifier tube will usually blow the amp's fuse without causing any other damage, but I would still tend to be careful. Likewise, a tube tester won't always show up a bad one either.

 

[swbo101]

Thanks...I have a Mesa Boogie Mark IV A. I'd be a be nervous "trying" out a power tube in that anyway. However if a pre-amp tube won't harm it, then np. The only place I'd use it for would be the V1 position. Thanks again...I'll see what I have when I go back to pull the tubes.

 

[94Tremoverb]

That will work well. The MkIV uses both halves of V1 as the first two gain stages, and there are no cathode followers in the amp. It's also capable of very high gain so it will really show up any noise and microphonics - don't throw out any tube that fails for one of those reasons just yet though, since it may be perfectly usable in the later positions, especially V5 (phase inverter). The only one to be careful of is V4 which is the reverb driver - which has a fairly high plate voltage on one half, and isn't really a 'tone' position anyway so you probably won't get the most benefit from an old tube there (although I still think it does make a difference, if you have any 12AT7s in the haul).

 

[swbo101]

Hey thanks for the followup!! Yeah I usually stay away from the V4 slot since it's a lower voltage tube. I usually just put stock mesa 12AT7's there. I'll let you know what I find. Much appreciated