A Blues Harp Player's Guide to Tube Amplifiers
For blues harp there are several considerations for tubes.
A. Rectifier tube: There should be a rectifier tube. It should be one of the following: 5Y3, 5U4, 5AR4 or GZ34. The tubes numbers might be followed by letters, as in 5U4GB, but that is a technicality. There are other choices like 6X4, but the vintage tube amps that we are interested in use the tubes above.
The 5Y3 is the lowest power and has the most impact on the sound of an amp. It's called sag. You play a note and the amp is overpowered and can provide the volume at first. It then catches up and gives the amp a nice bark. The 5AR4/GZ34, I am told, is the cleanest sounding. The 5U4 can handle the most power.
B. Power Output Tubes. You have two choices, either a 6V6 or a 6L6 (5881). If there is only one of these, the amp is usually called (wrong or right) a Class "A" amp or single ended amp. If you have two of these tubes, they are usually sharing the output power and can be called class AB (it's called an AB, but it might not be - it's all too technical for me). These Tubes provide the watts. A rough rule of thumb is that each 6V6 can push maybe 10-14 watts through the output transformer into the speakers and that a 6L6 can push 18-20 watts or more.
EL84's or 7591 tubes are roughly the same as a 6L6 in a different package. They are usually found in amps from the 1970's and indicate that the amp manufacturer was trying to find ways to cut costs. These tubes can be used to make great amps, but were put into too many bad amps, so avoid them. Also the EL84 is used in amps like the Marshall that have different design goals than the average vintage amp. For harp players, Marshalls sound nasal; they offer a greater range of frequency response that enhances the "tin toy" origins of the harmonica. Good for Jimi - bad for Walter.
C. Preamp Tubes. These are the tubes that take the micro-voltage from the guitar or mic and multiply it thousands of times and hand it off to the power output tubes. The tubes numbered 12AX7 or 12AU7 or 12AT7 (any 12A ending in a 7 is good) are the tubes that a harp player wants. Early tube amps used 6SN7 or 6SJ7 instead of this tube. These are nice tubes, but are not as powerful and don't give the same overdrive as the 12AX7 type tubes. An amp with a 6SN7 is a good amp, but it may be too tame for a harp player. You have to play through the amp to see if it's right for you. I like 6SN7s, but I like the sound from a 12AX7 better.
The more 12AX7 tubes, the more preamp stages. This is good for a guitar player, but bad for a harp player. The feedback demon thrives in the preamp stages of an amp. One or two 12AX7 tubes are enough. Three is the maximum.
D. Tremolo or effects tube. Sometimes mixed up with the 12AX7's is a 6AU6 or a 6SN7 tube which us used to drive the oscillator in the tremolo or the preamp for the reverb. Don't worry too much about this tube.
Tubes to be avoided at all costs!
35W4, 50C5, 50L6, etc. The first number on the tube is the filament voltage. The tubes with 35 and 50 on them indicate an amp that was made without a transformer to create the filament voltage, instead they use straight line voltage! This was a cost saving option for amplifiers made in the 1960's. These amps hum no matter what you do. 110 Volt Line voltage drives all of the components and if there is any failure or short in the amp, you will die of electrocution. Harp players have the mic right at their mouths and these amps will give you a good shock.
Starting in the 1960's and into the 70's some amp manufatures used solid state rectifiers instead of a tube rectifier like the 5U4GB. These amps are OK. Some of them sound good. But they are an indication that cost cutting has been going on and these amps have smaller power transformers and cheaper components. I avoid amps with solid state rectifiers. Solid State: It's a bad idea.