There are several modern EL84 amplifiers which have wiring violations on pin 1 of one or more EL84 sockets. NOS EL84s have internal connections between pins 1 and 2 where modern EL84s do not. Sometimes designers of EL84 amps assume that pin one is a safe location for a "tie point", a place to connect a grid resistor and the signal wire from the phase inverter. This will result in a NOS tube placed in such a socket furiously "red plating" and the amp humming. If run this way for long enough, damage to the EL84 tube and/or the component connected to this socket will result.

Hi Mike,

Valves arrived downunder today.

Awesome as always, your valves have proven to be the most reliable on the NOS market.

Thanks for great service.

Kind Regards Steve West

Tube Reviews by Amplifier

Written around 2000, the prices are out of date but the rest of the information is pertinant.
http://www.tonequest.com/articles/article3.htm

Power tubes are subjected to a 5 stage test before shipping.

• Each tube is measured for transconductance and current draw with 400 volts on the plates. Measuring at real amp voltages is the only way to assure a tube will work correctly in an amplifier. Many tube testers measure at 150 volts or less.
• Each tube is tested for noise and microphonics. Rejected tubes are discarded.

Please read the reviews of tube types and amplifier types. These are all real reviews submitted by guitar players and audiophiles. Remember, tone isn't a science and "one man's treasure is another man's garbage".

Rectifier Interchangeability and it's relation to Clean Headroom

Here's the list of the most common guitar amp rectifiers in order of most to least clean headroom (highest to lowest voltage output) and least to most sag:

All of the following tubes are interchangeable and may be used anywhere one of the other tubes was originally specified. The only possible exception is that 12AY7/6072As should be used only in circuits where the plate load resistor is rated for at least one watt.

Q: Do I need to rebias my amp when changing tubes?
A: Sometimes. When changing preamp tubes, biasing is not necessary. Preamp tubes are typically the smaller, 9 pin tubes in your amp. Typical preamp tube designations are 12AX7/A, 12AT7, 7025, 12AY7, 6072, 12AU7, 5751, etc.

When changing power amp tubes, whether to rebias or not depends on the type of amp you have. There are 3 general categories of amplifiers: