Help me understand impedance

To avoid the electrical engineering and skip the math.

The power tubes performances depends on many factors, plate voltage, plate resistance, internal resistance, plate current, screen voltage, cathode current, cathode current, grid voltage, etcetera, etcetera, etcetera.

The resistance the tubes plate (anode) sees affects it's performance. Let's look at the tube data for a 6L6GC for an example (I couldn't find a good example for the EL84 used by the Peavey).

The linked graph is the performance of a pair of 6L6GC's in a class AB, push-pull arrangement. You'll see the minimum distortion is around 6,000 ohms. The highest output is around 7,200 ohms. If you lower or increase the resistance, the tubes output diminishes and its THD increases. The latter is very important in hi-fi, less so in guitar amps.

The speaker load creates the resistance reflected to the tube plates, but it requires the output transformer to convert it to the right plate resistance.

If we want to show our pair of 6L6GC's a plate resistance 6,000 ohms and are using an 8-ohm speaker cab, we need a output transformer with a ratio of 6000:8; the OT takes the 8 ohms from the speaker cab and turns it into 6,000 ohms connected to the power tubes.

But it's a ratio after all and 6000:8 can also be written as 3000:4 or 12000:16. That means ...

  • If we use a 4 ohm speaker cab instead, our tubes only see 3,000 ohms of resistance...output drops from 50 watts to 30 watts (also bias rises, but we'll ignore that this time).
  • If we use a 16-ohm speaker cab, the tubes see 12,000 ohms of plate resistance. That goes off our graph, but we can estimate output drops to 40 watts (we also risk flyback AC voltage coming backwards through the OT and being amplified, this is dangerous top the tubes and OT in big amps)

Many amplifiers have a multi-tap output transformer, this means you can change the secondary (speaker) side of the transformer to match the speaker load you use. On the Peavey 30. It's deigned to use the 16-ohm tap when you use the internal speaker only (which is 16 ohms stock) but switch to 8- ohms when you plug in an extension cab (two 16-ohm speaker loads in parallel combine to a single 8-ohm load). If you plug in your 8-ohm cabinet with the internal speaker, the combined load is 5.3 ohms when the amp is expecting 8 ohms, plate resistance is drop by roughly a third and there will be decreased output and your tubes may run hotter. Ideally, you would unplug the internal speaker and just use the 8-ohm cab in the Ext Speaker jack. But that requires mods.

Personally, I wouldn't bother trying to combine them. I would either look for an 16-ohm ext cab or invest in a real head/cab half stack.

TL:DR Yes, they are compatible. You will suffer reduced output wattage from the amp, higher total harmonic distortion (not the musical kind) and your tubes may run hot, but it shouldn't damage the cab or the amp.

/r/GuitarAmps Thread