2-EL84 vs 4-El84 tone

[txbluesboy]

I tried my Rocket set at 16 ohm on the output selector into an 8 ohm cab. I like it better, it seems clearer and has more punch.

[j-po]

Hi guys,
New to the forum. Must say you have lots of interesting topics here.

Anyway, one factor that has been overlooked is the amount of tubes the PI has to drive. Could this affect tone? I would expect it would make a difference at least when the power tubes are overdriven as the grid starts to pull more current.

[Merlinb]

Hi guys,
New to the forum. Must say you have lots of interesting topics here.

Anyway, one factor that has been overlooked is the amount of tubes the PI has to drive. Could this affect tone? I would expect it would make a difference at least when the power tubes are overdriven as the grid starts to pull more current.

That's a good point, you have to deal with twice the grid current, so you can expect a more crunchy, 'driven' tone with 4 EL84s.

The main reason for the sparkle with 2 EL84s is the bass roll-off caused by the OT. With 4 valves you normally use a 4k primary, so with the same primary inductance you'll get a little more than 1 extra octave of bass out of the OT.

[dave g]

I tried my Rocket set at 16 ohm on the output selector into an 8 ohm cab. I like it better, it seems clearer and has more punch.

Umm, you will ruin your amp by doing that...

[fishy]

I tried my Rocket set at 16 ohm on the output selector into an 8 ohm cab. I like it better, it seems clearer and has more punch.

Umm, you will ruin your amp by doing that...

Can you help me understand why its bad for the amp. I would like to understand this better.
It appears manufacturers use mismatching for tone shaping, KOC suggests a correctly selected tranny will cope with a mismatch fine and can be used for tone shaping.
Speaker impedance is not constant anyway with speakers going significantly higher than nominal impedance in most instances so why
I have seen some speakers with fairly flat impedance curves but they are geared to audio nuts and generally low power tube stuff.

Are you suggesting mismatching to a higher impedance is bad or mismatching as a whole?

Pete

[Jana]

Mismatching with, for example, an 16 ohm speaker on the 8 ohm tap doubles the impedance that the tubes are seeing. So let's say the transformer is supposed to be 4k. With the mismatch described, the impedance is now 8k. Tubes will much better tolerate a mismatch in the opposite direction. Going the direction in this example, however, is kind of like stepping on the gas in your car without it in gear... WEEEEEEE! What can happen is arcing of the tubes and/or the tube socket pins. Occasionally the result of this is then a chain reaction with arcing in the transformer. Most times you will be okay but it can happen. It is more likely to happen in amps with higher plate voltages.

You can intentionally mismatch if you know what you are doing and why you are doing it. I would avoid, however, arbitrarily mismatching just to haphazardly see what sounds better. Yeah, it might sound great, they often do just before things go nuclear.