Speaker/ AMP Impedance question

[angelodp]

Hi, I have a Rocket Amp - with these specks

1 quad of EL84's = ~ 4 Ohms load impedance
1 pair of EL84's = ~ 8 Ohms load impedance

Output Transformer = ~ 5.2 K Ohms primary / 8 Ohms secondary (these are approximate and in reality they vary constantly)

The amp has an Impedance switch that allows 4/8/or 16 ohm settings and a 1/2 power switch that allows two of the four EL84's into the circuit.

Question is this. At full power what is the correct setting for the Impedance switch into an
8 ohm cab - 8 ohms ?

with the 1/2 power switch into the same cab ( 8 ohm ) should I then use the 16ohm setting?

Another question. Same amp, full power into a 16 ohm cab - what setting on the ohm switch?

Same amp. 1/2 power into a 16ohm cab, what setting on the ohm switch??

thanks Ange

[newey]

Ange-

This probably should have been posted in "Amps", but no problem.

I assume this is one of the Ampeg Rocket amps. There have been several flavors of this over the years including bass amps with the "Rocket" name. You might look for a manual for your specific model.

However, at first blush, I would think that the whole purpose of an impedance selector switch would be defeated if you had to change the impedance setting every time you changed the power switch setting. It's meant to be set at 8Ω for an 8Ω cab, 16Ω for a 16 cab, etc. I would be very surprised if it operated any other way.

But don't take my word for it, check it and see.

[JohnH]

I think some research is needed here on that specific amp. It sounds a bit like a built-in switchable version of the old trick of pulling two tubes. And when you do that, correct cab impedances need to be doubled, so an 8 Ohm setting with 4 tubes becomes a 16 ohm setting with 2 tubes. So what is not clear is whether this amp has a way around that built in, or not.

John

[sumgai]

John,

No research needed, newey's done the deed.



newey,

Stop being so bashful, you've hit the nail on the head much more often than not!



ange,

Your transformer is pretty much middle of the road, spec-wise. Some say it should be down around 4K for the primary for four tubes, but most agree that 5K is close enough to cause no harm (neither to the tonality, nor the tubes and their lifespan).

That said, Ampeg has always marked their output impedance selections for "real-world" use. IOW, a user shouldn't have to scratch their head during setup just to make sure that things don't blow up during the grand finale. And so it is......

For your case, alway select the output impedance that most closely matches your cabinet's rating. This applies even if you hit the so-called "half power" switch. The reason being, even when two of the tubes are idling (not receiving any signal to amplify), they are still connected to the output transformer. John's scenario is correct, when tubes are physically removed, but that's not the case here. The nominal impedance reflected by transformer's primary still sees all four tubes connected, thus, it changes very little, and hence, the output impedance remains almost the exact same as at full power.

HTH



sumgai

[newey]

SG-

newey,

Stop being so bashful,

We're talking about an Ampeg Rocket. I would love to own one of these, but I don't. I prefer to think of it as being "careful".

[angelodp]

Actually its a TW Rocket clone, the 1/2 power switch does take two tubes out of the circuit.
Here is the layout.

s260.photobucket.com/albums/ii9/angelodp1/?action=view¤t=LayoutH1dcopy.jpg

[sumgai]

ange,

While the cathode of the tube is now unbiased (via the ½-power switch), the tube itself is still "hot", in the thermal sense - the filaments are still on. This causes some leakage current, which the transformer primary does see. However, it's likely that this won't affect anything until you reach nearly full-drive levels. For most playing scenarios, the situation remains as I stated above - follow the markings on the chassis/switch, matching them to the rating of the cabinet(s).

Pragmatically speaking, even a mismatch of 2:1 (or 1:2, if you so wish) will not harm anything. You may notice a bit of tonal change, or even a slight volume change, but unless your sound levels have earned you the nickname of The Punisher, then that should be OK.

HTH



sumgai

[angelodp]

You are most kind.... thanks for great info.

best Ange