Using 2 power tubes instead of 4

[spuds]

I was thinking of retubing my amp and only using half the number of power tubes. I think I read somewhere on this forum that doing this will change the output impedence rating. Is this true? How does it change it?

 

[Lake Placid Blues]

Re: Using 2 power tubes instead of 4

What amp is it? On 100 watt Marshalls this is pretty simple, but adjusting the impedance may be problematic on some others.

You can run most class AB amps, such as a 100 watt Marshall, with either the outer two in, or the inner two in. The rule is to set the impedance selector to 1/2 of what it should be set to. For example, if your driving a 16 ohm load, set the selector to 8 ohms.

 

[spuds]

Re: Using 2 power tubes instead of 4

Its a fender super six reverb... essentially a twin. It doesn't have an impedence switch. So if use 2 power tubes, I guess it would mismatch my cab. Unless I can rewire my cab to DOUBLE the impedence? Is that right?

 

[The Golden Boy]

Re: Using 2 power tubes instead of 4

But why?

Halving your power will cut your volume by around 12%, and barely noticable in either overall volume or decreased headroom.

 

[spuds]

Re: Using 2 power tubes instead of 4

I'm aware its not much. But I figure that a little is better than nothing. Coz I play in a setting where I really can't turn it up very loud... and plus, its 2 less tubes I gotta buy. =)

 

[Lake Placid Blues]

Re: Using 2 power tubes instead of 4

Its a fender super six reverb... essentially a twin. It doesn't have an impedence switch. So if use 2 power tubes, I guess it would mismatch my cab. Unless I can rewire my cab to DOUBLE the impedence? Is that right?

Fenders handle impedance mismatches fairly well. On most Fenders you only have an 8 ohm tap for all speaker jacks. Even when a extension cab is used and you plug into the extra jack it doesn't change anything. People have been mismatching Fenders forever this way, and it rarely causes any problem. Don't worry about it.

Reducing wattage by half reduces volume 3 db's. This is the same as going from a G12H to a G12M. I noticed your other thread. By reducing wattage and using a lower efffeciecy speaker set, it's a 6db reduction. That's actually pretty noticable. Even a 3db reduction could be crucial in some gigging situations. However, the G12H has a tighter bass and usually works better than the GB in an open back cab. The GB allows for higher master setting and more pushing of power tubes before you get into trouble with volume though.

 

[spuds]

Re: Using 2 power tubes instead of 4

Fenders handle impedance mismatches fairly well. On most Fenders you only have an 8 ohm tap for all speaker jacks. Even when a extension cab is used and you plug into the extra jack it doesn't change anything. People have been mismatching Fenders forever this way, and it rarely causes any problem. Don't worry about it.

Reducing wattage by half reduces volume 3 db's. This is the same as going from a G12H to a G12M. I noticed your other thread. By reducing wattage and using a lower efffeciecy speaker set, it's a 6db reduction. That's actually pretty noticable. Even a 3db reduction could be crucial in some gigging situations. However, the G12H has a tighter bass and usually works better than the GB in an open back cab. The GB allows for higher master setting and more pushing of power tubes before you get into trouble with volume though.

Yeah? Cool, I didn't know that... Yeah I read that in my amp, there is a slight mismatch with the transformer or something like that. But Fender just thought "what the heck, its not gonna blow".

Thanks for your advice. Yeah, just trying to drop down the volume a little bit so I can just turn the master up that little bit more!! Is the volume difference between the G12M and G12H that significant?

 

[Lake Placid Blues]

Re: Using 2 power tubes instead of 4

It's noticable. Although 3 db's doesn't seem to be much just looking at the number, try A-B ing a 97 db speaker and 100db speaker back to back, and the difference in volume to your ears is more than most people would think.

BTW, you could use two 8 ohm speakers in a 2x12 wired in series for a 16 ohm cab, but wired in parallel it would give you 4 ohms. You wouldn't have a 8 ohm option this way, if you needed 8 ohm cab. Two 16 ohm speakers in parallel is the usual set up for 8 ohm 2x12's.

 

[ES350]

Re: Using 2 power tubes instead of 4

Fenders handle impedance mismatches fairly well. On most Fenders you only have an 8 ohm tap for all speaker jacks. Even when a extension cab is used and you plug into the extra jack it doesn't change anything.

umm, Fender used 8 ohm speakers almost exclusively. Ergo, any multi-speaker Fender amp could not be an 8 ohm load. Dual speaker Fenders have 4 ohm OT's, quad speaker amps have 2 ohm OT's. Plugging another 4x10" wired for 2 ohms into a Super Reverb gets you a 1 ohm load. A 50% load mismatch is a pretty good rule of thumb for Fenders...

 

[Lake Placid Blues]

Re: Using 2 power tubes instead of 4

Many of the heads, such as the bassman heads I used to use, are hard wired to 8 ohms. In a four speaker cabinet for the heads, 8 ohm speakers were wired series/parallel for 8 ohms. If you drove an additional 8 ohm extension cab you had a 4 ohm load on the 8 ohm tap.

 

[DesertRose]

Re: Using 2 power tubes instead of 4

Most of the answers are being mentioned already =P
I would think that using 2 tubes instead of 4 would cut the power by almost half