Need amp/cab configuration help

[snatchss454]

Ok guys and gals, this is my first separate head and Speaker rig and I'm confused on how to hook everything up. Here is the break down.

1 RM100M Head running ONLY 2 KT88s outside biased @ ~50
Cab1 GenzBenz Gflex 2x12
Cab2 Randall R212CX

Original setup:
Head Impedance = 8ohms
Head Main Out -->Cab1 16ohms
Head Extension Out -->Cab2 16ohms

After Blowing a fuse and removing 2 inner tubes:
Head Impedance left @ 8ohms
Cab1 - changed input from 16 to 8ohms
Cab2 - unplugged because of ohm options on rear

I read that only having 2 tubes in the head changes the amps impedance and now I'm unsure how to hook the cabs up in the back and what impedance to select on the amp.

I'm also curious if I should run the cabs in stereo? Is there any benefit? Any help would be welcome. Here are some pics of the back of the cabs to help out. I could also use a good online store to order more fuses if anyone has one.
Thanks Shane

 

[snatchss454]

Someone?? Anyone??

 

[JKD]

Main and Extension on the back are paralleled outputs.

It looks like both your cabs can run 16Ohm mono and therefore you went

1/16+1/16= 1/8 => 8Ohms

*BUT* by pulling half the tubes I think you need to go one step down in order to match and therefore the RM100 should be set to 4Ohms in the two tube configuration.

 

[snatchss454]

OK, so I should have left the cabs plugged into the 16 ohm inputs and just changed the switch on the RM100 to 4 ohms....correct?

 

[JKD]

OK, so I should have left the cabs plugged into the 16 ohm inputs and just changed the switch on the RM100 to 4 ohms....correct?

Yes. Hopefully no more blown fuses that way

 

[snatchss454]

Thanks a million!! Would you happen to know a good place to purchase replacement fuses?

 

[JKD]

Thanks a million!! Would you happen to know a good place to purchase replacement fuses?

Any good tube stockist has 'em usually....tubedepot, thetubestore etc. Also the giants like Mouser electronics, heck some have even had luck in Radio Shack.

I've not blown a fuse yet....lets hope it stays that way!

 

[JD]

If I have two 4x12 cabs and am using all 4 tubes in the RM100 can I plug in to the 4ohm mono (or 16ohm mono) on both cabs and leave the amp switched to 4ohms (or 16ohms) or do I need to do something different? Thanks.

 

[JKD]

If I have two 4x12 cabs and am using all 4 tubes in the RM100 can I plug in to the 4ohm mono (or 16ohm mono) on both cabs and leave the amp switched to 4ohms (or 16ohms) or do I need to do something different? Thanks.

If you are plugging two cabs into the RM100 Main and Extension then you should follow the formula for impedance added in parallel.

1/LoadA +1/LoadB = 1/Load

In your case 1/16+1/16 = 1/8 set the amp head to 8Ohms

You should not use 4 Ohms because 1/4+1/4=1/2 is only a 2Ohm load, which is too low.

 

[JD]

Thanks for clearing that up JKD. Hopefully I haven't hurt anything since I had everything running and switched to 4ohms.

 

[StratmanR2S]

I've seen on other forums where people have stated that removing half of your tubes will mean half the impedence. My Carvin V3 has a 100w/50w switch and it does not change the impedence and it is doing basically the same thing as removing two tubes. Also the output transformer is used to match your amp to your load. Different taps are used to to add or subtract the amount of windiings in the secondary of the output transformer adding or subtracting impedence or resistance.

Changing how many tubes are feeding the primary isn't gonna change the amount of windings in the secondary. It will change the power output and the votage and current (P=VI)in both the primary and the secondary but the output impedence should remain fairly constant. Output impedance will very some depending on current and frequentcy but it shouldn't very anymore than it does when you adjust your master volume knob.

Also Granger stocks a good assortment of fuses.

Back to the cab problem. Two cabs of the same value in parallel divides the impedence in half. Series adds it together.

You could run the two 16ohm cabs parallel for 8ohms or get 16ohm speakers. then you will have 2 8ohm 212 cabs.