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Speaker Jack Question / Headroom Question

1/26/2006 2:16 PM
Aaron Walker Speaker Jack Question / Headroom Question
I'm working on an Octal Fatness amp with an el34 power section.  
I have a Hammond ESE125 OT.  
I want to be able to swap between power tubes...probably 6v6, 6l6, el34, and either a 6550 or KT88. I think the iron will be alright for the big boys as long as i keep the bias under 80ma (thats the rating on the OT). I still need to install a bias pot in place of the cathode resistor, but I'll also want to change the load on the OT Primary. Since I'll only be using an 8 ohm load on this, i can set it up two jacks (one for 2.5k load, the other for 5k load)  
refer to wiring diagram at the end of this link:  
How would i wire the extra speaker jack? Is it just in parallel with the other jack? Should I use shorting or non-shorting jacks? I may order a rotary switch later on, but I just wanna fool around with it tonight.  
And about the headroom thing.....what kind of difference can I expect using kt88 or 6500 in there?
1/27/2006 10:52 AM

To use switching a jack you need one that has both NO and NC contacts. The NC connects to white. The NO connects to yellow. Black goes to the sleeve. The jacks are then wired in parallel. An 8ohm load plugged into the switched jack will give a 5K primary Z. If the switched jack is unplugged then the other jack will give 2.5K with 8ohm load.  
If you don't have the correct jack then don't parallel the jacks. Just connect both sleeves to black and the tip of one to white and the tip of the other to yellow.  
A benefit of the switched jack setup is you can run two 8 ohm speakers, one in each jack, and get 2.5K primary.  
As for headroom, a 6v6 will be about half the power of the other tubes (-6dB I think). I don't think a 6550 will give you any headroom over 6l6 because 80mA current will not be enough to get full power from the larger tube.  
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