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EL34s in a 5F6A circuit

3/2/2005 10:53 AM
EL34s in a 5F6A circuit
I am starting to get the urge to build a 5F6A circuit, but I want the option to swap in KT66s or EL34s if it is possible. If my memory serves me, the EL34s require a different power transformer than 6L6s, with a 6.3 V rather than a 5 V, or another tap, or something like that. Can anyone help me? Are there any other considerations for building a Bassman that will take either 6L6, KT66, or EL34s?  
3/2/2005 12:35 PM

Sounds like you want to go in the Marshall JTM-45 direction. You definitely will need a bigger power transformer, likely something that can handle the higher B+ current and heater current of the EL34s. The current and voltage handling capability of your B+ rail resistors, filter caps and output transformer should be up to par as well, so maybe you should go with an output tranny made for EL34s. I would also get an output tranny with an adjustable secondary load so that you can accommodate the different ideal primary impedances for the different tubes. Make sure you make your bias circuit plenty flexible to accomodate the different biasing requirements for 6L6s, KT66s and EL34s. Even though the drive requirements of the different tubes are not night and day, you will probably have to compromise on what kind of drive you have going into the power tubes. Just pick one level of drive and go with it for all tubes. If you are getting a custom chassis or homebrew chassis, allow sufficient space between those big bottle KT66s. Good luck!
3/2/2005 1:22 PM

I thought about just building a JTM-45, but I really want to build a Bassman, with a 4x10 cabinet and all that. I am looking at the Weber kit, which is everything you need to build a 5F6A Bassman for $600. I just wanted to have the luxury of using EL34s if I want, and I figured if it was an easy swap, then it would be cool. Maybe I shoul wait to see if Weber puts out a Bluesbreaker kit?  
Thanks for the good information!
3/2/2005 4:21 PM

Using the EL34 vs a 6L6 isn't a big deal for most amps. The biggest problem with this swap is the fact that the EL34 heaters draw more current than those of a 6L6. Most Fender transformers can handle the extra load without problems, I've got a modified Bandmaster head that's had them installed for years.  
The EL34 requires a slightly different tube socket wiring but it's no big deal, you have to simply connect a couple of pins that are not normally connected on a 6L6 amp, it's fine to leave it that way if you want to switch back to the 6L6 later. Check out the Groove Tubes book, there's a detailed write up on what needs to be changed to make the swap.  
The EL34 will generally require slightly less bias than a 6L6 or KT66 in the same circuit. My Marshall for instance had about -34 volts with EL34's and about -45 volts with a set of KT66HP's which are spectacular by the way...
3/2/2005 6:06 PM
Al Lang

I have an Allen Old Flame, his version of a Super Reverb. I set it up to run EL34's. A few wiring changes on the sockets, ground pin 8, up the grit resistors your ready to go. I have a diagram of the tube sockets of a black face conversion to run El34s. I can scan it.  
You have to set up your amp to have a wide range to bias them along with 6L6s.  
The tone was nice, creamy with crunch at high volume, a great blues amp.  
Good luck,  
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