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Drake 784-389 OT
Posted: Tue Dec 29, 2015 5:19 am
by acdsee
Hi i have a jtm45 '89 first year production amp with Drake 784-389 OT and 5881 tubes and i want to convert it to jtm50 specs with el34 tubes but i cant find any info about OT specs.Could anyone help with some info about this transformer primary impendance and if can support el34 tubes?Thanks!!!
Re: Drake 784-389 OT
Posted: Tue Dec 29, 2015 7:20 pm
by axeman
Keep searching
Re: Drake 784-389 OT
Posted: Wed Dec 30, 2015 6:02 am
by neikeel
Yes you can use this OT with EL34s.
You will need to change the screen resistors to 1k (they are 470R as stock). You will probably need to change the bias range resistor (the one in series with the bias pot) either by substitiution (risks damaging the traces of the pcb or piggy-backing onto the existing one to get your new tubes in range.
The existing OT, if set up for 5881/KT66s I believe will have the same primary impedence as the Drake 784-74 and Drake 784-103 which were 8k. Generally EL34s in A-B push-pull pair operate best in the 3-4k range (a Drake 784-128 was the OT used in JTM50 has a 3.4k OT0 so if you do go this way set your amp at 16ohms and use an 8ohm cab that will give you the correct reflected impedence.
Re: Drake 784-389 OT
Posted: Wed Dec 30, 2015 11:45 am
by acdsee
Thanks for the help its difficult to find some technical info for the specific transformer.