View Full Version : Ideal primary impedance for new build
11-07-2011, 09:47 AM
Howdy all, I'm going to be working on a bass head and wanted your thoughts on an ideal output transformer, particularly the primary impedance. This is one area where I'm not expert. Basically I'll have 440Vdc on the plates running a pair of EL34 tubes. I'd like to get absolute maximum grunt out of this and hope that 50Watts is attainable. Curiously enough, Wikipedia shows the following for a pair of EL34/6CA7 tubes:
Power output: 55W
Anode resistance: (anode to anode) 3,4k Ohms
Anode voltage: 420V
Anode current: 2*120mA
Screen voltage: 375V
Bias voltage: -38V
This is pretty close to what I'll be running, but that 3.4k seems a little low when compared with other sources.
11-07-2011, 02:12 PM
Those are the spec's for a 50 watt Marshall Output Tranny.
11-10-2011, 09:56 AM
I think I'm going with the Hammond 1750G. It seems to be the optimum value and gives me the 4-8-16 Ω options I'll need to run various cabinets. Thanks for the input folks.
11-11-2011, 04:45 AM
When I had a Vox AC50 in for repair a couple of years ago (a choke winding to its terminal had broken off), I gave it the final test by using it for the bass amp, swapped it in place of the usual 100W Fender, for a session.
It sounded fantastic, it really filled the room, gave the impression of having headroom over the Fender, even though on the bench the Vox could only manage about 40W clean.
I'm not sure whether it's due to the mixed bias on the power tubes or the open loop power amp, or something else again.
The OT had been replaced with a Marshall / Drake type.
It may benefit from a higher B+ than 440V though - say ~480V at 'no signal'.
So although it may not be the obvious design to use as inspiration for your build, I recommend it for your consideration.
As a comparison, when I've tried the same test with similar EL34 amps, eg Marshall 50W JCM 800 or Traynor YB1A, the Fender walks all over them.
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