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Drake 784-139 50W OT?
Posted: Sat 03/01/08 3:21 pm
by sub
Guys,
I have a old ('76) Drake 784-139 OT.
What do you think, is it good for a 36W build.
I know i can put this OT in a 36W, but in your opinion the sound will be very good, or good, or not bad........
....or i should build other amp, say cathode biased EL34 with 18W preamp, in other words 18W with EL34....
Plus i have a 50W PT, but with low secondary voltage 310-0-310 (i measured it is about 300VAC).
Any opinion would be appeciated.
Posted: Mon 03/03/08 3:26 am
by sub
Nobody tried 36W amp with 50W OT (with 3400 ohm pimary)?
Posted: Mon 03/03/08 10:27 am
by Phil_S
Sometimes the stated spec on a tranny is not consistent with the turns ratio and other times it is. It certainly seems to me that 8K for the 18W suggests 3k4 for the 36W is reasonable. I don't believe things need to be all that precise. We are discussing static ratings for a dynamic appliance.
If I had one, and were building a 36W-er, I wouldn't think twice about giving it a try. If you like it, good and if not, you get something else.
Posted: Mon 03/03/08 1:47 pm
by sub
OK! Thank you Phil!
Posted: Mon 08/17/09 6:38 pm
by manic
I built a cathode biased amp using EL34's and the very tranny Your askin about.
Its sounds and work great.
I actually prefer 6l6's for the band Im currently in.
Im running 400v on the plates and biasing with a single 240ohm cathode resistor.
Sure its not by the book I suppose, but I really dont mind.
It could be cooking the tubes, but it seems to be around 80% diss.
It works and in guitar amps that is really the bottom line!
The Chieftain uses EL34's cathode biased with about a 400v B+ on the plates. They claim its class "A".
One thing though I have no idea what the primary impedance is on a Chiefs OT.
Posted: Mon 08/17/09 9:08 pm
by zaphod_phil
This transformer could be used either with a pair of EL34s or a quad of EL84s. Your choice
80% is pretty good for a cathode-biased Class AB amp.
manic wrote:The Chieftain uses EL34's cathode biased with about a 400v B+ on the plates. They claim its class "A".
Which everyone knows is Vox/Matchless marketing speak for cathode-biased Class AB.
Posted: Wed 09/16/09 9:49 am
by txbluesboy
Matchless uses 4K primary impedence OTs in both 18 watt and 36 watt Amps. Terry
Posted: Thu 09/17/09 3:52 am
by JamesHealey
i've seen this OT used in an Vox AC30 before so i guess it works!
Not sure how it sounded though.
Posted: Thu 09/17/09 8:02 am
by darkbluemurder
manic wrote:I built a cathode biased amp using EL34's and the very tranny Your askin about.
Its sounds and work great.
I actually prefer 6l6's for the band Im currently in.
Im running 400v on the plates and biasing with a single 240ohm cathode resistor.
Sure its not by the book I suppose, but I really dont mind.
It could be cooking the tubes, but it seems to be around 80% diss.
It works and in guitar amps that is really the bottom line!
The Chieftain uses EL34's cathode biased with about a 400v B+ on the plates. They claim its class "A".
One thing though I have no idea what the primary impedance is on a Chiefs OT.
I just finished a Chieftain build this week and used a Marstran 50W Marshall output transformer. Primary Z is 3k4 AFAIK. I have about 410V B+ on the plates but I upped the cathode resistors from the specified 270R to 390R (I had planned to use 330R but 390R was what I had lying around). Seems to work well although it still gets a bit hot.
Bottom line: it should work.
Cheers Stephan