Drake 784-139 50W OT?

sub · Post by **sub** » Sat 03/01/08 3:21 pm

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.

sub · Post by **sub** » Mon 03/03/08 3:26 am

Nobody tried 36W amp with 50W OT (with 3400 ohm pimary)?

Phil_S · Post by **Phil_S** » Mon 03/03/08 10:27 am

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.

sub · Post by **sub** » Mon 03/03/08 1:47 pm

OK! Thank you Phil!

manic · Post by **manic** » Mon 08/17/09 6:38 pm

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.

Post by **zaphod_phil** » Mon 08/17/09 9:08 pm

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.

txbluesboy · Post by **txbluesboy** » Wed 09/16/09 9:49 am

Matchless uses 4K primary impedence OTs in both 18 watt and 36 watt Amps. Terry

JamesHealey · Post by **JamesHealey** » Thu 09/17/09 3:52 am

i've seen this OT used in an Vox AC30 before so i guess it works!

Not sure how it sounded though.

darkbluemurder · Post by **darkbluemurder** » Thu 09/17/09 8:02 am

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

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Drake 784-139 50W OT?

Drake 784-139 50W OT?