EL84's in Twin Reverb

[SiriusBlues]

I'm interested in running 2 EL84's in my 75 Twin Reverb using the THD Yellowjackets with a pair of the existing 6L6's. THD advertised that the amp would then run as class AB. Not being an amp tech, my question is which of the octal tube sockets should I put the Yellow Jackets in ?

Thanks in advance.

[mlp-mx6]

outer two or inner two. NEVER in the "left pair" or the "right pair".

[Structo]

I wonder how the yellow jackets affect the output impedance?
Anybody know?

[Johnhenry]

I don't really know, they should have told you the proper placement i have a Question for you, i don't know anything about your yellow jacket's ? how do you propose to run those EL 84's on the existing B+ which i know will be to much voltage for those tube's and how does it fix the Bais problem, did you install dual bais pot's ?
I'm interested if the yellow jacket makes all this possible, cool if it does ! but if it don't, your gonna have problem's.
John

[ampgeek]

Looks like they do it all! THD literature excerpt below:

Class-A With Any Amp!
Yellow Jacket Converters not only
rearrange the pin locations of the 9-pin
EL84 tube bases to permit them to work
in 8-pin sockets, but also perform a
number of essential electronic
functions that permit the EL84s to
function well and sound great in your
amplifier. These functions include: 1)
Reducing the voltage on the plates of
the tubes, 2) Reducing the voltage and
limiting the current on the screens of
the tubes, 3) Blocking the amplifier’s
DC bias voltage from reaching the
control grids of the EL84s, 4) Giving
the control grids of the EL84s their own
ground reference resistor for gain
control, 5) Giving the EL84 its own
cathode-bias (self-bias) circuit to
enable it to run safely and squarely in
the Class-A range. (Sorry if this is a
bit more information than you needed,
but some people like this kind of thing.)

The resultant impedance of running mixed breeds is an interesting question. For a first pass SWAG it seems safe to view the tubes as variable resistors. Since you have two "resistors" in parallel on each side of the OT primary you could calculate as you would with any other parallel resistor arrangement (1/Rt = 1/R1 +1/R2) using the apparant plate resistance of each tube type as R. The "optimum" reflected load across the OT (in the shoot from the hip approach!) should be ~1/10th of that.

Does that seem reasonable?

Dave O.

[SiriusBlues]

Thanks for your replies.

[fperron_kt88]

To me, the "literature" version translates into:

-we added a safety resistor in series with the screens
-we added a resistor in series with the cathode (thus biasing the tube hopefully somewhere into class A)
-we added a cap in series with the grid connection and some high value resistor from the grid to the ground
-the plate is just the plate, as it was.

I don't know if it makes sense, but to me this arrangement would turn a typical class AB output stage into a class A output stage, given the proper power enveloppe and bias conditions for the tube and circuit...

Do they sell specific yellow jackets for specific amps? Or B+ ranges?

I would really like to know how it turned out in your amp. If I understand correctly, you are basically reconfiguring your amp into something akin to simul-class... Clever if it works ... and sounds right!

[Tubetwang]

I have a pair of these that i sometimes put on me Tweedy...

You may put two in your Twin, leaving two 6L6 out, for a warmer compressed sound.