OT Firefly output tube options

[John Lyons]

Hey there.

There has been some mention of adding two El 84s (3 tube sockets) in parallel with the 12AU7 in the firefly amps output (with a higher output wattage transformer of course) . Either to switch between the output tubes via  switch or to pull out the 12AU7 and plug in 2 EL 84's in the other two sockets when you want more volume. Does anyone out there know the pin connections/conversion to acomplish this, since the 12AU7 is a triode and the EL84 is a pentode. I just need to know how to wire up the pentodes in parallel with the dual triode.

It would be nice to have it range from 1.5 watts to 6 when you switch to the EL84s.

Thanks for your time

[Peter Snowberg]

Welcome Basicaudio,

Try this schematic:

http://www.triodeel.com/compact.html

For an in-between choice, there is also a tube called the 12BH7 which is like an oversized 12AU7.

For the best one-stop source of basic tube data, download the Tube Data Sheet Locator from http://www.duncanamps.com/

Best of luck on your build!

Take care,
-Peter

[Ansil]

not a bad idea, but whynot just incorporate some 6dx8's as they are four watts each on the pentode side and also have half a 12ax7 in it or a single triode too.  seems like it would be a little easier.

keep it as the outputsection and just put a voltage drop on it for a lesser wattage amp..

i original did this to with my second firefly for the b+ drop

i did the two tranny thing back to back and had the coiltapwired up too. so wheni hit the swtich the whole amp went from 150or so to around 280 or so..

nice little trick. the next time i added the option to do it to either the preamp or poweramp section.  or both.. so you oculd run

pre     150+    280+  280+  150+
power 280+    150+  280+  150+

nice little trick and if you add in a bias adjustment it works like a charm

[John Lyons]

Ansil

Hmmm..not quite sure I got the grasp of what you were taliking about. I was figuring on changing the 3 watt Hammond 125A for a 125 that can handle maybe 10 watts or so and wiring in 3 sockets (two for the el84's and 1 for the 12ax7.) then I could change out the 12ax7 for the dual 84s when I needed more juice. Sounds simple at least!

Peter.
I have the article on the self split EL84 you linked to. I'll have to take another look at it. I need to loo into the 12BH7 as well. Thanks for the info.

JOhn

[Ansil]

the tube will put out less wattage with less power in a nutshell

the tube i was refering to is a single triode ala a 12ax7 adn a single pentode ala el34 or such

the pentode is rated at 300v four watts output,
so u could use two of these that will give u the two halves of the 12ax7's for preamp, and still leave in the 12au7 for the smaller wattage,

also u will have to change the output tranny..

but look at it this way..

yours gives you four tube sockets

1 12ax7  1 12au7 2 el84's that you wish to remove when you dont' need as much power.

mine uses

1 12au7  and 2 6dx8's

and you simply remove the power from either the 12au7 or the half of the 6dx8 you are not using.  so you can leave all the tubes in place. and still have the flexability.  wire in a bias for each that can be adjusted if nesc.

and even without using the 6dx8's for poweramp you can still use the triode half for the preamp.

if i am still unclear here is the datasheet on the 6dx8 if someone wants to help out with the explanation please do so

http://hereford.ampr.org/cgi-bin/tube?tube=6dx8


http://hereford.ampr.org/cgi-bin/tube?tube=el84

http://hereford.ampr.org/cgi-bin/tube?tube=12ax7

http://hereford.ampr.org/cgi-bin/tube?tube=12Au7

hope that helps in orderthe datasheets for the 6dx8  el84  12ax7 12au7

[John Lyons]

Ok, I get it now (had to look up the 6DX8) It's a single triode with a single pentode in one tube package. Pretty clever.

I have already build the firefly so rather than rework it too much I'll go with trying to figure out trying to add two El84s in self split.

I Wonder what the crosstalk between the pre and power section on the 6DX8 especially in using it as pre an power in an amp. Wouldn't it feedback or occilate being so close (same glass envelope) to the triode/pre amp section?

Can you substitute two El84s or 6DX8s for the 12AU7 (wired in parallel)without any extra components in the circuit. Ansil mentiond bias circuitry...

The high octane was built be a few folks using selfsplitting, I need to get my head around this.

[John Lyons]

By the way. I'm not stuck on using the 2 el84s so much as wanting to get an arangement where I can use the regular split 12au7 for jamming quietly and switching to another tube or tubes when I need a few watts more. Nothing too loud, just 3 to 6 watts or so. Whatever I can do to get this would be nice. Either pulling tubes or switching via the front panel.

Can I use the same split method as cris hurley's High Octane in the fire fly? Maybe with just a single el84 then...I'm pretty green at this so bear with my questions, I need it spelled out a bit.

Thanks!

[Ansil]

i havent had any problems with it.