Cornford Hurricane Build: 10lbs of crap in a 5lb Chassis

[Reeltarded]

Love the whole idea, Tony! It's killer!

[xtian]

Good to hear!

[dorrisant]

Thanks again for the kind words...

I had someone question just what is the cathode bypass attenuation anyway... Well, here is where I read about it...

http://el34world.com/Forum/index.php?topic=4401.0

http://el34world.com/Forum/index.php?topic=4437.0

I actually backed into this idea by researching the Hurricane... There was a Hispanic site that had a member detail how to separate the cathodes and bypass caps etc.. Further research brought me to the above links and I was like "What??? Why have I not done this before? And, why have I never heard mention of this on TAG?" Maybe it has been discussed here somewhere... But it is here now!! Lol!

Anyways, everyone even slightly interested should try it. It is too easy not to.

The main doc I used is posted below (Three cheers for Da Geezer)... Please try it and enjoy!

[dorrisant]

Final pictures....

[Malcolm Irving]

The main effect of removing bypass cap(s) on a class AB push-pull power stage is to reduce the gain of the output stage. If you leave all the other controls set the same that will indeed give you a power reduction. But, if you can crank up the volume to get enough output voltage swing from the PI you can get almost all of the power back again. There will be a slight loss in power as the bias resistor(s) are now contributing to the AC load on the output tubes. However, this power reduction will be fairly small as the bias resistor(s) are a much lower impedance than the speaker load reflected through the OT.

[Malcolm Irving]

Using Merlin’s EL84 push-pull example:
http://www.valvewizard.co.uk/pp.html
With the bypass cap in place, a positive going swing of 12V on the EL84 grid is needed to push the plate current up to its maximum of 135mA.
Neglecting screen current to keep it simple:
Merlin shows a quiescent plate current of 30mA, for a cathode bias of about 12V. So a bias resistor (for each EL84, not shared) of about 400 ohms is required (=12/0.03). (Probably more like 350 ohms if we allowed for screen current.)
Without the bypass cap, a positive going swing of 54V (=400x0.135) is now needed to get Vg1 to zero and the plate current up to 135mA.
Can a typical 12ax7 long tail pair PI provide this much swing? Maybe, just about.
If so, the power reduction is only about 17% (= 400/(2000+400)). (The 400 ohm bias resistor now adds to the 2k reflected load and the current is the same in both, again neglecting screen current, so the power is reduced in proportion.)
If the PI can’t provide the necessary swing, then we do get more power reduction and the PI will clip before the power stage does.

[TUBEDUDE]

She's a beauty! Nice face, wood and plate. Did you etch the plate?

[dorrisant]

She's a beauty! Nice face, wood and plate. Did you etch the plate?

Yes, all plates are acid etched by hand... and thanks!

[Phil_S]

Does it come with a roadie? Kidding aside, nice job, Tony.

[dorrisant]

Does it come with a roadie? Kidding aside, nice job, Tony.

Phil, did you mean a road case or a roadie to carry it?

It is pretty heavy for the small size...

Malcom, I had to read that a few times to get it... and it makes sense.

[Phil_S]

We don't need a case, we just need someone to carry it. Roadies don't drop stuff, do they?

[RJ Guitars]

Hmmm... I missed this when the thread was fresh but it has some cool innovative stuff going on. Thanks for posting...