Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on July 15, 2017, 08:06:35 pm

Title: EL84 SE BIAS
Post by: TIMBO on July 15, 2017, 08:06:35 pm

Hi guys, Building a SE El84 clone of a much sort after little amp http://el34world.com/Forum/index.php?topic=19477.msg201773#msg201773 (http://el34world.com/Forum/index.php?topic=19477.msg201773#msg201773)
Using what I have on hand transformer wise I have come up with this.
(http://i.imgur.com/csfX5nX.jpg)
I originally did not have a valve rectifier.
With the SS rectifier I was hoping my voltages were close to this.
(http://i.imgur.com/TkQyvLD.jpg)
Knowing that I'm not going to be able to get the voltages close to the original amp I've increased the cathode resistor to 390ohms.
As per the voltage chart the EL84 is at about 90% plate dissipation (just under 11W).
With the valve rect in the plate dissipation is lower as expected.
(http://i.imgur.com/a1b4Oot.jpg)

Should I increase the cathode resistor to 470 ohms just to relieve a bit of stress???

Also I'm thinking that a DPDT switch to add in a 100ohm resistor in series to the 390ohm would work when in SS rect mode.

Title: Re: EL84 SE BIAS
Post by: shooter on July 15, 2017, 08:35:43 pm

I'd say ship it.  you using modern tubes?  I did a PSE el84 at about 11w a tube and to my knowledge she's still out in the world making music

Title: Re: EL84 SE BIAS
Post by: TIMBO on July 15, 2017, 08:40:23 pm

Hey shooter, Just did the calcs on the original amp
Plate-264v
Screen-266v
Cathode-7.3v
Plate-cathode 256v
Plate diss-12.7w

Title: Re: EL84 SE BIAS
Post by: labb on July 15, 2017, 09:24:52 pm

It is pretty common to bias single end amps at 90% dissipation. I think that if you look at some of the old Fenders you will find some at 100%. I have built several SE amps and all are at 90%. Not a problem.

Title: Re: EL84 SE BIAS
Post by: EKDENTON on July 15, 2017, 10:57:01 pm

I have the el84's biased hot in my Orange amp with a 120 ohm resistor.  I have not had any problems at all. It sounds awesome and I use that amp a lot. I tried a larger value resistor and the amp did not sound very good. Of coarse this one is PP and your doing a single ended but I just wanted to say i am using the JJ brand tubes and they must be pretty stout, the data sheet suggests 270 ohms for pp class ab.

Title: Re: EL84 SE BIAS
Post by: TIMBO on July 16, 2017, 04:27:25 am

Thanks guys, A good day in the shed and I think this one is sorted.
(http://i.imgur.com/3Z3w9Pk.jpg)
(http://i.imgur.com/Sv3UDc2.jpg)

On to the fun bit...........the cab  :icon_biggrin:

Title: Re: EL84 SE BIAS
Post by: TIMBO on July 22, 2017, 06:47:10 pm

Hi guys, CAB is done!!
(http://i.imgur.com/c7qYu3b.jpg)
(http://i.imgur.com/V5l5bLX.jpg)
(http://i.imgur.com/mpt7T4s.jpg)
(http://i.imgur.com/qUfW0Xs.jpg)

Title: Re: EL84 SE BIAS
Post by: shooter on July 22, 2017, 06:54:03 pm

very fine as always!, I like how you stacked one on the other, so the bottom amp is next :laugh:

Title: Re: EL84 SE BIAS
Post by: TIMBO on July 22, 2017, 07:03:40 pm

Hey shooter, WHICH one do you want me to do next  :help:
(http://i.imgur.com/WLmIxi8.jpg)

Title: Re: EL84 SE BIAS
Post by: TIMBO on July 22, 2017, 07:08:43 pm

What about one of these.............
(http://i.imgur.com/EkMmsPs.jpg)

Title: Re: EL84 SE BIAS
Post by: shooter on July 23, 2017, 10:21:11 am

 :think1:
Son, you win :laugh:
you need to go play in the desert for awhile!

Title: Re: EL84 SE BIAS
Post by: TIMBO on September 15, 2017, 08:02:19 pm

Hey shooter, A mate did a clip for me.
https://www.youtube.com/watch?v=V3FmZ2_1ppE (https://www.youtube.com/watch?v=V3FmZ2_1ppE)

Title: Re: EL84 SE BIAS
Post by: shooter on September 15, 2017, 08:39:06 pm

sounds great! although I do lean pretty hard SE, PSE, QSE :icon_biggrin:

Title: Re: EL84 SE BIAS
Post by: super&plexi on September 15, 2017, 09:08:22 pm

Dug the tube rect. Clips....its got more breath, like a songwriting amp. Cool as hell. And loved your presentation