Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on March 16, 2012, 10:25:05 am

Title: Need help with an amp running el84 at 240v on the plates!
Post by: bruno on March 16, 2012, 10:25:05 am

I trying an output stage that uses 2 el84 and a power transformer with 185-0-185 VAC, I'm expecting around 240V on the el84 plates.

I'm using a 8k primary OT, and I'd like to keep the amp's response and dynamic despite the low voltage. However I'm not really sure 8k for the primary will be fine, running the output stage at such a low plate voltage.

I need to go through the exercise of calculating my screen voltages and cathode voltage/bias current, in order to squeeze 15 watts out of this output stage.

Help would be greatly appreciated!

Title: Re: Need help with an amp running el84 at 240v on the plates!
Post by: rzenc on March 16, 2012, 11:53:07 am

Check out page 02 on Mullard EL84 datasheet:

http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf)

There are info regarding 250V on plates AND screen. Rl is 8K. Rk 270R. Pout 11W.

185VAC into SS recto might deliver 260V unloaded. Pretty close to your specs (8%>).

I built a 4xEL84 4K plate to plate running plates and screen @ 285V, 280V respc.. Rk: 82R Rg2:1K.

Smoke test like this set up and adjust Rg2 and Rk to taste...
Add Ck...change it...I ended up with 100uF||0.1uF.

The higher Rg2 added compression effects...

Hope this helps!
Best Regards
R.

Title: Re: Need help with an amp running el84 at 240v on the plates!
Post by: PRR on March 17, 2012, 09:14:10 pm

> squeeze 15 watts out of this

Can't do that.

I say almost 12W, Philips says 11W.

Changing supply voltage changes BOTH voltage-swing AND maximum current. Optimum load changes only slightly.

Power output changes nearly as Square of voltage. From 300V to 240V seems to be 0.64 times the power. (It's actually worse.)

Why do you need a specific Watts level?

Title: Re: Need help with an amp running el84 at 240v on the plates!
Post by: bruno on March 18, 2012, 05:37:21 am

Don't really need to PRR. But since some 2xel84 are not loud enough for gigging applications, I just wanted to make sure the amp would be loud enough.

But I guess I'll only know for sure once it's built.

Title: Re: Need help with an amp running el84 at 240v on the plates!
Post by: rzenc on March 18, 2012, 08:40:20 am

2xel84 are not loud enough for gigging applications

if you drive sensitive speakers, >=100dB SPL @ 1W, you may be able to use your 11W amp...Actually, 2x100dB SPL speakers might add up to 103dB SPL @ 1W...each time you double up W applied, i.e.: from 1W to 2W you get double power...it's only 3dB more...with 2x100dB SPL speakers + 11W amp...you may squeeze ~110dB SPL, enough if drummer is not superloud..

Which speakers do you plan to use?

Title: Re: Need help with an amp running el84 at 240v on the plates!
Post by: bruno on March 18, 2012, 10:42:46 am

I'm using a celestion alnico gold which is has a high level of db.