Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Paulycoo on June 24, 2019, 01:52:57 pm

Title: Schedule 40 voltages
Post by: Paulycoo on June 24, 2019, 01:52:57 pm

Hello All,
I've just finished the schedule 40 and it sounds great! But I'm sure it's biased too hot. Can anyone help with the voltages please? I can't be sure of my maths being thick.
El84:
Pin 3= 11v
Pin 7= 326v
Pin 9= 324v

B+ is 330v using a 5y3 at moment.
So, 11v/250ohm=0.044amps x 315v = 13.8w

Is this right? It's 115percent class A.

Amp distorts almost immediately.
Any ideas?
Many thanks Paul McNamara

Title: Re: Schedule 40 voltages
Post by: sluckey on June 24, 2019, 02:08:38 pm

Math is good.

Title: Re: Schedule 40 voltages
Post by: Paulycoo on June 24, 2019, 02:22:06 pm

Thank you Steve
How do you recommend I bring it to 95 percent or So?
Cheers Paul

Title: Re: Schedule 40 voltages
Post by: shooter on June 24, 2019, 03:35:58 pm

changing bias on an SE requires doing a balancing act with diminishing returns.

increase the Cathode R by 100 - 200 ohms, re-measure, recalculate.  what you'll find is plate vdc will go up because current will go down and plate dissipation will do little.\

Quote

Amp distorts almost immediately.

I would tweak here 1st.  If you want more "headroom" you are probably overdriving the 84/6V6?, drop preamp gain, either changing voltage dividers, putting a lower gain tube in, all the above.  Once you're kinda sorta happy, go back n play with the PA some.

Title: Re: Schedule 40 voltages
Post by: PRR on June 24, 2019, 06:53:22 pm

14W won't kill an EL84 this year. And they cheap.

I just had a small surgery and more thoughts can come in a few days.

Title: Re: Schedule 40 voltages
Post by: Paulycoo on June 25, 2019, 01:48:11 am

Thank you all very much.
I hope you recover well from your operation.

When my 5v4 rectifier arrives, I'll test it all again exactly as schedule 40 spec.
Is there a limit to the el84 voltages? Is 400v too much?

Many thanks Paul

Title: Re: Schedule 40 voltages
Post by: PRR on June 25, 2019, 12:05:07 pm

> Is there a limit to the el84 voltages? Is 400v too much?

http://www.mif.pg.gda.pl/homepages/frank/vs.html