Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kingplank on October 10, 2016, 10:22:18 pm

Title: New EL84 15 watt build
Post by: kingplank on October 10, 2016, 10:22:18 pm

Hi,
It 's been 3 years since I build a tube amp. This time a little high gain amp with cathode bias and EL84 tubes.
The schematic is unchecked and unproven, but I am going to take a go at it. maybe the good folks here can give an advice or two.


For transformers, I have a 6.3v 5amps x 2 heater transformer, and a mains trans with 120v and 220v output tabs. For OT, it is an old Dynaco A470 pulled from a ST-70


The schematic shows 238v and 173v output vs my 220v and 120v . My question is: If I run with the low power mode with so low voltage do I need to switch R47?


Second question: Is it a good idea to insert another Volume pot, in front or behind V3A?


If you see any drastic errors of this schematic, please let me know


Thanks in advance
KP

Title: Re: New EL84 15 watt build
Post by: tubenit on October 11, 2016, 06:55:55 am

I would be inclined to put in a PPIMV.   I would replace R27 and R28 with a 500ka pot.

All of that is a matter of personal taste and not essential to do.

With respect, Tubenit

Title: Re: New EL84 15 watt build
Post by: kingplank on October 15, 2016, 11:57:49 am

Components layout, with 60 pin turret board.
Just in case someone is interested

Title: Re: New EL84 15 watt build
Post by: kingplank on October 17, 2016, 08:06:36 pm

Wondering if the mains transformer is big enough, 60VA at primary at 220V,   @120V=  136ma

Title: Re: New EL84 15 watt build
Post by: shooter on October 17, 2016, 08:35:46 pm

Quote

Wondering if the mains transformer is big enough

I'd give the 2 '84s 100mA, 2mA each 12XX tubes.  you'll have to look up heater draw for all your tubes from tube datasheets

Title: Re: New EL84 15 watt build
Post by: kingplank on October 19, 2016, 08:49:07 am

Thanks Shooter. I took your advise and exchange the xfmr , now I have 500ma.
The heater transformer is good for 5A x 2. Its going to run cool.

Any predictions how this amp will sound at the 120v transformer secondary tap?

Title: Re: New EL84 15 watt build
Post by: shooter on October 19, 2016, 09:38:41 am

Quote

Any predictions how this amp will sound

that's pretty subjective, B+ is part of the equation, but pre-amp tone shaping has lots to do with it.  I've build 3-4 EL84's PSE not PP, and they don't mind being overdriven, they are easy to overdrive. once it's up and playing, there are wise folks here that have spent many hours bending and shaping tone, you'll have fun!

Title: Re: New EL84 15 watt build
Post by: kingplank on November 07, 2016, 09:07:34 am

Power supply and heaters wired up

Title: Re: New EL84 15 watt build
Post by: kingplank on December 01, 2016, 09:10:42 am

Corrected layout drawing, amp is 100% working. Other than speaker/cabinet differences, it definitely has the that signature Dark Terror tone.

Will post sound clips later