Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sean_thornton on April 14, 2016, 01:25:52 pm

Title: ECF82 for overdrive section in a princeton(ish) design
Post by: sean_thornton on April 14, 2016, 01:25:52 pm

Has anybody used the ECF82 (a triode-pentode) for an OD section? I am working on a schematic based on the Fender Princeton non-reverb and wanted to add a switchable overdrive section similar to some of Tubenit and Geezer's designs but using a single ECF82. I have a sleeve of NOS ones I picked up for another project.
Any help would be greatly appreciated. They sound wonderful in a preamp with the triode driving the pentode.

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: PRR on April 14, 2016, 06:41:31 pm

The whaaat?

Oh! 6U8! Not the application you want, but I've seen many in TV (as 5U8) and radio duty. Also seen it suggested for the Dynacos which used an odd pentode-triode in years when tubes were scarce.

Just do it. Fiddle cathode resistors to get semi-sane plate voltages. When in deep doubt, use Resistance Coupled Amplifier Table values for 6AU6 as your starting point for the pentode.

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: Paul1453 on April 14, 2016, 07:52:39 pm

Savage Croaker

Google search that and you will find a 6U8/ECF82 preamp.

I played with it on my breadboard, and as PRR pointed out, I was not happy with the sound of it.  :sad2:

A simple 6AU6 or 12AX7 produced much better results IMO.  :icon_biggrin:

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: sean_thornton on April 15, 2016, 11:44:26 am

I am familiar with the Savage Croaker, a friend of mine built one, great sounding amp. I am wondering if I should just use the preamp from that as the OD section  :dontknow:

I have never used a 6au6, I only know it is a 7-pin sharp-cutoff pentode. Can anybody suggest any schematics I should look at? Also, am I going to need a triode driving the 6au6 or is it enough on its own?

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: Paul1453 on April 15, 2016, 12:19:03 pm

If you thought the Croaker sounded great you should check out Dummyload's Bogen K10 mod:

http://el34world.com/Forum/index.php?topic=7404.0 (http://el34world.com/Forum/index.php?topic=7404.0)

This thing ROARS instead of croaks!  6AU6 input preamp, 12AX7, PP 6AQ5 output @ 10W.  :icon_biggrin:

You wouldn't need a K10 to start from, it could be built from scratch.

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: VMS on April 15, 2016, 02:12:53 pm

Vox ac10 uses ecf82 triode as a input stage:

http://el34world.com/charts/Schematics/files/Vox/Vox_ac10_2.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_ac10_2.pdf)

Maybe you could start with those values, or if you don't like the sound of that triode, it could be used as a phase splitter.

Is this the non-reverb princeton you're building?

http://el34world.com/charts/Schematics/files/fender/Fender_Princeton-AA964-schematic.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_Princeton-AA964-schematic.pdf)

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: sean_thornton on April 15, 2016, 03:45:21 pm

Yeah, it's the non-reverb model, it stays cleaner further up the dial. I want to try adding an OD section to that circuit. .

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: VMS on April 15, 2016, 05:31:32 pm

So just for fun I did some cuttin and pasting how I would implement the od section and then tweak it.

Triode values are from ac10 and pentode values from this thread:

http://www.ax84.com/index.php/bbs/dm.php?thread=309556 (http://www.ax84.com/index.php/bbs/dm.php?thread=309556)

Hopefully other forum members will chime in and critique this.

Title: Re: ECF82 for overdrive section in a princeton(ish) design
Post by: sean_thornton on April 17, 2016, 03:02:50 am

VMS, that is REALLY close to what I came up with, thanks for your work on that. I have to breadboard the OD section and then graft it into one of my other amps. I will post my schematic tomorrow.