Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: john_t on October 27, 2014, 12:50:24 pm

Title: Little losses on 5 f1 negative feedback
Post by: john_t on October 27, 2014, 12:50:24 pm

I am a little losed on my Hoffman style 5f1 build. In particular the Negative feedback. I see the schematic has the secondary of the output transformer to the cathode of the preamp tube and to the tip of the speaker jack. I can not see or visulize it on the layout photos. Am I missing something


Thanks John.

Title: Re: Little losses on 5 f1 negative feedback
Post by: Willabe on October 27, 2014, 12:58:13 pm

Look in here;

http://el34world.com/Hoffman/files/Hoffman_5F1.pdf (http://el34world.com/Hoffman/files/Hoffman_5F1.pdf)

http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1_schem.pdf)

http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1-layout.png (http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1-layout.png)

                       Brad    :icon_biggrin:

Title: Re: Little losses on 5 f1 negative feedback
Post by: john_t on October 27, 2014, 01:12:45 pm

Yeah, that's my problem I see the negative feedback going from the board to pin 8. On the layout diagram. I  don't see or understand the conection between the ot tranny  to the  jack tip to pin 8 I see in the schematic

Title: Re: Little losses on 5 f1 negative feedback
Post by: sluckey on October 27, 2014, 01:42:17 pm

Oops! Looks like Doug forgot the 22K NFB resistor on the layouts. I'll call his attention to it.

Title: Re: Little losses on 5 f1 negative feedback
Post by: EL34 on October 27, 2014, 01:59:33 pm

Sorry guys.
I had the note on the board but forgot to add the resistor


I'll fix the layout diagram
thanks for the heads up

Title: Re: Little losses on 5 f1 negative feedback
Post by: EL34 on October 27, 2014, 02:20:45 pm

I fixed the layout on the 5F1 and 5F2A

You have to hit refresh on your browser to see the new version

http://el34world.com/Hoffman/files/Hoffman_5F1.pdf (http://el34world.com/Hoffman/files/Hoffman_5F1.pdf)

http://el34world.com/Hoffman/files/Hoffman_5F2A.pdf (http://el34world.com/Hoffman/files/Hoffman_5F2A.pdf)


You can just attach the 22k to the existing lug and use some heat shrink and a wire to reach the speaker jack
I will add one more lug to the existing layouts for new boards

Title: Re: Little losses on 5 f1 negative feedback
Post by: sluckey on October 27, 2014, 02:28:56 pm

Doug, you may want to mention NFB near the speaker jack on page 2 also.

Title: Re: Little losses on 5 f1 negative feedback
Post by: EL34 on October 27, 2014, 02:47:48 pm

will do, thanks

Title: Re: Little losses on 5 f1 negative feedback
Post by: EL34 on October 28, 2014, 07:48:20 am

I uploaded new PDF files for 5F1 and 5F2A this morning
Page 2 shows has a feed back note at the speaker jack


I also uploaded new BOMs for 5f1 and 5F2A


Go here to get all the new files
http://el34world.com/schematics.htm (http://el34world.com/schematics.htm)


make sure you hit refresh on your browser after clicking on the links


Thanks for the heads up guys