Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jrkrauss on November 25, 2020, 06:27:22 am

Title: Brown Concert hiss
Post by: jrkrauss on November 25, 2020, 06:27:22 am

I’m trying to chase down hiss in my 1960 Concert Amp. There’s no hiss  when it’s on standby. When it’s off standby the hiss is constant. Volume controls don’t affect it. Tone controls change the tone of it slightly. The doghouse was updated before i got the amp and those metal resistors test good. I feel like I’ve checked the usual suspects. Just wondering if I’m missing something obvious.

Title: Re: Brown Concert hiss
Post by: Latole on November 25, 2020, 07:05:24 am

Stanby ; no high voltage is send in the amp. Amp is near to OFF

Did you test with no guitar ? You must test amp noise with nothing plug to the amp.

1- Check preamp tubes with know good one

2-Hiss;
    Input jacks shorting contact to ground are dirty or broken ; to test them, plug 2 guitars in the 2 input jack. Guitars volume to 0
    No more hiss, issue are from input jacks.

3- Replace all 12XX7  100K plate resistor ; when old some plate resistor come noisy.

4- Clean all tubes socket and tubes's pins

Nothing to do with the hiss; on second had amp, I always check if fuse is the right one. With a too powerful fuse, amp is not protected

6G12 Schematic ;

http://ampwares.com/schematics/concert_6g12.pdf

Title: Re: Brown Concert hiss
Post by: jrkrauss on November 25, 2020, 07:52:03 am

I've got new tubes. I've replaced the plate resistors. I'll take a look at the jacks.

I can say, this morning I pulled v2 and there was no hiss. (I swear I did this before and didn't notice a change.) So I suppose that narrows the field a bit.

Title: Re: Brown Concert hiss
Post by: Latole on November 25, 2020, 07:55:33 am

4- Clean all tubes socket and tubes's pins

Title: Re: Brown Concert hiss
Post by: jrkrauss on November 25, 2020, 08:21:03 am

Well...I think I found it. I found a resistor that didn't match the schematic. There was a 10K resistor where the 6800 is supposed to be and a tiny cap bridging the 100K and 6800 resistors. I removed the cap and swapped in the right resistor. And the hiss is gone.

Thanks for the ideas.

Title: Re: Brown Concert hiss
Post by: Latole on November 25, 2020, 08:38:16 am

So the hiss was always there not only since few days ?