Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on February 11, 2025, 04:48:03 pm

Title: Hiss at idle ... ideas?
Post by: Diverted on February 11, 2025, 04:48:03 pm: Hi,

I am troubleshooting a single channel AB763 (deluxe reverb) build. Single channel, with opto trem removed and bias shift tremolo instead.
The amp behaves perfectly ... great trem, great and quiet reverb, very smooth and clean, no hum all the way to 10. I have zero complaints EXCEPT for the white noise I am getting at idle. Doesn't matter if an instrument is plugged in or not, it's a low grade soft fuzzy hiss that is noticeable.
Pulling V1 (preamp) and V2 (reverb driver) make no difference. Pulling V3 (reverb recovery/gain stage) kills it. Specifically, grounding out gain stage grid (pin 7) kills it; grounding out reverb recovery grid has no effect.
I have tried mulitple 12AX7s in that spot, none show noticeable differences.
So my problem is before that grid. I started poking around prior to that point and tried to trace the signal back, chopsticking and reflowig solder joints along the way, cleaning up lead dress etc. At this point I don't know my next step ... there's not much between V1B and V3B and if I've eliminated the reverb tubes as a source (pretty sure I have) there's not many places to look. Any ideas? Thanks!
Title: Re: Hiss at idle ... ideas?
Post by: shooter on February 11, 2025, 05:28:00 pm: unshare point E, temp in a 1.5k and a ~~~1-10uF cap, cathode to ground, gain stage
Title: Re: Hiss at idle ... ideas?
Post by: Diverted on February 11, 2025, 05:50:36 pm: Quote from: shooter on February 11, 2025, 05:28:00 pm
unshare point E, temp in a 1.5k and a ~~~1-10uF cap, cathode to ground, gain stage
Will do, thanks. The wife has called me in for the night but I will split the cathodes up tomorriw and report back. Thanks Shooter.
Title: Re: Hiss at idle ... ideas?
Post by: Merlin on February 11, 2025, 05:55:30 pm: It will be the 3.3M reverb mix resistor. It looks like you don't have the 10pF cap in parallel with it; that cap is there to shunt some of the high frequency hiss produced by that resistor.
Title: Re: Hiss at idle ... ideas?
Post by: Diverted on February 11, 2025, 06:10:38 pm: Quote from: Merlin on February 11, 2025, 05:55:30 pm
It will be the 3.3M reverb mix resistor. It looks like you don't have the 10pF cap in parallel with it; that cap is there to shunt some of the high frequency hiss produced by that resistor.

Thanks. Sorry for the inaccurate photo. It wasn’t most recent - I took it maybe an hour before I put a 10pf cap in parallel with the resistor. Unfortunately no change.
Title: Re: Hiss at idle ... ideas?
Post by: Merlin on February 18, 2025, 09:01:49 am: Did you sort it?
Title: Re: Hiss at idle ... ideas?
Post by: tubeswell on February 18, 2025, 09:32:58 am: Is the hiss occurring
A) with or without a guitar input cable plugged in?
B) With all control knobs at minimum rotation?
Title: Re: Hiss at idle ... ideas?
Post by: Latole on February 18, 2025, 02:08:35 pm: Quote from: tubeswell on February 18, 2025, 09:32:58 am
Is the hiss occurring
A) with or without a guitar input cable plugged in?
B) With all control knobs at minimum rotation?

".....Doesn't matter if an instrument is plugged in or not, "
Title: Re: Hiss at idle ... ideas?
Post by: Latole on February 18, 2025, 02:13:13 pm: IMO the filter cap may not go at this ground.
And the bare twisted wire should not be grounded a is left side. I can't see if it is.

You do a great job !
Title: Re: Hiss at idle ... ideas?
Post by: Willabe on February 18, 2025, 02:48:32 pm: Quote from: Latole on February 18, 2025, 02:08:35 pm
Quote from: tubeswell on February 18, 2025, 09:32:58 am
Is the hiss occurring
A) with or without a guitar input cable plugged in?
B) With all control knobs at minimum rotation?

".....Doesn't matter if an instrument is plugged in or not, "

Yes it does matter.

It could be 2 different causes.