Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on April 08, 2025, 11:35:31 am

Title: help with channel debug. massive hiss
Post by: joesatch on April 08, 2025, 11:35:31 am

i have this channel which I've isolated from the other preamp channel and have it running into the phase inverter directly  (2204 power section). i am getting massive background hiss on this (even with no guitar plugged in). i have tried moving ground locations around and swapped preamp tubes to no avail. Another channel in the amp doesn't have this hiss and if i ground the P.I. input the hiss goes away completely. With the guitar plugged in the channel works fine and tone/volume controls work fine (aside from the massive hiss). I have also tried swapping out the pots. Measured all the resistors and measured for DC coming out of there coupling caps (there is none)

Title: Re: help with channel debug. massive hiss
Post by: shooter on April 08, 2025, 12:43:05 pm

Quote

have it running into the phase inverter directly

how is it "running" to the PI?  guitar cable, wire?
Is the preamp "standalone" or part of an amp?


what happens when you ground the grid of the 2nd stage, WITH the PI connected?

Title: Re: help with channel debug. massive hiss
Post by: joesatch on April 08, 2025, 12:47:45 pm

I have disconnected the other channel in the amp (also removed preamp tubes for it)
I have an elevated shielded cable running from the 220k/220k voltage divider to the PI input.  I noticed this channel is very similar to the AB763 normal channel

Title: Re: help with channel debug. massive hiss
Post by: Merlin on April 08, 2025, 04:01:42 pm

What happens to the hiss when you turn down the vol pot?

Title: Re: help with channel debug. massive hiss
Post by: joesatch on April 08, 2025, 04:26:48 pm

hiss is there with the volume on zero volume adjustment make no difference in the hiss. To me it is a missing ground but i've gone over this a dozen times and there are no missing grounds. could it be a bad coupling cap or cathode cap?

https://youtu.be/FwMx88D4eNI (https://youtu.be/FwMx88D4eNI)

Title: Re: help with channel debug. massive hiss
Post by: shooter on April 08, 2025, 05:25:21 pm

Quote

hiss is there with the volume on zero volume

that indicate the "hiss" is in this area.

Title: Re: help with channel debug. massive hiss
Post by: shooter on April 08, 2025, 05:27:07 pm

Quote

I have disconnected the other channel in the amp (also removed preamp tubes for it)

did you ever use this channel into the PI??

Title: Re: help with channel debug. massive hiss
Post by: joesatch on April 08, 2025, 07:47:35 pm

yes and it has zero hiss. hiss is coming from this channel i posted the schematic of

Title: Re: help with channel debug. massive hiss
Post by: joesatch on April 08, 2025, 10:25:27 pm

Quote from: shooter on April 08, 2025, 05:25:21 pm

Quote

hiss is there with the volume on zero volume

that indicate the "hiss" is in this area.

yup this 220k/220k divider has too much output. that seems to be the issue.  The P.I. needs to have a pot feeding it. I moved some things around in the circuit. this seems to have solved it