Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on March 21, 2017, 09:20:09 pm

Title: Is this what blocking distortion sounds like?
Post by: TubeGeek on March 21, 2017, 09:20:09 pm

Troubleshooting a deluxe reverb clone and when I turn the volume up to max on both channels, this noise appears and is driving me nuts:   https://soundcloud.com/glacier/noise-from-ab763-deluxe-reverb-clone (https://soundcloud.com/glacier/noise-from-ab763-deluxe-reverb-clone)


I have modified the circuit so reverb and tremolo are on both channels using the method pulled from Rob Robinette's AB763 mods page. See attachment.


I have also voiced the normal channel for more gain with 220k plate resistors, 56k slope, 0.022uF tone stack caps and a 0.68/2k7 cathode configuration.


Is this blocking distortion?


Any pointers on what to try to get rid of it?

Title: Re: Is this what blocking distortion sounds like?
Post by: SILVERGUN on March 22, 2017, 08:35:46 am

Scope the grids and plates of your gain stages and see if you can see where it's happening.
Watch closely as you increase volume and see where it starts to get ugly.
Look closely at the PI , due to the fact that you have increased the gain preceeding it.
-You might have to put in a voltage divider before the PI
-Maybe increase negative feedback
 
It's hard to tell from the sound clip exactly what the offending noise is...especially through my cheap computer speakers.
 
Have you tried another speaker?...it kinda sounds like you might be getting some vibration from the cab or baffle board
 

Title: Re: Is this what blocking distortion sounds like?
Post by: FranciscoPerez on March 22, 2017, 11:59:10 am

I was wondering the same, have you tried another speaker?
I recently had a speaker that got hit and needed a recone which did exactly that sound.

Title: Re: Is this what blocking distortion sounds like?
Post by: SILVERGUN on March 23, 2017, 08:26:07 am

Any luck?

Title: Re: Is this what blocking distortion sounds like?
Post by: TubeGeek on March 23, 2017, 08:51:37 pm

Did some more poking around, changed the 220k power tube grid stoppers to 100k, replaced the normal channel treble cap and that seems to have got rid of the noise for now.  I'll play it hard for a few days and see if it comes back.


I also thought it may be a speaker issue but it is a brand new speaker (cannabis rex) and brand spankin new mojo 1x12 cabinet.  The noise did happen with multiple speakers.


I need to brush up on using my oscope....I don't reach for it often enough.

Title: Re: Is this what blocking distortion sounds like?
Post by: Merlin on March 24, 2017, 07:26:22 am

Sounds like a speaker issue or something loose vibrating/resonating, to me.

Title: Re: Is this what blocking distortion sounds like?
Post by: Leevi on March 24, 2017, 09:10:21 am

IMO that is not blocking distortion. In blocking distortion you
can clearly hear when the signal breaks and the result is much worse than this.
/Leevi

Title: Re: Is this what blocking distortion sounds like?
Post by: TubeGeek on March 24, 2017, 11:26:21 am

update:


I found what was needed.  I needed to add a 500pF cap in parallel with v1b 220k plate resistor.  Once I added this, the noise completely disappeared.  I went back to my notes on this mod and sure enough this cap was part of the mod. I missed it when I wired it up.  Apparently it is an important part of the mod.


At first I thought it was a speaker issue, trust me, I have had that issue too. I tried 4 different speakers and cabs and all had the same sound.

Title: Re: Is this what blocking distortion sounds like?
Post by: TubeGeek on March 24, 2017, 11:29:17 am

Quote from: Leevi on March 24, 2017, 09:10:21 am

IMO that is not blocking distortion. In blocking distortion you
can clearly hear when the signal breaks and the result is much worse than this.
/Leevi

good to know!