Bad amp grounding??

[Sea Chief]

Hi chaps,

I need to start afresh on a problem I have: I'd be extremely grateful for help on this. It might well be fairly simple to many.

I have a very prominent HUMM!! noise (like a LOUD bad grounding issue). Vox ac15Tb. I've shorted across 2 caps so far, without successs/ the noise remains. Helmholtz has kindly helped so far, but I'm currently stuck how to proceed. So to recap (another one of my fabulous puns):

If I remove the 1st preamp tube (V5 here, V1 is the PI).. the noise ceases. Putting V5 back, no guitar heard behind the loud HUMM!! Loud HUMM!! noise increaces with turning MV knob (very loud on 1, so god only knows what it's like on 10). If I turn vol knob to 0, & turn up MV.. noise not heard.

That is all I have by way of symptoms/ help.

-------

Is it the case, that by pulling V5 & the noise stopping, that I have at least achieved the "advantage" of significantly narrowing the possible culprit components down? I need to get the idea of what's achieved by pulling V5 1st. I need to go back/ strip everything back to think how to solve this it seems.

Thanks, Sea Chief.

[Jon Snell]

V5 is or rather should be the first pre amp valve.
If removing this valve stops the hum then the issue is in that area alone so disregard anything further down the line.

Not a lot to look at except jack socket wiring unless C4 or the valve itself.
I presume VR7 is what you call MV which narrows it down even further to one section of the valve.
Try shorting a 1k resistor from pin 2 to the -ve side of C12 ... what happens.

[Sea Chief]

V5 is or rather should be the first pre amp valve.
If removing this valve stops the hum then the issue is in that area alone so disregard anything further down the line.

Not a lot to look at except jack socket wiring unless C4 or the valve itself.
I presume VR7 is what you call MV which narrows it down even further to one section of the valve.
Try shorting a 1k resistor from pin 2 to the -ve side of C12 ... what happens.

Hi Jon- I think you put up the preamp schematic before, that's great/ appreciated.

Yes V5 is the 1st valve (seems opposite to fender numbering). Ok so if I'm disregarding everything further down the line.. does that imply I'm luckier/ less possible options/ easier in theory to trace the issue, rather than were to I have found that the HUMM!! continues up until say I removed V4-?

I can only think VR7 would be the 'Volume' knob- which I think is the "preamp volume". So no, I don't think VR7 would be the MV (would it?): I thought MV's were to do with the "power amp volume", & so wouldn't be found at such an early position in the amp 'chain'..? but I don't know.

I will try your shorting suggestion. Thanks alot, SC

[Helmholtz]

We already tried a few things in your original thread: https://music-electronics-forum.com/...-ac15-tb/page8

You said that shorting R25 killed the noise. Is that still the case?

[Jon Snell]

Hi Jon- I think you put up the preamp schematic before, that's great/ appreciated.

Yes V5 is the 1st valve (seems opposite to fender numbering). Ok so if I'm disregarding everything further down the line.. does that imply I'm luckier/ less possible options/ easier in theory to trace the issue, rather than were to I have found that the HUMM!! continues up until say I removed V4-?

I can only think VR7 would be the 'Volume' knob- which I think is the "preamp volume". So no, I don't think VR7 would be the MV (would it?): I thought MV's were to do with the "power amp volume", & so wouldn't be found at such an early position in the amp 'chain'..? but I don't know.

I will try your shorting suggestion. Thanks alot, SC

It means you are luckier than say V4. If V4 then the components will include all of of V4s and V5s.

[Chuck H]

Here's an observation. SC, haven't you had trouble with the input jack on this amp at one time or another? Has anyone yet suggested a ground fault for the input jack? Seems like the first thing to eliminate to me. With the amp off, plug a cord into the input and take a resistance reading from the sleeve to either end of R24. What is the reading?

[nosaj]

Here's an observation. SC, haven't you had trouble with the input jack on this amp at one time or another? Has anyone yet suggested a ground fault for the input jack? Seems like the first thing to eliminate to me. With the amp off, plug a cord into the input and take a resistance reading from the sleeve to either end of R24. What is the reading?

Or if the input jack has been fiddled with , does this model require an insulating washer? Some do and some don't
nosaj

[Greg Robinson]

Or if the input jack has been fiddled with , does this model require an insulating washer? Some do and some don't
nosaj

Note the ground-loop breaking components R25, C24, D1&2. A non-insulated jack would short these out and render them useless. Also note the S2-PC designation, S2 almost certainly indicates a Cliff S2 type jack, which is plastic bodied/insulated.

[DrGonz78]

Could removing the V5 tube then tack soldering a wire from V5 pin 2 to a connection point between C3 and R22 help isolate everything before V5's connections?


Edit: Also it's not just one input jack that could be in question but two of them. Sea Chief have you tried both input jacks?

[Jon Snell]

Could removing the V5 tube then tack soldering a wire from V5 pin 2 to a connection point between C3 and R22 help isolate everything before V5's connections?


Edit: Also it's not just one input jack that could be in question but two of them. Sea Chief have you tried both input jacks?

That will reduce any hum because it removes the pre amplifier section that is amplifying the hum.
Ensure there is a good earth connection as that can cause your hum if all is good the only thing left is to fit the correct jack sockets and make sure they are wired correctly. The chassis/ground point must be to the correct positioning.

[Sea Chief]

We already tried a few things in your original thread: https://music-electronics-forum.com/...-ac15-tb/page8

You said that shorting R25 killed the noise. Is that still the case?

Hi HH.

Indeed we tried a few things (noise prob started on pg 9 of the other gremlins thread.. hence just needing a fresh start/ new thread for this).

No unfortunately shorting R25 did not kill the noise (I tested amp without turning both vol & MV knobs up together [noise only occurs with -both- knobs rotated].. so I was fooled into thinking the noise had stopped).

Thanks, SC

[Chuck H]

Is this the same noise you had when one of your input jacks had bad solder joints?

[Sea Chief]

Or if the input jack has been fiddled with , does this model require an insulating washer? Some do and some don't
nosaj

Hi nosaj,

the 2x inputs are those black plastic 'marshall' style into-the-pcb ones.

Its been fiddled with: Chuck H I did indeed resolder input 1's 4x tags into the pcb, which sorted the amp's innitial 1st problem out fine, this problem being the reason for the other gremlins thread (making sure the washers on both sockets put back.. a non-conductive black insulating washer I assume).

But I do see the logic of looking here/ input 1 afresh, for this new noise issue. I will certainly go over it again & do the continuity reading Chuck H suggests today.


Thanks to both of you for the ideas- SC

[Sea Chief]

Is this the same noise you had when one of your input jacks had bad solder joints?

Cant recall Chuck, possibly.. but a sensible avenue. Nothing physically n/g with my solders tho.

[Sea Chief]

Here's an observation. SC, haven't you had trouble with the input jack on this amp at one time or another? Has anyone yet suggested a ground fault for the input jack? Seems like the first thing to eliminate to me. With the amp off, plug a cord into the input and take a resistance reading from the sleeve to either end of R24. What is the reading?

Ok I get continuity both ends of R24, to the sleeve of input jack. Cord plugged in. 0.6 ohms one side of r24.. 1.1 ohms other side.

Thanks SC