Relay channel switch - help needed

[txstrat]

I thought of that too but on the other hand I think there must be a way to get it done with the extra winding on the PT as well. Before I start to rip everything out (just kidding) I'll make the battery test. That should at least proof whether the hum comes from the 12v supply or the channel itself (although I don't wanna believe that it's the channel).
BTW when I lift the 0 point of the 14.5v winding and use it like ENZO proposed shouldn't that be similar to having a separate transformer?

[txstrat]

Okay, here we go.
I lifted the connections of the PT to the relay and put a 9v battery to the pins. Even the gain channel makes only a slight hiss when turned fully up and NO hum at all.
That proofs the 12v DC from my PT are not as DC as I would like them . I checked the ground and found out that I had already put a wire from the relay to the star ground so the hum cannot be from interfering into the preamp ground (at least I hope so). The footswitch has no more connection to the speakers ground. Only to the chassic via it's housing.
I checked my desk but don't have a bigger filter cap for the 7812 at hand. I'm gonna buy some tomorrow. Think I'll get those 1000uF that "voxrules" advised.
As I remember the hum got way less loud when I changed the caps from 22/25 to 100/25. I thought that would be enough but I do know for sure after I put the 1000uF in.
Any other suggestions?

Matt

[olddawg]

Okay, here we go.
I lifted the connections of the PT to the relay and put a 9v battery to the pins. Even the gain channel makes only a slight hiss when turned fully up and NO hum at all.
That proofs the 12v DC from my PT are not as DC as I would like them . I checked the ground and found out that I had already put a wire from the relay to the star ground so the hum cannot be from interfering into the preamp ground (at least I hope so). The footswitch has no more connection to the speakers ground. Only to the chassic via it's housing.
I checked my desk but don't have a bigger filter cap for the 7812 at hand. I'm gonna buy some tomorrow. Think I'll get those 1000uF that "voxrules" advised.
As I remember the hum got way less loud when I changed the caps from 22/25 to 100/25. I thought that would be enough but I do know for sure after I put the 1000uF in.
Any other suggestions?

Matt

A small isolated 12v transformer with a perf board filter, rectifier and regulator circuit and twisted wires to the relay. It will cost very little, take very little room on the chassis and will do the same thing as a battery. You already know that it works. Why keep messing around with ground loops and unknown interactions? There is a reason why this scenario is commonly done. If you do it any other way you may get another unwelcome suprize when you try to use effects pedals.

[Robert M. Martinelli]

Matt,
now that you proved that hum IS coming from the relay's coil ( God bless batteries.... )you' re almost there; I just want to add a thought.... you probably have your channel switching relay close to some very sensitive stage of the amp, ( we' ve already discussed this ) so, you could also try to change the relay orientation...there is a chance that you' ll find a "sweet spot", an angle where hum decreases.( This of course has to do with electromagnetic laws and it' s the reason why transformers are usually mounted with a 90 degrees angle with respect to each other ).

Anyway, I' m pretty sure ( grounding being not an issue ) that properly filtering the 7812 will fix the problem , if not, adding a separate supply as per Olddawg' s advice is the "next step", but I would say you' re pretty close to your goal...

Best regards

Bob

[txstrat]

Bob,

I bought the 1000uF caps and will change them as soon as I can. If the hum will not disappear I'm gonna lift both the secondary 14.5 v windings completely as ENZO proposed and put the 14.5v lead to the (+) of the 7812 and the filter cap (of course with a diode in between) and the 0 to the (-) of the filter cap and the IC (-). Maybe I'll drill the wires too, as olddawg proposed. That should be very similar to using a different transformer, shouldn't it?
I didn't think of the orientation of the relay, but wouldn't you agree, that the hum should also be there when using a battery IF the orientation was the fault?
I added a pic from inside the amp (don't blame me for the rats nest, cause I tried to orientate myself on the layout of the Fender Deluxe Reverb II). Relay No 1 is for the channel switching, No 2 is for switchable master volume.
And as I said: with a battery dead quiet and even fully open just some hiss from the channels. The Fender layout can not be THAT bad, huh?

Matt

[Robert M. Martinelli]

Bob,

I bought the 1000uF caps and will change them as soon as I can. If the hum will not disappear I'm gonna lift both the secondary 14.5 v windings completely as ENZO proposed and put the 14.5v lead to the (+) of the 7812 and the filter cap (of course with a diode in between) and the 0 to the (-) of the filter cap and the IC (-). Maybe I'll drill the wires too, as olddawg proposed. That should be very similar to using a different transformer, shouldn't it?
I didn't think of the orientation of the relay, but wouldn't you agree, that the hum should also be there when using a battery IF the orientation was the fault?
Matt

Hi again Matt,
well, instead of a diode alone, you' d better put in a couple of 'em ( full wave ) or better still a rectifier bridge, the reason for this being that it' s more difficult for the caps to catch the peak voltage and keep it with no fluctuation with only one diode.....with a full wave array you' ll find smoothing the voltage will be easier - be aware that, using a rectifier bridge and a big enough capacitor, your output DC voltage will be approximately 1,414 times the input AC voltage.

As to the orientation, no, the coil-inducted hum cannot be there when battery testing, because batteries supply pure DC voltage, so there' s absolutely no ripple ( this is the reason why I suggested this test in the first place, to see if hum was coming from the ripple over the coil ) your hum is INDEED coming from the AC component ( ripple ) found over the DC supply voltage, that' s the reason you should render the relay's DC supply as smooth as possible...to make it as close as possible to a battery. Orientation becomes an issue only when the DC voltage is not properly filtered.
Hope this clarifies the matter.....
Good luck!
Bob

[txstrat]

Orientation becomes an issue only when the DC voltage is not properly filtered.

That's what I meant. Just couldn't put it that way.
If I use a bridge rectifier, where would I put the ground for the bridge? I mean in case that I want to lift the 14.5v leads off the ground as I mentioned before.
Can I put the ground for the bridge straight to the (-) of the filter caps?

[Robert M. Martinelli]

Hi again Matt,
If you use a bridge you' ll have to connect the bridge's (-) directly to the filter cap's (-) together with the 7812 center pin and the output 0V. With 14,5 VAC at the input you' ll find about 20,5 VDC at the 7812's input pin, this means that the voltage drop across the 7812 will be 8,5 VDC, and, as long as you' re using that for the relay coils only, you' ll need no heatsink, as the power dissipation will be well below 0,5 W.
Waiting for some good news.....
Take care
Bob

[txstrat]

BINGO
It was the filter cap.
I changed it and the darn thing went dead quiet. Can't believe my ears.
When I bought the caps I was about to buy a small 12v transformer too, but I decided what the heck, one more try and then I'll buy it. Luck was on my side.
Thank you so much guys for your help and patience.

Matt

[Enzo]

Glad you sorted it out.

[Robert M. Martinelli]

Hi Matt,
glad to hear that!
Rock on!
Bob