down on this killing ( noise ) floor.

[pedro]

Hi,

I'm not happy with the noise floor on my build - its just too much. I thought it was OK at first but comparing to my other amps its not and I need to sort it out.

Does anyone have a systematic way of going about seeking out the sources of hum and hiss using basic test equipment ??

I have an Oscilloscope and sig generator recently acquired but I'm a novice using them.

Its pretty much Hybrid A with a few value tweaks.

cheers
Pete

[Funkalicousgroove]

First thing to check is your preamp tubes, they can cause ALOT of noise, Next look at your ground scheme and make sure that no part of the circuit has multiple paths to ground. Next check your plate and grid leads, Plate leads should be long, grid leads should be as short as possible and the two should NEVER run paralell, if they must cross it should be at 90 degree angles. If that doesn't fix it check all of your solder joints, then all of your electrolytics.

If you rectified your fillament tap to power your relays make sure there isn't more than one ground reference, that will make alot of hum, so will using both the CT and 2 100 ohm resistors.

[ElectronAvalanche]

One approach to find the culprit would be this:

Take an alligator clip and ground the PI input cap (on the input side). If there is no hum after doing this, your source of noise is upstream. If you still get hum, there is noise in the PI and Power Amp.

Do you have seperate Bias pots for the Push and the Pull Output tube pair? If not, this can help you dial out the hum in the PP section.

Next step would be to ground the output of the Clean channel after the CL2 decoupling cap. Still got hum? Then it might be introduced by the channel switching relay. Switch to OD mode with the CL2 output cap still grounded:

You should here only white noise and slight hum when putting the OD pot on full.

If the noise stops by grounding the CL2 output cap, then the problem is upstream.

Make sure you terminate the shielding of the wire running from the inputs to the grid stopper on CL1 near the CL1 tube to a ground point.

Make sure you have the correct grounding scheme:

All grounds of one tube system (cathode ground) should go to a unique grounding point. Then connect the grounding points of the tube systems to a buss and run that to the filter section. Ground it there.

Ground the PT CT to the chassis close to the earth ground. Run a wire to your filter cap ground.

Make sure your Relay power supply is NOT grounded to the chassis or the ckt.

Do NOT use a Relay being fed from the 6.3V heater voltage of the preamp tubes. Use a seperate transfrormer for the Relay supply.

You can IMHO ground the FX Loop jacks and Output jacks to the chassis.

Isolate the input jacks from the chassis and ground them to the first tube system ground.

The Pots are grounded by means of a ground buss wire. DO not ground the pots to the chassis. This pot ground bus then goes to the ground of the first filter cap.

At is all for now.

Hope this helps!

Electron

[pedro]

thanks fellas - i'll check all of those things and try changes where needed


- my relays are driven from a rectified heater circuit

- I have two seperate heater supplies from my mains TX - both with about 4A supply.

At mo I have one winding supplying preamp ( including relay rectifier ) and the other doing the power tubes.
Maybe I'll split off the relay supply onto one winding, and merge power amp and preamp onto the other - guess I should have done that to start with !

I'll try all the other tips too.

Many thanks
Pete

[jelle]

Pedro,
is your noise hum or hiss?
In the case of hum the things mentioned will help.
If it is hiss then the preamptubes or the pre-amp is the problem.
That's another story fixing.
I use 2W metal film plate and cathode resistors, solder the grid stopper (carbon/carbonfoil) resistor to the tube socket and keep the unrectified heater wires as far away from the signal path as possible. NFB on V1b helps too! A 67/1 ratio for the plate to cathode resistor ratio helps. Too much current increases hiss. Carbon resistors for the plates especially the low wattage ones increase the noise. Think blackface fenders: if it doesn't hiss it probably doesn't work... That's why I use metal films.
IRRC this phenomenon is described at aikenamps.com in the technical articles section.

I hope this will help you,
Good luck!

Jelle,
The Netherlands

[pedro]

Electron,
I'm now using one 6.3v winding exclusively to power the rectifier and filtering for the relay PS. Hum seems much improved, but still going to make the additional changes you and funk described.

question - how can i not have it ( the relay power supply ) grounded to the chassis or circuit as you suggested? Can I just have the bridge rectifier floating, or do i need a blocking cap between the ground and rectifier ground point ?

cheers
Pete

[Funkalicousgroove]

If you're using a separate trannie it's fine to ground, but if you're using a filament winding that already has another ground reference you have to isolate the relay circuit. Ground doesn't have to be chassis ground, it is only a point of reference, one method that has worked for me when using the filament tap for both filaments and relays is to use a FWB rectifier, and instead of connecting the - side to ground connect it to a resistor (10R or so) and connect all of your other relay circuit grounds to the other end of the resistor. Be absoluteley sure that your footswitch jack is isolated from the chassis. This is the method that the little weber relay boards use.

[BobW]

Funk,

I can't say I fully agree with this particular suggestion:

"Do NOT use a Relay being fed from the 6.3V heater voltage of the preamp tubes. Use a seperate transfrormer for the Relay supply. "

My amp is dead quiet using the 6.3 vac heaters as the relay power source, but I am also using a voltage doubler since all I had at the time was a 12V relay. Probably the additional capacitance in the voltage circuit is what keeps the relay quiet.

Pedro, Shielding your guitar can also help tame noise that may seem like it's the amps fault. This can make a big difference since these amps have a higher gain than a stock Fender. hth

[Funkalicousgroove]

I didn't say not to, Electronavalanche did, I told him a way TO use the heaters.

[ElectronAvalanche]

Indeed,

that was me....

Of course you can get away with using the preamp heater rail, but I had to learn the hard way, that this is a safeway to introduce hum. It just worked better to use a seperate transformer like Dumble did.

Cheers,

Electron

[pedro]

I'm lucky to have two distinct heater windings on my TX which are not gnd connected with each other.

having shifted relay supply onto one on its own away from heater circuit I have a much quieter amp.

cheers
Guys

[Tdale]

You have problably tried this, but I had some noise problems in my Matchless clone, and after lot's of trying, it turned out to be the grounding point that wasn't screwed tight enough to the chassis....

simple solution, but very effective...

Tommy