Premier 90 Reverb Clone HUM

[Jhtjon]

I recently built a premier 90 reverb. I am having a lot of hum problems. When I scope the reverb I get a clean signal from end to end but when I connect it to another device I get HUM!

I have tried lifting the ground on the reverb and it actually made it worse.






The only changes I have made since the picture was taken was to change the rectifier from half wave tube to hybrid full wave bridge, add a grid stopper to the power tube and I revised my grounding a little. But the reverb scopes out perfect.

[terminalgs]

a schematic would be helpful.


do you have a CT for the filament circuit?  either from the PT or the an artificial CT?   where is that CT grounded?


You said you lifted the ground, do you mean with a 3-prong/2-prong adapter?

[terminalgs]

I see the gray/brown pair is your filament circuit.  I don't see a CT (wire from PT or otherwise).   I'd add an artificial with two resistors (100 or 150ohm).  I'd also play with the grounding location of the CT.   try these locations: the cathode of the pentode, the neg. side of the first electrolytic in the power supply.

[HotBluePlates]

When I scope the reverb I get a clean signal from end to end but when I connect it to another device I get HUM!

Because either your reverb or the other device needs to have a proper ground lift. Otherwise, the chassis ground in your reverb connects to the chassis ground in the other device through the shield of the connecting cable and voila! Ground loop through the connecting cables, through the chassis, through the 3rd wire of both devices and to the breaker panel.

Breaking off the power cord green wire lug is not a safe way to do a ground lift (though the hum will stop). I described a known-good ground lift in this post.

If you don't want to rewire to isolate every circuit ground from the chassis, there is an alternative: Use a plastic output jack (Cliff-style). Get a 1:1 transformer (they're pretty small, available in a lot of places). Connect your existing hot & ground output wires to the primary of the transformer (including connecting the ground wire to whatever ground it has now in your circuit, including chassis). Connect the 2 secondary wires to the 2 lugs on the output jack.

The transformer and isolated output jack will break the ground loop at the output of your device. I believe there are pedal-sized boxes that do the same thing (having their own internal transformer) meant to sit between the connecting cable out of your reverb and the cable going into the other device. You can use these wherever, without it being permanently in a piece of gear (or to troubleshoot this exact problem, by identifying the need for a correct ground-lift).

[ac427v]

Hi Jon,
Here is my posting from the other forum. Maybe someone here can figure this out:
“If you can fix this I plan to build one for myself. The deep cavernous reverb sound is outstanding!
 I spent a month working on my brother's Premier 90 hum. I replaced almost all of the resistors and caps--no improvement. Hard wired all the ground connections--no improvement. Replaced the jacks and shielded cables--NI. Used an ungrounded ac adapter (3 prong to 2 prong with ground clip unattached) to eliminate ground loops with other equipment--NI. Did the trick with the heater ground attached to the power tube cathode (elevated ground)--some improvement! Replaced the single diode power supply with a bridge rectifier--Some improvement. Turned down the reverb level--big improvement but...
 I attached my modified schematic hoping that some of the better minds here can help you solve this. The sound of the reverb is truly spectacular!”

My current theory is that this is a design problem inherent to all Premier 90s. The Accutronics site emphasizes that the springs must be driven hard to prevent hum. The small power tube driven by a low voltage would result in a very small signal to the springs. The recovery triode must amplify the snot out of the spring output--including residual hum-- to get the signal large enough to send on to your guitar amplifier. If that theory is correct, the design needs a bigger power tube running at higher voltages. Then the recovery stage could be tweaked to provide less gain. The Premier 90 is not on my immediate build list but if you or someone else can get it work right a 90 would be my next project.
Craig

[HotBluePlates]

When I scope the reverb I get a clean signal from end to end ...

Don't know about your situation ac427v, but the OP says there's no hum until an external device is connected.