Premier 90 Reverb Clone HUM

[jon]

I recently built a Premier 90 Reverb clone. I did make a few minor modifications to the circuit. I used a EZ80 Rectifier, I added a .1uF cap to the input to stop DC flowing back to the guitar pots, and I am running the power tube in Triode mode. I am getting a substantial amount of hum. After carefully isolating the unit stage by stage I found that the hum is coming from the recover stage. If I ground the grid of the recovery stage the HUM does not go away, if I ground the normal level pot the hum does not go away. If I ground the output signal the hum goes away. On the original units there was a shielded 1/4" plug exiting the device to provide connection to the amp. I obviously used a 1/4" female chassis mounted. I am at a loss at this point. Any thoughts

[IMG768]http://i82.photobucket.com/albums/j250/Jhtjon/IMG_3553_zps24c73458.jpg[/img]

P.S. Ignore the missing dropping resistor in the power supply. I was trying different resistors to see the effects of lower voltage on the preamp tone.

[Stevem]

How is your layout for the pan, if the output side of the pan is too close to the power transformer it will pick up 60 hz hum.
Remember the output side of the pan is nothing more than a small single coil pick up!
Is this what you have 60 hz hum, or is it 120 hz as in from the power supply?

[jon]

Sorry it is 120htz

[Stevem]

Is the pans case bolted to the chassis?

[jon]

No

[Cliff Schecht]

Typically these tube reverbs have a ground loop formed between the reverb unit, the amp and the wall socket. You get a common ground through the guitars ground but also through the wall socket, hence the large ground loop. You can break the ground loop at either place but for safety you really should break it at the inter-amp cable. Make a cable that has only hot, no cold/ground connection, and see if that helps..

[jon]

Will Do

[jon]

No change

Here is the trace with no input signal

[FUCHSAUDIO]

You realize this Premier model (the original) had a hum issue from day one ? 120-Hz hum is usually either of three things" bad filters, a ground loop (s), or elevating the filament line with respect to ground and thus also the cathode. I've seen many techs claim " it can't be eliminated ". Wish I could be more positive.....but I do hope you sort it out.

[jon]

I think it can work. All my trouble seems to be in the recovery stage. The signal is clean and quiet till the plate of the recovery triode.

[jjman]

The posted schematic shows a half wave diode rectifier. I see that the scheme is not original but I'm thinking the original could not have 120hz hum with half-wave rectification.

A GZ80 is full wave so 120hz PS hum is certainly possible, assuming it's wired as such.

The trace looks like more than 120hz + 60hz hum but I wouldn't know how to interpret that extra "stuff" on it.

The grounds look to be spread thru the chassis. I would consider a single star for everything except the wall safety ground and the heater CT. A complete star would include isolating the jacks and intensity pot from the chassis and bringing their grounds to the star. Perhaps a less radical multi-star approach as a 1st step.

[Craig B]

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 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!
Craig

[FUCHSAUDIO]

Maybe switch to 12AX7's (simple rewire and most values need not change) ? Perhaps 6EU7's are not happy in this circuit. Glad you made progress. Maybe DC filaments or adding a hum balance in addition to the lift circuit ?

[shoggoth]

That 47uf/160v filter cap is going to be seeing 170+ volts before the tubes warm up. Maybe it's fried ? Could try replacing it with a 250v rated cap

Also you could try increasing the value of the 47uf/500v to 330uf or 470uf (voltage rating doesn't have to be 500v, 250v is probably fine)

EDIT - Oh I see, Craig B's schematic is his solution for hum compared to Jon's original hummy version. I would do what Craig B did w/ the power supply, and you've already got 450v caps when I look at the pic. You can increase the 470R dropping resistor to improve filtering and get voltages down where they were in the original. Half-wave power supplies stink.

[jon]

Switching to a 12ax7 seems to have done the trick

Thanks everyone