How to Quiet a Pro Junior

The Pro Junior is a ballsy little amp, but it's marred by hum, hiss, and buzz--each with a different cause. I bought a 2006 PJ at a Guitar Center mega-sale, and took some time going through it. It's now much quieter. It's not perfect--that would take a more serious reworking--but hum is gone, buzz is barely there, and hiss is well within acceptable levels for guitar amplifiers. This is not the amp you would use for closely mic'ed recordings of delicate fingerstyle work, but at least it won't embarrass you with rude noise and antisocial behavior.

You can see just about everything I've done here. The blue resistors on the right are plate resistors for the preamp and phase inverter. I've changed all of them to metal film 1% resistors. There's still some residual hiss; changing the cathode resistors to metal film would probably help, too. Other resistors, such as grid leaks, are unlikely to help because they're not really in the signal path. The voltage divider in the phase inverter might help, and a couple of resistors in the tone circuit might benefit from metal film. But the plate resistors are typically the biggest contributors to hiss.

I also changed the coupling caps and one of the tone caps (which you can't see in this pic) to Orange Drops, using the stock values. It didn't change the hiss, but made the tone a bit rounder and warmer. C6 is a tight fit.

I removed the Illinois Capacitor (IC) 47uF cap in the first filter stage and replaced it with a pair of 47uF 450V Xicons. The Xicons have lower ESR (equivalent series resistance) and better ripple rejection. Doubling the first filter stage reduces the 120Hz sawtooth voltage from 4V to 2V. The 120Hz frequency finds its way into other parts of the amp and is heard as buzz.

I also removed the 470K bleeder resistor because it was dumping a faint echo of that sawtooth voltage right into the ground plane under the tone caps. I felt it was a potential noise source.
I made the bias adjustable because the factory bias is just too hot on Pro Juniors and Blues Juniors. This particular amp was idling at just under 12 watts, which is supposed to be the maximum dissipation for the output tubes. As a result, it went much higher when you actually played. This "High AB" biasing seems to be fashionable among some EL84 designers, supposedly giving the same sonic attributes as a Class A amp. It doesn't. It's just more hiss, more heat, and shorter tube life.

I changed R28 from 56K to 47K (1/2 watt flameproof), but I found on subsequent PJs that there's no need to do this. I replaced R29 with a 50K 25-turn trimpot, configured as an adjustable resistor. This gives me a huge adjustment range for the bias. I set the idle at 9 watts. It sounds just as ballsy as it ever did, but the temperature on the glass of the EL84s is 70 degrees lower.

The lower leg of the pot is in the lower R29 hole and I drilled holes for the other two legs. The wiper leg is bent and soldered to the lower leg and a jumper wire connects the upper leg to the upper hole for R29. Simple.  Note that I bent CR5 just a bit to make room.
You can see here how easy it is to do the bias mod on the back of the circuit board. One leg of the trimpot goes into the existing R29 hole.

Note that the bottom edge of the circuit board in the picture above is now the top edge in this picture.

The offset hole comes out pretty close to the existing R29 hole, so all you need to to is bend the lead over so it touches the solder pad. I haven't soldered them yet in this photo, so you can see them clearly.

For the other lead, a simple jumper does the job. Make a loop in one end and place it over the trimpot lead. Bend the other end 90 degrees and place in the top hole of R29. I crimped the loop a little tighter before I soldered this.

After you've put everything back together, it's best to set the bias voltage to around -12V before you insert the output tubes. That way, you know they won't immediately red-plate.
I determined that the AC filament supply was coupling an unusual amount of hum and buzz into the cathodes of the tubes. There are two cures for this--a DC filament supply or a DC offset for the AC filaments. DC filament supplies tend to run hot and tax the power transformer, so I opted for the DC offset. It's also smaller, with fewer components. It raises the filament voltage around 50V, so that the 6.3VAC can't affect the charge on the cathodes.

Here you can see the three components--a 470K 1/2 W resistor that isolates and drops the voltage from the power supply's 294VDC "Y" tap and a 100K resistor and 22uF 100V cap to filter the voltage further.

On the left, the trace that used to connect the two balance or virtual ground resistors (on the opposite side of the board) is cut. I drilled a hole in the stub of the trace to connect the 50V supply to the virtual ground.

I placed the components on the foil side of the board to keep the capacitor away from the fierce heat of the EL84s.
The connections on the other side of the tube board are crude but effective. The high voltage comes in and connects to the 470K resistor. At the other end of the 470K, the 100K resistor, the + side of the 22uF cap, and the wire to the filament virtual ground resistors are connected together. The 100K resistor connects at its other end to the - side of the 22uF cap and to the ground wire.

I used Teflon-coated wire for the connection to the virtual ground resistors because of the heat from the EL84s.

The + wire goes to the "Y" point in the power supply; the - goes to the ground side of the first filter caps for maximum quietness.

This circuit takes over the role of the bleeder resistor that I removed from the B+ point in the power supply. It will still self-discharge when you shut down the amp.
Here's the circuit diagram for the DC offset. It takes high voltage from the power supply, reduces it, filters it, and feeds it to R35 and R36 instead of the stock ground connection. There's little current consumption, mostly just a DC charge that keeps electrons from migrating from the filament to the cathode when the AC filament voltage swings negative.

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