Isolation transformer upgrade for old guitar amps

Step 9 Option C (busting the hum)

OK, it's a year later, and then some...

These changes do seem to introduce hum to some AC/DC tube circuits. For a few reasons: SS rectifiers are more efficient, the filtration is a little lacking, and fullwave rectification shifts the PS wave peaks from 60Hz to 120Hz.

So in the quest for a hum-free amp, I've modified the circuit somewhat. This has made the little Gregory amp almost totally free of nasty hum. Your mileage may vary--each amp is a little different.

NOTE about this section:
There's cost for converting to higher voltage DC filaments--increased power consumption. The power draw for the 120V AC filaments is 18 watts; 25.2 watts for 168V DC filaments. Keep that in mind. Note also that this mod may raise the plate voltage for the 50C5 output pentode somewhat higher than the recommended voltage...this has worked fine for me, but YMMV.


Option C

This option Inserts another filter cap after the SS rectifier. It's a little odd, as the additional filter cap is placed between the two rectifiers. Nothing technically wrong here, just unusual...(as are two rectifiers, but we know that works.) We're just feeding the second rectifier a current source that's less...wavy .

However, Option C introduces a complication: With even a moderate filter cap, the filament voltage is much closer to DC than the original AC.

That's good, right? DC is quieter. Yeah, but the DC voltage resulting from rectifying and filtering AC is closer to the peak AC voltage, and can't be treated as an "average"... So the new DC voltage is higher--TOO high, in fact. The old AC-to-DC formula is in play...the DC voltage is approx 1.4 times the AC RMS, approx 168V. This would surely burn out the filaments.


Taming the Higher Filament Voltage

But there's already a series resistor inserted with the three filaments to drop the voltage--for line AC (115-120V). We just need to increase that resistance so it can handle the higher voltage.

So how do we figure the new resistance value for Rv? A few facts...

-- the three tubes (12AU6, 35W4, 50C5) drop a total of 97 volts (12 + 35 + 50 = 97).
-- each tube draws 150 mA (0.150 Amps ). That's important.
-- the stock Rv value is 160 ohms (for 120V).
-- the new filament supply voltage is 168V .

Hmmm, each tube draws 150 mA. AaaHa! Current is equal for all components in a series circuit. So the current draw of Rv must match.

Time for good ol' Ohm's Law (R = E / I, or resistance = voltage / current). Let's check the original value:

120 - 97 = 23 extra volts to drop.

To achieve the same current draw for Rv: 23 / .150 = 153 ohms. Good! That's almost spot-on to the 160 ohm spec'ed value.


The New Rv Value

Estimated DC voltage for the filaments: 120 * 1.4 = 168V
168 - 97 = 71 volts to drop.

71 / .150 = 473 ohms. That's SO close to a standard value...

470 ohms is the new Rv resistor value. Rv is dissipating 10.5 watts, 15 watter is required.

This has been tested, and worked perfectly--the very first time (yeah!)

Yes, this ups the current draw (total wattage) of the amp, without increasing the output power. The higher filament voltage is drawing about 7 additional watts. The iso transformer does get a little hotter.

The New Filter Cap

Pick a reasonable value here. I used 22uF / 250V, but upped that to 100uF / 250V. It works just fine, and obviously the 100 uF cap is a bit quieter.


Other Anti-Hum Mods

I've moved the initial SS rectifier ground directly to the bolt that holds the rectifier to the chassis. Probably helps a bit. The first (filament) filter cap is also grounded here.

Also moved the isolation transformer a little farther away from the speaker voice coil. It's easy to experiment with this...just clamp the transformer "module" in different spots and test. Didn't have much effect, but it can't hurt.

Don't forget to clean and reseat input jacks, especially if they are grounded directly to the chassis. That's a common source of hum.