No such thing as good hum

[RevD]

Are there any fluorescent fixtures near where your playing this amp and single coil pickup near a computer monitor for example? I know these sound ridiculous but have seen it happen before. Have you tried other guitars through it? A stock strat with little shielding will cause some hum IMHO. Oh well, just throwing them out there. Maybe it just don't know the words...

Regards,

Don

[LarryLarry]

I'm starting to wonder if I just need to adjust my cathode resistor value. I tried a 5u4GB and it brought my voltages down to those with the 'normal' Rocket PT (mine is 285-0-285). It also cut the hum way down.

I also notice that the PT is pretty warm and forget about getting near the EL84s or GZ34! No red-plating though, but I'd rather them be a bit cooler!

I took some measurements and pluggged them into the Weber bias calculator (see photo) and it looks like I'm running the EL84s at 18 watts!

I entered 75 into the 'Voltage across cathode resistor' and it brought it down to 12 watts, but if I change the cathode resistor then the voltage across it will change too, right?

Maybe I'm getting close???

[Firestorm]

I entered 75 into the 'Voltage across cathode resistor' and it brought it down to 12 watts, but if I change the cathode resistor then the voltage across it will change too, right?

Yeah, it'll go up. Try that. It hums with no PI tube, so (part of) the hum is strictly output stage. Underbiased outputs hum.

[Firestorm]

I entered 75 into the 'Voltage across cathode resistor' and it brought it down to 12 watts, but if I change the cathode resistor then the voltage across it will change too, right?

Yeah, it'll go up. Try that. It hums with no PI tube, so (part of) the hum is strictly output stage. Underbiased outputs hum.

[Firestorm]

I entered 75 into the 'Voltage across cathode resistor' and it brought it down to 12 watts, but if I change the cathode resistor then the voltage across it will change too, right?

Yeah, it'll go up. Try that. It hums with no PI tube, so (part of) the hum is strictly output stage. Underbiased outputs hum.

[Firestorm]

I entered 75 into the 'Voltage across cathode resistor' and it brought it down to 12 watts, but if I change the cathode resistor then the voltage across it will change too, right?

Yeah, it'll go up. Try that. It hums with no PI tube, so (part of) the hum is strictly output stage. Underbiased outputs hum.

[Firestorm]

Wow. Three hiccups. A new record?

[LarryLarry]

No kidding
No kidding
No kidding

I've got a 25 watt 100r I'll try in place of the 25 watt 50r in there now. It's the closest I've got in my junk boxes...

[LarryLarry]

I tried the 25 watt 100 ohm cathode resistor and am getting realistic dissapation readings now. The hum has been cut at least 75%, still just a little bit there.

The 100r is a ceramic, how should I mount that to the chassis? It will get too hot for hot-glue, right?

[Firestorm]

You could just put it on a terminal strip. It should throw off quite a bit less than 5 watts so a resistor that big won't need heat sinking. Do keep the bypass cap well away from it though.

[LarryLarry]

Well, I had to go a bit MacGyver on fabricating a halfway decent mounting of the ceramic 25 watt 100r cathode resistor.

Played through the amp for a couple of hours and could still put my hand on the PT without risk of life or limb and the hum is barely there. No heat on the OT.

[rooster]

Well this is good news. I would look into some chassis mount single connection standoffs to do that a little better maybe, hey but McGyver is cool, too!

[hans-jörg]

Fine, you got it.
What is the screen voltage? If you get it a little more down aprox. 300 V, and plate 316 V, (this are my readings on the Liverpool), so you will be more in line.
I had 330 too, but I hooked some Zener Diodes at the CT (10V/5Watt) and I´m right there. You would need maximum 2 of them.
Filament: thouhg I suggest to put in a trimmer (at least for test. But remove the 2x 100R in this case). You will see - you can dial the rest of the humm out.

Hans-Jörg

[LarryLarry]

2 Zeners in series on the CT of the PT mains to ground then? I've never used them but I'm curious what the difference in sound would be with the voltages a little lower.

On the hum balance pot, 250 ohm? Can I use a normal pot or does it need to be a higher wattage?

[martin manning]

Re the hum balance pot, 7 volts across 250 ohms is 0.2W, so a normal pot (they are typically 0.25W) should be okay, but a 0.5W cermet would be better, or use a 500 ohm pot.

To sum up, you've lowered the B+ by going to a 5U4, and adjusted the bias current downward by installing a larger (2x!) cathode resistor. I think perhaps you were overstepping the PT's current rating.