Dr Z. Stang Ray build.... Video, pics and that crappy Hum

[yalesmith]

My brother wanted me to build him a clean amp so i built a Dr. Z stang Ray and I must say that i am very happy with the the tone and it is fairly simple build. I also need to thank someone on this board but not sure if he wants a public acknowledgement. The one thing that stands out to me is the humongous AC 30 Vox transformer and that horribly hum. See link below for a video of the HUM and gut shots.

Awesome and thanks in advance.

http://youtu.be/pYbape6JXjU

[M Fowler]

My first thought is no center tap on the filament supply so you would need a 100r resistor of each leg of the filament wires to act as artificial center tap.

Otherwise there is a ground loop or defective B+ filtering.

[yalesmith]

oh shoot i did not notice there was no center tap on the filaments! i will try that, if not i will install a hum pot/

thanks

[Firestorm]

I noticed in the video that you seem to have your PT and OT mounted parallel. Hum from that wouldn't respond to the volume control, though. Certainly add the center tap, but it also appears you've got your input jacks and preamp circuitry at the noisy end of the amp.

[jjman]

Am I correct that the hum initially goes down as you turn the volume up from zero? If yes, this implies that the hum is (at least) on both the 1st and 2nd stage since it phase-cancels there from the inversion between the stages. The missing heater CT would affect each stage. The parallel OPT and PT could also be an issue but would affect only the output section. Fix the heater 1st for sure.

[Cygnus X1]

Lead dress?
Input wiring runs real close to a canister cap.
Preamp tubes very close to speaker jack.
Things like that.

[Structo]

That's what I was thinking, OT too close to AC wiring.

[yalesmith]

hey was wondering if anyone had a schematic for a hum balancing pot, just want to play it safe and not ruin my pt. I tried the 100ohm resistor to ground and it reduce the hum a lot but it is not entirely gone. I also moved the input to the other side of the chassis and moved the 2 filter caps off the turret board and towards the back of the amp.

thanks

[Structo]

Did you determine what frequency the hum is?
60Hz = heater hum
120Hz = power supply hum

Also, not sure if it is a problem on this amp but verify the phasing of the heater wires.
IOW, make sure the same wire goes to pins 4&5 on preamp tubes and the other wire goes to pin 9.
That is why I like to use two different colored wires.
Fender got it write on about 50% of their amps due to using green wire.

Hopefully you used two 100R resistors, one from each leg of the heater circuit to ground.
And, do not use the heater center tap if you do use the artificial (resistors) tap.

The Fender style "Humdinger", or hum balance circuit is simply a pot that has each outside leg connected to the heater circuit with the center (wiper) to ground.
But usually two 100R resistors work pretty well.

Or in other amps they used a hum balance pot feeding the two grid leak resistors to balance the bias between the power tubes.
Instead of an adjustable bias current pot.

[yalesmith]

not sure if it is pt hum or 60 cycle.....Don't really know the difference. I used an artificial tap and it reduced the hum a lot but there is still hum. I have used the artificial tap before and it worked perfect. I also moved the OT off the chassis to see if there was some sort of interference between the PT and OT because the proximity to each other and it made no difference. I also elevated the voltage on the heaters (Artificial center tap to Power Tube Cathode) and that made no difference, maybe even worse. I ordered a Hum Pot and will try that. This maybe a stupid question, but why does the hum pot need to be 2watt when it is dealing with such low voltage? I guess the fact that it is AC has something to do with it.

Thanks

[M Fowler]

not sure if it is pt hum or 60 cycle.....Don't really know the difference. I used an artificial tap and it reduced the hum a lot but there is still hum. I have used the artificial tap before and it worked perfect. I also moved the OT off the chassis to see if there was some sort of interference between the PT and OT because the proximity to each other and it made no difference. I also elevated the voltage on the heaters (Artificial center tap to Power Tube Cathode) and that made no difference, maybe even worse. I ordered a Hum Pot and will try that. This maybe a stupid question, but why does the hum pot need to be 2watt when it is dealing with such low voltage? I guess the fact that it is AC has something to do with it.

Thanks

Because of high amperage in the filament supply.

I would make sure you don't have any ground loops. Connect filter caps and grounding to one spot not random grounding.

[jjman]

The hum pot does not carry any of the heaters' current. It only passes some current thru itself based on ohms law. I have not found heater hum balance pots to be more useful than using 2 resistors, yet.

Example:

100R=6.3v/amp

amps=0.063

0.063 x 6.3 = 0.4 watts

Higher voltage will mean more wattage. 5watt 100R is commonly available.

[yalesmith]

I realized what type of hum it is and it is the hum you get when you turn on an amp and have the standby switch on. When i turn the standby switch off the same hum is magnified and increase with the amount of volume. I did notice that when the amp is turned on with the standby switch on the hum is a lot louder than any amp i own. Any ideas? and what type of hum is that?

Thanks

[yalesmith]

Any ideas? This so frustrating!

[Firestorm]

Any ideas? This so frustrating!

Maybe update the pics so we know what we're dealing with.