First question on my build

[J Luth]

Tested today with the junction of C13 and R15 grounded with a jumper, the 100R is in place and the amp connected to a speaker cab.

Without the ground jumper the hum is there, with the ground jumper connected most of the hum is gone. There is a very slight hum from the speaker. With the PI pulled there is a very slight hum from the speaker but barely noticeable.

Also tried a couple of other things, not sure if they will help.

The 100R for right now is connected to a switch, it makes it easier than disconnecting and reconnecting when asked to do so. With the junction of C13 and R15 grounded and the 100R in circuit as stated the hum is almost gone but no completely. If I take the 100R out of circuit with the switch and the jumper in place any hum if there is any is masked due to the amp starts to pulsate, motorboats.

With regards to the difference in gain I did the following rather than relying on my ears.

With the ground jumper disconnected, I connected the oscilloscope to a dummy load on the speaker jack, set the scope to .5mv per division and set the amp volume to 4, all tone controls are set to the center position. Connected the small oscillator to the input jack, set it to a 1K signal and turned the amplitude up until the trace was covering 5 divisions peak to peak. When the jumper is reconnected the trace goes flat so there is no output with the C13 and R15 grounded.
Sorry for the confusion.

One further test with the oscilloscope without C13 an R15 grounded. No changes made to the amp, the oscillator or the oscilloscope settings. I had mentioned in a previous post about a volume difference with the 100R connected and disconnected. What the scope shows, if I did it correctly. With the 100R in circuit the trace is at 5 divisions peak to peak, with the 100R lifted by the switch the trace drops to almost exactly 1 division peak to peak.

[J Luth]

Dave,
Rechecked levels today using the oscilloscope.

[J Luth]

Chuck,

You are correct about the gain being zero. Test results are posted.

[eschertron]

With the 100R in circuit the trace is at 5 divisions peak to peak, with the 100R lifted by the switch the trace drops to almost exactly 1 division peak to peak.

That's quite a bit of gain change between the two! No wonder you hear a bunch more hum with the 100R in place. Have you done any real-world simulation, for example: stand across the room from the speaker cab, have a radio or TV on (bar noise), and play at normal gig volume? How do you need to change the amp's controls when the 100R is in vs. out? Once setting the amp to be a 'normal' volume for each situation, then compare the hum. With and without the masking noise. Is it still too much?

[J Luth]

I have not done the test the way you described but will try when I get a chance.

As of now without playing through it with the 100R connected the hum does not change with the volume control until the volume control is turned up to half or more then there is a large increase in the hum.

Also, just had friend here with a better ear than me. I hooked up the oscillator to the amp and tried different tones. It appears to be a 120 cycle hum.

[eschertron]

120Hz ripple at the PI might be addressed/fixed by lead dress or better filtering.

I hate to say it, but adding wiring and a switch on that resistor changes the experimental design somewhat.

[Enzo]

Or grounding scheme.

[J Luth]

The switch can be undone easily.

[J Luth]

I have tried numerous grounding arrangements with very little difference. There was some difference but not much.

The main power supply, filter caps and line in are star grounded at the same location, the rectifier is grounded near it's location and the grounds off the main board for the low switch, the tone stack, preamp tubes and PI are grounded close to the input jack.

[J Luth]

I am thinking about doing a rewire on the amp with some different wire. I've gone this far so why not?

Noticed that if I get anywhere near the wire, not touching it, that is connected to pin 2 of the PI I get a loud buzzing which increases the closer I get to it. I can start to hear it when I am about 1 1/2" away and it gets louder as I get closer to the wire. There are a couple of others also plus if I pluck a couple of wires in the amp or the capacitor connected to pin 6 of V1 with the plastic stick I can hear it through the speaker. I tried a different cap but still the same.

This is getting long and a bit frustrating on my end and probably confusing on the end of everyone trying to help. By no means am I giving up so don't think that.
I appreciate all the help and will continue to try anything that is suggested. If I knew more about this it would help.

[Dave H]

Also, just had friend here with a better ear than me. I hooked up the oscillator to the amp and tried different tones. It appears to be a 120 cycle hum.

120Hz hum is from the B+ supply full wave rectifier. The centre tap wire from the PT B+ winding carries nasty high current charging pulses to the first filter cap. Check that it is not running next to any other wires. It should really connect directly to the first filter cap -ve terminal not the chassis.

[J Luth]

If I remember correctly, I am not near the amp right now, that would be the red/yellow wire out of the PT. It was moved from chassis ground to the ground of the first filter cap a while ago. It is not close to any other wire but I'll double check.

[Chuck H]

I'm still of the belief that the amp has a hum issue that is only worse with the 100R connected because the amp is producing it's full wattage. So that, the 120Hz noise and the motor boating have me thinking there may be a mistake in your filter wiring. Believe it or not it's entirely possible to miss something like this even when you've been through the grounding scheme, etc. I can't remember if you're using a doghouse (the filters would not be visible) but I'll go back to the photos and have a look.

[J Luth]

No doghouse the filter caps are inside the amp. Here's the filter picture. There are ground jumpers under the board connecting the 22UF's, a lead under the board connecting the 2, 100UF's and the resistors between the 100UF's are connected under the board.
It's not as messy as it looks. Although it is getting messy with all that is being done but that can be cleaned up later.



Wire from the positive of the second 100Uf connects to a terminal board along with the rectifier output and one side of the standby switch.
Wire from the first 22Uf on the left connects to the junction of the 2 100K resistors after the tone caps.
Wire from the second 22Uf connects to the junction of the 82K and 100K at the far right of the main board.
Wires from the 3rd 22Uf, wire to pin 6 of all power tubes, one lead from the choke is connected there as well, the other lead from the choke connects to a terminal board where the primary of the OT and the other side of the standby switch is connected.

Also the rectifier if you need it.

[Enzo]

Is that still even a question? The 100 ohm resistor needs to be there for the circuit to work. No doubt removing it drops the gain substantially, and so you hear less hum. But the hum needs to be cured with the resistor in place.