Troubleshooting hum in ST35

[Shannon Parks]

Then there is "buzz" , which is more of an issue with linestages and preamps. THis can be 60 oir 120 Hz "hum" based, but with a more raspy or high pitched character to it. THis is more likely pickup of externam magnetic or electrical fields, or pickup of noise from the AC power line - light dimmers, TVs, CRTs and switching power supplies are a real factor here).

I was up at my uncle's a few weeks ago and he asked me 'what's that noise in my amps?'. He has a pair of the first Eiclones on his work bench in the garage, and just above the bench is a flourescent work light. Turned that off and the Ikes were silent. I think he just re-routed the power hook-ups. That said, my work lights have never caused this phenomenon. Probably due to the ballast type?

Shannon

[mesherm]

Ashok, on your RCA input jacks do you have their grounds tied together at the jacks themselves or do you have a dedicated pair (signal+ground) traveling from the jacks to the PCB solder holes. If the latter case, ground the two RCA jacks together at the back where they enter and run a shielded stereo cable (red+white+shield) from the junction at the jacks to the PCB inputs. Connect the shield wire to either the L or R ground point on the PCB. On my ST35 I also ran the inputs from the back to the front past trannies and other wires but I used some commercial instrumention cable designed for strain gaged load cells.

On the subject of hum, I had a hum problem on an ST70 type amp I had built using a power tranny without filament center taps. I had to solder a 0.15 mfd cap on each 6.3v line to ground at the end of the tube trains.
I discovered quite a high AC voltage from the filament lines to chassis/signal ground. Installing the caps removed a good 95% of the hum.

[Shannon Parks]

On the subject of hum, I had a hum problem on an ST70 type amp I had built using a power tranny without filament center taps. I had to solder a 0.15 mfd cap on each 6.3v line to ground at the end of the tube trains.
I discovered quite a high AC voltage from the filament lines to chassis/signal ground. Installing the caps removed a good 95% of the hum.

Yeah, I'm interested to hear the results of Ashok's DC filament testing. The diy ST35 uses two 100 ohm resistors for this DC reference like what you did with the caps. Maybe switching to the center tap (if available) or directly grounding one of the sides (leaving the other float) might make an improvement. Of course, a hum pot would be an interesting exercise if available.

Shannon

[ashok]

Thanks for the advice and hints every one:

Each RCA jack has a dedicated pair traveling from the jack to the input terminals on the PCB. I am waiting on some shielded pair cable to see if shielding solves the problem.

Filament Testing:

I first lifted R40 and R41, and connected the center tap of the 6.3V to ground. That did not reduce the hum. Next, leaving the center tap open, I grounded one side of the 6.3V secondary, and that definitely reduced the hum by a good bit, although it did not eliminate it.

Can I do the same thing on the 5V secondary also? Pins 2 and 8 on the 5AR4 are connected to the 5V supply. Also, the the HVDC is obtained from Pin 8. Does this mean I can ground pin 2?

Thanks,

Ashok

[Shannon Parks]

Filament Testing:

I first lifted R40 and R41, and connected the center tap of the 6.3V to ground. That did not reduce the hum. Next, leaving the center tap open, I grounded one side of the 6.3V secondary, and that definitely reduced the hum by a good bit, although it did not eliminate it.

Anyhow, this test familiarized you with 60Hz hum. Did grounding one side make it by far the best hummm operation?

Can I do the same thing on the 5V secondary also? Pins 2 and 8 on the 5AR4 are connected to the 5V supply. Also, the the HVDC is obtained from Pin 8. Does this mean I can ground pin 2?

Doesn't sound like a good idea. Also, I forget the particular use of a 5V center tap - I'm sure someone will chime in.

Shannon

[mesherm]

One side of the 5v filament is already at power supply ground through the 5AR4 pin 8 so it should be ok. Grounding pin 2 will SHORT the filament! If you have 6.3 v center taps connect them to the same ground with a 0.02mf ceramic capacitor. If no center taps then I would recommend a 0.02mf ceramic on each 6.3 leg to ground.
You input jacks should be connected together to ground at the back. The shield of your cable should connect at the same place. If your not using a volume control on the front then don't connect the other end of the shield at the PCB inputs, just connect the R and L leaving the R and L grounds with nothing attached. (they are attached to ground at the rear)
Whenever you have multiple paths to ground that are looped in some way there is hum potential hence the star grounding technique.
happy loop hunting
Mike

[EWBrown]

With a 5AR4/GZ34 , it would not be a good idea to use the 5V Center Tap, as this would impose an extra 2.5VAC of unwanted hum on the HV DC.
The cathode is directly connected to pin 8, so the DC should be taken from that point.

With older style direct heated rectifiers (5U4) , it would be best to use the 5V CT if there is one, or else just use one side of the filament to supply the B+ voltage.

If the rectifier tube has a cathode that is not connected to the filament, then just take the B+ off the cathode, and don't use the CT at all.
In any case, the 5V filament CT should never be grounded.

/ed B in NH