Hum in PH14

I have an SP12 with a PH14 in the same chassis (see attached pic). I am trying to track down and eliminate hum in the Phono section. The SP12 has an ever so slight hum thru the woofer that does NOT change with volume. If you are more than a foot away from the woofer you can’t hear it. The PH14 has a hum that is also thru the woofer that DOES increase with volume. It may be a 60Hz hum. I’m using it with an ST-120. The ST-120 and TT are dead quiet when using my SS Pre-amp. I run the amp off a variac as my house voltage can exceed 122VAC. Everything is plugged into the same outlet but only the amp is driven thru the variac. I have the variac plugged into a GFI device ( https://www.amazon.com/gp/product/B000XVG72G/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1 ) as we have frequent ‘flickering’ in the electrical: branches falling on lines and people hitting electric poles. It trips and needs to be reset when it loses normal voltage.

What I’ve done so far: Added shorting plugs to all unused inputs. Seems to have brought the hum level down. Added red silicone tube dampers to all tubes except the rectifier. Typically run the Phono with the (L) and (R) gain knobs turned down 5 clicks. I’ve tried it with and without the variac. No difference noticed. I mostly listen to jazz on vinyl and not very loud. Typically I can only hear the hum in between tracks and in quiet passages but if I turn the volume up so its loud it is present in the music.

I like to use NOS tubes from the 1950’s and 60s. I find the new production tubes do not sound toobie to me. However, Roy suggested I try some JJ 12AX7 and add on the low microphonics/noise option. Same hum. Some tubes seem to cause more hum than others. The best combo so far is a GE 7025 in V1, a GE 12AX7A short plate in V2 and a GE JG5157 in V3. The GE 7025 has pins that are a tad too big and its difficult to install and I can only get in in half way. I have an NEC 12AD7 coming for V1.

Will tube hum decrease with use i.e. ‘burn-in’ time ?

Is it tube hum or an internal or external source being picked up via EMI,  RF, ground loop ?

What else should I try ?

Hi Dave, I've spent many hours chasing similar hum with my SP12/PH12 and I did have some success. Perhaps some of what I learned could help you as well. Most of my testing was with all inputs shorted on the bench connected to a spectrum analyzer. It was definitely 120Hz, and most evident in the phono stage like yours.

First thing I would do is a very basic test. Short the phono inputs and see if the hum decreases. This eliminates the turntable and its wiring from the equation.

EDIT: One question. Can you verify that the ground bus wire on all the rear RCA jacks is not connected to the phono inputs? I can't tell from the picture. The Phono inputs should be grounded only at the phono board.

Thanks for the response rjpjnk.
I'll have to pull the unit from my rack and take a look at the grounding.
I have shorted the Phono inputs and still had the hum. I will do it again and see if it diminishes at all.

If it is 120Hz wouldn't that indicate a capacitor issue?

The bond wire is cut between the Phono jacks and CD jacks. The ground wire is used on the wiring from the phono RCA jacks to Phono board. However, the solder point for the ground on the board does not indicate that its a ground point (other points on the board do indicate ground). (see attached pics) Looks to me like the ground is tied to the Left channel.

I haven't done the shorting plugs in the phono jacks yet in case I need to solder something.

I have continuity from both phono RCA center pin wires to ground.
Why would that be?

I want to start of by saying that I don't have the equipment you are using or a schematic of it but if you have continuity between your two RCA center jacks would it be possible that the red and white wires are connected to the ground on the board and the ground is connected to the left in? Maybe someone that is built these boards can chime in.

DavidR wrote:I have continuity from both phono RCA center pin wires to ground.
Why would that be?

With nothing plugged in to them? Are you sure? This sounds impossible unless they were wired backwards as explained above. However, I looked up your PH14 board and the wiring appears correct as you have it. The schematic I found shows 47K from RCA centers to ground.

Nothing plugged in. The Pre/Phono unit is on the bench with no wiring going to or from it.

Also, I found this in the Build Notes:
FINAL WIRING AND CHECKS Once you are sure all the wiring is correct, screw the board down to the chassis mounts.
Insert all the tubes. Hook up a DVM (digital volt meter) set on AC on a 400vac scale and turn the power switch on. You should measure around 290 vac across the two HV points. Then set the meter on a 20vac scale, and you should measure 12vac where those two wires connect.
If nothing smoked or blew up due to incorrect wiring, those voltages should all check OK. Now set your meter for 400vDC and connect the meter leads between ground (center right end of PCB) and the point next to D3 marked B+    It should measure 254vdc. Then put your meter leads on each end of D3 and you should measure somewhere between 2040vDC across D3.   If this voltage is lower than 20vdc you will have HUM when you try to listen to music.  Also check the voltage from H- to H+  it should be 12vDC. If any of these are incorrect, recheck your wiring.   Email me if you have any questions or problems, if the D3 voltage is low email me for resistor change on R23.
Once all this is correct, you should be able to connect a music source to an input, and your amplifier and speakers to the output, and enjoy music with no hum.   If you have any hum at all, It is either a wiring error in the grounding, or is a low B+ voltage (across D3). Email me if you have any questions or problems.

I did not do the build but I've had issues with the Phono section since Day1.

DavidR wrote:Nothing plugged in. The Pre/Phono unit is on the bench with no wiring going to or from it.

Hmm... Is there also continuity from the outside of the phono RCA jacks to ground?

I'll have to check.

Are you saying that both the inner and outer conductors of the Phono RCA input jacks have 0 ohms resistance to the chassis? I find this hard to believe given that the preamp is working. It is working, right? Phono?

I didn't do resistance but I can. The Phono section has not worked right from day 1.

Yes, definitely measure the actual resistance rather than using a continuity beeper. Measure with and without the first tube inserted (the one near where the phono input wires attach to the board).

OK I used both a Fluke 15B+ and an older 8021B.
The L&R center pin hole was OL on both meters with and w/o tube V1.
The outer area was 0.1 ohm on both meters with and w/o V1.
The 15B+ would work its way down to 0.5 quickly and finally settle at 0.1 ohms after maybe 30 secs.

OL means Open Circuit. So this contradicts your observation "I have continuity from both phono RCA center pin wires to ground."  
Right?

Yes, Open Line on the center pin would indicate that I do not have continuity. However, the older Fluke can do just continuity and is not thru the resistance circuit and it indicates (beeps) continuity. So I'm confused and the damn unit still hums.

Ok, so put that old Fluke in a drawer somewhere and never use it for continuity again

Do you have a schematic? I couldn't find a reliable one online, but from what I could find it looks like there should be an input load resistor of around 47K, but your measurements show OL. Maybe you have a different circuit than the one I found.

rjpjnk wrote:Ok, so put that old Fluke in a drawer somewhere and never use it for continuity again

Do you have a schematic? I couldn't find a reliable one online, but from what I could find it looks like there should be an input load resistor of around 47K, but your measurements show OL. Maybe you have a different circuit than the one I found.

I do not have a schematic just a picture of the blank board.

You are correct. I had the resistance setting on low (up to 400ohm) Doh. They both read 46.9 ohms.

I also put the old Fluke on resistance with the beep and it did not beep for continuity on the center pin. The resistance needs to be less than 70 ohms to beep.

What's next?

So after all this we can say the inputs are wired correctly.

Now what was the original problem? Hum? Since you are not too familiar with using a meter it might be really difficult to troubleshoot like this. Hopefully Roy will chime in. You may need to send it in for repair.

Most hums are due to grounding issues, but your amp look so well built I suspect whoever built it knew what they were doing. Another common source of hum on these boards is insufficient voltage drop across the B+ regulator circuits. Roy has a whole section of this in the assembly manual and has often reviewed it in in these forums, but it involves connecting a meter to some pretty high DC voltages so might not be a good idea for the inexperienced. If this is the case, it is a simple mater to fix it with a resistor swap.

Finally, the main source of hum in my case was insufficient capacitance in the B+ filter. I was able to drop my residual 120Hz hum more than 10dB by increasing the capacitance here. In theory it shouldn't be necessary given the regulator circuit, but it was.

When I last spoke to Roy I think he said some older boards had less capacitance. The designs are always improving and I don't know which you have. Without a schematic, there is nothing much I can recommend at this point.

Well, Roy built it and he has been little help other than to say it didn't hum for him. It hums for me and the hum increases with volume.

Many of the solder points could use touch up.

I do know how to use the meter. Just an oversight. No, I do not feel like sticking my hands in to test B+ and diode.

Thank you for the help offered.

Ah, I see. Well if Roy built it and tested it it is no doubt wired correctly and has the B+ set properly. So probably it is already as quiet as you're going to get it. I wish I could help more. I understand how frustrating hum can be.

Since it increases with the vol you know it is getting in through the phono board somehow. That board looks like it has it's own B+ power supply too, so it's not like mine where my PH12 board shared the SP12 power supply and "borrowed" the hum that way. Have you tried different tubes in PH14?

Btw, you can get a spectrum analyzer app for an iPhone that will allow you to measure the noise at your speaker and see if it is at 60 or 120Hz. I am guessing it's 120, which means rectification noise from power supply which might be fixable with additional capacitance as in my case.

I'd test it myself but I know when its really not something I should tackle. Being legally blind in one eye causes a very bad depth perception and to top that off the pollen this year has my allergies at an all-time worst and my doctor has me taking an albeuterol inhaler which makes me Mr. Jittery. Actually that's how I stumbled upon getting a beep tone when checking to see if the ground wire from the RCA plugs was actually a ground point. I accidentally bumped the solder point next to it and got the continuity beep. My hope is that I can find someone in the Massachusetts area that knows what to do.

I bought some JJ tubes Roy suggested and paid the extra fee for low noise and microphonics. At first I thought I had the solution but later realized I had the gain knobs turned way down. One tube went white from vac leak and I sent them back. I then took my tubes to Viva Tubes in western MA and had them test them for noise. All but one were very quiet and not microphonic. I had that tube replaced by the vendor. But again, thank you for your time and help. Much appreciated.

So I took the time to go thru all the capacitors and resistors on the board and make sure all values are correct. I only had trouble reading R25 and R26. R26 color bands looked brown to me but when my son took a picture with his iPhone the bands did appear red which made the value correct. R25 had brown bands (brown = 1% tolerance or a value of 1) on each end and if reading from L to R it was wrong but decided it was mounted opposite way of others so was correct when reading R to L.

That leaves the caps. I was unable to ID the value of C6 but will assume it is correct as all the other caps EXCEPT one labelled C21 on the signal input end. Seeing C21 is a 470uF 25V electrolytic on the power supply end I'm assuming it is actually C25 in the audio signal. It has a jumper wire and not a 0.01uF cap. (pic attached below)

Does anyone have an opinion on the C25 cap as to why it has a jumper wire and not a capacitor ?

I built both the VTA ST70 and SP12.  I run only the ST70 at 118v from the variac.

I discovered that I MUST run the SP12 at full line voltage 122v. I tried the SP12 at lower voltages from the variac and always got a humming noise.

I only run the ST120 thru the variac. Pre board is quiet; just the slightest hum with your ear right up against speaker. My big question right now is about C25. Why do I have a jumper wire and not a cap?