loud noise when i probe the EF86 plate

[jewishjay]

I built a Selmer Zodiac 30 clone, and it sounds great but I need to work out a few bugs. There's a loud hum, possibly a ground loop or bad ground scheme. When probing to check voltages and bias I found that when I probe the plate of the EF86 I get a loud CRACK at the speakers, louder than anywhere else. Is that normal?

VOLTAGES:
12ax7  145  & 158
ef86     280
12ax7   167 & 254
el34 plates  379
el34 screens  375
gz34 rectifier  391


INPUT JACK GROUND BUS:
preamp cathodes
tone controls
tremolo oscillator
preamp filter caps

TRANSFORMER GROUND LUG:
center taps
first stage filter caps
mains earth

I know its hard to really diagnose anything over the internet, but do those voltages, and that ground plan seem ok to you? Is the loud popping noise at the ef86 plate a problem?

[tubeswell]

Seeing as how the EF896 is early-on in the signal path, it isn't unusual for the pop test on that stage to make a loud noise - because the pop is being amplified by each subsequent stage in the amplifier. (Whereas if you popped the output tube grids, the pop wouldn't be as loud).


Re the hum: if you unplug the EF86, does it still hum the same loudness? (if not, the problem might be your grounding around the EF86/pre-amp filter cap stage. If so, then look for the source of hum elsewhere in the amp circuit, each lead dress, EMF coupling, or grounding elsewhere etc)

[jewishjay]

If you unplug the EF86, does it still hum?

When I unplug V2 (ef86) the hum stops
But if I unplug V1 (12ax7) the hum persists
does that mean the problem is near there?
AND the volume pot is scratchy like there's DC on it
so I measured and found  0.14 vdc at the volume pot
not sure if that's a problem or THE problem or how it's getting there
leaky couple cap i guess?

[tubeswell]

Re: the scratchiness, there shouldn't be any DC on the vol pot - so suspect the coupling cap.


Re: the hum, yes it sounds from your reply that its around V2 (the EF86). I take it you ran Pin 2 (Faraday shield) to ground? How have you done the signal grounding?

[jewishjay]

Here's the schematic I worked from. The original was 2 channels with a complex push button tone selector on one channel. I simplified it to one channel, perhaps I made a mistake before I even started building?

[tubeswell]

I see you’re running the EF86 from the  same PS filter cap node as the PI. It may be better (‘less hummy’) to run it from the same filter cap node as V1.

[Willabe]

Re: the hum, yes it sounds from your reply that its around V2 (the EF86). I take it you ran Pin 2 (Faraday shield) to ground? How have you done the signal grounding?

Did you ground pin 2 (or pin7)?

How have you done the signal grounding?

And this too is important. 

[jewishjay]

I see you’re running the EF86 from the  same PS filter cap node as the PI. It may be better (‘less hummy’) to run it from the same filter cap node as V1.

I made this change, and brought the power tube heaters in phase as per a suggestion from the "tube amp builders" Facebook group. No change.

Originally pins 2&7 were grounded, but since they are also internally connected, I lifted pin 2 just in case that was causing a tiny ground loop. This also changed nothing.

Reminder, the hum goes away then you pull out the ef86 (v2)

So I measured all DC voltages on the ef86...see if this tells you anything....

1   19.5v
2   0v
3   .5v
4   heater
5   heater
6    263v
7    0v
8    .5v (tied to pin 3)
9   between 0v and 0 .26v depending on the volume knob

[tubeswell]

Have you tried subbing another EF86 (to see if it’s a ‘hummy’ tube)?

[sluckey]

1   19.5v

Pin 1 is way too low. Replace that .068 cap. Use a .1 if you don't have another .068. Does pin 1 voltage increase?

Pin 3 is also low but that's probably related to pin 1 being way low.

[tubeswell]

^Yes^ Pin 1 should be at least 60V-80V. Either change the voltage divider or ditch the 1M lower resistor and replace the 6M8 with a 1M

[jewishjay]

Have you tried subbing another EF86 (to see if it’s a ‘hummy’ tube)?

Yes, I tried replacing ALL the tubes. Not a bad tube.

^Yes^ Pin 1 should be at least 60V-80V. Either change the voltage divider or ditch the 1M lower resistor and replace the 6M8 with a 1M

I changed the 6.8M to 1M and that increased the voltage, but didn't chase the hum away.

I've been over and over it a million times. I got really frustrated with it and sent it to a local amp tech with decades in the biz. Maybe he can find my mistake....

[HotBluePlates]

... sent it to a local amp tech with decades in the biz. Maybe he can find my mistake....

I read this then chuckled when I thought, "You've been talking to folks with "decades in the biz" (of troubleshooting amp builds over an Internet forum).

Sluckey's up to ~23k posts.  If he sometimes seems grumpy, it's because probably 12k of those posts were telling people the same thing about common oversights, or needing more care in their measurements, or being told "but I already checked and everything is correct."

[Willabe]

... sent it to a local amp tech with decades in the biz. Maybe he can find my mistake....

I read this then chuckled when I thought, "You've been talking to folks with "decades in the biz" (of troubleshooting amp builds over an Internet forum).

Sluckey's up to ~23k posts.  If he sometimes seems grumpy, it's because probably 12k of those posts were telling people the same thing about common oversights, or needing more care in their measurements, or being told "but I already checked and everything is correct."

 

[jewishjay]

12k of those posts were telling people the same thing about common oversights, or needing more care in their measurements, or being told "but I already checked and everything is correct."

And I really appreciate all your help! Your collective wisdom is valuable. Thank you for your patience with us newbs.

After applying some suggestions from  this thread, I'm still experiencing a 60hz hum somewhere in the vicinity of the EF86. When you pull out that tube, the hum stops. I replaced all the preamp grid wires with shielded wire. I changed the 6.8M screen resistor to 3M and deleted the 1M to ground, which raised the screen voltage to 50, so now the little pentode is dissipating 1W as per the datasheet. Still hums.

Here are all the voltages on V2 now:

plate 199v
screen 50v
cathode 1.2v

And one more thing....when I probe the cathode there's a faint 120hz buzz at the speaker. Normal?

[jewishjay]

SOLVED by elevating the heater CT to the power tube cathodes.

As a side note...I ended up changing back to the 6.8M screen resistor we talked about. Changing it to 3M killed the tremolo! I guess it needs to be at that lower operating point in order for the oscillator to effect it. Weird.