Meter Question

[trem]

I have a hum problem.
Basically a Fender BF preamp...into a LTP Inverter...into a 2x 6K6 power amp.
The hum is effected by the volume pot, and it goes away if I pull V1.
If I set my meter to Hz, and out it on the speaker jack, it says 120.....but it does not say 120 anywhere further back.
So that is the problem I need to find.

My meter question is...when you have the meter on Hz...are the probes in series with the meter.?
If I try to get a Hertz reading off the positive lug of the preamp power supply cap (red probe on cap+, black probe at chassis ground) the whole amp goes dead quiet.
I do not think that is what happens if I try to get a Hertz reading from the other PS Caps.
Thank You

[g1]

When you say "the whole amp goes dead quiet" do you mean it quits working? Or it's working and the hum is gone?
The meter should not stop the amp from working, maybe the pressure from the probe is temporarily fixing a bad connection or solder joint?
Try to see if the amp passes signal while your meter is making it "dead quiet".

[Gtr_tech]

2 possibilities: Gnd loop, and hum pickup from incorrect lead dress.

[trem]

When you say "the whole amp goes dead quiet" do you mean it quits working? Or it's working and the hum is gone?
The meter should not stop the amp from working, maybe the pressure from the probe is temporarily fixing a bad connection or solder joint?
Try to see if the amp passes signal while your meter is making it "dead quiet".

This is with no instrument plugged in.
I do not know what is happening with the power tubes when I check Hertz on that one cap.
But is this normal....to see 120Hz across the speaker jack, but I cannot see 120 Hz in the preamp.?
Thanks

[nickb]

This is with no instrument plugged in.
I do not know what is happening with the power tubes when I check Hertz on that one cap.
But is this normal....to see 120Hz across the speaker jack, but I cannot see 120 Hz in the preamp.?
Thanks

What is the make and model no of your meter? Or post the datasheet. It's pretty hard to be specific without knowing more about it.

[trem]

35XP-A Compact Digital Multimeter | Amprobe
This is the DMM I was using.

[trem]

2 possibilities: Gnd loop, and hum pickup from incorrect lead dress.

Yeah...good points.
This chassis is REAL Tight. I dealt with lead dress as best I could.
I have the bias circuit and the power supply filters (except for the power tube cap) mounted on "top" of the chassis.
In the preamp, I have 4 grounds all in one place. Those 4 are:
Input Jack
V1 Cathode
Volume Pot Ground
B+ For V1
Do those ^^^^^ sound like a problem.?
Thank You

[nickb]

35XP-A Compact Digital Multimeter | Amprobe
This is the DMM I was using.

The datasheet is under the 'resources' tab. That meter needs at least 2.5V rms to indicate frequency. The data doesn't say but I'd be very surprised if the input wasn't both AC coupled and high impedance. So, there's no explanation on why the preamp dies when you use the frequency range on the filter cap.

2.5V rms is a pretty loud hum on the speaker. OTOH 2.5V is quite a big signal on on the first stage of the preamp so that explains why you might not see it there.

You'll get better information using the AC volts scale to probe the hum as it's more sensitive.

BTW, you stumbled on a common technique used by engineers to track down noise sources - look at it from a frequency perspective using a spectrum analyzer. Clever trem!

[trem]

nickb...Thank You
I never would have even looked at that data sheet, nor probably have gleaned the importance of it.
I see what you are saying...with no instrument plugged in (the jacks short to ground) I will not be anywhere close to 2.5 Volts.
I will give the AC a shot like you say.
I do have a scope...and a small Sig Gen. I can use it in a basic manner, with a Freq to the amp input, and a dummy load, and then hunt for a dirty (or clean) signal. I am never sure when is the best time to use it.
BTW...if I did something clever, I can assure you it was just dumb luck.....
Thanks Again

[Enzo]

Another possibility I like is oscillation - instability. If your amp is sitting there oscillating at RF, the audible result is generally a hum and the sound of the audio tends to be kinda washed out sounding. Perhaps touching your mete across those points is enough to stop the oscillation, thus killing the hum. Hey, it could happen...

A scope on the output would tell us that quickly.

[trem]

Another possibility I like is oscillation - instability. If your amp is sitting there oscillating at RF, the audible result is generally a hum and the sound of the audio tends to be kinda washed out sounding. Perhaps touching your mete across those points is enough to stop the oscillation, thus killing the hum. Hey, it could happen...

A scope on the output would tell us that quickly.

OK...but like I say...my scope skills are lacking.
Would a 1k 1 Volt signal be a good place to start...and then scope the speaker jack.?

But one more question...my Speaker Jack/OT Is Not grounded...so I will need to ground the scope to the chassis, and then scope one of the Jack/OT lugs.
Is there a problem with not grounding the OT/Speaker Jack.?
This is just a single 8 Ohm OT...it goes right to the speaker jack, with no ground connection.
Thanks

[Jazz P Bass]

OK...but like I say...my scope skills are lacking.
Would a 1k 1 Volt signal be a good place to start...and then scope the speaker jack.?

1K/ 100 mv's would be more appropriate.

Then again, if the amp is oscillating, you will not need an input signal.

[soundguruman]

Get all of you power supply grounds over to the PT side of the chassis.
Get all of the preamp grounds to the opposite side of the chassis.
Your 1 preamp ground should be on the input jack side of the chassis.
Don't connect them together except thru the resistance of the steel chassis.
Get your heater wires tightly twisted, and as far away from the audio path as possible.

For your first power supply filter, you want to use 22-33uF instead of 10.
Old design did not pay attention to hum and noise as much as you would like.
So, you are better with 33 uF coming from the rectifier tube.

What's happening is that the first preamp tube is picking up filament and amplifying it.
The remaining noise ripple in the B+ is being amplified also.

So depending upon which direction the first preamp tube is facing,
turning it a different direction can reduce noise.

Oscillations: the input to the first preamp tube should be directly under the first plate resistor.

[g1]

Inject a signal with the speaker connected so you can hear it.
Put your meter set for frequency across that same cap. Do you still hear the signal?

[trem]

Get all of you power supply grounds over to the PT side of the chassis.
Get all of the preamp grounds to the opposite side of the chassis.
Your 1 preamp ground should be on the input jack side of the chassis.
Don't connect them together except thru the resistance of the steel chassis.
Get your heater wires tightly twisted, and as far away from the audio path as possible.

For your first power supply filter, you want to use 22-33uF instead of 10.

That is how my amp is now, except the B+ for my preamp is grounded with the V1 cathode.
Are you saying the V1 B+ should be over at the PT ground.?