Noise problem - any idea ? (solved) (soundclip added)

[mat]

Hi,

I was putting two relays + diodebridge + regulator in my clone. When finished it got terrible noise (I had the bridge and regulator grounded to the chasis which I should not have done..). I have searched the problem for 3 days now without solution

Turning the following knobs makes the noise worse: reverb, presence, master, drive, drive volume, tone and boost and bright switches.

The switchable treblecap switch makes a 'pop' sound and also sometimes the midcapswitch does that also.

The amp was working beautifully before the relay project. What I have done now:

I removed the relays + bridge + regulator - no help.

I measured continuity on the pots ground points, they were ok.

I removed and measured the presence pot, it was ok.

Removed the channel switch and did put a new in - no help.

Removed the boost switch and put a new in - no help.

Checked every ground point on the amp, all were ok.

Did change the presence cap to tantalum - no help.

Did change the presence resistor to ground - no help.

Changed different PI + V1 + V2 tube in - no help.

I took preamp tubes out one by one. Clean - no help, od - no help, PI - silence.

I had earlier a similar noise problem when I put the ground wire on the wrong side of the presence pot, it gave same kind of noise, but it is not the case now.

Feel kind of frustrated now

Any kind of info much appreciated,

[skyboltone]

Start at the power supply and check voltages B1,2,3,4,5. Then the plates on the pre tubes. Then check that the cathodes on the pre tubes are up a volt or two. Then the PI, plates and Cathodes. Finally the PA but it ain't gonna be there. Something is fried in the Power supply or pre amp section. Work methodically. If a voltage is missing or way wrong, work with the resistors around it.

Sorry if I had any part in this. When you get back to point one. Wire the relay instead of the panel switch. Then its way simpler to accomplish what you need.

Good Luck
Dan

[mat]

Start at the power supply and check voltages B1,2,3,4,5. Then the plates on the pre tubes. Then check that the cathodes on the pre tubes are up a volt or two. Then the PI, plates and Cathodes. Finally the PA but it ain't gonna be there. Something is fried in the Power supply or pre amp section. Work methodically. If a voltage is missing or way wrong, work with the resistors around it.

Sorry if I had any part in this. When you get back to point one. Wire the relay instead of the panel switch. Then its way simpler to accomplish what you need.

Good Luck
Dan

Hi Dan and thanks for the checklist I already checked the plate voltages that seemed to be ok. I'll check all other voltages tomorrow.

Hey, this is in no way Your fault You have been very helpful on the relay wiring

Will report tomorrow. Thanks,

[mat]

Hello,

I measured the voltages and there seems to be any differencies to the earlier measurements.

B1 455
B2 454
B3 431
B4 339
B5 327

V1 plates 185,197 ,cathodes 1.76,1.68
V2 plates 196,206 ,cathodes 1.78,1.60
PI plates 275,277 ,cathodes 61 ,grids 22,21
Reverb 448,186 ,cathodes 2.8,1.8

Output tubes:

plate 455
grid2 452
grid1 -39.4
cat 0.04

plate 456
grid2 455
grid1 -53.5
cat 0.04

How does them look ?

[mat]

I tried to put back the shielded wires on input and V2 grid to drive but same noise stays.

Is there a way to bypass the presence pot, cap and resistor to know they are not the cause of the problem ?

I noticed that I have had 50V caps on the treble cap switch (it is connected to V1a plate !). Changed it to 500V without help to the problem.

[Normster]

I wouldn't spend too much time in the preamp area since the amp is still noisy with V1 and V2 pulled. If the noise only goes away when you pull the PI, that's where I would start. Most of your voltages are in the same ballpark as mine with the exception of the PI grids. Most (if not all) of my amps typically have a 20V difference between the PI grids and cathodes, i.e., 37V/57V. That may not be your problem but I thought I'd point it out anyway.

Besides pulling tubes to isolate the problem, you may also want to desolder some key components to really nail it down. Since the noise is still there with V1, V2, and V3 pulled, try removing the 100k resistor that feeds the PI. That will isolate the signal side of the preamp. If the noise is gone, then it's very likely in your master volume circuit (or switching). If the noise is still there, then it's most likely the PI.

[mat]

Hi Normster and thanks for joining this thread

Most (if not all) of my amps typically have a 20V difference between the PI grids and cathodes, i.e., 37V/57V. That may not be your problem but I thought I'd point it out anyway.

Ok, I have had about 40V difference in there for a while now and it's been working ok.

try removing the 100k resistor that feeds the PI. That will isolate the signal side of the preamp. If the noise is gone, then it's very likely in your master volume circuit (or switching). If the noise is still there, then it's most likely the PI.

I lifted the 100k (between master pot viper and reverb pot right lug) and the noise got guite a bit louder

I tried to change the presence pot and the wire going to NFB and also removed the 22k trimmer + two 100k resistors going to the plates of PI and replaced with 100k and 120k resistors - no help...

Thanks,

[Normster]

I guess it makes sense that the noise got louder when lifting the 100k. Most likely if you grounded the 100k it would have reduced the noise. I wonder if you're getting AC into the PI from a bad filter cap? I'm not an amp tech so I don't even know how to check for that. It should be possible with a DVOM though. (I don't want to send you on a parts swapping binge. I guess this is where having a scope comes in handy.)

[mat]

I guess it makes sense that the noise got louder when lifting the 100k. Most likely if you grounded the 100k it would have reduced the noise. I wonder if you're getting AC into the PI from a bad filter cap? I'm not an amp tech so I don't even know how to check for that. It should be possible with a DVOM though. (I don't want to send you on a parts swapping binge. I guess this is where having a scope comes in handy.)

Would be really nice to know how to measure them with DVM. Should I measure across the caps with DVM on ac-setting ? Hope someone knows this one.

I don't have a scope. Quite big investment, but would be cool to learn how to use one.

By filter cap I think You mean the power supply caps ?

Thanks,

[tictac]

Here's one of the best and easiest ways to troubleshoot noise problems.
Make a probe from a chopstick and a .1 600V cap. one lead of the cap is your probe tip the other end is attached to a lead with an aligator clip on the other end.

Clip the aligator clip to ground and with the amplifier powered up, touch the probe to various power sources and signal paths throughout the amp. I'd start with the AC heater circuit, then check the preamp tube plates, then all grid circuits.

When you touch the probe to the source of the noise, the noise will stop. Now you know what area to focus on to solve your noise problem.

If you do this let us know what you discover.

TT

[mat]

Here's one of the best and easiest ways to troubleshoot noise problems.
Make a probe from a chopstick and a .1 600V cap. one lead of the cap is your probe tip the other end is attached to a lead with an aligator clip on the other end.

Clip the aligator clip to ground and with the amplifier powered up, touch the probe to various power sources and signal paths throughout the amp. I'd start with the AC heater circuit, then check the preamp tube plates, then all grid circuits.

When you touch the probe to the source of the noise, the noise will stop. Now you know what area to focus on to solve your noise problem.

If you do this let us know what you discover.

TT

Hi TT,

I will do this immediatly, THANKS

[mat]

Hi TT,

I'd start with the AC heater circuit

The noise dissapeared as soon as I touched the pilot lamps lug. I examined the heater wiring and it looked ok everywhere. I got 6.1VAC in there.

, then check the preamp tube plates, then all grid circuits.

The PI's first plate made also the noise to disappear, no other plate or grid silenced it. Not sure how to continue..

Thanks,

[mlp-mx6]

Is the heater circuit properly grounded? (either a center tap on the power xfmr, or with an "artificial ground" of 100 ohm resistors from each side of the supply to ground - usually attached to the pilot light terminals and ground)

[mat]

Is the heater circuit properly grounded? (either a center tap on the power xfmr, or with an "artificial ground" of 100 ohm resistors from each side of the supply to ground - usually attached to the pilot light terminals and ground)

I have no heater centertap on my PT. Also no 100R's to the ground ? Earlier it worked ok ?

[mlp-mx6]

If you do not have a center tap, I would HIGHLY recommend that you set up the 100 ohms from each side to ground. (100 ohms value is not exactly critical, but a relatively low value to ground from each side) Clip a pair in, if you can, and see if the hum diminishes.