Ampeg B-15N - Consistent "Tick" Sound Has Got Me In A Mood!

[parentheticalfact]

Hello! I've got an Ampeg B15N that belongs to a local studio on the bench. I've worked on this amp before for noise issues got a lot of issues sorted at that time which I felt called me to replace most resistors in the preamp section as well as coupling caps. After several months the amp has developed a new noise problem where there is a defined "tick" or pop sound that occurs between 10 seconds to 30 seconds apart.

I pulled preamp tubes to try to isolate the issue and the amp continues the sound with the preamp tubes removed but the noise stops when the phase inverter tube is removed.

The octal socket for the phase inverter was in awful shape (crumbling, possibly unstable material) so I replaced the socket, cleaned the board and replaced the remaining original resistors in the phase inverter section. This job was called as a rush and it was easier to do a bit of "shotgun" approach when it came to possibly noisy resistors - they are very easy to get to on this board.

After rebuilding the phase inverter section the noise remained. I tried different output tubes, swapping octal preamp tube positions, different rectifier tube...I even tried disconnecting the wires that lead into R22 and R18 to the mixing network into the phase inverter.

As well as these things I have removed the filter cap lead that feed the phase inverter section and subbed a single F&T cap in place - no change and no improvement.

Simple amp and it seems like it's got me baffled. All coupling caps have been replaced except for what exists internally to the PEC module.

Maybe something related to the negative feedback? Can't think of what could cause the issue but I feel it's something to do with an interaction of the phase inverter and power amp section.

[nevetslab]

Have you tried swapping out the 5AR4 Rectifier tube? As you've isolated the preamp section from being the cause, it does seem to point to the Power Supply or the PI circuit. Bypass cap across R24 cathode resistor could be involved, though I'd think it's power supply related. Do you have a scope, in the event swapping the rectifier tube doesn't cure it? I'd be monitoring the supplies at first stage (OT C/T tap), then the Screen Supplies, then the Plate supplies for the PI. Also check the bias supply for these periodic tic/pop signal.

[nickb]

Can you at least isolate it to one half of the PI by removing one 6L6 at a time?

You can also short pin 1 of the PI to ground to eliminate all the stuff prior to the PI ( not the same as removing preamp tubes),

[parentheticalfact]

Have you tried swapping out the 5AR4 Rectifier tube? As you've isolated the preamp section from being the cause, it does seem to point to the Power Supply or the PI circuit. Bypass cap across R24 cathode resistor could be involved, though I'd think it's power supply related. Do you have a scope, in the event swapping the rectifier tube doesn't cure it? I'd be monitoring the supplies at first stage (OT C/T tap), then the Screen Supplies, then the Plate supplies for the PI. Also check the bias supply for these periodic tic/pop signal.

I have swapped out the rectifier. No help although I did start getting arcing in a JJ GZ34 i used for subistution so I put the vintage mullard back in. Arcing occurred during standby flips which I've encountered before. I should perhaps double check the first filter stage. But the noise was the same with both rectifier tubes. Scope shows a blip at phase inverter grid and plates when the noise occurs. Maybe the board is conducting? In a previous servicing a few months ago I replaced the bypass cap on the phase inverter but just to double check I replaced again with a fresh stock Sprague Atom and no change. I'll use the scope to see if it's found in the supply line or just signal path.

[parentheticalfact]

Can you at least isolate it to one half of the PI by removing one 6L6 at a time?

You can also short pin 1 of the PI to ground to eliminate all the stuff prior to the PI ( not the same as removing preamp tubes),

This is a good idea, I will try pulling a tube (probably with bias meter hooked up to make sure nothing goes super whack in regards to current through the remaining tube) and/or shorting pin one of the phase inverter.

[boroman]

I have the same thing going on on a '68 B15NF. I've recapped the amp, changed tubes, coupling caps, bias section, cleaned sockets and pots and ticks are still there, but it's so minor that I left it that way. You don't hear it on the recording anyway. This is not related to the preamp because those "ticks" are heard even when both volumes are down. Also, I would not change preamp resistors and caps because this is all about the sound of this amp...

Of course you can replace everything but I think the problem is not so easy to track down. Maybe it has something to do with PT/OT.

[Jon Snell]

To further isolate the issue, remove the PI listen to what happens, it may stop, in which case it could be an anode load resistor on the PI R26, 7, 8 or 9.
A poor wiper connection on VR7 can cause an intermittent popping sound.

[parentheticalfact]

To further isolate the issue, remove the PI listen to what happens, it may stop, in which case it could be an anode load resistor on the PI R26, 7, 8 or 9.
A poor wiper connection on VR7 can cause an intermittent popping sound.

I did mention that the phase inverter section was where it was most notable - removing the phase inverter tube almost completely eliminated the noise issue. I even replaced the socket (it was crumbling) and replaced every component directly related to the phase inverter's operation including caps and resistors.

You brought up a great point though with VR7 - the hum balance pot. Cleaning this pot very thoroughly and dealing with some sloppy previous work on the bias section (about the only part I hadn't rebuild on this amp yet!) eliminated the issue.

Some points interesting to note
1) The noise was just as loud with the preamp tubes removed.
2) Volume controls had no affect on the level of the popping sound.
3) Removing the phase inverter eliminated the popping sound

I would have thought the preamp section to be very sensitive to this issue but the phase inverter for some reason was the most affected.

Hope this info helps someone in the future! Thanks for all who chimed in!

[nickb]

Glad you found it but your reporting of the issue was contradictory.

In the first post "the noise stops when the phase inverter tube is removed."
Above you said "removing the phase inverter tube almost completely eliminated the noise issue"
and also above "Removing the phase inverter eliminated the popping sound"

When doing remote diagnostics it's essential to be accurate in reporting any findings. Knowing that it still happened with the PI removed would have led in a different direction from the outset.

[Helmholtz]

Very strange! This amp employs the opposite of heater elevation as the the heater voltage is referenced to the negative bias voltage (-50V).
Maybe this coupling emphasized the audible symptoms.

[nickb]

Very strange! This amp employs the opposite of heater elevation as the the heater voltage is referenced to the negative bias voltage (-50V).
Maybe this coupling emphasized the audible symptoms.

Positive or negative works equally well. For small voltage differences the rate of change of leakage current is high. Once you get past a few volts in either direction it reduces rapidly. Here is a plot of leakage current vs H-C voltage from the 1904- RCA Vacuum Tube design manual, chapter 2, pg 28:

[Helmholtz]

Thanks Nick.
So the heater-cathode impedance Zhc = d(Vhc)/d(Ihc) is lowest around zero heater-cathode bias (Vhc).