Help Troubleshooting 60's Deluxe Reverb AB163 Amp

[jlatrace]

I am a hobby amp builder, but not an amp tech. I have a friend with an old Fender DR AB163 circuit with an intermitant problem that I have never run into in my limited experience. The problem is intermitant noise or static (almost sounds like a bad shorting jack but it makes the sound regardless of whether a guitar is plugged in or not) and sometimes sounds like static on an AM radio. The sound varies in intensity rushing louder and then stops. Later to return. I found it will occur more often when switching on standby after the preamp filters are fully drained of voltage.

Here is what I have done and some observations:
1. Checked all B+ voltages (pin 1 & 6 of preamp section, pins 1 & 6 of PI and pins 3 & 6 of the PT) and the bias voltage. B+1 was 415 vdc and the rest were all relatively in line with schematic. Bias voltage checked out too.
2. After replicating the problem; I pulled all the preamp tubes one at a time; but the noise continued
3. However it stopped when I pulled the PI tube.
4. Replaced PI tube; but the noise continued.
5. Inspected electrolytics. They had all been recapped before my friend purchased the amp; they appear to be in good shape.
6. Checked the amp's circuit board and other grounds, they appear to be solid, but I have not pulled the board to inspect the underside and someone in the past has replaced some resisters.
7. The non-electrolytic caps in the amp are all original.
8. I can make the problem worse by flipping the standby swtich off and back on. Also, when turning off standby the amp sometimes will make a "pop" sound & when it does, I usually can get the problem to re-occur, even when the preamp electrolytics are charged, by flipping the switch back on.

When the problem is not present, the amp plays like it should and frankly sounds awesome.

On my list to check are:
1. Heater solder joints
2. Change out power & rectifier tubes (I have a test pair of 6v6's and a test rectifier)

After that I have exhausted my ability to find the problem. Before I have my friend take the amp to one of you, I wanted to ask for insight and suggestion. Thanks, I haven't posted for a while because I have been learning about and building D-clone amps with the help of folks at TAG.

[Jazz P Bass]

Just a few thoughts.
The PI coupling caps may be leaking in to the 6V6 grids.
The output transformer may be arcing internally.

[Prairie Dawg]

I am a hobby amp builder, but not an amp tech. I have a friend with an old Fender DR AB163 circuit with an intermitant problem that I have never run into in my limited experience. The problem is intermitant noise or static (almost sounds like a bad shorting jack but it makes the sound regardless of whether a guitar is plugged in or not) and sometimes sounds like static on an AM radio. The sound varies in intensity rushing louder and then stops. Later to return. I found it will occur more often when switching on standby after the preamp filters are fully drained of voltage.

Here is what I have done and some observations:
1. Checked all B+ voltages (pin 1 & 6 of preamp section, pins 1 & 6 of PI and pins 3 & 6 of the PT) and the bias voltage. B+1 was 415 vdc and the rest were all relatively in line with schematic. Bias voltage checked out too.
2. After replicating the problem; I pulled all the preamp tubes one at a time; but the noise continued
3. However it stopped when I pulled the PI tube.
4. Replaced PI tube; but the noise continued.
5. Inspected electrolytics. They had all been recapped before my friend purchased the amp; they appear to be in good shape.
6. Checked the amp's circuit board and other grounds, they appear to be solid, but I have not pulled the board to inspect the underside and someone in the past has replaced some resisters.
7. The non-electrolytic caps in the amp are all original.
8. I can make the problem worse by flipping the standby swtich off and back on. Also, when turning off standby the amp sometimes will make a "pop" sound & when it does, I usually can get the problem to re-occur, even when the preamp electrolytics are charged, by flipping the switch back on.

When the problem is not present, the amp plays like it should and frankly sounds awesome.

On my list to check are:
1. Heater solder joints
2. Change out power & rectifier tubes (I have a test pair of 6v6's and a test rectifier)

After that I have exhausted my ability to find the problem. Before I have my friend take the amp to one of you, I wanted to ask for insight and suggestion. Thanks, I haven't posted for a while because I have been learning about and building D-clone amps with the help of folks at TAG.

Plate load resistors. They're the 100k carbon comps. Replace with metal film or carbon film.

[JoeM]

Besides the noisy carbon comps, I've had similar noise problems due to a socket that wasn't making perfect contact. Make sure the socket pins are clean and retightened, especially the socket for the PI tube. Even some tubes with dirty pins can have noise, de-oxit gold works real well to fix this.

[edited to add]

Did you try another PI tube? The tube itself may be bad.

[jlatrace]

Someone has already replaced all plate load resisters in the preamp section. I will pull the phase inverter plate resisters and replace with metal film. How about the B+ dropping resisters, could they be the problem? They are carbon comp too.

[jlatrace]

Just a few thoughts.
The PI coupling caps may be leaking in to the 6V6 grids.
The output transformer may be arcing internally.

I'll pull them and test test.

[jlatrace]

Besides the noisy carbon comps, I've had similar noise problems due to a socket that wasn't making perfect contact. Make sure the socket pins are clean and retightened, especially the socket for the PI tube. Even some tubes with dirty pins can have noise, de-oxit gold works real well to fix this.

[edited to add]

Did you try another PI tube? The tube itself may be bad.

Thanks, yes I did try a new PI tube without success. Hopefully it's the plate load resisters or coupling caps. I am thinking it's the plate load resisters because soemone has already pulled and replaced them in all stages between the input and the PI (and I assume it was done because they were bad).

[jlatrace]

One additional symptom is that the tremo optocoupler flashes when the amp is acting up.

[acorkos]

On my list to check are:

2. Change out power & rectifier tubes (I have a test pair of 6v6's and a test rectifier)

have you swapped in some new 6V6 power tubes yet? that would have been the first thing i would have done....

[Bruce / Mission Amps]

Bad solder joint.
Frequently it's the ground wire from the PI area on the eyelet board from the board to the brass plate or at the brass plate.
The solder joints look good but have oxidized after 50 years.
Reflow them all and look particularly at the ground wires to the brass plate....
Note where the opto vibrato bug is grounded.....

[jlatrace]

Bad solder joint.
Frequently it's the ground wire from the PI area on the eyelet board from the board to the brass plate or at the brass plate.
The solder joints look good but have oxidized after 50 years.
Reflow them all and look particularly at the ground wires to the brass plate....
Note where the opto vibrato bug is grounded.....

Thanks Bruce and everyone else who have commented. So far in response I have reflowed the ground; reflowed the heaters; and swapped out the power tubes. No change.

So I metered the power tubes screens, plates and ma (mv measured across a 1 ohm resister to ground). The noise is a sputtering sound and when it occurs you cannot get any guitar sound out of the amp, only noise. I noticed that when the sputtering starts, the power tubes lose their bias. The plate voltage drops and varies from about 225 to 275; the screen voltage drops and varies in a similar way: the current drops and varies from 5ma to 14ma; the sputter continues until the tubes regain bias, voltages return to normal and the amp begins to transmit guitar sounds again.

I have not pulled the phase inverter plate resistors, dropping string resistors or the coupling caps. Could this be a bias circut problem? If so I'll check before I start pulling and replacing resisters and caps.

[Chuck H]

Have you tried replacing the other preamp tubes? Have you cleaned the socket contacts yet?

[Ted]

The Vibrato on these amps modulates the output stage bias voltage. When the neon in the drive circuit is lit it pulls the bias as close as it can to 0V. If for some reason the neon is lighting up continually, without being pulsed by the vibrato oscillator, it may very well reduce the bias to the point where the output stage starts to pull down the HT.

Try replacing V5 (and definitely clean contacts)



Check component values and soldering around the modulator valve V5b. I'd be particularly suspicious of the electrolytic across the cathode resistor.

Does the vibrato work OK?
It it doesn't check components around V5a

[jlatrace]

Have you tried replacing the other preamp tubes? Have you cleaned the socket contacts yet?

Thanks for the suggestions. All preamp, PI, power and rectifier tubes have been replaced and the tube socket contacts have been cleaned, no chenge.

[jlatrace]

The Vibrato on these amps modulates the output stage bias voltage. When the neon in the drive circuit is lit it pulls the bias as close as it can to 0V. If for some reason the neon is lighting up continually, without being pulsed by the vibrato oscillator, it may very well reduce the bias to the point where the output stage starts to pull down the HT.

Try replacing V5 (and definitely clean contacts)


Check component values and soldering around the modulator valve V5b. I'd be particularly suspicious of the electrolytic across the cathode resistor.

Does the vibrato work OK?
It it doesn't check components around V5a

Thanks Ted, very helpful explanation. The vibrato does work, I have replaced v5, reflowed solder joints in the entire vibrato and phase inverter circuit and cleaned the tube socket contacts, no change.

I am going to re-check voltages to make sure I didn't miss something.