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Oddball amp noises


Scratchy Pots

See Cleaning the pots - Page 2 section first. If a pot is still scratchy sounding after cleaning, they may be worn out and need replacing.

 First we will check to see that there is no direct current on the pot. Using your multi-meter set on DC voltage, clip the black probe to a suitable ground such as the screw next to the power transformer where the transformer center tap grounds to the chassis. Touch the red probe to the right tab of the pot. Any sort of DC reading indicates that direct current is leaking through a capacitor and is present on the pot. See the direct current on pot section below. If no direct current is present, then replacing the pot is probably in order. Each pot has a value stamped on it. The pot must be replaced with the same value and taper.

 The taper may be audio (250ka = 250,000 ohms audio taper), or linear (100kl = 100,000 ohms linear taper) or reverse audio (50kra = 50,000 ohms reverse audio). An audio taper pot changes resistance in a way that is closer to the way a human ear hears. A 500k linear pot set half way will read 250k. A 500k audio pot set half way may only read 75k.

 First make a diagram of where each wire is connected. Unsolder the wires attached to the pot. Install the new pot. Solder the wires to the new pot using your diagram you drew.


 Usually the only pots in a tube amplifier that will have direct current flowing through it will be a presence pot or a bias pot. You can hear a slight scratchy noise when you rotate a pot with DC on it. A pot that reads direct current may be reading lower DC than is actually present because some of the pots have a connection to ground and the meter is just reading the voltage drop across that part of the circuit. Several pots linked in a tone chain may all have DC readings. The voltage may also show up on pin seven of the first two pre-amp tubes because the volume controls center tab is connected to pin seven.

 Somewhere there is a capacitor that has a large DC voltage on one leg and is leaking DC through to the other leg. Capacitors pass AC signals and block DC. To find the leaking capacitor you must unsolder one leg of all caps that are connected to the pot or network of pots. For example on the super reverb schematic, we will look at channel two. A DC reading was present on the volume, treble, middle and bass pots of channel two. The wires on the tone pots lead to the circuit board. At the circuit board you can see that the treble pot is connected to a 250pf cap on the right tab and a .1 cap on the left tab. The middle pot is connected to a .022 cap by it’s center tab. The volume pot has no cap leading directly to it but it is connected to the treble cap by a jumper wire that goes from the center tab of the treble pot to the right tab of the volume pot. The bass pot is connected to the middle pot and the treble pot. The three tone caps are the suspects here.

 Make a diagram of the wires leading to the three caps and then unsolder all three wires leading to the tone caps. Pull the wires out of the eyelet’s on the circuit board. Now the tone caps are connected to a very high DC voltage on one side and connected to nothing on the other side. Using your multi-meter set on DC volts, connect the negative (black) probe to ground. Touch the red probe to the unsoldered leg of each tone cap. If a DC voltage is present, then the cap is leaking DC and needs to be replaced. You can replace just the bad capacitor or you can upgrade all three caps in the tone section to better quality caps. After replacing the bad cap, re-solder the wires to the circuit board and check for DC on the pots again. Using the super reverb schematic you can also see that the reverb pot has a .003 cap leading to it. The speed pot has a .02 cap leading to it and the intensity pot has a .1 cap leading to it. These caps would be likely suspects if direct current were present on any of these pots.


 Also see DC on the pots section. The caps in the tone section are not the only caps that can leak DC and cause problems. Using the super reverb schematic and looking at channel one, you will see that there is a .047 cap that leads from one of the 100k resistors to point x. This is where the AC signal leaves channel one and goes to one of the two 220k resistors to be blended with channel two. Channel two has a .1 cap that leads from one of its 100k resistors to point y. This is where the AC signal leaves channel two and goes to the other 220k resistor and is blended with channel one. A DC reading at point X or Y may indicate that either the .047 on channel one or the .1 on channel two is leaking. Unsoldering both points X and Y and lifting the wires out of the eyelet’s will allow you to take a DC reading at those two points. If a DC reading is present, then the cap should be replaced or upgraded. There are several more coupling caps that can leak DC. On channel two’s first half, the .02 that is connected to one of the 100k resistors and the 500pf cap that is connected to the .02 meet at point z. If a DC reading is present at point Z you must unsolder the wire that leaves point Z and goes to the 3.3meg and 10pf cap. You can unsolder and lift this wire off the board at the 3.3m and 10pf junction. You must also unsolder and lift the 500pf cap where it joins the 1m resistor.

 Take a DC reading at point Z where the .02 cap and 500pf cap meet. A DC reading here indicates that the .02 cap is leaking DC and needs to be replaced. Now check the 500pf cap by taking a DC reading on the unsoldered leg where the 500pf cap used to be soldered to the 1m resistor. A DC reading here indicates that the 500 pf cap is also leaking DC and needs to be replaced. At the extreme left side of the circuit board you will also find two .1 caps that are connected to a 100k and 82k resistor at one end and two 220k resistors on the other end. If these caps are leaking DC they will upset the bias voltage and cause trouble with the power tubes. To take a DC reading here you must unsolder and lift the two .1 caps where they are connected to the 220k resistors. If a DC reading is present at either of these caps I suggest replacing both caps.

(PART 1), replacing the 1/2 watt plate resistors.
 A low level crackling noise or a bacon frying noise, can usually be traced directly to the ? watt carbon plate resistors. The current for some pre-amp tubes must flow through these plate resistors. They become hot internally because of the current flowing through them and the resistance will increase as they slowly burn up. Changing them is easy and a very good way to prevent future servicing of the amplifier. Using the super reverb layout diagram we will locate all four V shaped plate resistor groups. Starting from the right side of the diagram, right in front of the first pre-amp tube there are two 100k resistors that are joined at point D and branch out in a V shape. These resistors are the plate load resistors for pre-amp tube one. The second set of 100k resistors are located between tube two and three and are the plate load resistors for pre-amp tube two. The third set of 100k resistors are located between tube three and four and are the plate load resistors for tube four. The fourth set of resistors are located just slightly to the left of tube six and are the plate load resistors for tube six, the phase inverter. This set usually is a 100k resistor on the left and a 82k resistor on the right in black face and some silver face amps. You may also find 47k resistors in this location. Some smaller amps have a different phase inverter configuration and do not have a V shape here. The easiest way to change these resistors without damaging the components that are connected to them is to first unsolder the two points where the components join them. Not the V end where the resistors join together. For example, channel one on the super reverb schematic. Unsolder and lift up one end of the .047 cap that connects with the left 100k resistor. Unsolder and lift the 100k resistor that branches off and goes to the tone caps on the right 100k resistor. Now the two 100k resistors are standing alone with only wires in the eyelet’s. Unsolder the wires making note where they belong. Now the resistors are standing completely alone. Unsolder and remove both resistors from the board. Use a solder sucker if you have one to remove all extra solder from the three eyelet’s on the circuit board. Now it’s time to install the replacement resistors. Two one watt 100k resistors would work here but I prefer three watt metal film resistors. Take your replacement resistors and bend the legs over so that they line up with the eyelet’s on the board. Trim the legs of the resistors so they are able to fit in the eyelet and not be in the way of .047 cap that joins together with the left 100k resistor. Insert the resistors into the eyelet’s along with the .047 cap and the other 100k resistor that branches off to the tone caps. There is also a wire for each eyelet that goes to the tube. The wire that goes to the left eyelet goes to pin six on the pre-amp tube. The wire that goes to the right eyelet goes to pin one. After you have all the components and wires inserted into their proper eyelet, solder both eyelet’s. Now examine the rear eyelet where the two 100k resistors join together. There should be a wire that also joins the two resistors at this point. Insert the wire into the eyelet with the two resistors and solder the eyelet. This is the basic technique that you will use for the other plate load resistors also. Some of the other V shaped areas have more components than others. Some of them have hidden wires that are inserted into the eyelet from under the board. Channel two has two wires that enter a hole in the board and come up through the eyelet where the two 100k resistors join together. If you want to be sure these two wires are being re-soldered correctly you can unsolder and pull them out from under the board. Then when you reconnect everything you can just push them into the eyelet like channel one and you will be sure of making a good connection.

- (PART 2).
 Another low level crackling noise that I have encountered is caused by the circuit board itself. I believe the circuit board absorbs moisture and becomes a conductor. If you have replaced all of your ? watt plate load resistors and still have a tiny crackling noise then you may want to try this. Power on the amp and take it off standby after it warms up. The amp must be making the crackling noise at this time for this experiment to work. Find the three screws on the board that bolt it down to the chassis. Carefully unscrew the three circuit board screws and remove them. Using a magnetic screw driver helps here. Do not touch any high voltage points with the screw driver. Carefully lift the board up a little so it does not touch the board below it. If the crackling noise stops or becomes much better then your board probably contains moisture. In Florida where my shop was, I used to see this occasionally because of the high humidity here. I have heard of repairmen putting the amplifier head into an oven set on warm or low and baking the head to remove the moisture. I don’t recommend this, even if it does work, because chances are the humidity will just re-enter the board and the problem will start all over again. I recommend cutting a piece of poster board to the same size as the circuit board and slipping it between the top board and bottom board. This can be done by slipping it in at the transformer end of the circuit board. Sometimes this is enough to stop the problem. Sometimes when you tighten the three board screws down the problem comes back. This means that you must insulate the screws from the top board. First ream out the hole in the top circuit board slightly so that the screw will not touch the board when installed into the hole. Then you must locate some fiber or plastic washers to put on the screw so the screw does not touch the top of the board. You could probably make your own washers by cutting up some plastic and drilling some holes in it. This is a very strange problem and does not have to be done that often, but it does occur and can drive you crazy trying to locate it.


 Usually a intermittent popping noise is caused by a bad solder joint somewhere. Sometimes the amp loses power when this occurs. If you hit the amp and it pops it will probably be easy to find. Lets start by looking at the ground straps along the brass face plate. There are several wires that come from the circuit board and are soldered to this brass plate. With the amp on and off standby, gently tug on these wires with a pair of insulated needle nose pliers. If a wire is loose you will usually hear a loud pop and the wire must be re-soldered. This is difficult without a high powered soldering iron because the brass plate will suck the heat right out of your smaller soldering iron and it will not be able to recover. The only thing I can help you with here is that if you put a screwdriver between the brass plate and metal chassis, slightly to the side of the solder blob, you can solder easier because the brass is not touching the chassis and therefore not sucking as much heat out of your iron. You will have to have at least a forty watt iron to be able to do this. You can also use a Variac to jack up your soldering iron temperature. If all the brass plate ground wires were tight, then you should look around the chassis for other ground wires that are soldered or screwed down to the chassis. Gently pull on them and see what happens. Be extremely careful not to touch any high voltage wires. If all the ground points are secure then we will move on to the board and look around. Use a non-conductive item like a small wooden dowel rod, chopstick or a plastic pen to poke components and push down on the circuit board. Start at one end and thoroughly cover the board. Gently push on all the components to see what happens. Push down on the circuit board and listen for bad solder joints. If you hear something, grab your insulated needle nose pliers and put one hand behind your back. Gently pull on any wires and components in that area of the board. If you find a bad joint, re-solder it and continue on poking and pushing the entire board. The most common place these bad solder joints occur is where a wire enters an eyelet. The wires sometimes can be pulled right out of the solder joint. If you find a bad solder joint where a wire joins the board then shut off the amp and drain the filter caps. Chances are the wire is corroded or the solder is not sticking to the wire. I find that older wires, especially cloth covered wires, will not solder properly and must be sanded clean before being re-soldered. Additional soldering flux also helps the joint re-solder properly. Never use acid flux on an electrical joint. If you have the time, the most effective way to eliminate any potential bad solder joints is to re-solder every eyelet on the board. This takes a long time but is well worth the effort. To be able to tell which joints you have done and which are not done you can first take a red sharpie pen and put a little red dot on every joint. After a joint has been re-soldered, the red dot will not be there and you can see where you have been. When you are re-soldering the joints you do not want to put excessive heat on the components. A quick re-solder with a tiny bit of new solder will do the job. If you put the red dots on the joints, you can do one end of a component and then come back to it after it has cooled to do the other end. After re-soldering all the joints on a circuit board hopefully your crackling or popping noise will be fixed. If not, go on to the next section.

- (PART 3).
 Another area to check if you have intermittent popping or crackling noises is where the wires are soldered to the tube sockets. Sometimes this is very difficult to spot so you have to look very carefully. Don’t do this with the amp on and please drain the filter caps first. Sometimes a wire looks like it is soldered to a tube socket pin but if you grab it with your pliers a give it a gentle tug you will see the solder joint crack. Again you may have to unsolder the wire, sand the end and then re-solder it to the tube socket pin. Separate the pins from each other so that there is no danger of them touching each other. Do this by gently bending them out away from the center of the socket. You can only do this to the pins that are not soldered to another pin on the tube socket. After examining all tube socket pins and you still have some noises then try rotating the tubes in their sockets. If a tube makes any scratchy noises while being rotated you will have to clean and retention the tube socket pins. See the clean and retention tube socket section.


 Humming noises can be caused by several things. First it helps to know if the noise is 60 or 120 cycle hum. A very low frequency hum indicates 60 cycle hum. 120 cycle hum sounds more like a buzz than a hum. 60 cycle hum can be very difficult to find and correct. Grounds are usually the culprit. Check all the grounds on the amp. Check the grounds for the power transformer. They are soldered to the chassis next to the transformer on most amps. Having all the grounds for the whole amp soldered to the grounded lugs next to the power transformer can help the hum problem, but this is usually not required. Find all the grid and cathode wires and lift them up away from the metal chassis. On pre-amp tubes, pins two and seven are the grids and pins three and eight are the cathodes. Pins 1 and 6 are the high voltage plates. Press these wires down next to the metal chassis. Pins four, five and nine are the heaters and carry a 6.3 volt AC heater current. These wires are usually green and are twisted together. They should run directly above the tube sockets and then drop straight down and go to the tube socket. These wires should be kept away from all other wires. The two 100 ohm resistors that are soldered across the back of the pilot light are a hum control. See the section on replacing a hum balance control. Adding shielded cable from the input jacks to the pre-amp tubes and back to the volume control is a good idea and can eliminate some noise problems. See the adding shielded cable section. Tubes can also cause humming noises. If you have extra tubes try removing and replacing each pre-amp tube and replace it with a good tube. Input jacks not grounding properly can cause hum, see the input jack section. Adding a three wire grounded power cord can also help eliminate hum. See the section on installing a grounded power cord. The power supply filter caps cause all kinds of noise problems and should always be replaced if there is any doubt about their condition. See replacing the filter caps section. Faulty reverb cables can cause some humming problems also. Check the condition of the reverb tank and cables with a meter. See the reverb tank section. 



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