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The Heater system |
| General service of the heater system: It is always a good idea when an amp is being serviced
to straighten up the 6.3volt heater wires. Take a look at the entire length of the heater wire string. Make sure the wires
are not close to any wires that may pick up hum from the heater wires. The heater wires should go through the air directly
above the tube sockets and then drop straight down to each tube socket. Keep the wires twisted together and as far away
from other wires as you can. Make sure that the heater wires on the power tubes stay in phase. Pin two should go to pin two and pin seven should go to pin seven. If you see that they are crossing over from pin 2 on one power tube to pin 7 on the next power tube, you need to unsolder them at one of the power tubes and reverse them. This has the effect of canceling hum in the power amp. You will find them wired wrong all the time. If the amp has a 120 cycle hum, check the two 100-ohm resistors that are soldered to the back of the pilot lamp assembly on some models. Check their value with a meter and also check for bad connections. The meter will only read 50 ohms because both resistors are in parallel. I have seen connections that look fine at first but upon closer examination with a magnifier you can see solder cracks and broken resistor leads. I have also found the two tabs of the pilot light bulb holder slightly touching each other and causing a hum problem. If there are no 100-ohm resistors on the back of the pilot light, you may want to install them to see if there is any difference in the quietness of the amp. See the hum balance pot section below for details on installing the resistors. Make sure you re-tension the tube socket pins so that the heaters are getting good pin contact. I have encountered a problem with the heaters that may baffle you and fool you into thinking that the amp has output transformer trouble or bad tubes. If an amplifier ever starts to lose power and distort or is intermittently doing this, reach back and carefully check each power tube to see if it is hot or as hot as the other power tubes. If a tube is not hot and you can’t see the heaters light up, pins two and seven may not be making a good contact. The heaters draw so much current that even the slightest bad contact or corrosion will cause a high resistance and no voltage will enter the tube. Sometimes the pins of the tube socket are so wimpy they will make good contact for a while and then do it again. If this happens more than once you should think about replacing the tube sockets. |
| Heater system basics. |
| The heaters are a coil of wire inside the tube that heats up like the coils of a space heater.
They heat the cathodes of the tubes so they will emit electrons. The system basically works like this. The transformer has two or three AC heater wires, usually colored green, that put out around 6.3 vac. If the amp has three green wires, one will be the center tap between the two 6.3 vac windings. This center tap is usually soldered to the chassis right next to the power transformer. The two 6.3 vac windings go from the transformer to the 6.3 volt pilot light bulb. At this point there is sometimes two 100-ohm resistors that are soldered to each winding and then the other end of each resistor is soldered to ground. These resistors make an artificial center tap ground and prevent the amp from humming. The two 6.3 vac wires leave the pilot bulb assembly and then go to pins two and seven of the first power tube. The green wires are twisted together because they carry AC and this helps to prevent hum in the amp. A twisted pair of wires is better at canceling the hum than two wires run separated. The 6.3 vac windings then go to all the power tubes in a twisted pair fashion. The wire that went to pin two of the first power tube goes to pin two of the next power tube and so on. The 6.3 vac windings then go to the pre-amp tubes in the same way except that the pin numbers change. On the 12AX7 and 12AT7 tubes, pins four and five are soldered together as one connection and pin nine is the other connection. Pin two on the last power tube goes to pins four and five on the pre amp tube that is closest to the power tubes. Pin seven on the last power tube goes to pin nine on the pre amp tube. The 6.3 vac winding continues down the line in a twisted pair fashion to the rest of the pre-amp tubes. In later silver face amps you may encounter a hum balance control pot near the last pre-amp tube. This pot is supposed to replace the two one hundred ohm resistors that usually are found on the pilot lamp assembly. I have had trouble with this hum balance pot and I usually disconnect it and solder in the two one hundred ohm resistors for more reliability. See the section below. |
| Replacing a hum balance pot |
| If your amp has a hum balance pot I would consider getting rid of it and replacing it with
two 100 ohm resistors across the back of the lamp assembly. Replacing the hum balance pot with two 100 ohm resistors
is just something I do because I don’t like the hum balance pot system and I don’t trust the pot. I have seen too many
of the pots fail. To do this all you have to do is disconnect the two wires that go from the pre-amp tube to the pot. Then remove all wires from the pot and leave the pot in the chassis as a hole plug. Make sure that the last pre-amp tube heater wires are connected to the second to last pre-amp tube heater wires. Pins four and five go to pins four and five and pin nine goes to pin nine. See the next section to add 2 x 100 ohm resistors to replace the hum pot. |
| Adding 2 x 100 ohm resistors to the heater system |
| If your power transformer has a green wire with a yellow stripe and it is soldered to the chassis
ground you already have a heater supply center tap and you do not have to install the 100 ohm resistors. I actually prefer the two 100 ohm resistor method because they act like fuses if a power tube plate wire ever shorts
out from pin 3 over to pin 2. Pin 2 is one of the heater wires and is a direct short to ground for the high voltage that
is found on pin 3. This happens all the time and you can save your power transformer by having the pair of 100 ohm resistors
instead of a transformer center tap. The 100 ohm resistors will melt in half if the plate wires ever short out to pin
2. This is way cheaper than replacing a power transformer. If you want to get rid of your heater supply center tap and replace it with two 100 ohm resistors, all you have to do is unsolder or clip the green/yellow center tap wire and heat shrink or tape it off. You must make sure this wire is tapped off and cannot touch or make contact with the chassis or any other wires. It will just be coiled up inside the amp and not used any more. Add the two resistors by following the instructions below. How to add an artificial heater supply center tap. The two 100 ohm 1/2 watt resistors are connected to the lamp assembly and then to ground. Solder a 100 ohm 1/2 watt resistor from each green winding on the bulb holder to the chassis ground lug next to the pilot light and you are done. In other words, you will have a 100 ohm resistor soldered to each green heater wire and the other end of each resistor is connected to the chassis ground. The 100 ohm resistors create an artificial center tap for the heater system. If you do not have a heater supply center tap, you will get a 120 cycle hum. Most people confuse a 120 cycle hum with a 60 cycle hum. You must listen carefully. A 120 cycle hum sounds more like a buzz than a low pitched hum. |
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