So I got a new Fender Musicmaster Bass amp with 2 6AQ5's and a 12AX7 a week ago from a guitarist friend. He said, "Go fix it up. All I know is that when I turn it on it smokes." So after some inspection, the speaker had blown, the 1k resistor for the power tube grids had blown, and one of the power tube sockets had melted and burned around the pins for the plate. I replaced the rectifier diodes, the input jacks with better ones, and the r12 resistor with a 2 watt 1k one. The problem is, whenever I turn it on, it's still sparking at the sockets. I don't really know what to replace. The voltage from the diodes by themselves, without anything else connected, is about 330 volts, and there are no problems until I connect the 1k resistor and the output transformer's center tap back in place. What is going on with this thing? It's not a shorted rectifier, and I can't tell if it's a shorted power transformer or output transformer. I can provide a few pictures if you all need them but I added the schematic.
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Exactly. Your tube socket has arced, and will continue to do so. If an arc forms, it carbonizes the socket, which makes it conductive. An arc is a plasma flow of current, and has very little resistance. Pin 3 is the high voltage on the tube, and pin 2, right next to it is the heater, which most likely has a connection to ground either through a center tap or a couple resistors. So any time you connect the center tap of the output transformer, B+ voltage will be on pin 3, which will arc to pin 2 and thus ground. The arc is a dead short to ground for pin 3 then.
Replace the socket.
Replace the socket.
Enzo is correct except that for a 6AQ5, the plate is pin 5 and the heater is pin 4. Replace the sockets with ceramic ones.
Thank you guys so much for the help. I'll get those sockets replaced as soon as I can! But how do sockets get into that condition to start arcing like that?
A heavily overdriven amp creates voltage spikes at the speaker that can be in the hundreds of volts. On the other side of the output transformer the spikes are in the thousands of volts. These spikes will breakdown tube sockets and the insulation within the output transformer. A common cure is to put diodes from the plates of the output tubes to ground. Another is a conjunctive filter (RC snubber) between the output tube plates or a zobel across the speaker. A resistor of prehaps 200 ohms across the speaker also helps limit the spikes.
Oops... Sorry, I am so used to thinking octals...
As to why the socket arcs THIS time and not yesterday, it is usually something we never know. A tiny spider might have stepped between them and that was enough to allow a spark to fly. Or a little excess humidity combined with a speck of dust. Once an arc strikes, it will perpetuate itself. It can be an very tiny thing that makes the difference between an arc and not. Some printed circuit boards even have slots cut through them to make an air space, more reliable in preventing arcs than just plain circuit board.
As to why the socket arcs THIS time and not yesterday, it is usually something we never know. A tiny spider might have stepped between them and that was enough to allow a spark to fly. Or a little excess humidity combined with a speck of dust. Once an arc strikes, it will perpetuate itself. It can be an very tiny thing that makes the difference between an arc and not. Some printed circuit boards even have slots cut through them to make an air space, more reliable in preventing arcs than just plain circuit board.
I have seen this happen live, and I have seen it totally fry an amp. If the amp was cranked full up when the speaker blew (why else would the speaker blow) the full power output of the amp has nowhere to go. This causes the voltage to rise at the plates of the output tubes (simplified explanation). In some cases the voltage can go into the kilovolt range. Voltage in this range will cause an arc somewhere, usually inside the OPT, but a dirty tube socket or PC board is often fair game too. Once the arc starts it is fed by the full output of the power supply, and the only thing trying to slow it down is the OPT, often frying it. If the arc is allowed to continue for more than a second or so it will burn the insulation, plastic, or PC board material, turning it into conductive carbon. This is why it continues to smoke even though the original fault is gone. Do not power it up again until the socket is replaced. You risk frying a transformer or two!
Note, for those thinking this is a simplified explanation, it is. In reality there was a current flowing in an inductor (plate current through the OPT). Inductance is the electrical property which tends to oppose a change in current flow. When the speaker blew the load was removed from the amp and the current through the OPT secondary was abruptly interrupted. The energy stored in the inductance caused the voltage to rise in an attempt to keep the current flowing. The voltage will rise UNTIL CURRENT FLOWS!
I have autopsied several amps that have died in this manner. Some were simple repairs (replace the fried socket) some were totaly toasted (an Ampeg SVT at full song into a low powered Traynor cabinet when all 8 speakers blew! OPT, Power transformer, a few 6550s, a preamp tube, some resistors and caps.....all fried. The bigger the amp, the higher the voltage, and the bigger the lightning bolt that jumps through the circuits.
This is a small amp, so replace the socket, and maybe the other 6AQ5 socket, connect up a good speaker and try it. It is possible that the fried socket saved the OPT. The heater circuit is grounded in this amp. That is good, because sometimes the voltage jumps from the plate to the heater circuit, goes through that and blows up all the semiconductors in the preamp. It can take out a cathode bypass cap too, and any of the tubes.
Guys, thanks so much for all the advice. The tech who worked on this before replaced one of the sockets with a ceramic and left the fried one so the ceramic is fine. I have an order of tubes and new sockets on the way. I would assume the arcing also explains the fried resistor that I've replaced so that's good. Thank you guys.
Alright guys, I got it working. It hums a bit but it's not bad. I switched out the 6AQ5's for a nice pair of EL84s I had since I had good 9 pin sockets just laying around. This little guy really can roar. If anyone wants an audio clip I can set up my recording stuff later.
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