Marshall re-issue

[Valvehead]

on the 1987-X re-issue they added a hum balance circuit to the heaters.

Has anyone removed this and im assuming the amp would function just fine without it?

This one burned up the pot and the 2 100 ohm resistors in the past --now i see the 2 resistors are open again.

[Enzo]

That is almost always a sign of a bad power tube.

If the pot burns up, I don;t know why the resistors wouldn;t too.

Look at the circuit, a 6VAC winding, with CT left unconnected. there are 100 ohm resistors to ground through the 100 ohm pot. There is no circuit path for current to ground. The three 100 ohm resistances are effectively in series across the 6VAC. 300 ohms across 6v, looks to me like 0.12 watt. Not much.

Since the circuit is the winding, with the tube heaters and those 300 ohms across it, we can connect the loop to ground at any one point we want, and no current will flow to ground. There has to be a complete circuit for current to flow, and there is not one here.

Now take a power tube and short high voltage to the heater inside. Now we are basically putting the B+ supply across the little circuit. The winding and tube heaters have almost no resistance, so all the B+ sees trying to get to ground is those resistors. And since the winding has so little resistance, basically the two resistor ends are wired together, so it becomes two 150 ohm paths in parallel. 75 ohms.

Even try to measure one of the two 100 ohm resistors on a Fender while in circuit? You get about 50 ohms.


Now figuring about 480v B+, trying to flow through that 75 ohms, it will try to draw about 6amps or so. The B+ canl;t handle that much, but it will give all it can. At least until a fuse blows. But however much the B+ can allow, those resistors will TRY to disipate about 3000 watts. They burn out.


Of course the B+ will load way down, but still those are just little resistors. Sure you could put in 5w resistors or something, but the real problem is eliminating the source of the burnout current.

In fact, one very common reason for this is when someone breaks off the plastic center post of a power tube. They can then shove the tube in the socket facing the wrong way. The heater betwen pins 2 and 7 of the tube is basically a piece of wire, so those pins are shorted together.

[Valvehead]

i might as well eliminate the circuit then ....the original 1987 didnt use it...

[pdf64]

You will need to reference the heaters to something.
The original had a grounded centre-tap on the heater winding.
If you don't you'll get hum, and if a tube shorts again as Enzo describes, then it will destoy the insulation between heaters and cathodes on all your tubes.
The fact that the resistors/pot is burning out is telling you that something bad is happening. Resolve the root cause and then replace the pot or resistors.

[Enzo]

Exactly, under normal circumstances there is negligible stress on those parts. Putting heftier resistors in there is like wrapping a fuse with aluminum foil to keep it from blowing. And agreed with not removing the circuit. It does a job you need. The trouble is elsewhere.

[Valvehead]

ok , i will try and find the problem, Im not seeing anything yet..sockets look clean , etc...

but.....

cant i remove the heater "hum" circuit" and just ground the CT ?


the last episode there was a bad power tube and it did take out the screen resistor--im assuming that led to the 2 hum resistors burning as well

[Enzo]

Sure, you can revert to the center tap. They did it this way so you could tweak the hum balance. That won;t cure whatever is shorting to your heaters though.

[Valvehead]

ok, thanks guys .

its hard to tell if the sockets were arcing--they are dark brown. I might replace them as a precaution

[Enzo]

Turn out the room lights and power the amp - if it is arcing you should see a little blue spark.

[JerryP]

This can also be caused by high voltage damage due to running into the wrong load, no load, or running it into an attenuator and decking the amp. I've seen it here several times.
Jerry