Gibsonette Filter Resistor Cooking

[g1]

That is what I initially did per the schematic in my prior post. The 2W cooked.

As said a 2W resistor is not sufficient. From the cathode voltage the power tubes alone draw 80mA at idle.
Also current will somewhat increase with output.

Jon's suggestion was to also move the OT feed to the other side of the 470, so power tube plate current is not going through it. Look closer at his diagram in post #10.

[Helmholtz]

Jon's suggestion was to also move the OT feed to the other side of the 470, so power tube plate current is not going through it. Look closer at his diagram in post #10.

That would significantly increase hum with this single ended amp.

[g1]

That would significantly increase hum with this single ended amp.

Yes, I agree. And for that same reason I think that choke that has been installed should still be preferable to a resistor.
Unless it is shorted as you wondered about earlier. But I'm not sure if he tried it with the choke after rearranging the filter nodes, so the choke may not have been what was causing the 120 Hz hum.

[misterc57]

Yes, I agree. And for that same reason I think that choke that has been installed should still be preferable to a resistor.
Unless it is shorted as you wondered about earlier. But I'm not sure if he tried it with the choke after rearranging the filter nodes, so the choke may not have been what was causing the 120 Hz hum.

Amp started with high level loud 120 hum with choke and 3 filter nodes. The choke was in circuit when I first replaced all the filter node caps, at that time 3 nodes. I then had low volume 120 hum.

I then followed the amp schematic that used a 12AX7 V1, removed choke from circuit as it was not in any schematic I found, installed the additional filter node with the 2W 470 R. No more hum but cooking the 470R 2W. Now have a 470R 10W in there.

[mozz]

How was the choke mounted? You should be able to tell if it was factory or someone did that mod years ago. A 60's amp should not have had a field coil speaker from the factory, 50's maybe. If you buy one of those cheap MK328 semiconductor tester, they will test chokes.

[misterc57]

How was the choke mounted? You should be able to tell if it was factory or someone did that mod years ago. A 60's amp should not have had a field coil speaker from the factory, 50's maybe. If you buy one of those cheap MK328 semiconductor tester, they will test chokes.

An "early" version schematic of this amp shows a field coil and a 6SJ7 V1 tube. The V1 chassis hole on this amp is octal size, looks like the 12AX7 socked was not original.

The choke on this amp is mounted on the end side of the chassis, bolted on one end, welded on the other end. Does not look original.

[misterc57]

As all is working now and I believe the 10W cement R can tolerate up to 200 C, is there any reason to consider the choke being omitted from this circuit?

Thank you all.

[mozz]

I think it is shorted with those low readings.

[g1]

Should be fine with the 10W 470R in there.

[Helmholtz]

Should be fine with the 10W 470R in there.

As said earlier.

[g1]

As said earlier.

I was responding to post #22 thank you.

[Helmholtz]

An "early" version schematic of this amp shows a field coil and a 6SJ7 V1 tube. The V1 chassis hole on this amp is octal size, looks like the 12AX7 socked was not original.

The choke on this amp is mounted on the end side of the chassis, bolted on one end, welded on the other end. Does not look original.

This "early" version shows a different amp using a push-pull output being less sensitive to ripple than the parallel single ended output stage in the other schematic.

The choke measurements indicate that choke is not shorted and should work well in place of the 470R resistor.

[mozz]

14.6 ohms and 700mh sure seams shorted to me. Most any power supply choke I have seen is at least 5H and 100-500 ohms.

[Helmholtz]

14.6 ohms and 700mh sure seams shorted to me.

733mH means an impedance of 552 Ohm at 120Hz, so should work at least as good as the 470R resistor in the same position.

Those 5H chokes are typically wired after the screen node, so have to pass far less DC.
A lower inductance choke will naturally have a lower DCR.

[mozz]

So it's a whole system choke? Didn't look at the schematic due to so many variations.