Gibsonette Filter Resistor Cooking

[misterc57]

I serviced a 1960ish Gibsonette. Had a hard time finding the matching schematic. This amp has 1 volume pot, a 12AX7 preamp tube and a choke (which I think is a field coil choke).

I think this amp may have originally had a field coil speaker and a 6SJ7 preamp tube.

It had very loud 120 cycle hum when brought up slow on the variac. I replaced all the filter caps and resistors. After which I had a background 120 cycle hum.

I ended up re-wiring the filtering section per for following GA-8 schematic. This included removing the field coil choke from the circuit and adding an additional filter stage.

The only difference I could see between the GA-8 schematic and this Gibsonette is the GA-8 has a tone pot.





The amp was returned to me and I can see that the added filter stage 2 watt 470 ohm resistor has been cooking. Seen in the center of the below pic (below the blue filter cap).





I also found the following schematics. First shows an "un-valued" resistor where I have the 500 ohm (really 470 ohm) 2 watt.




So I had a bunch of confusion on what schematic to use and why the field coil was still in circuit. Anyhow, what do you think of the changes and why is the 2 watter cooking? Do I just need to use a 5 watt or could there be other issues to consider?

Thank you

[misterc57]

More reference shots Choke on far right of last pic.

[mozz]

Nothing there is a field coil. You can't really remove a field coil from a speaker and call it a choke, it would not have any iron. They may be regular chokes. Good luck finding Gibson schematics that match anything.

[The Dude]

Yes, I think just regular chokes. I would just leave the choke instead of the resistor you added. Can you use one of the schematics that's close to your amp and just modify it to show how yours is different? It would make things easier if we could see a schematic of the amp you have in front of you.

[misterc57]

I never found a schematic showing a filter choke. The choke looks like an after thought, placed on the end of the chassis, bolted on one end and welded at the other end. This choke reads 15 ohms across its leads.

The amp still had 120 hum so I followed the schematic below, removing the choke from the circuit and having 4 filter nodes. After which no more hum and a narly sounding amp. all seemed well until that 470 R cooked.

I have since replaced the burnt 470 R 2W with a 600 R 5W. I am sensing a very hot temp of 175 F at the first filter node. I have never measured temp at filter nodes before but this seems way to HOT.

Thank you for any suggestions on this.

[Helmholtz]

It is special to this wiring that power tube plate currents run through the 470R.
Average dissipation is around 4W. I'd recommend a 10W rating.

[misterc57]

It is special to this wiring that power tube plate currents run through the 470R.
Average dissipation is around 4W. I'd recommend a 10W rating.

Okay. I installed a 470R 10W in its place. Amp plays great, no burning. Temp on that 470R is 180F on the hot side and 90F at the other end. Not too hot?

[Helmholtz]

What type of resistor is it?
I think something like 80°C should be ok with a cement wirewound resistor.
Best check the datasheet.

What's the voltage drop across the resistor?

[g1]

I never found a schematic showing a filter choke. The choke looks like an after thought, placed on the end of the chassis, bolted on one end and welded at the other end. This choke reads 15 ohms across its leads.

The filter choke will have been installed when someone replaced the field coil speaker with a regular type. The added choke takes the place of the field coil, which was acting as a filter choke.

[Jon Snell]

If it is the 500Ohm, circled, that has to supply the pair of output valves (possibly 20Watts) and the rest of the amplifier.
If you cannot put the old loudspeaker back in and wire it up as it was, I would connect the output transformer feed to the 20uF and fit a standard 470Ohm 2Watt in the place circled.

[misterc57]

What type of resistor is it?
I think something like 80°C should be ok with a cement wirewound resistor.
Best check the datasheet.

What's the voltage dropacross the resistor?

10W cement. 34V drop across it. 360 VDC at this node.

[misterc57]

If it is the 500Ohm, circled, that has to supply the pair of output valves (possibly 20Watts) and the rest of the amplifier.
If you cannot put the old loudspeaker back in and wire it up as it was, I would connect the output transformer feed to the 20uF and fit a standard 470Ohm 2Watt in the place circled.

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

[misterc57]

More ref pics

[Helmholtz]

If the choke in place of the 470R resistor results in more hum it might have a short.

[Helmholtz]

If it is the 500Ohm, circled, that has to supply the pair of output valves (possibly 20Watts) and the rest of the amplifier.
If you cannot put the old loudspeaker back in and wire it up as it was, I would connect the output transformer feed to the 20uF and fit a standard 470Ohm 2Watt in the place circled.

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.