Stoppers for plate cap tubes

[kstagger]

I'm building a PSE 1625 amplifier (12HG7 driver, regulated screens, plate-to-pate feedback) and seem to be running into some RF issues when the two output tubes are plugged in. The amplifier does work better with one output tube installed, but hum and audible distortion greatly increases with two in parallel.

Thinking of adding plate stoppers in an attempt to handle some of this nastiness. Also more R-C separation for the screen supplies.

Question - what's the best way to mount a carbon comp resistor to a plate cap? Pictures would be nice!

 

[Kay Pirinha]

I'd prefer damping chokes of some µH's, as e.g. McIntosh did in their MC-3500.
Best regards!

 

[DF96]

Use a lossy inductor, not a resistor. Wind a coil on the body of a CC resistor.

 

[artosalo]

Like this:

 

[kstagger]

I'm not keen on having exposed voltage so I'll have to snug it close to the ceramic top cap and then use some shrink tubing. I'm sure that will do wonders for the cooling ability of the resistor.

 

[DF96]

The resistor should not dissipate any power. DC and audio goes through the coil. Only RF goes through the resistor but if it is a successful stopper then there isn't any RF.

 

[Kay Pirinha]

Artosalo's resistor has burned, though...

Best regards!

 

[rdf]

Like this:

Preferably with a resistor that hasn't cracked from overheat though..

 

[artosalo]

Fortunately it is not mine. The picture is from QRZ forum.
And I would not do such bad soldering .

 

[kaputt]

I have had good success with resistor wire wound over a resistor, about 10 turns. Inductors doen't get much lossier than that

 

[rdf]

And I would not do such bad soldering .

Tinning is for wimps.

 

[DF96]

Fortunately it is not mine. The picture is from QRZ forum.

It is probably an RF PA. Hence the low coil inductance. Some of the RF will go through the resistor, and it will probably suffer from temperature cycling during a typical QSO. However, even a cracked CC resistor will add some loss to the coil.

 

[kstagger]

Thanks, everyone. I'll do the inductor/resistor thing once I got the carbon comps in.

 

[trobbins]

Slip ferrite beads over the wiring going to the top cap. Are your grid and screen stoppers directly on the socket terminals?