Choke or no choke?

[stephenhartley]

My beloved Tremolux started smoking last week. Having successfully built my own 5e3 based amp, I thought I'd have a go at fixing it myself.

After discharging the power caps, I checked all the resistors and transformer coils - all fine. I couldn't find anything wrong, but going off the burning smell, the PT is on its way out. I played through the amp and it was fine.The B+ was 348v.

The amp has been re-capped. The PT although circa 1960s, is not original & the OT has been replaced. The choke is missing altogether.

I've bought a set of Hammond transformers (including choke) and I'm going to put it back to original spec.

Can anyone tell me what a choke actually does? Why would someone have removed it (the amp has worked fine for 20 years without it)?

[GregS]

Without getting into great detail or specifics it acts as a filter for the screen supply. A power resistor can be used in its place between the plate and screen nodes and I expect this is what it was replaced with. The choke tends to result in a stiffer response as the coil is very low resistance and drops very little voltage on signal swings compared with a resistor.

[stephenhartley]

Thanks very much for that - there's a 120 ohm 1/2 watt resistor across there. I presume that's the power resistor?

[GregS]

If it's between the plate and screen supply nodes then it's serving that purpose. 1/2 watt is a very low rating for this resistor. It should be rated significantly higher than that depending on the current through it and the voltage it's dropping.

[Tage]

You will probably want a 3 watt or better resistor there, you are definatly dissipating around .5W if you run the amp all out. but the higher power rating will help keep the resistor running nice and cool, no point stressing a .5W resister and having it short, that would be a bad smoke to let out.

[rathmann]

My beloved Tremolux started smoking last week.

You should try to get it to switch to gum or hard candies...