Simms-Watts AP100 mkII 1972

[JohnnyCreepy]

Start of the story.

Couple years ago I was on hunt for a new amplifier and found Simms-Watts AP100 sale nearby. Amp was in good shape, played regulary and power side was recapped within 10 years. Majority of the components were original and circuit compared to schematic available.

Topology was two channels both with their own plate driven TB tone stacks, followed by two gain stages before LTP and ultralinear power amp with EL34.





Played few months with it before one power tube gave up and started modding this thing. To be continued.

 

[JohnnyCreepy]

Played few months without bigger problems. Accuired 2x15” speakers and they revealed a need to reduce PA coupling cap values. OTX pushes lows efficiently to the point overexcursion was happening with modern speakers. Also grid blocking was occuring. Blocked spkr cab’s reflex ports and reduced PA coupled from 100nf to 22nf. Was all good for a while.

So the first bigger problem came when year 2025 started. One of the power tubes gave up and went dud with a cool zap. Fly high in tube heaven with your C-wings you poor SED EL34 tube!

So it was new power tube time and matched quartet of JJ EL34s went in. Also beefed up wattage for resistors around tubes. Changed power supplys first pair of big can 240uF electros which were the original ones.

Notice the safety of the original build with with exposed HV connections. And yeah, when chassis is in amp cab EXTRA attention is needed when doing tube rolling…


To be continued with HUM, tweaks and rebuilds…