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…

 

[JohnnyCreepy]

I just made a long post about last year tinkering with the amp, but accidentaly closed my browser window. Poof, all gone. Shit.

So let’s keep this short:

1. ruined vintage value with new caps and resistors



2. Hum and pcb falling apart bothers - rebuild on tag strips with first timers messy looking layout


3. Hum still bothers and second channel feels bit useless - rebuild like Model-Tish



After last pic there’s been small changes like input resistors, fine tuning layout and wiring

And it sounds like this. https://on.soundcloud.com/ZF55qVirSzezMZGGEa

 

[Nostradoomus]

Send it to Robert to trace

 

[JohnnyCreepy]

Send it to Robert to trace

Pcbs would be named ”Model Tea&Biscuits”?

Oh and latest build mostly solved hum problem. It was either reducing grid leak values from 200K to 126K or amount of NFB fed to PI. Also ditched shielded input wiring to V1 grid and raised stoppers from 10K to 40-50K. Response for dirt pedals is now better.

 

[Asdrael]

Could you diagnose where the hum came from before the changes btw? I would say a shielded input to v1 grid stopper is mandatory. Raising the grid stoppers probably did a lot for you there.

 

[JohnnyCreepy]

Could you diagnose where the hum came from before the changes btw? I would say a shielded input to v1 grid stopper is mandatory. Raising the grid stoppers probably did a lot for you there.

It was happening on somewhere between V2 and PI+NFB.

NFBs first resistor from speaker feed has a pot wired as variable resistor series onto it. If I turn pot to reduce NFB amount the hum is more aubile. Now the NFB section is valued 20K resistor+50K (or was it 100K pot) from 16 ohm to 10K resistor parallel with cap leading to second lug of a 10K pot. So maybe that?

To be honest exact reason hasn’t revealed for me. Heaters are elevated, NFB feed is shielded etc. But currently my mains freq hum is low enough to keep me satisfied for a while.

Gonna just play with the amp for a while, but next time when chassis is opened I should raise heater elevation DC voltage from 50 to near 100v to make sure CF tubes c/k limit isn’t exceeded and feed elevation from PI power feed instead of the first cap leading to OT primary CT.