Simms-Watts AP100 mkII 1972

New PT and 4H choke with 400mA rating arrived today, but pity me about figuring PT lead as no tap markings on this one. DC resistance readings only confused, because secondary has HV taps with bias and center tap there too.

I know I could power it with low AC trough another trafo, but there’s also extra lead on supposed primary side and I’m not sure what it is. That lead wasn’t mentioned on specs. Shield maybe?

So only choke goes in today and gonna email Uraltone about color coding on PT leads. Better to be safe than sorry.
 
Okay, choke went in as planned and when I was chassis open had a thought to add ceramic 100pf bypass caps for V1 anode resistor. It helped with excessive hiss taming and the choke tamed down 50Hz hum FINALLY, but there wasn’t such thing as free lunch here. Hiss and 50Hz hum were masking 150Hz hum which came audible. Thats’s for sure better than the rumble I had earlier.

Did some freq analyzing with my phone yeasterday (good typo, won’t fix it!) after rehearsals. Channel volume or master down the amp is completely silent, but as soon either is opened the 150Hz creeps more audible as pot is turned.

IMG_8801.png

This volume pot turning led me to realize, I’ve made a mistake with grounding stages for their own separate stars. I’ve not followed hooking grid leak resistor gnd side to stages cathode on preamp. Neither with vol&tone pots. Gonna fix that next!

Also choke did gooood, amp is meaner than ever and low chord transients really push air.
 
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