I had an eidetic (photographic) memory, but it never developed. As for the tube, check plate voltage and grid bias voltage to see if they are in spec. Be sure cathode has a connection to ground, either directly or through a resistor. Check the value of the cathode resistor if there is one. Be careful as you would be working around potentially lethal voltages.
I’ve posted this elsewhere, or, deja vu, here before:
The portion of the circuit below the 5Z3 I haven’t unfolded yet, but it’s NOT in this amp, which predates this schematic by about 5 years, if the seller (local guy who
runs one of the annual HiFi shows in early December here in DC-MD), it was a dealer sample, without schematic, sounded AWESOME through a pair of Tannoy speakers (don’t know the model, but he mentioned a selling price of around $9k, IIRC) despite playing the original Sophia 2.5v 300b tubes at 5 VAC (I changed this later at home) without serial number, nothing signed inside or dated.
All capacitors check out okay (although I’ve bought replacements for the PS at the suggestion of a friend who works on tube amps - he bought my heathkit 1500 W Linear and MFJ 1000W HF amplifiers I had rebuilt so I might know my way around high voltage), thinking reducing ripple a little more might help - I haven’t gone there yet.
Okay, there is NO “stand by“ switch circuit, NO “pot per regulare la corrento della valvola finale” (potentiometer for regulating the current of the final output tube) also no “negativo di griglia” (negative feedback?) NOR is there a current meter, and the 1k 5% resistor/100 uF/200V cap precedes the hum pot on each side where I’ve sketched it in. The PS wiring is even more bizarre, and I’ve sketched it out twice, going to open this up again and take more photos for later review.
Voltages throughout seem in line with those on the schematic and published elsewhere (other 300b SET designs), a little high at the filaments of the 300b tubes - my notes showed 2.73 mV in one spot, another, second check showed higher. My notes are a mess - I need to do this over again.
As I’ve mentioned before, this was ALL the factory could supply, or would supply through their NA distributor.
From several different sources I hear:
1 - it’s an AC filament design, I’ll have hum (but I didn’t hear this when I bought it - might reconnect Ohm 4’s but I doubt this is the issue)
2 - run DC power to filaments - a little complicated, but doable, if only for 300b tubes, I can rewire later). I breadboarded Stephe‘s little 5 VDC circuit
and it runs CLEAN DC power at 5.01VDC with about 0.2 mVAC
3 - Buy better tubes (EMLs didn’t change this, why pay twice as much for WE 300b’s?)
4 - There shouldn’t BE any hum.
So, while I appreciate the help, I’ll likely just “deal with it” for now, finish building the Skunkie Designs amp I’ve got going and puzzle this out later.
Thanks again.
