Latest Rabbit Hole project....Started with adding a line isolation transformer and morphed into a complete circuit rebuild... The original circuit was pretty much a widow maker design, with direct line voltage feed the amp.
I was asked to add a line isolation transformer and proper grounding. Also The owner wants a external speaker jack added. Firing up the amp before the mods it worked, but the 60Hz hum was substantial. I competed the mods and still found the 60Hz hum too much. The wring was a mess and it had several potential common loops, which grounding it properly seemed to agraviate.
SO......I decided to rewire the circuit.... and fix the half wave rectified power supply while I was at it
and a new turret board for the main circuit is in the works....
Comments
paulears Mon, 09/13/2021 -…
paulears Mon, 09/13/2021 - 23:04
I’m confused. Your mains input has a 150 ohm resistor between line and neutral and the switch is connecting that path to chassis? Where do the heaters get their feeds?
Yes sir I was also saying WTF when I first started looking at the schematic. This amp was made as cheaply as possible, and what they did to say parts was actually quite interesting. They ran the heaters of the three tubes in series. The heaters are fed from the 120VAC through a single series resistor (150 Ohm 10W). But the interesting part is the heater voltages required:
12AX7 Heater Voltage 12.6 VAC or VDC (Series), 0.15A Current
50C5 Heater Voltage 50 VAC or VDC (Series), 0.15A Current
35W4 Heater Voltage 35VAC or VDC (Series), 0.15A Current
So crudely ....50VAC+35VAC+12.6VAC =97.6VAC
Vresistor =120VAC - 97.6VAC= 22.4VAC across the 150 Ohm Resistor
Therefore the current through the resistor is:
22.4VAC/150 Ohm = 0.15A
Pretty cute if you ask me....certainly not ideal.
So I added two transformers and two full bridge rectifiers and removed the 35W4. But kept the heater concept with DC.
Slowly get the Amp back together now.
Wiring Almost done...
Just need to need make the star point common to ground connection. I tested the the circuit without the tubes and measured the bias voltages, everything was as expected. I plugged in the tubes and watched the heater voltages. As the 50C5 warms up its voltage rises slowly, unfortunately the 12AX7 during that time sees ~17VDC across it...I thinking about ways to clamp that, but I am pretty sure the original circuit must have done the same thing.
A little more wiring then I will give the circuit a test drive. If that goes well, I will clean up and secure the wiring, add any last support silicon and box the amp up.
The Rabbit Hole Goes Deeper....
The amp is now pretty much done.....but I don't like the stock tone control.....
so I added a 4PDT switch to change the tone section into a Big Muff PI style tone control.
But now I want a make up gain stage....so I am thinking I will add a 2nd 12AX7 where the 35W4 was.....
More to come....
I added the BIg Muff PI Filter Section
It sounds great but has about about 12 db of loss across it, as expected. So Now I want to add in another 12AX7 make up gain stage. I started to play around with a simple 12 db single Triode with feedback, but ran was causing instability in the the 50C5.... So that's the next rabbit hole to follow.
I am also noticing a weird oscillation as the tubes heat up, my hunch is saying in may because of the overvoltage on the heater….but I will to chase that one down too…
After many twists and turns...its done..
So the amp is not exactly the same anymore. I changed the 50C5 to 6V6. This allowed me to simplify the heater power supply. I added 2 toroidal power transformers one for 6.3V and one 180V. Both are being rectified and heavily filtered.
The almost final Schematic is here:
I added a switchable second tone filter cap option as well a gain boost mode. As well as a external speaker option.
oh and I added a power light...Done for now!
Sweet!!