1st from scratch build - AB763 With sprinkles

[sluckey]

For the heater string install a hum balance 5 watt pot like used in Fenders.

Added thanks

The hum pot should be connected across the heater winding, not in series with the pilot lamp.

[cdemike]

The PT you spec'd is center tapped, so IMHO no need to do both a humdinger and heater elevation; I think one or the other should be adequate to control hum. There are a few ways to elevate your heaters, but they key consideration from a noise perspective is to ensure the reference you're feeding the center tap is above 6.3v so that the potential remains above ground during entire cycle 50/60hz cycle. The key limit is the heater-cathode voltage limitation for the tubes you're using, which you can find in datasheets. 6L6's maximum h-k voltage is 180V, and 12AX7's is 200V, so as long as you're not elevating your heaters more than 180v, you're okay. If your B+ was lower, I'd recommend connecting the heater CT to the junction between the balancing resistors on your reservoir, but that'd be above the h-k voltage maximum. So you could just set up another voltage divider at the same node to get something like 50v (arbitrary figure -- if you had a DC cathode follower, it'd be something to pay closer attention to, but not applicable here). Something like B+ > 1M > heater CT > 150K should give you about 50V without drawing too much current. I'd go with elevated heaters since you already have a center tapped heater secondary, but that again might just be a matter of personal preference.

Almost all of HT fuses I've seen go right after the rectifier, i.e., between the rectifier and the reservoir cap. Some fuse the center tap (Hiwatt comes to mind: https://hiwatt.org/Schematics/DR_50wPS.pdf) but I don't think doing so offers the same level of protection as the more common arrangement.

Were you planning on the same reservoir cap values as Rob Robinette's?

[Helmholtz]

You did not add the essential ground connections I mentioned earlier.
Your output jacks don't look to be wired to the OT secondary.
It makes a difference to gain and presence which output tap is used for global NFB.

[plokij88]

You did not add the essential ground connections I mentioned earlier.

Yes sorry I did take note I'm just upset with DIYLC and am not redrawing the OT

Were you planning on the same reservoir cap values as Rob Robinette'

The answer is yes but are they not on the schematic?

Almost all of HT fuses I've seen go right after the rectifier

Seems like a good place for them!

MHO no need to do both a humdinger and heater elevation

Humdinger it is

[plokij88]

[/quote]The hum pot should be connected across the heater winding, not in series with the pilot lamp.
[/quote]

blacknick schematic 0.5.png

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