Autoformer vs Split Primary windings on Power Xfmrs

[nevetslab]

Totally off topic, but the schematic in the first post is missing a 1M resistor from the grid of the PI voltage reference tube to ground.

We've made quite a number of amps using the traditional schematic and haven't run into the cathode-heater buzz problem so far.

Wouldn't that 100k cathode resistor of the second half of the preceeding stage suffice for the grid of the LTPI input stage?

I'm just getting started with the client in handling their service needs in the LA area, or possibly on the West Coast as was originally stated. I've only looked at/heard four of their current products, and every one of them has this rectifier buzz spikes on the output, and thus far, only thing I've found to cure it is adding a humdinger to null out the rectifier buzz. Then had to sort thru different ECC83S and 12AX7 tubes from various mfgrs to find one that yielded the lowest noise and minimal hum. The J/J ECC83S, being the selected tubes for their preamps stages in this instance wasn't as good as some other tubes. I've been told this is related to the tube's cahthode/heater capacitance. Once the humdinger pot was in place and adjusted, THEN I found swapping in different tubes yielded surprisingly different results. J/J wasn't the best choice there.

[mhuss]

The 1M8/1M voltage divider sets the DC voltage, and the triode is just a buffer to provide a low impedance voltage source/reference voltage to the PI. Only the Hiwatts do this twist on the LTP PI AFAIK...

[Helmholtz]

The 1M8/1M voltage divider sets the DC voltage, and the triode is just a buffer to provide a low impedance voltage source/reference voltage to the PI. Only the Hiwatts do this twist on the LTP PI AFAIK...

You're right.
Without a resistor from V4A grid to ground Its cathode and the PI input grid would sit at around 220VDC.

There's another strange thing in the schematic: V4A and V4B output signals seem to get mixed at the PI input.
Is that wiring correct?

I think the low output impedance of the CF (below 1k) will essentially short the plate signal of V4B.

[mhuss]

Good catch, that's another schematic mistake. The schematic above looks mostly like this, but with errors.
https://hiwatt.org/Schematics/DR_Pre4Input_v1a.pdf

[nevetslab]

You're right.
Without a resistor from V4A grid to ground Its cathode and the PI input grid would sit at around 220VDC.

There's another strange thing in the schematic: V4A and V4B output signals seem to get mixed at the PI input.
Is that wiring correct?

I think the low output impedance of the CF (below 1k) will essentially short the plate signal of V4B.

The schematic is incorrect, according to the layout drawing I have for the Preamp wiring. That 47nF cap is NOT shown on the turret board from the 1000pF cap that feeds the Presence control. AND, I DO see a 1M grid resistor from Pin 2, where it was fed from Pin 6 via 1M8/22nF. I'll advise their technician in Toronto on that.