AC Balance

[erichayes]

Hi All,

I was perusing the ST70 board to see how similar it is to a prototype I came up with in 1999 (frighteningly, except for the CCS--an improvement), and noticed something that those of you who bought into the Boonton thing might appreciate.

Replace R19 thru R22 with 39k 2W MOs, and put a 10K 2W type J or RV4 pot between the fixed resistor leads on each channel that would go to B+ (one to each end of the pot) and connect the wiper to B+. This will allow you to balance the signal going to the grids of the output tubes and get minimum THD.

Adjusting the AC balance can be a little frustrating, as it takes miniscule adjustments and several seconds of waiting for the analyzer to stabilize to see the results.

If you have a 400~ high pass filter option, check distortion with it in and out. The difference in readings will be your hum component.

I don't know if the "Monitor" jack on the rear apron lets you see what's going on in the distortion mode (my 1120's not on my active bench) but if it does, you'll get an immediate response to your tweaking, rather than having to wait for the nulling circuit in the analyzer to do its thing.

Happy hunting.