Single Ended Stereo Preamp - Shared power supply

[StarTrucker]

On Power amps, I've seen sharing the B+ directly between channels, but what about on Single ended amps?

This preamp has a mono schematic and I housed 2 of them in the same chassis to run in stereo off the same power supply. They're single ended and I get oscillation/interference between the channels. The problem is in channel 2 but I get junk in channel 1. Disconnect ch 2's B+ and the problem goes away in both channels.

The B+ ends with a cap to ground (last PSU Cap) between the channels, so I figured any junk would go to ground.

This is troubleshooting, but the real question is how to share a power supply between 2 SET amps, so I posted it here in the drawing board forum.

 

[Rusan]

Which tube type are you using? Parallelling two triodes effectively creates a new, single triode with twice the transconductance and half the anode resistance. Which, of course, makes it much more susceptible to funny business like oscillations, on top of what was already a worst-case scenario for power supply rejection. It'll only be aggravated further by low Rp, high Gm types like 6922/6DJ8, 5687, ECC99, etc. If using such types, grid stopper resistors of adequate value are of double importance, with one per grid connection and very short leads.

How well is your B+ filtered and decoupled? How long are the cathode resistor connections to the ground side of the filter cap? Do the grounds run through the chassis? Do the grounds or the signal leads run close by the output transformers, which could be inducing a feedback loop?

In such a simple circuit (if you don't have any hum going on), it's quite possibly a lead dress/wiring issue of some type in Channel 2.

 

[StarTrucker]

Which tube type are you using?

12AT7 triode with both halves parallel'd. Ua100d I the circuit, and the PSU is in a separate enclosure with a 2foot run to the audio circuit which carries B+, signal and chassis grounds, and heaters.

How well is your B+ filtered?

4 stages RC


Is the schemo I posted correct where you can just link the two output transformers together from the B+ supply? Nothing needed to knock down cross talk?

 

[Rusan]

Is the schemo I posted correct where you can just link the two output transformers together from the B+ supply? Nothing needed to knock down cross talk?

No issue there. However....

....the PSU is in a separate enclosure with a 2foot run to the audio circuit which carries B+, signal and chassis grounds, and heaters.

Is the decoupling cap that the output transformer primaries are connected to housed in the PSU chassis, or the audio chassis? A two-foot long wire from the grid leak and cathode resistors of those two amplifier stages to the ground side of that cap could be an issue.