Transistors on regulator ground pin

[JohnRoberts]

yes indeed, however if the issue is too much voltage being dropped across the regulators (leading to excessive power dissipation and heat), then if feels unlikely the approach would be problematic, although I'm sure a weather eye on the voltage rails when the desk is running at full load would be sage.

it's the average voltage that causes heat, minimum voltage that leads to dropping out of regulation.

I hadn't thought of that ... ... but isn't it also likely to induce ripple noise

dropping the C will increase the ripple voltage a proportionate amount. Whether that affects noise floor depends on the circuits PSRR.

as well as reducing the MTBF for the smoothing capacitors because they're now seeing far higher ripple voltages and thus greater ripple current?

Higher ripple voltage but ripple current will be pretty similar (ripple current is an average heating phenomenon also).

You correctly note it'd also reduce the drop out voltage but by how much is probably harder to calculate whereas the diode is quite predictable from its forward volt drop.

Great example of different circuit designs which achieve the same result though!

many ways to skin every cat..

Another way to scrub off excess transformer voltage is a resistor in series with the transformer winding (before the rectifier)... I used the resistor trick in a small headphone amp that drew too much current if I connected it to drive a loudspeaker. The headphone amp was happy driving a loudspeaker load ( perhaps over designed) but it would overload the thermal fuse in our 1A wall wart. Adding a small resistor in series with the transformer effectively power limited it so the wall wart would not melt playing loud into a loudspeaker. The headphone amp was not intended to drive loud speakers but I routinely did that at trade shows, because I could.

JR

 

[vintageampfixer]

it's the average voltage that causes heat

it's the average voltage "drop" across the regulator, thus:

(V out - V in) x I = W dissipated

 

[EmilFrid]

Sorry I didn't see this earllier. It would be very interesting to know what the AC voltages are between:

J1-4 and J1-5
J1-4 and J1-6

With these AC voltages known, it's possible to calculate the DC voltages we should expect across C5 and C7. Please also measure these DC voltages:

Across C5
Across C7

Lastly the output voltage of the two regulators REG1 and REG2:

Across C11
Across C12

If you'd measure these and reply, we'll see what we can come up with that might help

If it interests you, I most certainly could make these measurements! As stated before though, this doesn't have to be much of a problem. It's been hot since day one, and never failed. I've changed some caps, but other than that, it looks like the PSU:s virginity has been kept intact since the 80's.

I'll go make some measurements.