any ideals what these diodes do in this design

[iccaros]

This is a preamp of an old radio, I am betting primitive noise gate, the author at http://antiqueradios.com/forums/viewtopic.php?f=6&t=193450&p=1614588#p1614588 thinks pop filter..

[gmoon]

Peavey used a similar schema for a noise gate on the Bravo and Triumph amps. The Peavey implementation evolved slightly, eventually they used a resistor in parallel with the diodes to make the gate somewhat less abrupt. It's common to just jumper and remove them from the circuit...

A pop filter's a noise gate, too, I recon. This schematic has some RC filtration working with the diodes.

[Earthscum]

wouldn't a pop filter shunt excess voltage to ground? If this was in a stompbox, I'd be guessing some kind of crossover distortion if you drive it hard enough.

[artifus]

fuzzy headed pre  coffee guess - frequency dependant limiter?

[teemuk]

Yes. As indicated even by the linked forums discussion it seems to be a "ghetto" rumble / hum filter.

The hi-pass circuit represents a very low impedance for frequencies roughly above 100 Hz, practically bypassing the diode gate. At lower frequencies the path through the diodes begins to have a notable effect and the signal begins to pass through the diodes, assuming there's enough forward voltage to make them conduct that is. Note that they are Selenium diodes with low threshold voltage and soft "knee" so the crossover gating effect is somewhat minimal.

The difference to a basic bridge-T with a resistor is that resistance of the diodes is not constant but amplitude variant. I don't know about rumble filters / phono preamps / turntables enough to tell what the trick in that feature actually is but my guess is that rumble (or hum) is assumed to be somewhat low amplitude in comparison to low frequency signals actually found from the media. So, the circuit introduces attenuation to low-amplitude low-frequency signals but practically no mentionable attenuation to higher amplitude low-frequency signals. Could be a noise-gatish style operation too: Higher amplitude signal portions sort of hide the rumble and hum but it would be heard during quieter passages.