On my 500B, the AF amplifier stage immediately before the PI does not have bypass caps on the cathode resistors. The 500C has 50uF caps but the cathode and feedback circuitry is a bit different. 500B Service Manual with schematic: http://akdatabase.com/AKview/albums/userpics/10004/Fisher 500-B Service.pdf And Dave G wrote this in one of his threads where he added 22uF caps to his 400 receiver: "First, with the output stage now operating under the control of EFB, the grid bias voltage for this stage has been reduced from typically -18 vdc or more, down to ~ -15 volts. Less bias voltage equals less drive voltage for the same power output, meaning that the output stage has more gain. Then, the rest of the lost gain can be recovered by bypassing the 1200 ohm cathode resistor of the AF Amplifier stage with a 22 uF cap. These two measures then allow the revised circuit to display the same amount of open loop gain (OLG) as the original design did, allowing the same level of NFB and overall sensitivity to be achieved as well. Except that now, with the revised circuit, there is plenty of low distortion drive available for the output stage, that the original design was simply not capable of producing." On my 500B I have added EFB and IBAM along with all the other standard safety improvements. I also removed the PI noose including feeding the AF stage with a lower B+ (node F). So...... I believe adding bypass caps would be appropriate and if true what size?