quadzilla,
the tunnel diodes in the 453 are TD3A 4.7 mA parts, I assume that Roger Evans is correct and that they are Germanium diodes. If you were to follow the link provided by Roger you would find a list of Russian made tunnel diodes in a large table. All you need to do is find a part whose material is "Ge", whose use is "Switching", and whose Ip exceeds 4.7 mA (preferably something close to 4.7 mA). Several diodes early in the list seem to match these criteria:
1I304A (1§ª304§¡) / GI304A (§¤§ª304§¡) Ge Switching 4.5-5.1 mA
1I304B (1§ª304§¢) / GI304B (§¤§ª304§¢) Ge Switching 4.8-5.4 mA
1I308A (1§ª308§¡) / GI308A (§¤§ª308§¡) Ge Switching 4.5-5.5 mA
1I308B (1§ª308§¢) / GI308B (§¤§ª308§¢) Ge Switching 4.5-5.5 mA
I would go with the 1I304B/GI304B (1§ª304§¢/§¤§ª304§¢) since it's minimum Ip exceeds the spec for the TD3A. The 1I308A/B (1§ª308§¡/§¢) parts have no leads (you would see this as soon as you searched for these parts on eBay), so I would avoid them. Adapting a leadless part into a circuit using a leaded part is not impossible, but it is extra work.
The 1I304B parts are available on eBay at reasonable prices (quantity four for $12.75 including shipping). I'm not sure how much work it will be to adapt the ribbon-style case to the circuit in the 453.
I also see that the exact part you are looking for (Tektronix 152-0125-01 tunnel diode 4.7 mA, 15 pF) is on offer on eBay. It's the first hit I get using the search string "tektronix 152-0125 tunnel diode". This part is much more expensive than the Russian TDs, but has the advantage of being a drop-in replacement.
-- Jeff Dutky
