Hi r89. Do your 7236s have many hours on them yet? Are your TS/Chathams used, with many more hours on 'em?...more time should indeed clarify things a bit further... I used to wonder (worry) about such discolouration of the flashing...until I read that it can in fact be a good sign, in that it has done/is still doing its job of absorbing unwanted gases. My own examples have always been just as 'strong' as those with perfect silvering. My NOS QTL-branded Mullard ECC31 also has a small patch on the glass (inside) above one of the heaters, even though the flashing is actually on the bottom...and it performs brilliantly LOL......so you should be OK. If your TS were in fact nearing its end, I should think you would notice an imbalance in the output volume...(but hope it isn't! ). CHEERS!