tubefan9 wrote:could the two 50uf bias caps be causing this? Those are original.. I just thought if that was the case it would be affecting both channels
Well yes, both channels.
No, the observed phenomena would not necessarily be identical in each channel.
One of the great weaknesses (of several) in the OEM Laurent design is that the bias pots serve as voltage dividers in this application, from a single voltage source. When the source is well-behaved, the pots will adjust nicely. If ths source should be producing too much voltage, one side might bias correctly, but the other side will run high. To anyone who has biased a significant number of these beasts, this will be obvious as it is a constant adjustment until both sides are correct. Taking one side, only to the "correct" 1.56 V, and then not checking the other is not going to work out so well. In the OEM design, the selenium diode was part of that overall circuit, inserting a silicon diode throws that out. Follow this thought:
a) The voltage was already drifting too high - any number of reasons - but the selenium diode was simultaneously increasing in internal resistance - which is, by the way, what they do.
b) So, correct bias was possible.
c) All of a sudden the the selenium diode goes away, a silicon diode is in its place.
d) *POOF*, correct bias is no longer possible.
I will make one very basic assumption about the unit in question: That the tubes are all good and verified as such. NOTE: Good or bad tubes would have little or nothing to do with hum or bias unless internal shorts or other obvious failures obtained. And that would be counter-intuitive to the narrative so far. Not impossible, but unlikely.
Accordingly I will make two suggestions to Tubefan9:
a) Replace the bias capacitors. Minimum 47 uf, maximum 60 uF, minimum 80 VDC. 105 C. Example: https://www.mouser.com/ProductDetail/Vishay-BC-Components/MAL211990518E3?qs=sGAEpiMZZMu3dWSqd4Tl0AYVZSvceoL0OhTm97d%2fefw%3d
b) Check, and consider replacing, all sections of the Can Cap. Either with an aftermarket board (suggested), or with a new 4-section cap - most authentic, but costly.