ST70 Bias Circuit resistor replacement (2 watt?)

[snitch56]

On a stock ST70 can I use 2 watt/10K resistors on the terminal strip or should I upgrade to a higher watt resistor?
Thanks in advance,
Brad

[erichayes]

2 watts is plenty of dissipation. The original Stereo 70 used ½ watt resistors without any ill effects. What you want to deal with is the selenium bias rectifier diode. If it hasn't already been replaced, just take a 1N4007 and hook it in parallel with the selenium, using the latter's lugs as tie points. Seleniums don't short; they open up or get really crappy front-to-back ratios, so there's no problem with bridging them with silicons.

The other components that go South are the 50 µF filter caps in the bias supply. Replace them with 100 µF 100 V caps, if they haven't already been yanked.

[snitch56]

Thanks for the advice Eric.
I replaced the resistors, selenium rectifier and capacitors. The capacitors were replaced with some 50 uF 125 V I had in my stock pile. Is there any advantage to using a 100 uF capacitor over the 50 uF in the bias supply?
Thanks in advance
Brad

[Shannon Parks]

If upgrading from scratch, sure, 100uF/160V makes a pretty good all purpose bias cap, but I think 50uF will be fine. I just added a hybrid fixed bias to a SET amp I'm tinkering with (a GSG with 300Bs). I had some 33uF/160V and they work great. No added noise.

[EWBrown]

One of my ST-70 rebuilds, I did a bit of overkill, and replaced the two pots with some nice old surplus Bourns 10-turn 10K pots, and replaced the 15.6 ohm resistors with 10 ohm, 1%. Definitely makes bias-setting "to a gnat's arse" very easy Yellow_Light_Colorz_PDT_03

When a selenium rectifier fails, it would stink out loud, like a dozen rotten eggs, combined with the "sweet" scent of a week-old a dead skunk. Yellow_Light_Colorz_PDT_21

Been there, done that. The old "thyrectors" were the selenium spike-suppressor predecessors to MOVs, and they could really kick up a nasty rotten stench :parp: :urgh:

/ed B in NH