I purchased a used ST120 seated all the tubes in there previous positions (previous owner marked them) Plugged in 121V. Turned on and all the heaters glowed except for the 5AR4. No bias voltage. Turned off and re seated the 5AR4, turned back on and got heaters in the rectifier. Checked bias and all but the right rear were at .476. The right rear was at 375mv. Could not lower it. Re seated tubes turned back on and the 5AR4 looked like a 4th of July sparkler. So I’m guessing a new dual rectifier board and a new JJ6550 are in my future… Any other thoughts?
2 posters
New to me ST120 blown 6550 & 5AR4
batman7349- Posts : 21
Join date : 2014-12-08
Age : 66
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
why did you want to lower the bias? at any rate, check to make sure the 10 ohm resistor on the output tube socket has a good connection and still reads 10 ohms.
Always wait 2-3 minutes before restarting an amplifier, a hot rectifier tube will blow up as you've found out. Dual rectifiers recommended.
Always wait 2-3 minutes before restarting an amplifier, a hot rectifier tube will blow up as you've found out. Dual rectifiers recommended.
batman7349- Posts : 21
Join date : 2014-12-08
Age : 66
Roy, thank you. I have ordered the dual kit. When I was setting the bias 3 tubes were at .476 close to .5. The 4th tube right rear was at 375. mv, note the position of the decimal point. I should have taken pictures… I use a Klein MM600 DVM
batman7349- Posts : 21
Join date : 2014-12-08
Age : 66
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