B52 AT100 coming in

[shooter]

I’m taking on a “once over” look at a B52_AT100.
 
Going over the schematic while I wait for the amp;
Getting hung on SW5, Location 5_A/B, which go to location 7_B   (bias and To mode switch)
What I’m “seeing”
SW5 section B puts ground on the power tubes grid R’s (112,113) “Bias”(cathode bias) or (fixed bias) volts via  B2 B3
 SW5 section C1,2,3 “to Mode switch” “appears to always apply ground to the power tube cathodes??   

[sluckey]

Not what I see. I see the cathodes are only grounded in mode positions 2 or 3.

[Johntb]

I bought one of these last year and love it. It was listed as "not working". After some research I found they run VERY HOT. I got it a d it worked fine but shutting lights off I could see it red plating. Got new tubes and it was fine. Also found they switch in/out on the reverb and now that works fine. I have read a lot of people put fans in for cooling. (The transformers are what gets really hot and causes all kinds of issues).

Anyway, on the back of the amp there is a switch for "solid state/AB/A". Pretty sure SW5 is the bias circuit for the different modes of rectifier. But I'm no expert. Looked again and notice when SW5 is in position 2 there is an extra 6k8 resistor before the 5k bias pot. When SW5 is in position 3 it goes straight to the 5k bias pot. Maybe to make it run hotter??

[sluckey]

Pretty sure SW5 is the bias circuit for the different modes of rectifier. But I'm no expert.

Yes. In addition, position 1 switches the output tubes to cathode bias. That's probably the "A" label on the switch.

[shooter]

in the normal position C and C1 are at ground.  C is always grounded at location A_4 so when the switch goes to C2 and 3 there's still ground.


I would think C1 would be "open" for cathode bias 

[sluckey]

C is hard wired to ground. C1 is also hard wired to ground. C1 has nothing to do with cathode bias.

[Johntb]

One other thing I have found in my reasearch for mine is be really careful with the bias pot. They have been known to shatter.

[shooter]

C1 has nothing to do with cathode bias.

As I was driving to the Beerbaitngas, that dawned on me, verified when I got home, schematic says (c2/c3) 
that's why I  spend hours on the schematic so I only need minutes in the amp!


thanks for getting me past my blindspot!


As for the bias pot, I don't plan on doing anything except note "anomalies" since it's a working amp, according to the owner. but he did say the switch (SW5) was not working except in one position.
pretty sure I'll be back in a couple days after I crack it open


 

[sluckey]

Nothing like a drive to the Beerbaitngas to get your mind right!   

[Willabe]

Nothing like a drive to the Beerbaitngas to get your mind right!   

[shooter]

 


It's to hot to fish, I haven't drank a beer in 30yrs, and I had a 1/2 tank of democrat gas already so;


I bought tea!!! and a scratch off, won $40!!
now I just need another $40 to break even 

[Willabe]

I bought tea!!! and a scratch off, won $40!!
now I just need another $40 to break even