B-52 AT100 Schematic

[tekhed7]

AT-100 series bias circuit

Well... I just finished Fixing 1 of these sorry behemoths.
Sure enough, contacts at the input jacks & impedence selector slide switch were both crapped, but the real problem with this combo was that a hot-running, common cathode resistor for the power stage... that white, 15 watt 110-ohm power resistor on the power supply board, had cracked free of the foil on one end.
The young guitarist had changed All the tubes with 'GT' tubes in an effort to "combat" the resultant major power drop (to almost nothing) when the resistor post went "open". Silly boy.
After I got the input jack board repaired for a busted HI jack, and the Line Out jack remounted back up on the rear panel with a new "front-nut", I went probing & found that loose power R. R95 I believe.
After the head was all spiffed up, the cab rewarded me by having it's entire front (dual 12") speaker panel simply flop out of the cabinet! They freagin' STAPLE those front speaker panels in!
Ahhh, well. A bit of wood glue & thin screws & she's solid.
Customer was told by "everyone" that his power drop was all probably a bias issue! (??)
Never ceases to amaze me the lack of understanding that musicians can have of their equipment. And it seems there's lotsa people to chime in with their 2 cents worth. (More than what that advice was worth)
BUT... that's what keeps us techies in business, Huh?

[glsharum]

b52 bad power supply

I got one of these beasts with a bad power supply. I got it at a good price and did not want to put much into it. So I replaced the PS with after market ps to run the plates and heaters. I got a small low voltage supply from hammond to power the 12 volt needs. I then removed the bias selector switch, which was a disaster, removed the tube rectifier (I like SS), and disconnected the bias circuit. I put marshall type of bias circuit on a small board power by PS bias tap. I am getting ready to put it back together. At least the bias circuit will be more reliable. I think the stock system is what killed the PS to start with.

[johnhoef]

Hi everyone, I just now registered, so totally new to this forum. Looks like this thread is perfect to help me solve my problem. I run an electronics repair shop here in Portland Maine, and currently have two amps with the same problem, and with both I am stumped as to what the fix is. One of the amps is the B-52 AT-100, and the other is a fender super bassman. Both amps use four 6l6s, that should produce 100 watts output. Not the case! I can drive both amps to the verge of clipping into an 8 ohm load, with the resultant output being around 50 watts. Reducing the load to 4 ohms, resultant output is less that 50 watts. All four 6l6s are brand new, as well as the driving 12ax7. All voltages are correct, including bias. All resistors in the power amp section check good, and darned if I can find any bad components. I am observing the output with my scope, and have my fluke multimeter in AC mode measuring voltage output. E squared, divided by output load ohms should give me wattage output. Am I doing something wrong here? These power amp circuits are not complicated, leading me to believe that I am doing something wrong??? Anyone??

[J M Fahey]

1) welcome to the Forum

2) do yourself a favour and open *another* thread dedicated to your problem.
This one is 10 years old and has not been visited since 2010

[johnhoef]

Thank you, Juan...will do.

[Enzo]

I agree^^^

Hi john, welcome

This section is for requesting schematics.

Over in the repair section I suggest you start a new thread for each of your amps.

Remember, these are guitar amps, no hifi amps. At the very least, for power output test, apply your signal to the power amp directly, usually through the FX return.