AOR 100 watt. one side redplating

[joesatch]

Been at this for a couple days.  I essentially shotgunned all the coupling caps on the board and checked all the resistors (swapped the 220k bias ones as well). One side of the amp is redplating.  Also 100vdc+ gets measured at the .1u cap from the phase inverter.  Bias voltage checks the same on both sides. Plate voltage checks the same on both sides.  It's not the tubes or the tube sockets as i have isolated the issue to the two sockets driven by one side of the OT. using one tube for each side bringing it up with a variac, one side  redplates. I've already nuked a set of new JJ's. Any ideas? i'm going to disconnect the OT primaries and measure the windings

[joesatch]

OT Primary windings measure fine. One side 16 ohm the other 17 ohm.

[shooter]

 I've already nuked a set of new JJ's.


did the redplate stay on the same side?
did you ohm the screen R's
did you measure the bias -vdc at the tube socket, tube side, WITHOUT tubes installed?  was it stable over time?

[joesatch]

screen resistors are fine . grid resistors are fine. each bias negative voltage reads the same and maintains negative voltage without the tubes installed. I cant understand why only one side of the amp redplates. the redplate doesnt follow the tubes so its the left pair in the pic on my initial post. This amp has been worked on, not by me so i've been tracing and measuring everything. Gonna clean these two tube sockets. they both likely arent bad on this side. chasing my tail

[joesatch]

Solved by google gemini. It had me take a series of measurements.  The phase inverter had no reference to ground so was pushing out 130vdc on it's cathode pin 3/8. Turns out the presence pot was missing the 4.7k resistor.  who knew an ungrounded presence pot could nuke your tubes?

[jbrew73]

How did the amp sound before you found the problem?  Was it somewhat normal or were there signs of a problem?


Would the missing resistor have caused excessive negative feedback and oscillation?

[joesatch]

this was someone else's basket case that was full of errors. came with the OT ground connected to the speaker positive terminal. bad solder joints, all kinds of issues. i'm rebuilding it

[passaloutre]

Got a schematic?

[joesatch]

if you come across a clearer copy, let me know. thanks

[glass54]

Hi Joe
Not quite the same sch BUT same power Amp and cleaner sch.
Note: no VT1 and VT2 in Preamp.
Might be of use 
Regards
Mirek

[elams1894]

The Laney AOR series are interesting beasts. Cool circuits if you disregard the thermonuclear Bass Boost. Its a shame they were so poorly built as the circuit is solid. From memory, the opamp power supply dropping resistor sits right on the board and burns a big nasty patch on the PCB. Replacing that and lifting it off the board is a good move as well.

I rebuilt an AOR50 long ago in the original chassis with original transformers and choke and still have it. Although it works well, I'm not entirely happy with my layout design back in the day. Its always been on the project list to redesign the AOR50 again, however without the boost circuit which I think is entirely unnecessary. These amps can do a good 'Hot For Teacher' tone. Thanks for the memories of this super cool unusual sleeper amp. You do not see many around these days and the Power Transformer is a beast of a thing. Well worth hanging on to and restoring.