sort of Diary read.... to put you to sleep (in 2 parts) Serial No: 96030520 - Mar 1966 Very Early AU-111 model with Staggered Preamp Valves. Highcut28, posted on AK on how to resolve the Anaemic sound being produced on his AU-111 early this year. Following a few PM's the unit landed in UK at the end of May. The AU-111 is one of the really early units, with 2-tone controls and staggered pre-amp valve section. The Amp was shipped with all the Tubes/Valves removed and safely wrapped inside the Amp. Issues: 1) Tone controls non-effective 2) Sound reproduction thin and anaemic 3) Occasional drop out from the Aux input 4) Balance not working 5) previous Attempts made to repair and given up. 6) Output Valves accidentally killed by incorrect speaker connection. The removed valves on inspection had a pair of JJ 6L6GC and a pair of metal canned RCA-6L6 and the usual newer preamp and driver stage valves. All the control knobs were also removed for transit. Mid-May With the underside removed, evidence of previous work and signal cables left cut and tucked away as well as volume control re-wired, balance control not connected, newish RS branded coupling caps and insertion of Ceramic disc caps, some expensive looking yellow/Copper capacitors in the signal path. etc.. slightly molested. Bit of history of the Amp suggests one of the previous owner did not like the facility of the tone control being active in the Amp and had it re-wired to bypass that section. I remember this phase of HiFi during the late 70's. (I am sure the merits of this has been discussed to death, revived and more.) Back to the Amp, nice little challenge. Few hours tinkering during each weekends. Usual Visual checks dd not show anything that may cause fire or be electrocuted. Resistance checks on the chassis isolation to mains and transformer checked all good and set to 240V. The main 15W on the B+ was loose and flapping as the mounting insulating washers have broken up. May be the Transit from Malaysia to UK was too much for its age. End Of May 1) So powered up with a Variac slowly with none of the valves in and all the DC potentials looked good and stable. (pheww) 2) Inserted the pre-amp valves in, powered up and left it for few mins, still good. Inserted a bit of audio into the Tuner input, audio coming out on the pre-amp output, very low and tinny. Volume control works, nothing else. 3) Powered off, inserted the JJ and a couple of spare 6L6GC in, slowly powered up. Half way through, the 1.5A fuse on the Variac failed. Removed the fuse from the Amp, it was not the correct 2A fuse but a re-soldered lump wire for a fuse, which would never have popped. (Forgot to check this early on, early stage dementia setting in, need to start a todo list, me thinks). Inserted a correct 2Amp fuse. 3) Tried again, this time with an Amp meter in place, the excess current being drawn is only apparent when the 6L6GC start to come alive. So, wound down the bias to the absolute minimum (cold-bias), better, however one of the JJ starts to red-plate. 4) Removed the JJ's tried, all OK. Checked pin5 with the power on, sitting just over 52V. May be the coupling capacitor breaking down.. Checked the Capacitor, seems ok. 5) Checked the JJ's, not matched at all, no shorts internally,lets try it again. No red-plating, all stable. Very strange... Something is not right. 6) Replace the U link between the pre-amp and Amp section, sound is there but as HighCut28 stated not what you expected or want to listen to. 7) Inserted the Audio signal straight into the Power-Amp section, lot, lot better. Atleast the output stage is decent and good, So its repairable..... and passed the good news to HighCut28 and started on the bits to replace for the suspect main coupling caps as well as all the paper-oil caps in the pre-amp and tone section. So a list of Orangedrops for the signal section, set of Solen's for the Coupling and a set of JJ's for the Output stage. Some Ceramic Washers for the Power Resistors. 1st Wknd June: Lets start on the Power Amp, section. 1) Removed the existing RS badged Coupling caps. Instead of the values being 0.33uF, they were 0.22uF. Adds to the anaemic sound issue. Replaced them with the set of Solens. 2) Repowered up, to check, red-plating again on the same position. Power-down, wait for a while and started to poke around the socket where the red-plating occurred. The 1K resistor feeding pin 5, 0.25W carbon resistor of that time, where the resistor barrel is held by compressed caps with leads to solder. The compressed cap was just being held by the tension of the soldering on the lead. Replaced all the resistors in this area of the bias. 3) while I was plumbing the Solen's I spotted a real serious issue. As this Amp has gone through a few repair and modification cycles, solder has been used excessively, while the amp was upside down and as a result, the solder being applied has flowed down from the 9pin valve socket tags and bridging to the chassis. Easy to do, difficult to spot. Sucked it up, checked around all Valve bases, specially where new solder has been applied.Further overflow's were found, re-heated, sucked up and fresh solder applied. 4) Powered back up, all stable, sound good just being driven from the pre-amp/amp split sockets. 2nd Wknd June: 1) the amp is a 2-pot bias scheme. With authorisation from HighCut28, changed to 4 pot version, however used some Vishay panel-mount trimmers and test-points, these are now presented so, it can be checked and adjustable from the top of the Amp. No new holes, used the existing venting holes around the 6L6GC area. Should not affect the ventilation too much. Yes used a bit of vero board, easily removable for restoring the originality, however, its a well worth useful tweek/deformation. 3rd Wknd June: 1) OrangeDrops are now being exchanged for the oil-paper caps. The OrangeDrops, just about able to squeeze & Tuck it into position. Really fiddly job, Cutout old, remove and clean old lead, cleanup/suckup excess solder on tags. Wholesale change of caps as HightCut28 wanted this to be the last time and wanted the amp to last. 2) This is a time consuming activity, each capacitor replacement can easily take up to 15 to 20 mins of slaving over the Amp. 3) For the Coupling caps, I used fibre based high temperature sleeving. Great, only problem with these are they start to fray at the cut ends and will be a potential issue in tight areas. Looked around, and found a suitable rubber/silicone based sleeving which melts at +180C. Had to wait a few more days for this to arrive. 4) Finished replacing all the orangedrop 5) Re-wired the tone section 6) Made up a replacement Balance control using 23step attenuator. The Original was replaced with a standard dual pot. Which is not the same as the original pot in-terms of how the resistive track is made up. The originals are 500K pots, however, the 500K was only half way up the potentiometer track and then remaining was Zero Ohm and opposite layout for the other channel. Very hard to find these, so 23 step attenuator re-worked. 7) Re-wired the Volume, Balance and Tape Monitor input. Tested the outcome, blast, still anaemic, but Tone control's now work, no more spurious discharge noises, tone control pots quite. 8) OK, get the scope out, 200Hz Signal, signal through C17, fine, via C19 attenuated very badly. This is one of the expensive looking Yellow bodied capacitor with a copper foil around it. Whip it out, remove the copper foil, .001uF instead of 0.5uF. Stuff in a temporary .33uF had lying around, sound million times better. back to the shopping list, identify the values on the Circuit Diagram, order some more orange-drops to replace the yellow/copper caps in the signal path.