Oh dear god......
When I saw this I wasn't sure what to make of it. I was pretty sure that whatever happened it was not repaired to code and it was not going to look nice under the hood. Furthermore some 1970's air conditioner is missing a power cord. This transformer is HORRIFIC!!!!!
The amp itself though is pretty cool, this particular one is a rare breed of B-15-N. You can tell from the 5U4 rectifier..
Opening the back cover we see the electrolytic quad cap that will need to be replaced followed by the removal of all of the tubes for testing. This is the LAST project to exclusively use the Century tube tester for testing as i transition in the Hickok 6000 this month.
Since my Hickock 6000 was not ready, I did go down to cornerstone and borrow theirs, also providing for some comedy relief. This was brought about by my inability to test the 6SL7 tubes on the century. and also because my Hickok is also ready. After testing, the amp was simply to disgusting to work on so I had to stop and clean it, and vacuum it.... and re-clean it.... and then disassemble it. Hey, theres a treasure map inside under a layer of dirt! This map tells us that the quad cap is actually wrong! the first of many wrong things in the amp. Jesus!!!! Look at the power cable, its not even soldered. Its tied in!
Theres also a soldered in fuse just hanging in-line, no rhyme or reason... no mount either, bet its high voltage that can short onto other connections.
Impressively, cathode bias is tied to the humdinger circuit.... cool!
This is the time to Deoxit everything.... I don't like this part, but it is necessary.....
The replacement monstrosity that was installed in the cabinet was drilled through. Whoever did this should find a new line of work. So i decided to remove EVERYTHING that didn't belong in the amp.
Starting with that nasty transformer.....
The schematic mentions a thermistor, which this device is missing. This is a curiosity but many of these actually did not come with them. It was not so important with tube rectifiers as it was with solid state. With new CAPS its even less important.
Due to mental health issues I decided to measure ESR for all of the coupling caps. But then common sense prevailed and I moved testing to the IT-11
Hey that 7.5k resistor is 1.5K.... Hmmm looks like components on the supply side were swapped to accommodate the Abrams tank that was powering this unit previously.
Work continues now that the parts have arrived to include the new power transformer and capacitors!!!!!! Apparently the transformer does not come with any SAE mounting hardware so its off to a trip to ACE.
This begins the mounting process for the transformer, having pulled all of the wires through. Finally I get to see what it looks like when it doesn't look hideous. Now the work begins; since I didn't swap an old one out ill have to take extra time to ensure which wire does where, and then match that up to the schematic. Having accomplished this I start making my connections. The ones I'm not 100% on yet i'm, leaving extra service loop in. Also the remnants of the old connections were removed and cleaned up.
The next job is the replacement of the quad electrolytic cap, which will have to be replaced and also inspected to ensure that the wiring is correct. The old mount is carefully removed and reused.
Also, that 1.5K has to go....... so Im putting in the 7.5K resistor as specified, along with the two remaining electrolytic caps. Also, finally installing the missing 20uf in the finals of the 6L6.
Finally the new power cord goes in to include a chassis ground off of the transformer bolt. Shorting tests are then done to ground and then through the power switch on the plug.
Ive decided at this point that AC would be measured from the tube sockets at a fraction of the voltage (40vac) using a Variac to ensure everything was wired correctly. This confirmed the wiring to be good.
The next test introduces the rectifier. But it did not produce a voltage. I was able to find that their is a missing section that forces the amp to standby when the speaker is missing. I had to make a jumper for testing.
With that, all tubes in, B+ is looking good.....
In this the final video of the series, the power supply testing is completed and its time to clean up the wiring. Also the 20uf cap was dressed in properly onto the board and the hanging fuse was removed from the cabinet.
After all of this work.... and 2 new pilot bulbs, i decided to have a look in the cable that goes to the speaker cabinet.
Yup.... Another horror show.... wired like crap and not even on the correct terminals. So I cleaned everything up and rewired it according to the diagram.... How did this amp ever work???
Yeah, so Cathode bias, is kinda interesting for me in this setup, I find it a bit on the high side, as I would have been happy with maybe 48mA. But this is the exact values and a bit on the low so ok.
It was worth testing though, seems a bit in Class A here. Also strangely channel 1 has no grounding plug.
Noise tests at 200hz confirmed nothing weird going on in the circuitry.
So I went back to the whole tube current thing, as I wanted to see current in full operation to confirm my findings. I didn't expect it to change provided an input.... and I was right. Though, too much input and it will sag due to plate voltage dropping. Again, this amp might definitely be operating in class A given what Ive looked up on the tube specifications.
A quick test with the guitar confirms no noise on a descent speaker and a natural vibrato. Now its time to pass to the wife for inspection.
Inspection passed , so now its time to reassemble and pack up. with one final adjustment, the hum knob.
And thats it..... Complete!
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