Hi everyone, I have on my bench a pair of old Japanese DIY monoblocks. They are fairly standard with 6189 input, Sylvania 6350 LTP phase inverter and 2A3 push pull output. Built with Lux kit transformers that are very common in Japan. Only one thing really has me stumped. The power supply is diode rectified, but the unused 5V 3A winding on the mains transformer appears to be connected to the secondary of the output transformer through a 3K resistor like this:
0----------16 ohm
3A Secondary of OT
5---3K----0
I am completely stumped as to why this has been done, I have worked on a lot a tube amps and never seen this arrangement anywhere. Can anyone explain? Many thanks in advance
0----------16 ohm
3A Secondary of OT
5---3K----0
I am completely stumped as to why this has been done, I have worked on a lot a tube amps and never seen this arrangement anywhere. Can anyone explain? Many thanks in advance
Sorry for the poor attempt at asci, I have taken some pictures and tried to sketch the wiring of the mains / OT. In the close-up of the mains transformer, the black wire is the "0" and the white wires are the 4, 8 and 16 ohm taps.
I agree, but I've triple checked the wiring and verified with meter that the wires do indeed go back to the OT 0 and 16 taps. I'm prepared to accept that this may simply have been a mistake on the part of whoever built this a long time ago and move on, it just looks very deliberate and is done exactly the same in both monoblocks... perhaps the builder did this to exact revenge on posterity or something 🙂
Could that have been done as an attempt at hum cancellation?
What is the exact kit model? These schematics are available.
If mine, I would disconnect the 16 ohm end of the 5V winding, and then connect it to ground.
Even if it does reduce hum. Why risk injecting power transformer noise into the OT?
If mine, I would disconnect the 16 ohm end of the 5V winding, and then connect it to ground.
Even if it does reduce hum. Why risk injecting power transformer noise into the OT?
Works out to 27 mV being injected into output. Could cancel 27 mV of hum, (pretty bad) at 60 Hz. Output transformer is a good distance from PT, but possibly oriented wrong, coupling with PT?
'Make Your Own Hum' trick circuit?
Best to unhook the top end, leave other end on common.
Remove the problem, it may be something stuffed in after the build, who knows for what purpose.🙂
Does the 5V also power the 2A3 filaments in series? If so, it is likely a hum cancellation device. Might still be even if the 2A3s are heated with AC from somewhere else. You've drawn cathode plates on your schematic but 2A3s are DHTs.
In any event, it's kind of a 'hack' design IMHO.
In any event, it's kind of a 'hack' design IMHO.
In Hitchhikers' Guide to the Galaxy a great computer was built to find the answer to, you know, Life, the Universe, and Everything, which was eventually revealed to be "42". Then, a bigger computer was built to calculate the question, ultimately revealed to be "What's 6 times 9?" ****-up.
All good fortune,
Chris
All good fortune,
Chris
In the books, Earth was built as a subroutine of the great computer, called Deep Thought, where white mice were performing elaborately subtle experiments on humans by running around in cages and squeaking, etc. It's a desert dry farce, comparable to John Cleese, if that's a useful reference.
All good fortune,
Chris
All good fortune,
Chris
It was one of the older methods of providing a load and subtracting hum at the same time.
Thanks everyone, I think the suggestions that this is some kind of attempt at hum reduction are correct. I've clipped out the connection and have one of the amps on the test bench now, can get the hum down to 1,2 mV which seems respectable. I think with some wiring cleanup etc it could come down further.