Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Misc Schematics and Layouts => Topic started by: Jalmeida on August 24, 2024, 08:41:37 pm
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*****REVISED to R3****
Corrections to layout:
-Add 750ma fuse to layout in different location
-Correct 16uf caps connections
-Correct V1 connections
-Correct wire colors
Hello all,
I took some time to put together a layout for the MKI single Channel AC50 JMI amp original layout. I have matched it against schematics and the best pictures I could find. But if you see something off, feel free to let me know and I will revise.
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Revised to R2. See above for current files and explanation of changes.
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I can't see a CT for the mains transformer HT winding?
Each lug of the corresponding poles of the DPDT output load impedance switch need linking across.
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I can't see a CT for the mains transformer HT winding?
Each lug of the corresponding poles of the DPDT output load impedance switch need linking across.
All of the CTs are internally grounded to the one common ground internal to the PT. So the ground that goes to the fuse is the CT grounds.
The MKi did not have an impedance load switch like the latter large box MK4 units had. So the speaker connections was my attempt to show the speaker connection barrier terminal blocks used on these.
Some of these early 2 input and diamond input amps had one XLR speaker connection and some had two XLR connections. Often confused by the one XLR looking connector on the rear RH which was actually not XLR and was a power plug connection.
Hopefully that covered your observations?
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Each lug of the corresponding poles of the DPDT output load impedance switch need linking across.
To your point, I will revise the barrier Terminal block to show links for continuity to make that more clear.
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Revision 3 loaded. Let me know if I missed anything.
Again, this is the JMI layout minus the voltage selector. So right or wrong for how it should be, this is the JMI layout.
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*****REVISED to R3****
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But if you see something off, feel free to let me know and I will revise.
An old post, I know, but many thanks for taking the time to do this really useful layout.
One note - both VR4 and VR5 are shown as 1M log pots on OS/044 (and are so in my amp).
Either the treble or the bass pot is wired backwards - if you want it wired original Vox style (so that clockwise reduces treble/bass) then treble is backwards; if you want it wired the modern way (so that clockwise increases treble and bass) then the bass pot is reversed. Wiring it this way makes the treble work the modern way and the bass the other way.
Interestingly, the 470R load resistor across the OT is not shown on OS/044 but may have been added during manufacture (JMI certainly put those on Mk II amps, so not unlikely they started adding them to Mk Is). It’s a good idea, so I think I will add one to the restoration I’m doing atm.
Digsby