Decca DMI-61 Schematic

[Aerostoon]

A small little amp called a Decca DMI-61 is in my repair shop for a tune up and I thought I would post the well preserved schematic from inside the amp for anyone in need.

[Enzo]

yep, another hot-chassis widow maker.

If you are going to use this for guitar, you REALLY ought to put an isolation transformer on the main input.

[Aerostoon]

Thanks Enzo, my next step is to convince the owner that it needs one. I ussualy use a N-68 Triad Isolation transformer for these little amps. I also add an inline fuse as well.

[Aerostoon]

Here is the basic wiring schematic I use when I install the Isolation Transformer. I am not the author, so thanks to GMoon:

[g1]

I'm not sure what chassis the are referring to where it says "ground lug on chassis", I'm assuming the chassis that holds the transformer and switch?
Also, there is a note on the schematic "please read the section on half-wave rectifiers before proceeding", can you post that info or a link to it?

[Aerostoon]

Hi g1, here is the link to the thread. I think the fellow (Gmoon) did a pretty good job at explaining the steps and importance of having an isolated chassis. At the top of the page click on "show all steps" and you will get the entire article.

Isolation transformer upgrade for old guitar amps

I didn't add a solid state rectifier to the amps that I have upgraded as they have all been small single power tube types 2.5 to 5 watt output amps and the N-68X has been easily able to supply them.

[g1]

I didn't add a solid state rectifier to the amps that I have upgraded as they have all been small single power tube types 2.5 to 5 watt output amps and the N-68X has been easily able to supply them.

As long as you checked and made sure there are no heating issues with the transformer. The amp in the instructable was only a 2W amplifier and it gave him overheating issues. It is not the power output of the amp that matters, it is the power consumption from the mains.

If I recall correctly, it was said somewhere that when half wave rectified, the N-68 should not be used with an amp that has consumption over 25W.

[Aerostoon]

I think you are correct. I checked and our local electronics supply store has a DF04 Full Wave Bridge rectifier 400 V 1amp for $0.57 each. I would be silly to not add one!