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 Post subject: First (small) Tramp mod
PostPosted: Sat Feb 15, 2020 1:07 pm 
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Joined: Fri Aug 16, 2019 4:17 pm
Posts: 38
Location: Detroit, MI, USA
Hi all. Thanks for the help in my build thread. Since then I've been learning a little about tube amp design, and I'm about ready to get the soldering iron out and start playing.

Starting small, my first modification is a safety resistor to prevent damage if I forget to plug the speaker in. Here's what I did:
Attachment:
File comment: This requires a power resistor and a shunt-tip output jack.
schematic.png
schematic.png [ 40.27 KiB | Viewed 130 times ]

The idea is that if the speaker is unplugged, the shunt tip in the output jack connects a resistor in its place. The resistor is downstream of the impedance selector switch, so it's only a good impedance match when you're on the 8 Ohm setting. I decided that putting it upstream of the switch wasn't worth the extra complexity. For completeness, here's what I think the "upstream" version of the circuit would look like:
Attachment:
File comment: If you have an isolated, break-circuit 1/4" jack handy, you could do this instead.
schematic2.png
schematic2.png [ 40.45 KiB | Viewed 130 times ]

Please ignore the fact that I drew a trace right through the middle of the impedance selector switch on this second schematic.

The 8.2 Ohm, 10W power resistor came from mouser.com. I had the replacement output jack lying around the shop, so I'm not sure where I got it.
Here's a photo of the result. The resistor is held in with some velcro tape for now. I'll probably use something more secure later.
Attachment:
File comment: Figuring out where to put the power resistor was harder than I expected.
twisted.jpg
twisted.jpg [ 203.31 KiB | Viewed 130 times ]

OK, that's all for now. Have a good weekend.


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