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 Post subject: Need advice in rebuilding a homebrew TRF
PostPosted: Feb Sun 18, 2018 2:21 am 
Member

Joined: Aug Fri 21, 2009 7:45 pm
Posts: 785
Location: Port Dover, Ontario
A while back I purchased a homebrew TRF which was a bit of a mess underneath and I would like to try to rebuild it.

The tube lineup is 25Z5, 6C6, 6D6, 43 and a ballast tube.

I found that the electrodynamic speaker has an open field coil and I hope to replace this with a PM speaker.

I happen to have a NOS Hammond output transformer and the spec sheet with it says that the primary has a 2500 ohm impedance and the secondary is 3.2 ohms.

Would this be suitable to use with the 43 output tube? I have some small 8 ohm speakers and wondered if one of these would be OK to use.

Thank you for your advice and help.

Joseph


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 Post subject: Re: Need advice in rebuilding a homebrew TRF
PostPosted: Feb Sun 18, 2018 6:07 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Jackson, TN
Hi Joseph,

The output transformer you mentioned should work ok for the 43 tube and an 8 ohm speaker. That tube likes 5k ohm load and that is what your transformer will present with an 8 ohm speaker on the secondary.

You'll want to review the circuit to be sure that the speaker field was not important to the plate power supply. As you know, also replace filter caps and any wax type caps.

Tim


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 Post subject: Re: Need advice in rebuilding a homebrew TRF
PostPosted: Feb Sun 18, 2018 4:11 pm 
Member

Joined: Aug Fri 21, 2009 7:45 pm
Posts: 785
Location: Port Dover, Ontario
Hello Tim,

Thank you for the reply. Very helpful!

I plan to strip the set down to the tube sockets and use one of the schematics for a TRF set using these tubes from the net.

There are a couple of hand wound coils in the set and I hope to save these. They have continuity as far as I can tell at the moment.

I may have to put in a new volume/on-off control as the switch part seems to be inoperable.

Someone hand built a nice looking cabinet to go with the chassis so I hope to get this thing up and running again.

Thank you again.

Joseph


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