Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 03, 2011, 01:52:42 pm
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Yesterday at a Ham Fest I got for € 5.00 part of an old Television Transmitting equipment
it was an unit that guaranteed the audio signal to be always "on the air"
(two copies of the audio from to different channels - the circuit was an automatic switch to put "on the air" the second copy of the audio if the first audio signal fail)
there were only 4 pieces I can retrieve, 2 nice relays for audio commutation and two audio transformers (A /cs 1 m W)
the transformers has
Primary: R 2 x 420 ohm (Z 5 - 20 Kohm)
Secondary: R 2 x 1200 ohm (Z 12.5 - 50 Kohm)
I've find also a schematic that use this transformer as input (you can see attached)
What can I do with this ?
are this fine for the Butterylicious microphone preamplifier ?
Waiting for your councils and indications
Thanks
Kagliostro
p.s.: the A/1394 transformer has same features of the A /cs 1 m W only different envelope
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I've find the exact data sheet
Nobody knows how to use this transformers ?
Kagliostro
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This may help: http://www.jensen-transformers.com/an/an002.pdf
If it does, then please explain it to me! :laugh:
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I believe it's a 1:2.5 interstage transformer. Wired fully differentially, Zin is 20k and Zout is 50k. If you parallel the primaries and secondaries Zin is 5k and Zout is 12.5k. Now if you parallel the primaries and series the secondaries, you can get a 1:10 ratio. Wired like this it might work as a microphone input transformer. Don't hold me to this because I still get confused by these little gems.
-Richard
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Mic input (as Richard said) or interstage - you can use them as the phase splitter by grounding the CT of the secondary.
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looking at the schematic it is a balanced or unbalanced (depending on primary strap) input to grid input transformer - the amp is full balanced. maybe it's a organ amp or used in pro-sound? sensitivity is decent - 600mV to make 20W - not quite modern guitar amp.
not bad input power handling specs +18dBm and respectable distortion numbers. it is manufactured with mumetal laminations - if my limited italian translation is correct, i guess that would mean they are high nickle content laminations.
depending on how you wired the primaries, it could make a input transformer for active balanced/unbalanced bass pickup system. and/or use as a PI. as per schematic kaglio attached.
for all you guys who want to build a lower powered P-P amp with all those 12AU7's you've got stashed away - the schematic kaglio posted might not be a bad place to start. scrap input tranny, add another 12AU7 use half for grounded cathode amp input stage, use the the second half as a concertina PI. kind-of-a-bastardized williamson ckt.
clever low-volt PS in that amp - makes DC for 12AU7 filaments, and since it's floating, does double duty as the (-) bias for the EL34s. EL34 bias point was chosen just so that would work. the downside is the scheme would make things difficult to modify to make more power.
the B+ supply is equally intriguing... the values are not something usually seen - those C and L values sure looks like they are making a ferroresonant PS,. that does not seem like the case here - IIRC, you needed a separate winding to build the resonance tank for ferroresonant PS. some expert guidance on how that PS ticks with those values would be appreciated...
--DL
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Thanks DummyLoad
it is manufactured with mumetal laminations - if my limited italian translation is correct, i guess that would mean they are high nickle content laminations.
not exactly, it is shielded with mumetal (the same metal used to shield the CRT in Oscilloscopes)
the amp is full balanced. maybe it's a organ amp or used in pro-sound?
sure not an organ amp i think it was designed to be a pro-amp
What about a full balanced mic preamp ? ......... Uhm.... :w2: ........... may be not, because in balanced configuration the input is not adequate to a mic impedance, isn't it ?
Many thanks to all for the given info
Kagliostro