Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 17, 2005, 10:05:58 pm
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Hoffman Amplifiers
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> Tranny center tap
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aerojazz
Senior tube assistant
Posts: 169
(3/9/04 9:24 am)
Tranny center tap
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My Plexi 100 is up and running, with a little help from my Friends.
I need to understand why I should bolt the CT of the pwr tranny to ground rather than to the junction of the two main filter caps though.
I seem to have learning dissability when it comes to what trannies do with voltages and currents.
any reading or instruction appreciated.
GroundhogKen
Forum Moderator
Posts: 2313
(3/9/04 10:14 am)
Re: Tranny center tap
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OK, let's wrap this up. I have a few questions to get everybody up to speed with this one amp.
What is the make and model of the power transformer?
Are you using a bridge rectifier?
Does the transformer have a bias winding?
Where is the standby switch?
Ken
aerojazz
Senior tube assistant
Posts: 170
(3/9/04 2:36 pm)
Re: Tranny center tap
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(Marshall 1959 schematic)
The PT is a Mercury Magnetics P100JM, thier Plexi 100 model(No bias windings).
I devised my own bias via a capacitor off of one of the HV secondaries. (it's working like a charm thanks to you guys)
I am using a bridge rect with no ground comming off the neg. terminal pin.(elimination of grnd. is when things started comming together)
I placed the plexi style standby switch imediately AFTER the bridge (B+ pin), I'm only using the switch to effect the B+ line.
I did try hooking the PT CT to the junction of the two filtercaps as per schematic and it seemed to work, though it did bring the voltage down a little at B+.
I didn't get to test a guitar through it at that point though, so I don't know if it actually worked.
Currently I'm seeing about 490 volt at the B+ pin of bridge rect. I had about 470 with the CT at fitercaps.
EDIT:
I want to also mention that while having the CT at the caps even though the B+ went lower(for sure), I seem to remember slightly HIGER voltages at plates of preamp tubes.
I will be happy to document voltages with both scenarios if you guys think it's safe for the circuit to do so.
Thanks
Edited by: aerojazz at: 3/9/04 5:18 pm
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