Powering up, here we go(100 watt plexi clone)

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aerojazz
Senior tube assistant
Posts: 160
(3/6/04 1:30 pm)
  Powering up, here we go(100 watt plexi clone)
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 Just finished my second amp.(100 watt Superlead), I decided to actually test this one before I put tubes in it.

I'm a little concerened, I'm bringing it up slowly(no tubes in) and just at a little past half way ( 70 volts ac) , I'm seeing 615 volts DC plus on the B+ at the second set of fiter caps. The first set seems to be deviding it. If these are 500 volt caps why aren't they blowing up.

Everything looks good except the voltages seem high to me.

Any guidance please




 

Edited by: aerojazz at: 3/6/04 3:09 pm
 
aerojazz
Senior tube assistant
Posts: 161
(3/6/04 2:26 pm)
  Re: Powering up, here we go(100 watt plexi clone)
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 I slowly brought the variac up to 110 volts, The B+ comming out of the + side of the bridge Rect is 945 volts.
This is twice what I thought I'd get. Is this normal?

EDIT:
I placed the tubes in and things look alot more normal. Still have 750 or so volts B+ at the bridge rect. but all tubes and caps seem to have fallen in line. Since I had to wire my own bias voltage I placed the standby switch AFTER the bridge rect. I assumed that instead of using all four terminals(standard plexi toggle) I would only want to use two of them. Am I correct here?
To make an already long story short, when I'm on standby all voltages seem ok, but 10 to 15 seconds after flipping off standby the main 4 amp fuse blows.
The first thing that come to mind is the standby switch.

Should I be using it to limit BOTH pos and grnd. of B+ ?


Edited by: aerojazz at: 3/6/04 6:44 pm
 
GroundhogKen
Forum Moderator
Posts: 2310
(3/6/04 4:37 pm)
  Re: Powering up, here we go(100 watt plexi clone)
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 What are your transformer specifications?

A bridge recitifier will give twice the B+ of a full wave rectifier with the same transformer.

Example:

B+ = ( 1.4 ) * 300 for a 300-0-300 Vrms secondary with full wave rectification.


B+ = ( 2.8 ) * 300 for a 300-0-300 Vrms secondary with BRIDGE rectification ( and non-grounded center tap) .


Ken
 
aerojazz
Senior tube assistant
Posts: 162
(3/6/04 4:45 pm)
  Re: Powering up, here we go(100 watt plexi clone)
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 I'll have to give a call to MM. I don'r think there web site has that info.

Did Marhall ever make a 100 watt plexi that didnt use a bridge rectifier?


Edited by: aerojazz at: 3/6/04 5:02 pm
 
Lucid Alice
Senior tube assistant
Posts: 176
(3/8/04 7:08 pm)
  Re: Powering up, here we go(100 watt plexi clone)
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 Don't take my word for it, but I understand that the 100 watt uses a PT with half the secondary voltage of the 50 watt PT. The bridge rectifier doubles the DC output.

I've never seen a 100 watt schematic without a bridge rectifier. No doubt it's because it's cheaper to build a PT with fewer windings of heavier gauge wire to get the current up, and the bridge isn't much more cost than some good diodes.




 
 
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