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#1 2013-01-27 20:20:22

ca_mccar
New member
Registered: 2011-09-20
Posts: 6

Ampeg B-15NF

Hey Everyone!

I recently purchased 1968 B-15NF. I bought it not working for $250 . The guy I bought it from said it needs an output transformer. Before I go out and buy a new output transformer I thought I would try and troubleshoot. I removed the chassis and found the following things:

- 30uf 600v filter cap was replaced with a 40uf 500v filter cap (should it be 600v??)
- the 40ufx3 Cap Can is stil the original
- the 100uf 100v bias cap was missing and had a piggyback resistor setup there
- the ground switch was unhooked and death cap was removed
- the standby switch path too ground is from the ground side of the 56k resistor ( I believe this is stock)
- all power section resistors meter within in spec

I plugged it in after rebuilding bias section to stock (used 100uf/100v sprague cap). Fuse immediately blew. replaced fuse , hooked the amp to a dim bulb tester. removed the power tubes and turned it on. amp turned on and produced a hum that slowly got louder. yes hum with no power tubes ( i think is a symptom of bad output transformer or bad cap among other things )

I tried to take some voltage readings and noticed there was no high voltage. like 25v on the plates of the preamp tubes. I turned the amp off, removed the rectifier tube and turned it back on. the hum was gone. I measured around 400v ac on each of the red HV wires going to the rectifier socket.

the tubes all look older (westinghouse, jan, etc.)

so I was wondering what my next move should be. Should I recap the filter caps. what do people recommend for the 40ufx3 can. I keep hearing about how bad the CE Manufacturing caps are. I saw somebody who dropped a tag board on top of the filter cap inside and just used axial caps. and then all new tubes and try to fire it up on the dim bulb tester and see what happens.

Any insight would be greatly appreciated!

Thanks!!

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#2 2013-01-27 21:19:03

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Ampeg B-15NF

You could pull the output transformer primary leads and measure their dc resistance.   The results could confirm the output transformer... Though it seems rather likely that is the issue

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