Reviving a old amp

bogzz

TDPRI Member
Joined
Feb 7, 2004
Posts
7
Location
Rochester,New Hampshire,USA
Hello all
I'm new to the forums. I have an amp that my brother built in the 60s based on a fender design. He was a keyboardist .
I would like to get it working again. I tried changing the 2 lead plug to a 3 lead plug. .All the tubes glow but when I plug in the guitar it just hums real loud.

So I set it aside until I figured thru this forum I might get some advice on getting it back in working order. I so have some electrical/electronical experience and understand the dangers of working on this amp.

thanks
 

jefrs

Doctor of Teleocity
Joined
Nov 20, 2007
Posts
13,331
Location
Newbury, England
Lights on but no-one at home? - try changing the valves.

Bad hum and no sound, usually a bottle. Start with the rectifier then o/p valves. Check HT fuses.

One amp I have has an R and a cap between the two anodes (to kill rf). It shorted and red plated the bottles: bad rasping hum and no sound.

Have you changed the psu capacitors? - if it has been left idle the electrolytics die. Can you test them? - if not then replace. The preferred series and shape of caps have changed since 1960 but values are rather arbitrary, anything in the region 30 to 50 µF will probably do.

Double check your mains 'polarity' and that you have earthed correctly and the power secondary is also earthed at its 0V. Use a star earth point. Hum can be lessened if the transformers are raised off the chassis by a couple of millimetres, not electrically isolated but it breaks electromagnetic induction to a steel chassis (that's why an aluminium chassis is better), a washer will do, ideally a sheet of Paxolin or GRP (blank pc board).
 

shadowfan

Tele-Meister
Joined
Dec 13, 2008
Posts
203
Location
Atlanta, GA US
Check the grounding of the input jack? your cable's ? Just a thought but missing grounds would give similar symptoms.
 




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