Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bruce on November 27, 2016, 08:11:09 pm
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Hey all. Have a fuchs 50 overdrive head,Great amp. No 1 gain - volume pot shaft broke.got part ,installed ,volume works well but I have lots of unwanted noise more gain more noise.. .The two big 6 amp diodes test .5 both directions. .the other 5 close to it tested fine 0 one way .560 other direction..so far so good ....Now I have two little germanium diodes each with its own tianbo relay ..these diodes test.5 both ways +to- and - to + is this test enough to determine it bad ..I can't get a schematic for this amp.. help on bad or not and how to get the rite diode ... any help would be outstanding thanks. Tianbo part no is jr 1-2c l-05v.. have two each with two germanium diodes
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Are you testing these diodes in circuit? You must disconnect one lead of any of those diodes to get a valid reading.
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Have a fuchs 50 overdrive head,Great amp. No 1 gain - volume pot shaft broke.got part ,installed ,volume works well but I have lots of unwanted noise more gain more noise.. .
I'm not sure what your saying.
Is this your amp?
Was it always noisy?
With the new pot it has more noise?
With the new pot it has more gain?
Is the new pot the same taper and resistance as the old pot?
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Yes my amp... was starting to make noise.did a visual under the hood,gain pot was shorting out that was broken.Replaced with pot from fuchs.work perfectly as a pot..but still have to much noise never had before pot problems..the 5 diodes in power supply i test in curcuit test goog the 2 6 amp diode did not test good,current flows both ways ,same with 2 germanium diodes,,.so test these 4 out of circuit ??? If so what about the 5 I think tested good ?? Why test out of circuit I don't get it ...shouldn't it still test 1 way no flow .? This amp is a little over my head with pcb relays a few ic ,not your basic non professional build ,trying to save time amp tec in sf area are backed up months..and a few went belly up. Please Gide me some where I am stuck
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You have to lift one leg on each diode to test it like Sluckey says. Your meter actually puts a small voltage across the diode, which is how it tests good or bad. One way it should read about .5 volts, other way should read infinity/OL or something like that. What I would do, *if it were me* is first build a light bulb current limiter so that after replacing some things you can test it safely without maybe shorting something out. Google it, I think geofex amp page has a good write up on the different tests you can do with it.
All that said, diodes are dirt cheap. Any doubts and I'd just replace 'em all. It'll give you practice soldering :icon_biggrin:
Be aware though! I wound up with a milk house full of amp stuff and old test equipment because my amp tech didn't have time to fix my amp the way I wanted it. And so the addiction began.
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Thanks for responding..Have current limiter light and variac ossioscope ect .this amp is different for me ,pcb tiny parts the pots are miniture.i am old school pop turret board kind of guy ,and over 60 years old. I thought maybe someone out there had this amp and fimilure with the circuit..did the lift to all diodes the two big 6 amp still read both way .ordered today 6 amp diodes ...one question on the chaissis there looks like foil or ground wire on screw hole of chassis can't tell if it went some where.or why it would be there..any thoughts ....I have made 6 amps and repair my own stock but no pro training mostly trial and error...I am one who loves tube amps and made a goal to build and repair my stuff I am retired and like spending my time doing mods and maintaining my amps and stereos that are tube...I just need coaching time to time.... I have a friend that worked at amped in Redwood City for 35 years who was a electronics engineer...but has forgotten most of what I ask of him .. also I am a ham operator KJ6DYW ..I am no pro but want as much nolage as I can .was hoping to learn on these sites but people are very judgmental.nowi have played Bill Graham gigs in sf ca.in early 80 ,I can play guitar well ,but if a friend is just learning guitar I by pas the BS for they can play with me first lesson ..There many short cuts in music and I am sure I don't know electronics like I do music or surfing but want too thanks all
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one question on the chaissis there looks like foil or ground wire on screw hole of chassis can't tell if it went some where.or why it would be there..any thoughts
Post a picture so we can get a better idea of what you are talking about.