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tubenit
Junior tube assistant
Posts: 16
(2/18/04 11:35 am)
hum resolved
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*see bottom of 2nd page To all who help me thanks!
I tried using Doug's fullwave bridge rectifier schematic and the hum did not change. I noticed that OTHER schematics also have a ground wire with resistor coming off the B+ side node. The other schematics show that as intregal current flow in making it a full (vs. half wave). I am a little nervous to try a wire to ground off the node which has an OT wire and output tube connected to it?! Sparks and smoke make me a little nervous and I don't know if that would happen.
Is it correct that I need to add a wire and resistor to ground off that node? If so, what resistor size would you suggest?
The node on the far left is going to ground which has the negative of the filter caps attached.
The diodes are headed/placed properly. Currently I have 2 40uf caps by the rectifier and 2 100uf caps connected to preamp circuit. I am using a PT with NO center tap.
Please advise and thanks!.
Edited by: tubenit at: 2/21/04 11:08 pm
GroundhogKen
Forum Moderator
Posts: 2280
(2/18/04 11:45 am)
Re: fullwavebridge rectifier Q
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A full wave BRIDGE ( four diode ) rectifier will not hum any less than a regular ( two diode ) full wave rectifier.
Can you post links to any of the schematic that you are talking about?
A resistor ( usually 220K or bigger) from B+ to ground does nothing except to bleed off a little bit of power or to dicharge the caps when the amp is off.
Ken
tubenit
Junior tube assistant
Posts: 17
(2/18/04 12:03 pm)
full wave bridge rectifier Q
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The schematic I looked at is:
www.infodotinc.com/neets/book7/27c.htmThe schematic looks like on the B+ node there is both the wire to B+ and one to ground with a resistor?
I have been using a Weber Copper Cap. Thanks!
EDIT: fixed link
Ken
Edited by: GroundhogKen at: 2/18/04 2:06 pm
GroundhogKen
Forum Moderator
Posts: 2282
(2/18/04 2:11 pm)
Re: full wave bridge rectifier Q
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The resistor ( RL ) in that circuit doesn't do anything. It merely represents the load placed on the rectifier--( ie. It represents the rest of the amp. )
Ken
tubenit
Junior tube assistant
Posts: 18
(2/18/04 4:52 pm)
Re: full wave bridge rectifier Q
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Thanks for the explanation! Is it reasonable to expect that a full wave bridge rectifier would have less hum (needing filtering) than a Weber Copper Cap in a half bridge mode?
Maybe I just need to add more filtering and boost the uf's from 40 to 100?
Again, I have 40 uf caps by the rectifier and 100 uf caps on the preamp. I tried switching the caps around and this seemed slightly less hum. Any suggestions? The hum remains the same regardless of whether a guitar is plugged in or whether the volume on the amp is turned down or way up. There is no increase in hum with the volume up.
GroundhogKen
Forum Moderator
Posts: 2283
(2/18/04 5:06 pm)
Re: full wave bridge rectifier Q
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You can try more filtering but...
I would think 40uF would be sufficient to have a quietly idling amp. You may need more filtering if the bass is mushy when you are wailing through the amp.
Sources of hum at idle are:
poor grounding,
under biased tubes ( too hot ),
old or leaky electrolytic filter caps,
messy lead dress,
misguided component ( transformer ) placement.
What kind of amp is this?
Ken
tubenit
Junior tube assistant
Posts: 19
(2/18/04 7:31 pm)
Re: full wave bridge rectifier Q
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Ken, I am truly appreciative of the help! Thanks for the interest in responding to my question.
It is the Gibson Scout that I posted (at length) on weeks ago. It has all new caps (filter and otherwise). (I've actually changed out the new filter caps with others thinking maybe that was the issue.) Tried new tubes also. I can't find a ground that I can diagnose as problematic. The solder joints are shiny and fluid in appearance and I've double and triple checked all of that. All caps and resistors replaced match the schematic and the voltages match the schematic very close including bias. It sounds fantastic and comparable to my Princeton Reverb and Vibro-Champ but with more compression and harmonics from the 6AQ5 output tubes. Except my PR and VC are almost dead quiet at idle. It has a Hammond 261-G6 PT, and a Princeton OT and Reverb Trans. Basically been totally rebuilt. It just seems to hum more than I wish and I can not figure out why the hum stays the same regardless of turning the volume and guitar plugged in or not? When I bumped up the filtering it did cut down some hum but not enough for me.
GroundhogKen
Forum Moderator
Posts: 2284
(2/18/04 9:03 pm)
Re: full wave bridge rectifier Q
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Gibson Scout...is it this amp?
www1.korksoft.com/~schem/...a17rvt.pdf
If it is, I would re-wire the heaters. Note that in the schematic one side of the heater winding on the power transformer is grounded. I would disconnect this ground. Connect two 100 ohm resistors--one on each leg--to the heater circuit and then ground the other end of the resistors.
In other words--make an artificial center tap.
Ken
tubenit
Junior tube assistant
Posts: 20
(2/18/04 11:31 pm)
Re: full wave bridge rectifier Q
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I have already rewired the heaters with the resistors making an artificial CT. Good thought though. I switched out the 40uf caps for 100uf caps with no difference in hum. It has got to be a grounding problem. All ground terminals connect with brown wires and ultimately lead back to the filter cap's negative which connects to one of the red PT wires. Everything showed grounded to the chassis...
EXCEPT neither of the output wires off the reverb trans were grounded. I tried grounding one side and it did not change the hum.
Does the fact that the hum is there and does not decrease or increase with the guitar plugged in or the volume on the amp turned up or down give some kind of a clue?
Edited by: tubenit at: 2/19/04 6:58 am
GroundhogKen
Forum Moderator
Posts: 2285
(2/19/04 8:12 am)
Re: full wave bridge rectifier Q
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It means that the hum is likely introduced somewhere after the volume control.
Ken
tubenit
Junior tube assistant
Posts: 21
(2/19/04 1:00 pm)
Re: full wave bridge rectifier Q
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I've made a lengthy list of things to try to resolve this hum beyond the lengthy list I've already tried. I will try pulling tubes one at a time, re-checking again the schematic, re-soldering all grounds, etc. Out of curiousity is there any chance that:
1) the physical proximity/position of the PT on the chassis
can create hum? It is currently at sort of a 30 degree
angle to the verticle/horizontal of the chassis?
2) that a pot even though "working" and not producing
a "scratchy/intermittent" sound could cause a hum OR
3) that my on/off "pot/switch" could cause a hum even
though it "works"
4) The heater wires have the resistors with a CT, but they
also have a ground at the "other end" of the chassis. I
will try removing that ground to see if anything happens.
5) The preamp tube is the only one with voltage different than
the schematic (lower) could that cause the hum? The amp
really sounds great but with hum at idle.
Again, everything I've checked pots, wire, caps, resistors,etc all show continuity with chassis.
I'd be open to some thoughts on any of these considerations?
Thanks!
Edited by: tubenit at: 2/19/04 1:03 pm
GroundhogKen
Forum Moderator
Posts: 2286
(2/19/04 1:09 pm)
Re: full wave bridge rectifier Q
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The 100 ohm resistors are useless if the filament circuit is grounded anywhere else.
Ken
EDIT: also make sure the heater wires are not cris-crossed on the power tubes. Make sure the same wire go to the same pin on each tube--say 4 to 4 and 5 to 5--Not 4 to 5 and 5 to 4.
Edited by: GroundhogKen at: 2/19/04 1:12 pm
ganzonimx
Hey get your own solder
Posts: 395
(2/19/04 1:33 pm)
Re: full wave bridge rectifier Q
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My first built had always a little hum at high volumes, I did - in my opinion - everything to reduce it, but it was still there, even more after the last mod (higher gain in the first stage).
Now I connected the center of the two 100 ohm resistors to to the cathode resistor (cathode side) of the power tubes and the hum has gone 100% away, absolutely NO hum, in the first moment I even thought something has to be wrong as I didn't hear anything when I switched it on and turned up the vol.
I'll do that to all my other amps...
Cris
GroundhogKen
Forum Moderator
Posts: 2287
(2/19/04 1:52 pm)
Re: full wave bridge rectifier Q
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That suggestion is fine--just be certain t
Topic: hum resolved (Read 3821 times)