Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on June 07, 2011, 12:10:46 pm
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After a build, I have more "hum" than is acceptable. I have tried everything I can think of in the way of lead dress, star, buss, elevated, center tap, virtual, etc. etc. I can not get the hum any better than it is. Just to rule out filtering, what would be the best route to take.?
Can I just parallel and additional cap to the first filter after the rectifier.? For trouble shooting purposes, if you guys wanted to add more filtering to this circuit, and it was already built, where would you do it.?
Thank You
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The octal 6SL7GT isn't humbucking - no center heater tap - and is thus much more susceptible to hum than common noval twin triodes. Noise is OK at the PI level, but can be a problem for low level stages.
You can modify the heater circuit and use well-filtered DC for V1. V2 and V3 are less critical, but you may also use DC there.
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The octal 6SL7GT isn't humbucking - no center heater tap - and is thus much more susceptible to hum than common noval twin triodes. Noise is OK at the PI level, but can be a problem for low level stages.
You can modify the heater circuit and use well-filtered DC for V1. V2 and V3 are less critical, but you may also use DC there.
It does seem like the noise is much worse at V1.......
I checked all my old Wall Warts, but the closest I have is 5 VDC. Is there a Fender or Marshall schematic that switches to DC down at V1.? Just wondering how they rectify the 6 VAC. Is there a schematic I can look at, maybe in the Hoffman Files somewhere.?
Thank You
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How close are your trannies to each other and/or other components? (just trying to rule that out too cause the other stuff won't matter much if this is an issue)
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How close are your trannies to each other and/or other components? (just trying to rule that out too cause the other stuff won't matter much if this is an issue)
The trannies are pretty close, but not out of the ordinary. I could post pictures, but this is a conversion of an old PA head, so I cannot do much about the layout. I thought filtering might be an easy thing to rule out real quick. I don't know, I am kind of at a loss at this point. The hum gets worse as the volume and tone pots increase.
Thanks
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With the 5Y3 there's a limit to how big the first filter cap can be. Data sheet says 20uF. So don't just add another cap parallel. Does it hum bad with first tube removed?
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I can help with the dc rec. part.
Bill
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jeff -
Let me pull tubes again and I will tell you if it is relieved with V1.
Tone Junkie -
Thanks for the heater rectifier schem. Pardon my ignorance, but is I.C.1 some type of voltage regulation.? Is that a part that Radio Shack might have, or is it more of a Mouser kind of thing.?
Thank You
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In my experience hum is usally a ground issue.
Also are your heater wires phycisally close to any other wires?
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Let's be sure this is heater hum which should be 60hz. Is the hum around the tone of the lowest Bb on the guitar or lower? If close to the Bb, it's 120hz and power supply/grounding.
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A quick way to figure out whether the noise is heater related is to disconnect the heaters from the PT and use a 6 Volt lantern battery.
jeff is correct about the data sheets for a maximum "reservoir" cap with a 5Y3GT, but there are thousands of Tweed Princetons with two 16uf caps in parallel right after a 5Y3 (effectively 32uf). With a push-pull poweramp, adding more filtering with a parallel cap to the second, screen grid node on the power rail might be more beneficial though.
With this particular circuit, it would be ideal to add another dropping resistor and filter cap so that the two halves of the 6SL7 have their own node on the power rail instead of sharing the same node as the phase inverter. Do you have room to do that?
Good luck,
Chip
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First, a confession..... I Do Not Play Guitar....
I know that must seem crazy to you guys, but it happens. Before all my surgeries, I played drums. Seemed like every guitar player I knew, either had no interest, and/or no ability to fix their amps. I have a general interest in all things electro-mechanical (not saying I am good at it) so I fell into amp "repair" by default.
With that in mind, I am not sure what Bb is. We have guitars and I know what string is the "Low B" but after that I am lost. So, would 60Hz be lower than the B string played open.?
Not saying I know more than any tube manual, or guys with electronics degrees.....just saying that I have built, repaired, observed, A Lot of amps using a 5Y3 that had a 42-47 reservoir cap and did not seem to have any problems after many years. But I know my RCA book says a max of 20 mics I believe.
Yes, I have room to add another drop resistor and filter cap. I do not know how to figure what value that drop resistor should be. Do you have a value in mind or know how I should calculate it.?
I have a power supply that will do 6VDC at 2 Amps. I am not sure what these octals draw for heaters, but I think I will be fine for a test.
Should I go ahead and add a parallel cap to the screen supply, or do you think I should wait on some of this other stuff.?
Sorry For All The Questions....
Thanks
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J, the B flat is marked on there for you. I play, but don't read music, so I don't know where most of the notes are either!
As far as the extra filter cap, the only firsthand experience I have is on my old Harmony. The schem calls for a 40 uf right after the 5y3, and it's got a (I think) 47 in it right now. And the 5y3 in there is a NOS, not the Sovtek.
Of course, it's that way because I didn't know any better when I was putting stuff in :dontknow: but it's still doing good, I play it a lot. Mine has developed a hum as well, not too bad, but I want to work on reducing that too. I think a better grounding scheme and better placement of heater wires will be the first thing I do to mine.
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Figure out what is humming first... Clarification for non-guitar player... Bb is on the lowest (fattest) string... 6 frets up from the headstock. :icon_biggrin:
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Dho! I thought that was A#.
Brad :laugh:
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That be the 6th fret on the 6th string. :-)
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> I am not sure what Bb is.
Apologies if you know this, OR if I screw it up.
When playing a single song in simple melody, we use a sub-set of 8 notes per octave. "Do Re Mi...". When we consider all melodies in many keys, we come up with 12 note-centers per octave. An unholy simplification gives 12 notes per octave. But we "name" them with 8 letters. The "other" notes are "sharp" (#) or "flat" (b) of the nearest lettered note.
In the USA, Canada, and other 60 Hertz lands:
Hum is NOT on the musical scale based on A=440Hz. It is near the lowest B-flat on piano, but a little lower. B-flat is also A-sharp. (Bb A#). To a musician, hum is "a flat B-flat". (I do not know why it isn't a flat A-sharp.... it is, but we don't say it that way.)
Pure hum is a pure mellow tone.
120 Hz buzz is also flat-Bb but an octave higher. Buzz usually has overtones. It is in fact hum run through an odd "fuzz box" then (usually) the treble rolled-off.
Guitar starts at 82Hz (standard tuning).
A guitar-amp speaker is typically NO bigger than needed for guitar, and has POOR performance below 80Hz. I have known amps with significant hum going to the speaker, yet the hum was inaudible a few feet away.
You can't "match" 60Hz hum with a standard-tuning guitar.... it would need about another foot on the low string. You can almost-match 120Hz; as they say, near the 6th fret on the low string.
Because guitar speakers have such poor 60Hz performance, and because the ear hears octave (1:2) intervals as "same flavor", it can be mighty hard to tell 60 from 120.
Do any of the knobs affect the hum/buzz?
CAREFULLY chopstick the heater wires while live: does hum/buzz change?
When you turn OFF, does the hum/buzz quit instantly or fade?
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Is your main filter cap returned correctly? To the PT, _NOT_ round-about through the preamp?
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Here it is my friend im not much help with any thing to technical but ive had to find these before.
http://www.allelectronics.com/index.php?page=search&search_query=7806&x=43&y=6 (http://www.allelectronics.com/index.php?page=search&search_query=7806&x=43&y=6)
Bill
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T J....thanks for the parts link.
Trying to get back on track......
I cannot play with lead dress and get any relief from this hum. There are no wires I can move that make it better or worse.
The Volume and Tone knob both have an effect on the hum.
I think I have my grounding scheme pretty well worked out, but maybe not......?
I have tried all kinds of different ground schemes with the pots, and nothing seems to be better or worse.
The hum is not really noticeable when playing, but you could never record with this head. It just seems like something must be "wrong" if the knobs interact with the noise.....
I pulled the heater wires from the PT. I connected a 6VDC PS to different points along the heater line, but the noise was worse with hooking up the heaters to the DC PS.
Perhaps my next step is to add filtering for V1. Sorry if I missed it, but what value would I use for the dropping resistor for that extra filter cap.?
Thanks
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Can I just parallel and additional cap to the first filter after the rectifier.? For trouble shooting purposes, if you guys wanted to add more filtering to this circuit, and it was already built, where would you do it.?
Thank You
Haven't worked with PP amps yet. I have single ended amps (several converted phonos & scratch builds). I have 22, 33,47 & 100 MF 450V caps with clips (insulated) on each end that I can parallel with any filter stage. Just takes seconds to simply clip them on (parallel) to the existing caps and see if it helps. If you want to try a big on the 5y3 and are worried about blowing it, you can clip it in and bring the amp up on a variac so the big cap charges slowly. If a giant cap on the 5y3 is the only thing that does it, you can replace the 5y3 with ss, use the giant cap and see how you like the sound. If you have sag, it will sound different.
Make sure the neg is to ground. Also, it is way safer & less sparky to unplug the amp & discharge the caps before clipping in the new ones.
The scratch builds are star grounded and always quieter than the conversions.