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Topic: Sano amp hum problem...is it layout? Choke? |
Steve Waltz
From:
USA
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Posted 2 Apr 2018 5:34 pm |
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I rebuilt this amp a while ago with all new caps, many new resistors, tubes . It sounds great overall other than a hum or buzz that I can't get rid of. This amp has the power amp section on the bottom and the on/off switch, volume, tone controls, input, and tremolo all on top of the amp.
Today I noticed that the filament wires aren!t twisted and it's AC heaters. Maybe you can see in the picture. There are so many wires running up to the top, I was thinking of converting heaters to D.C. With four 3amp ultra fast diodes in a rectifier bridge with standard caps for filtering and whatever resistor to get voltages correct. There is room and the coast is low. Seems like getting more ac out of it might help? There is also room for a choke after the rectifier and the shop down from me has NOS Chicago Choke that will work and its less than $20.
I would think extra filtering wouldn't hurt and neither would D.C. Filaments.
Any ideas? Thanks for any help.
Steve
if this helps, in the under side photo, what looks like a tube base on the Lower left side is connection to the upper part of the amp. The next one over to the right is also going out and that is humbalace pot below and between them, and the kind of rca is to the tone controls and all the way to the right is speaker out.
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J Fletcher
From:
London,Ont,Canada
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Posted 6 Apr 2018 6:19 am
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Hi Steve
Were all the filter caps replaced ? Can't tell from your pics.
Did it hum prior to being rebuilt? |
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Steve Waltz
From:
USA
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Posted 6 Apr 2018 9:28 am
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All caps have been changed.
Yesterday, I changed the filament wiring out to twisted pair. They were separate wires next to the chassis. It helped a little.
When I put my hand near the 12Sn7 and 12SL7 tubes there is a change in noise.
I'm wondering about those cables running from the amp to the control panel and possible noise. That grey bunch gets two wires from the plug to go up to the fuse/pilot light/ on switch......then back down to the Transformer while sitting next to filament wires in the same bunch.
The amp had hum before and after cap replacement.
Last edited by Steve Waltz on 6 Apr 2018 10:39 am; edited 1 time in total |
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Bill Sinclair
From:
Waynesboro, PA, USA
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Posted 6 Apr 2018 10:33 am
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| Octal preamp tubes have a reputation for being microphonic. I wonder if they might be amplifying transformer lamination buzz in the amp chassis? Is it a 60Hz, 120Hz or some other frequency hum? |
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Steve Waltz
From:
USA
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Posted 6 Apr 2018 10:47 am
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The transformer doesn't seem to have any mechanical type buzz. The octal tubes that seem to have a change in noise when I put my hand near them are up on top away from the transformer.
I also tried other tubes and it didn't seem to help. |
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Paul Arntson
From:
Washington, USA
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Posted 7 Apr 2018 9:43 am
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Steve, do 4 wires go to the speaker? Or is it a permanent magnet type?
_________________
Excel D10 8&4, Supro 8, Regal resonator, Peavey Powerslide, homemade lap 12(a work in progress) |
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Steve Waltz
From:
USA
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Posted 7 Apr 2018 11:05 am
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| Two wires go to the speaker. |
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Stefan Saal
From:
New Hampshire, USA
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Posted 7 Apr 2018 8:10 pm Sano amp
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I have a very, very similar Sano amp. The serial number on my power unit is 2833; power amp is identical to yours. It is also stenciled with a large 30 (yours seems steel-wooled in that spot). Same 2x el34s. My preamp (also up above) has 3x 12au7 tubes. Also has the Utah PA speaker (which i think is rather good, by the way, though it is considered a mid-quality item by some).
My amp has always had a slight hum. Does not drive me crazy. My brother, more of the electrical aficionado than I am, says that fitting a grounded plug properly to the amp would quiet down the hum a bit. Have you tried that?
I cannot see all your controls, but mine has a stereophonic, as well as guitar, and instrument inputs (all with individual gains), plus tremelo and tape output. Mine has no brand (yours says Giulietti) but I think mine is Sano-made for sure.
I love my amp, by the way...a truly great sound with lap steel. |
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Matthew Dawson
From:
Portland Oregon, USA
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Posted 8 Apr 2018 10:53 am
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| Pull each tube starting with the one closest to in the signal chain to the input jack and moving down the signal path towards the output tubes. See if/when the hum goes away. This may help you find where the hum is being introduced. If you find a stage where the hum is being introduced ground the input grids one at a time and see if that stops the hum. That will get you even closer. Check your heater center tap and maybe try adding an artificial one or hum balance pot. To check if you need more filtering get a 47 uF cap or so and clip it into the various nodes with test leads and see if that makes the hum go away. Be careful not to get shocked! Clean the jacks. That's probably where I would start and I hope that helps. Intuition tells me adding a choke or DC heaters is a little overkill in this situation. It's possible to "over-repair" a perfectly good vintage amp IMHO. |
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Steve Waltz
From:
USA
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Posted 8 Apr 2018 8:26 pm
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Stefan, what is your rectifier? Strange how close they are an yet you preamp tubes are 9 pin.
Matthew, thanks for the detailed suggestions. I'll try those and get back. So far pulling individual tubes has no effect. I tried a 5ar4 and there was a difference but I wasn't sure if that was smart swap so I'm checking on it now.
Steve |
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Carl Mesrobian
From:
Salem, Massachusetts, USA
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Posted 9 Apr 2018 6:53 am
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Are the signal wires at right angle to heater wires?
I'm not familiar with this amp. Is there a DC bias supply that might be injecting AC? Are the signals running through the harness next to other AC? Does the level of hum change with a change in controls?
_________________
--carl
"The better it gets, the fewer of us know it." Ray Brown
Last edited by Carl Mesrobian on 9 Apr 2018 6:56 am; edited 1 time in total |
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Stefan Saal
From:
New Hampshire, USA
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Posted 9 Apr 2018 6:54 am
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Steve, as you can tell, I am no expert, but it just so happens that my good friend Howard Rose, of Commercial Audio Systems in Chicopee, Mass., has my Sano in for cleaning while I am traveling out west. I wrote to him last night about your thread, and this is what he wrote back to me this morning:
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Stefan,
Undocumented engineering changes are commonplace with tube amplification.
For example, I have seen at least three variations of the Ampeg Jet from the 1960's; different preamp tubes, different power supply, different output tubes, but still sold as a Jet.
The model in the pictures shows a preamp section with "6" series tubes (6SN7, 6SJ7, etc). This refers to 6 volts on the heaters; tube used an octal or 8 pin tube socket. These were replaced with "12" series tubes beginning in the late 1950's (12AX7, 12AU7, etc.). This series uses a nine pin tube socket. I think your model is a later version of the 30WRT/50WRT series.
As to the power supply question, I get the impression that this person is thinking about changing his type of AC to DC conversion from a tube (yours has a 5U4GB) to solid state using either 2 diodes(half wave configuration) or 4 diodes (full wave rectifier).
The advantage to the diode approach is that it produces a higher DC voltage for a similar AC input, and generate less heat. Also, solid state rectifiers don't require the dedicated AC power supply secondary. A rectifier tube needs 5 volts AC.
Bottom line, these are slightly different amps built on a similar platform. Happens all the time.
In terms of the hum, several factors are possible:
1. Failing filter capacitors; meant to smooth the rectified DC power and eliminate "ripple".
2. Poor lead dress. Running AC signal wires near DC power causes all kinds of issues.
3. Failing power transformer. Over time, the metal plates that make up the core of a power transformers can begin to separate and contribute a 120hz "tone" to the amplified signal (sometimes referred to as a "singing transformer")
4. Tube failure is a possibility, but more often they get "microphonic" and produce squealing noises.
5. Bad circuit design. Yes, it happens.
6. Dirty connections. Tube sockets, input jacks, mating connectors between preamp and power amp sections. Anywhere that two conductors meet and have to pass on electrical signal is a potential noise source.
More later, I'll be digging into the amp later today or tomorrow morning.
HR |
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