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Small Tube Amp with Hum
Small Tube Amp with Hum
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Old 6th June 2010, 04:54 AM   #1
pchor is offline pchor  Canada
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Join Date: Jun 2010
Default Small Tube Amp with Hum

I recently acquired a small tube amp that Im trying to get to working condition. It is an old Pine/Pepco, the only markings on it are
"Made in Canada" and a paper at the bottom of the cab that shows all 3 tubes and says model 801.

The only reference to anything similar is on this page, but mine looks different then the one pictured, but the specs seem to be the same, its about halfway down the page:
Canadian Amps

On that website it states:

"Arc Guitar Amp. Model 801. Made in Canada in the 70's. This has an 8 inch spk. and was made by Pine Electronics. CSA # LR19952. "

The 3 tubes in the amp are 12AX7, 50C5, 35W4.

I have used Isolation transformer upgrade for old guitar amps
To add an isolation transformer as it originally didnt have a power transformer and it was 2 prong, which Ive converted to 3 now.

The amp sounds great, but the only problem is that there is a constant hum coming out of it, it gets quieter as I lower the volume and louder as I raise it. With the volume at 0 there is still a faint humming sound coming out of it.

So far, I have tried a different 12AX7 tube as I have a few from my Epi Valve Junior Amp, but this didnt change anything. I dont have replacements for the other 2 tubes to try, but will probably end up getting some if I cant find anything else wrong with it.

I have tried jumpering an extra capacitor over top the 2 big 47UF caps but that didnt do anything.

Also, of note is that adding the iso transformer did reduce the hum a bit, its not as loud, Id say it reduced it by 30% or so.

I cant find a schematic for the amp anywhere, the closest I have found is on step 5 of the instructables link above, but it uses a 12AU6 tube instead of 12AX7, the rest seems similar to mine.

I will get some pics of the amplifier tommorow if it will help, also if there is any other information that could help please let me know.

What would be the next steps to try? Ive read that star grounding helps with hum, should I try that? Or is there something else simple I can try that will help me find the faulty part/wiring?

Thank you!
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Old 6th June 2010, 05:53 AM   #2
bob91343 is offline bob91343  United States
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The hum may be because the filter capacitor needs replacing. However, this is a kluge and that isolation transformer modification is suspect. Especially when he says the transformer limits the current.

The 50C5 can put out about one Watt. The power supply is only about 100V. There are some tests you can run to see what might be causing the hum. You can pull the 12AX7 while you hear the hum and if it goes away, it's probably caused by the input circuit. I mean, if it goes away instantly. Of course it will go dead in a few seconds with that tube out. There are some other tests but let's see how far you get with this.
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Old 6th June 2010, 06:07 AM   #3
planet10 is offline planet10  Canada
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Small Tube Amp with Hum
With an amp that old, it is regular practice to replace all the electrolytic capacitors at the minimum.

community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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Old 6th June 2010, 01:49 PM   #4
SGregory is offline SGregory  United States
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I agree. if a cap is starting to go bad, the will not behave as originally intended which can result in hum or other more interesting effects.
Living Life Doing the Waltz in 4/4 meter.
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Old 6th June 2010, 02:24 PM   #5
Trout is offline Trout  United States
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Location: Midwest Madman

What did you use for the isolation transformer? I have a similar low power 3 tube amp in need of one.
Right now it is in need of a re-cap, but I want to find a cheap(low cost) isolation trans, heck the amp only cost me like $5.00.

I was thinking about a Triad N68-X or Triad N-77U but not sure if it will hold up.
My amp uses a 60FX5, 18GD6A and a 36AM3 rectifier.
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Old 6th June 2010, 04:21 PM   #6
Frank Berry is offline Frank Berry  United States
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You may need to put a shield over the 12AX7 tube.
The isolation transformer might be radiating hum into the amplifier.
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Old 6th June 2010, 04:49 PM   #7
pchor is offline pchor  Canada
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Join Date: Jun 2010
Thanks for all the replies.

The previous owner said he had the caps replaced, and they do look new to me. He also said he got all new tubes, which they do all look like they are newer, they are all westinghouse with a manufactured date of 1999.

The isolation transformer didnt introduce the hum, the hum was there to begin with, but with the transformer it does seem like the hum was reduced a little.

The isolation transformer I used is the one from the instructables, which is the Triad N68-X, which I got from mouser.com for $12 plus shipping.

Here are a few pictures of the amp:
Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old 6th June 2010, 05:53 PM   #8
bob91343 is offline bob91343  United States
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Ah so now the picture is more clear. Is there hum with nothing plugged into the input? Guitar amplifiers are always plagued with hum and there can be several causes. Since the gain must be high, only a trace of power line noise can result in significantly audible hum.

If there is no hum with the guitar unplugged, the problem is with the pickup in the guitar. If the hum is there with no guitar, it's a matter of pickup from the power line to the input. This is often caused by a poor grounding system in the amplifier. The specific point of connection of the filter capacitor is a very important issue. In your unit, the capacitors are going to a ground lug, not a good idea, unless the input power is also going to that lug, which it isn't. So you have ripple current going around the chassis, ripe for pickup by the high gain input tube.
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Old 6th June 2010, 06:09 PM   #9
pchor is offline pchor  Canada
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Join Date: Jun 2010

I think ive taken a large step back with this thing at this point With the isolation transformer and bridge diode that was added according to the instructables; The amp was playable but with major hum. I decided to remove all the additions and rewired it stock for simplicities sake. Now it really hums loudly, very loud with the volume turned up. The worst news is that with a guitar plugged in I am getting no output of the guitar signal. The hum is the same with both the guitar plugged in and not plugged in.

I did write down the original wiring before I changed anything so I believe it is stock according to the picture in my last post. That picture is with the wiring currently, which is stock. The only addition is the 3 prong power cord which has the middle prong grounded to the metal chassis.

Im guessing I actually did rewire something incorrectly, maybe I got the black and white wires backwards to AC power?

I went ahead and removed the 12AX7 tube while the amp was running and I heard a click and the amp went silent instantly.

The two large black capacitors do have a connection on that pink terminal board, but there is no connection to ground there. It is using the terminal next to the one that is connected to ground. There is also black wire going from that point to the left to the blue capacitor.

So at this point I think I will re-add the isolation transformer and see if itll run again like it did before. Unless there is something else that is clearly wired wrong that can be seen in the pic above that I can try first?

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Old 6th June 2010, 07:47 PM   #10
pchor is offline pchor  Canada
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Join Date: Jun 2010
I did reverse the black and white, after changing them the amp works again.

Ive reattached the iso transformer and rectifier bridge and the amp is working ok. Its actually bareable now compared to the humming it did when it was wired backwards

I think Ill leave it be for now and play it as is. As long as I keep the volume below about half its fine, I will just use a booster pedal if I need to go louder.

Later on Ill research some more on star grounding or other things to try and eliminate it completely. I think im just too used to how silent my Epiphone Valve Junior is compared to it. This one is 30-40 years older so its ok

Thanks for the help everyone. I will check back here to see if anyone has any other ideas for me to look at.
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