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 Post subject: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 4:09 pm 
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Joined: Dec Fri 28, 2018 7:24 am
Posts: 122
Location: Trabuco Canyon, CA
Hoping that My issue is related to the forums radio knowledge and you all can provide some suggestions on my noise dilemma

My Harmony H303 Guitar amp is quite similar to a radio except simpler with just preAmp, Output, and DC power section.
Its all octal tube with isolation transformer. (very cool)
Great little amp. Here's what I did.
Glued the torn speaker cone with flexible Tacky Glue
Replaced the filter caps C4, 5, 6 and C3 bias cap as they were bad.
Added a three prong.
Dressed the wires. Cleaned pot, chassis, Tube sockets
Checked the coupling caps C1, C2 - they are ok.
Pic and Schematic attached. The edits are my marks. The power supply is wired as I drew it (schematic wrong). The heaters pins of the schematic are also wrong. Measured voltages at about volume = 8

My dilemma is there's still slight amount of soft hum and there is an intermittent scratchy sound like AM radio interference.
No matter what I do cannot isolate it
Tried spare tubes, same sound issues
Remove Pre-amp Tube (Power tube installed), Issue remains until 20 sec, then issue gone.
Does this give us a hint? Can't figure out why noise is gone after short time. Maybe Cap discharge??

Any advice on noise dilemma will be greatly appreciated....
BTW - noise is not really an issue on tone as the amp sounds great. I just think it would be awesome to figure our why its there.
BTWW - my interest is both old tube radios and old tube guitar amps. Anyone else share these interests? Anyone know of a vintage amp forum that is a great as this vintage radio forum??
Thanks....

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 4:38 pm 
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Joined: Jul Thu 29, 2010 5:35 am
Posts: 1248
Location: Simonton, Texas
Both a bad cap and a bad resistor can cause noise like you describe. If the amp was mine I would replace the remaining old paper caps and try it again. Should the noise still be there I would start replacing resistors starting in the preamp section.
Just my 2 cents worth. Good luck.

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 4:43 pm 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 886
Location: 18424 PA
Change the wax caps, they are bad.


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 4:49 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 2613
Location: Austin, Texas
If the suggestions above don't solve the problem, you may need to add some bypass capacitors to remove the AM station noise.
This diagram shows my suggestion for adding bypass caps to the tube grids. This should not affect the audio but will get rid of radio signals.
The voltage rating of the caps can be 100VDC or higher.
Attachment:
Harmonyy h303 mod.jpg
Harmonyy h303 mod.jpg [ 274.29 KiB | Viewed 1159 times ]

Jay


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 5:26 pm 
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Joined: Dec Fri 28, 2018 7:24 am
Posts: 122
Location: Trabuco Canyon, CA
So what I did was to bypass the wax caps with new caps attached through 12" test leads. The noise was still there. Also tested the old caps and they were close to the original values. So a couple of questions:
can old wax caps still be "bad" even though they test good?
can the long test leads affect the result?
BTW - other than the elctrolytics, there are only two caps C2 is wax C1 is disk


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 5:42 pm 
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Joined: Jun Sun 15, 2014 11:04 pm
Posts: 1282
Location: Coos Bay, Oregon
Good advice from Koby and n3uvt. If after that, there is still crackling noises, and since the crackling noise diminishes after warm up, get some circuit chiller and spray each component until you find the one that makes more noise when chilled. Cheers, Russie.


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 6:48 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4912
Location: Sunnyvale CA
scstill wrote:
So what I did was to bypass the wax caps with new caps attached through 12" test leads. The noise was still there. Also tested the old caps and they were close to the original values. So a couple of questions:
can old wax caps still be "bad" even though they test good?


Did you test them for leakage? The value will typically read right, or higher capacitance than marked, if it is leaky. What is probably happening is that one or more of cap is breaking down periodically, causing the "scratchy" noise. A faulty/intermittent carbon comp resistor can cause a similar issue , but the overwhelming likelihood is that your paper capacitor is bad/leaky.

The hum is probably something else. A light hum is nearly unavoidable in this type of simple amplifier, strong hum means some sort of ground fault or miswiring somewhere.

Brett


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 6:57 pm 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 886
Location: 18424 PA
Cut the wax cap out, throw it away. You can't jumper across a component and expect that to mean pass or fail. If it is leaky, jumpering does nothing. Only if the failure mode was open will jumpering appear to work.


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 9:34 pm 
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Joined: Dec Fri 28, 2018 7:24 am
Posts: 122
Location: Trabuco Canyon, CA
What I meant was I unsoldered one end of the waxy and jumpered into its spot with a new cap.
Thanks all for your excellent advice will replace caps and then start into the carbon resistors.


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Wed 04, 2020 11:37 pm 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 886
Location: 18424 PA
You really only need to check the very high ones, they go higher. I don't think they are your problem unless you spot check and a bunch are way out of spec. Often these schematics are wrong, don't go rewiring unless it appears someone was in there. The disk cap is probably fine. Check J1 to make it is shorting when nothing is plugged in, otherwise you will of course get hum. I have a 300 series Harmony like this with AA5 tubes sitting here waiting for me to tackle it, someone was in there and it's a terrible mess.


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Thu 05, 2020 1:42 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4741
Location: Rochester NY USA
R4 would be my first guess - it has HV across it and an amplifying stage after, a noise would be amplified. Noisy resistors often read higher tha marked value..

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Thu 05, 2020 4:14 am 
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Location: Central PA 16801
deleted

mistake

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Thu 05, 2020 10:29 pm 
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Location: Vancouver Island, Canada
Notice jack J1 is grounded when not used, but J2 does not have the grounded input when using J1. Already mentioned, and good point, make sure the ground connection is not tarnished.

Some Harmony amps had isolation only on one preamp tube, a tiny little transformer. Yours is better.
We need a pic of the front.

Try moving around the house, as RF often leaks into the house wiring nowadays from power packs and wall warts.

A forum of dedicated guitar amp innovators is AX84.com. Lots of those guys have vintage amp know-how, and are not trying to sell you something.

I had a similar amp, a LectroLab, that was quiet at my place, but at a friend's house on top of a ridge, would morph into an AM radio. :x
It needed that cap Jay mentions to bleed RF to the chassis.
60 cycle hum can be internal or external. 120 cycle hum is obviously a higher pitch.
Modern electronics can be very noisy, and you can track them down with a portable transistor radio. Try your compute monitor for example.

The guitar cord can act as an antenna for noise. If the hum is there, shorting the input to the chassis with a jumper will cut the hum, amp powered up.
You could also try temporarily reversing polarity on your power cord using jumpers or an extension cord, not that you got it wrong, but to see if the power cord is part of the hum source, by way of the switch on the volume control.

Dirty volume control will arc a bit sometimes, and if so, the crackle noise will be affected by turning the volume up and down.

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Last edited by westcoastjohn on Mar Fri 06, 2020 1:43 am, edited 2 times in total.

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Fri 06, 2020 1:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4391
Location: Boston, MA USA
The reason the hum goes away when you pull the voltage amplifier tube is that its heater is in series with the rectifier tube heater. So when you pull the 12SQ7 voltage amplifier the B+ goes away. Since the 35Z5 rectifier is indirectly heated it will take a while for the B+ to diminish.

-David


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Fri 06, 2020 10:08 pm 
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Joined: Dec Fri 28, 2018 7:24 am
Posts: 122
Location: Trabuco Canyon, CA
dberman51 wrote:
The reason the hum goes away when you pull the voltage amplifier tube is that its heater is in series with the rectifier tube heater. So when you pull the 12SQ7 voltage amplifier the B+ goes away. Since the 35Z5 rectifier is indirectly heated it will take a while for the B+ to diminish.

-David

Thanks this is excellent insight....


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Fri 06, 2020 10:23 pm 
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Joined: Dec Fri 28, 2018 7:24 am
Posts: 122
Location: Trabuco Canyon, CA
westcoastjohn wrote:
Notice jack J1 is grounded when not used, but J2 does not have the grounded input when using J1. Already mentioned, and good point, make sure the ground connection is not tarnished.

Some Harmony amps had isolation only on one preamp tube, a tiny little transformer. Yours is better.
We need a pic of the front.

Try moving around the house, as RF often leaks into the house wiring nowadays from power packs and wall warts.

A forum of dedicated guitar amp innovators is AX84.com. Lots of those guys have vintage amp know-how, and are not trying to sell you something.

I had a similar amp, a LectroLab, that was quiet at my place, but at a friend's house on top of a ridge, would morph into an AM radio. :x
It needed that cap Jay mentions to bleed RF to the chassis.
60 cycle hum can be internal or external. 120 cycle hum is obviously a higher pitch.
Modern electronics can be very noisy, and you can track them down with a portable transistor radio. Try your compute monitor for example.

The guitar cord can act as an antenna for noise. If the hum is there, shorting the input to the chassis with a jumper will cut the hum, amp powered up.
You could also try temporarily reversing polarity on your power cord using jumpers or an extension cord, not that you got it wrong, but to see if the power cord is part of the hum source, by way of the switch on the volume control.

Dirty volume control will arc a bit sometimes, and if so, the crackle noise will be affected by turning the volume up and down.


Why did Harmony not ground J2? and ground J1?

This is likely not a locaton issue as it is used in different places with same issue.

Don't get me wrong this amp is not very noisy, after the stuff I did already it is really realtively quiet. Just looking to improve. The hum is very slight (maybe normal). The static pickup is not expected. Maybe the bypass caps will help. The tone of this is really excellent. I really like its low volume maybe 1w. Will be checking AX84.com for sure. The Hum and "AM static"noise are without the guitar plugged in. Until I added the three prong ground, the guitar was creating alot of noise. Now that is gone.

Volume control earlier detoxit sprayed

There is no other transformers (other than the power xfmr) on the chassis top (is the pic below what you suggested??)

Attachment:
H3036.jpg
H3036.jpg [ 209.3 KiB | Viewed 984 times ]


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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Sat 07, 2020 2:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4741
Location: Rochester NY USA
The input arrangement is a clever hack - Fender used it, marked the two inputs HI and LOW. When used alone, one input has a 2:1 voltage divider, so has half the gain of the other one. Use the one that gives the best volume range with your guitar. When two instruments are plugged in, they're equal in level.

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 Post subject: Re: Help with a Harmony Amp Noise
PostPosted: Mar Sun 08, 2020 5:44 am 
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Location: Vancouver Island, Canada
Yes, thanks for the pics. I just needed a memory jog. There is a version of that amp that has 3 input jacks, so the bass player could plug in too. 8)
Just kidding. If noise was a problem, you could slim down to just one jack and a new resistor to the 12SQ7 socket. But it sounds like you have the amp about as quiet as they ever get.

With a low plate voltage the output is a bit more than 1 Watt, but if you were to boost the high voltage to 200 vdc, the 50L6 can put out a whopping 3.8 Watts.

A neat feature of a low power amp is that you can crank it up. Your amp has just the one gain stage so it will sound clean and just bit loud, in a closet stuffed with pillows, maybe.
If you decide to build a clone, use a 9-pin socket and a 12AX7 for a two-stage preamp.
I had a couple of preamp designs that worked well with a 50L6.
With low power, you can sub a dummy load for the speaker and take a line level signal from there to another amp at low volume.

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Burl Ives, RIP, oldtimer.
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