Need advice replacing caps in Harmony H400a guitar amp - diyAudio
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Old 25th September 2008, 01:35 AM   #1
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Default Need advice replacing caps in Harmony H400a guitar amp

I've been learning slowly to work on tube amps, fixing up an old Harmony H400A amp (35W4, 12AU6, 50C5). At this point the power supply caps (C5A,C5B,C5C) have been replaced and I have re-routed some wiring - the amp sound pretty good, with very little hum. However, there is some crackling as the tubes heat up, and the overall output is lower than I think it should be. Some of what I've read points to considering replacing the other caps (C1,C2,C3,C4,C6) and making sure the old carbon comp resistors are still close to their rated values.

You can see the full schematic here and a photo of the original circuit with caps labeled is below.

I was wondering if I could get some advice on which caps to try replacing first (are some more critical or likely to change with age/use?) and what kind of caps to buy for replacement? I think C1,C3,C4 are ceramic disc caps (correct?), but I'm unclear what kind C2 and C6 are.

Thanks much in advance.


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Old 25th September 2008, 01:44 AM   #2
nhuwar is offline nhuwar  United States
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Wow scary design, no mains isolation there. You could easily electrocute your self with that one. Not to mention there's only half wave rectification.

Id try c1 and c3 to start, those are the coupling caps, I think



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Old 25th September 2008, 03:42 AM   #3
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first of all, you need an isolation transformer. No question about it. After you settle that issue, however, I would replace C2 and C6. Paper caps dry out and degrade over time. I wouldn't worry about the ceramics, though. they tend to last forever.
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Old 25th September 2008, 03:55 AM   #4
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Yes - I am add an isolation transformer and bridge rectifier - Eli Duttman provided instructions here.

AmpKiller - I'm not sure C6 is made of paper - it's more like a hard ceramic or something. But assuming C2 is paper, could you suggest an available part? The number of different capacitor types is dizzying for a newcomer and I'm not sure what to look for.
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Old 25th September 2008, 11:24 AM   #5
nhuwar is offline nhuwar  United States
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I was always under the impression that ceramics sounded not so good.
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Old 25th September 2008, 11:55 AM   #6
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C2 and C6 can be replaced with metalized polypropylene or polyester film caps. Make sure they are equal to or above the existing voltage rating.
I buy all my caps from these guys, but they are available anywhere.
http://www.justradios.com/capacitors.html
And yes, ceramics very rarely need replacing, along with mica caps.

Good luck!
Glenn
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Old 25th September 2008, 01:38 PM   #7
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Quote:
C2 and C6 can be replaced with metalized polypropylene or polyester film caps. Make sure they are equal to or above the existing voltage rating.
Hey Glenn - thanks much. Wanting to understand more: can you say what type of caps you think C2 and C6 might be? Especially since they are so visibly different from one another. And what are the properties of polypropylene and polyester caps that make them suitable for use as C2 and C6?
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Old 25th September 2008, 04:02 PM   #8
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I can't comment on what makes them suitable for this application, as I'm not a tube amp designer. I do restore a lot of vintage tube radios, so tone is not an issue if that's what you are getting at.
I just mention the poly caps because that's what are used to replace similar looking caps in radio applications. Maybe someone who has more understanding of how the location of the caps determines what materials are used could chime-in.
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Old 25th September 2008, 05:51 PM   #9
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Scary stuff indeed, I wouldn't want to on the end of that and hanging onto an earth. Why go to the trouble of having a heater transformer and not include the HT .... oh well!! I've had ceramics go up in smoke if they breakdown and that's even on low voltage stuff. If it were me I would replace C1 with a small polyprop and definitely change C3, it's connected to the anode of the gain stage onto the grid of the output valve, sorry about the English. Anode and Plate, same thing! It's surprising the amount improvement that can be gained by changing capacitors even when they appear to read OK.

Geoff.
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Old 25th September 2008, 06:20 PM   #10
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Quote:
If it were me I would replace C1 with a small polyprop and definitely change C3, it's connected to the anode of the gain stage onto the grid of the output valve, sorry about the English. Anode and Plate, same thing! It's surprising the amount improvement that can be gained by changing capacitors even when they appear to read OK.
Thanks Geoff. Again, for my education, can you say why you would choose polyprop for C1 rather than ceramic? Polyprop for C3 too or would you stick with ceramic? Thanks again.
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