Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rschiller on August 31, 2024, 05:53:44 pm
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Just an FYI. I normally would not use $15 capacitors as I find $1.50 Mallory-150s reliable and sound good. But the amp I'm rehabbing is a near mint 1958 Tweed Deluxe 5E3 and the original Astrons Yellows are all leaking significant amounts of DC voltage. So if for aesthetics only I emptied my wallet on a set of Red Jupiter coupling caps as later Astrons came in Red and the Jupiters look pretty. After I checked the Astrons I though to check the new Jupiters. The caps were clipped to B+ voltage after 100k drop on VIA; about 125vdc. Testing capacitors with both sides soldered in gives false readings.
Caps are .02 and .05s and all tested in the .008 to .001 range except one .05 on the "cold" side measured 119 vdc. Essentially I just spent $15 for 5 inches of wire with a fancy label. Though I had bought the set months ago, Antique Electronic Supply agreed with my test and sent me another one.
Is my experience one off or have others had this issue with Jupiters? Or other boutique brands?
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I haven’t had this issue with the Jupiters I’ve tried, but I know they’re a small shop, and would probably be interested to know about your trouble.
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Are you saying your testing the new Jupiter caps with 1 end of the cap connected to ~125dcv and then checking their uF value with the dcv on them?
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I was not testing for capacitance value, I was checking for the primary function of coupling capacitors: blocking DC voltage and allowing AC through.
I had asked Antique Elec. Supply to let Jupiter know of the issue. Yes, perhaps I should have contacted Jupiter directly.
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I was not testing for capacitance value, I was checking for the primary function of coupling capacitors: blocking DC voltage and allowing AC through.
I asked because this doesn't make sense;
Caps are .02 and .05s and all tested in the .008 to .001 range except one .05 on the "cold" side measured 119 vdc.
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I presumed its commonly understood that for coupling capacitors in tube guitar amplifiers: .02 uf to .1 or so are microfarads ("uf") so I did not included uf. .02 and .05 (600v rating) are the stated printed values on the caps and in my original post I did not address actual capacitance nor test for that. The clipped in hot side voltage is 125vdc. The open cold side (or what should be "cold" which is commonly understood electronic vernacular for no voltage), should be very close to zero dcvo as that is the fundamental function of a coupling capacitor so as not to change the value of the next stage bias. As previously stated, except for one capacitor all passed virtually no dcv and measured .001 vdc to .008 vdc on the "cold" side. The one bad capacitor let about 95% of the dc voltage through. The next stage preamp grid is likely biased at in the 1 to 2 volt dc range. Putting 119vdc on the grid is not adviseable.
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Ahh, I understand the issue now.
... Caps are .02 and .05s and all tested in the .008 to .001 range ...
I found the lack of differentiating uF and V DC readings above somewhat confusing / misleading.
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How long did you wait before checking the DC on the unconnected cap plate? (It takes a second or three for the change in charging current on the connected plate to stabilise after switching the amp on. While the charge is still changing, there will be a corresponding change in VDC on the unconnected cap plate, which should disappear when the charge on the cap is stabilised.)
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I presumed its commonly understood that for coupling capacitors in tube guitar amplifiers: .02 uf to .1 or so are microfarads ("uf") so I did not included uf.
1. There's a lot of new to tube electronics people here, so, it would be helpful to include the designation for what kind of numbers your posting, in this case, uF. And .008 and .001 could easily be a uF cap measurement that you took.
2. I didn't know what you were talking about because you posted 2 different sets of numbers right next to each other without any designation, uF/DCV. You can't do that and expect everyone to know what your posting. You know, but we don't. :think1:
The open cold side (or what should be "cold" which is commonly understood electronic vernacular for no voltage),
Yea, I've been here 14 years, read almost every post, have read at least a few dozen tube amp books and I don't recall that being 'commonly' used very much.
As previously stated, except for one capacitor all passed virtually no dcv and measured .001 vdc to .008 vdc on the "cold" side.
It wasn't 'previously stated', it was just some numbers not really attached to anything.
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I was not testing for capacitance value, ...
Your post wasn't clear on what you were testing for as already explained.
I was checking for the primary function of coupling capacitors: blocking DC voltage and allowing AC through.
Yeah, we know what a coupling cap does.