Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: llama on November 07, 2012, 02:59:10 pm

Title: pico farad scale caps
Post by: llama on November 07, 2012, 02:59:10 pm: Hi all,

I have two Silver mica Picofarad caps that are the only ones i have and measure way high with my DMM (set on capacitance).

I have a 250p that measures 520p, and a 47p that measure 200p.

Before i go further, is it likely my meter is reading false? It's specified to go down to 10p (harbor freight dmm).

the 47p is for a gain pot bleed and the 520p is for the treble cap in the tone stack. Are these value differences going to make a big difference?

Thanks!
Title: Re: pico farad scale caps
Post by: SILVERGUN on November 07, 2012, 03:10:12 pm: Quote from: llama on November 07, 2012, 02:59:10 pm
Before i go further, is it likely my meter is reading false? It's specified to go down to 10p (harbor freight dmm).
Yes, anything from Harbor Freight is suspect....the leads of your meter have an inherent capacitance before they are even connected to a cap....
My Fluke meter got a little squirly when reading small values like that so I imagine you're not getting a true reading with that meter

I bought this just for that pupose and it has been dead on: (it has 2 holes where you can insert the leads of a radial leaded cap, and two short test clips for axials)
http://www.newark.com/tenma/72-8150/capacitance-meter-with-resistance/dp/41M0998 (http://www.newark.com/tenma/72-8150/capacitance-meter-with-resistance/dp/41M0998)

Just for the record,,,it says the range only goes down to 200pf, but I've measured down to 10pf with precice readings....the lowest manual range setting is 200pf
Title: Re: picofarad scale caps
Post by: Willabe on November 07, 2012, 03:15:00 pm: Quote from: llama link=topic=14612.msg 139232#msg 139232 date=1352321950
the 47p is for a gain pot bleed and the 520p is for the treble cap in the tone stack. Are these value differences going to make a big difference?

Yes, those values are way different (IF they are correct readings) and will change the amps sound.

Brad :icon_biggrin:
Title: Re: pico farad scale caps
Post by: plexi50 on November 07, 2012, 03:31:41 pm: What amp are you working on?
Title: Re: pico farad scale caps
Post by: llama on November 07, 2012, 03:45:29 pm: Thanks again, Silvergun. That was my thought. I just bought these caps, so i suspect my methods. I just got them from Doug, he replied quickly to my mail, and mentioned that they are usually really close to the published value. I'll bring them over to your shop when i stop by.

Sorry that I did not mention what I'm working on. It's a custom amp with a 5879 tube.

feel free to see the schematic - C6 and C11 are the caps in question.

Thanks Willabe and Plexi50!!
Title: Re: pico farad scale caps
Post by: EL34 on November 07, 2012, 05:22:51 pm: The Silver mica caps you bought from me are + or - 5% tolerance

47p can be + or - 2.35p
250p cab be + or - 12.5p

I suspect the meter or method because out of 1000's of these, no one has ever emailed me about the values.

Meter leads have capacitance
Your fingers have capacitance

I would just install them and forget about trying to nail down an exact value
Title: Re: pico farad scale caps
Post by: llama on November 08, 2012, 07:33:01 am: Doug,

I'm sure your right. Sorry if my NOOB was showing....

Thanks for the quick response!
Title: Re: pico farad scale caps
Post by: Ed_Chambley on November 08, 2012, 09:26:08 am: Never got one from Doug that was even close to 5% drift. I have a meter that they insert into. Best this way. My problem is not measuring them, it is seeing them.