# How to read the code on vintage ceramic capacitors

#### Planobilly

Hi,

I am a bit perplexed by the codes on some old ceramic disk New Old Stock capacitors. Here are some examples.

This one measures close to 68 pf.
CM = MFG ????
U2J = ????
75J = I assume 75 pf and the J = the tolerance of 5%
1KV = 1000 volts

This one measures around 60 pf.
68J
N750
1KV

This one measures around 4.4 to 4.5 as the meter is bouncing around.
NPO
4.7D= 4,7 pf D= ???? tolerance
1KV

1KV ceramics were common in old Fender guitar amps from the 1950s. I assume 1KV was common in the tube/valve era for many high voltage tube applications.

Mouser and DigiKey don't stock 1KV caps and perhaps they are not being manufactured any longer . I have been given several thousand of these NOS caps by a friend who has an old electronics business. I need to label them. If stuff is not sorted out where I can find it, it is useless....lol

Cheers,

Billy

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#### davenn

Gold Member
This one measures close to 68 pf.
CM = MFG ????
U2J = ????
75J = I assume 75 pf and the J = the tolerance of 5%
1KV = 1000 volts
U2J = ????
I assume all that wasn't on one particular cap ?

U2J not sure, don't think I have seen that before 0.2pF ? seems too small ?
any character before a digit usually denotes a decimal point

yes 75pF

This one measures around 60 pf.
68J
N750
1KV
68pF

This one measures around 4.4 to 4.5 as the meter is bouncing around.
NPO
4.7D= 4,7 pf D= ???? tolerance
1KV

yes 4p7 (4.7pF) if ceramic or similar if paper or electrolytic it could be 4.7uF

Dave

#### Planobilly

Hi Dave,

That was three different ceramic caps. The three or four rows of numbers/letters are just like they appear on the cap, so yes to your first question. All are the typical dark orange ceramic caps you would see in any old electronics gear.

In example one I have no idea what the U21 means. It is marked 75J which I assume is the value in pf and it measures close to that.

In example three 4.7 D means 4.7 pf and D means a tolerance of + or - 0.5 pf. The NPO I am not sure about but I assume it is the MFG code.

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#### Planobilly

Actually I was wrong about the NPO. Here is what it means.

NPO Ceramic Capacitors are single-layer ceramic capacitors made from a mixture of titanates.

A NPO ceramic capacitor is an ultrastable or temperature compensating capacitor. It is one of the most highly stable capacitors. It has very predictable temperature coefficients (TCs) and, in general, does not age with time. This is a good thing for me, as the ones that were given to me are pretty old new stock.

NPO stands for negative-positive 0 ppm/°C, meaning that for negative or positive shifts in temperature, the capacitance changes 0 part per million, meaning that it has a flat response across a wide range of temperatures; the capacitance of the NPO capacitor stays constant (at the same value) despite variations in temperature.

NPO ceramic capacitors are suited for applications where stability over a wide range of temperatures and a high Q are needed.

Filter networks and most circuits associated with tuning and timing, as well as various types of resonant circuits, generally require ultrastable capacitors. They are also very suitable for oscillator construction in order to compensate for frequency drift with temperature"

I am not sure how long I will remember all this....lol Funny how memory works. I remember every mistake I make and forget all the good stuff...lol

Cheers,

Billy

#### davenn

Gold Member
A NPO ceramic capacitor is an ultrastable or temperature compensating capacitor. It is one of the most highly stable capacitors. It has very predictable temperature coefficients (TCs) and, in general, does not age with time. This is a good thing for me, as the ones that were given to me are pretty old new stock.
yes ... I forgot to answer the NPO for you sorry .... used in, particularly, RF oscillator circuits

#### jim hardy

Gold Member
2018 Award
Dearly Missed
Some Mil capacitors have a code that's assigned sequentially not related to value except through a lookup table.

i hope you didn't get a basketfull of them.

#### Planobilly

Little by little it is getting sorted out. No MIL stuff yet Jim, but I have only started to sort out all this. I don't normally use a lot of ceramic caps but lately I have been rebuilding a bunch of amps from the 1950s and 1960s. It is great to have the original caps that came in these amps. All the caps in the plastic bags are 1000V.

New 18 Watt Marshall style amp I just finished.

Things are a bit of a mess around here...lol

Cheers,

Billy

#### Planobilly

Speaking of needing to sort out stuff...lol I only have sorted a few of these by wattage.

I have a five gallon bucket of NOS resistors from the 1960s. I may go blind trying to read the color codes!!!!

Cheers,

Billy

#### Planobilly

That is not too surprising as I have seen a lot of different methods used to describe the values. I think until only very recently has there been a real set of standards put in place and agreed too.

#### davenn

Gold Member
I have a five gallon bucket of NOS resistors from the 1960s. I may go blind trying to read the color codes!!!!
unless you specifically wanted to use those to give a project the "old" look, they would be better binned
newer resistors, particularly metal film ones are going to be substantially less noisy than the old carbon ones

Dave

Gold Member
2018 Award
Dearly Missed

#### Planobilly

Dave, I only use the carbon composition resistors to make repairs to old guitar amps and also to build new amps that replicate the circuits of classic old amps.
I did some experiments recently with a classic Fender AB763 circuit. I built the circuit with new high quality metal film resistors and played the amp. It sounding nothing like a Deluxe Reverb should sound. I replaced all the resistors with carbon comp and everything went back to normal as too the sound. I am now slowing going through the amp and replacing "some" of the carbon comp with metal film. I am trying to determine just where they should and should not be used. The noise that the carbon comp resistors produce is part of the signature sound of these old amps. I suspect that them being non inductive also is implicated in how they change the sound.

I keep my eye open for NOS components that I can afford.

All of this experimenting has been an attempt on my part to separate the fact from the fiction that is so rampant within the guitar player world. Well...to the extent that I can. I am subject to my own biases and expectations just like anyone else and I am not doing real double blind studies.

Jim, those are really old amp schematics. I had not seen that link before. Thanks for posting. I will put it on my list.

Cheers,

Billy

#### tech99

Gold Member
Although NPO means zero temperature coefficient, such ceramic capacitors are reported to jump slightly in stable oscillator circuits, and the silver mica type are preferable. Use the physically biggest capacitor in oscillators to reduce heating, or put several in parallel.

#### rbelli1

Gold Member
The noise that the carbon comp resistors produce is part of the signature sound of these old amps.
Guitar amp designers jumped back through lots of hoops to make a specific noise that the hifi people jumped forward through to make the sound more "perfect".

Now everyone listens to 128k MP3 and it is all noise all the time. It's it's impossible to tell if the amp has transistors, vacuum tubes, or inner tubes to make it work.

BoB

#### Planobilly

LOL Bob....I am building all my TEXAS amps with barb wire inductors...LOL

"How to read the code on vintage ceramic capacitors"

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