I am repairing an old tube-powered audio signal generator (HeathKit AG-9) and want to replace the old wax/paper capacitors that control the frequency of the oscillator circuit. The unit uses a bridged T filter to set the oscillation frequency:


simulate this circuit – Schematic created using CircuitLab

And here is a photo of the caps in question:wax/paper and mica caps

Looking at mouser, I see Polyester, Polypropylene, and PET caps in suitable capacities (47pF-500nF) and voltage ratings (100V). Which of those would be suitable for this application? I am concerned about temp stability primarily, but also need a relatively low tolerance part because the exact capacitance is critical to the functioning of the circuit.

  • \$\begingroup\$ There's a chance this will be interpreted as either a repair question or a "tell me what to buy" question. To minimize that, you could perhaps modify your question to focus on what to look for when replacing a capacitor, parameter-wise. \$\endgroup\$ – pipe Aug 3 '16 at 20:00
  • \$\begingroup\$ I updated the wording a bit to focus on parameters, not parts #s @pipe Is that any better? \$\endgroup\$ – Frosty Aug 3 '16 at 20:22
  • 1
    \$\begingroup\$ C0G ceramics are the best for this applications. Very Low temperature drift and commonly available to 5% and less commonly to 2%. \$\endgroup\$ – Robert Endl Aug 3 '16 at 20:33
  • \$\begingroup\$ No need to buy expensive Polyester, Polypropylene, or PET capacitors. Those are for intense high-power pulse applications. The above comments should help. \$\endgroup\$ – Sparky256 Aug 3 '16 at 20:38
  • 1
    \$\begingroup\$ WHit3rd...Given the age, C1 & C2 could be mica. If so, I don't think they would go bad. Someone out there should know more. Wouldn't the cap accuracy affect dial accuracy? I'm thinking Heathkit used tighter capacitors than you think. I've seen 1/2% micas, so it's possible at least. If they are paper, those are famous for drift over time but you could buy them at high accuracy, if only as customs. \$\endgroup\$ – Robert Endl Aug 3 '16 at 22:38

The old Wax Paper capacitors For those who have never seen one, you can get an idea of their size by the size of the resistors. The large brown cylinders are the capacitors :

enter image description here

The tolerance on the capacitors wasn't so good. And they were huge physically. Not the best tolerance on temperature either.

You can safely choose among any capacitor that has same voltage rating, capacity, tolerance, and temperature dependence, when it comes to replacing old Wax Paper capacitors.

(Unless you are a super audiophile) winky winky.

EDIT 1 : Robert Endl pointed out in a comment that perhaps the capacitor might be Mica. In the photograph mica capacitors can also be seen. They would be the blocks that appear with color dots on them (similar to resistor color code). If indeed the OP's capacitor turned out to be a mica capacitor, they are often now days called "silver mica" capacitors. If the capacitor is a mica, then there are silver mica capacitors available.

EDIT 2 : With new photo from OP : The two red capacitors on the left do appear to be Mica capacitors. Just to understand Mica capacitors, they are very high Q ( high quality) capacitors that have very low loss. Very temperature stable too.

  • \$\begingroup\$ I would replace the paper caps, but I don't know how old micas aged. Could they be good yet? Plus, a little hunting might find the manual. This is the internet after all. \$\endgroup\$ – Robert Endl Aug 4 '16 at 2:17
  • \$\begingroup\$ @RobertEndl : I have worked on old tube units from 1935 vintage. A couple of times I replaced Mica capacitors because of mechanical problems. I have never seen an old tube unit Mica capacitor fail (but have heard anecdotal evidence). I would leave Mica caps in place. \$\endgroup\$ – Marla Aug 4 '16 at 2:19
  • \$\begingroup\$ Never mind, micas are famous for their stability. duh \$\endgroup\$ – Robert Endl Aug 4 '16 at 2:20
  • \$\begingroup\$ @RobertEndl , , and your early comment on Mica helped improve the answer for this question. As can be seen by the OP posting a photo that shows some of the caps are indeed Mica \$\endgroup\$ – Marla Aug 4 '16 at 2:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.