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I am building a guitar amp for a senior projects class and one of the amps I am modeling around has some electrolytic capacitors used in a way I don't understand (see picture below) and more importantly I have no idea where to buy some of these values anywhere which makes me think these aren't electrolytic capacitors. Basically, won't this kill this caps considering its AC?

full schematic can be found here, I have completely changed about everything but the capacitors are still relevant. http://www.mediafire.com/file/0jrroz0jx2ee8cj/Final+Version+Tremoverb+Diagrams+25+Oct+15.pdf

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  • \$\begingroup\$ 1. If you're shopping, 0.005 uF might also be marked as 5000 pF. 2. The signals are AC, but they may be riding on top of high enough biases that the instantaneous voltage across the capacitor is never reversed. \$\endgroup\$
    – The Photon
    Oct 13, 2016 at 4:46
  • \$\begingroup\$ I made sure to look at nF and pF. No luck... \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 4:48
  • \$\begingroup\$ On Digikey I found like 800 choices for 5000-60000 pF capacitors rated for 200 V or higher. (Under film capacitors, not electrolytics) \$\endgroup\$
    – The Photon
    Oct 13, 2016 at 4:50
  • \$\begingroup\$ Well really that's my question, can I use a non-electrolytic in this case? \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 5:41
  • \$\begingroup\$ Reminds me my own all-tube guitar amp design :) Anyway @DerivVr, cathode bypass caps should be electrolytic --as you might guess-- and the rest can be non-electrolytic. As the others stated, the caps you marked are all non-elcos. But be careful, the top-most two caps should have at least 200-250V rating because the voltage on V1's anode is about 180VDC. Likewise, the bottom-right (10nF) should have at least 200V rating because of the anode voltage of V4b. They all can be either ceramic or MKP or MMKP etc. doesn't matter for audio frequency. \$\endgroup\$ Oct 13, 2016 at 6:57

3 Answers 3

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These aren't electrolytic capacitors. They are film capacitors.

The electrolytic capacitor symbol has likely been abused to note which side of the capacitor to connects to the outer foil of the capacitor.

Back in the days capacitors have been built by having two metal foils separated with insulating film. If you roll them one metal film will be on the outside. This is usally marked on the part somehow because having the outer side connected to the lower impedance point in the circuit gives a slight advantage for noise immunity.

For buying parts, the classic brands for guitar amplifier capacitors still are:

  • Malloy 150 for british sounding amplifiers (Marshall).
  • Sprangue Orange Drops for a US sound (Mesa Boogie).

If you google for them you'll find online shops selling these.

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  • \$\begingroup\$ That you for the great info, I found several great orange drops from a vendor with appropriate values. \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 8:25
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If memory serves me correctly bearing in mind that I am an old guy who grew up with vacuum tubes. The B voltages are DC not AC i.e. +180 VDC. The signal on V1 pin 2 is ac and the output that triode is pulsating DC which must be blocked before entering V3. After those capacitors the signal is pure AC with no DC component. Those small value capacitors are disc ceramic. The 0.005 uF capacitors are used for bypassing any stray RF signals to ground.

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  • \$\begingroup\$ Well the "old guy" is exactly who I want advice from since I am make a tube amp :P And actually you are correct now that I look at it, it will be "AC" with a positive DC offset. So what you are saying is that I can use non-electrolytic caps? \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 5:57
  • \$\begingroup\$ @DerivVr: yes -those caps are disc ceramic type. You would find electrolytics on power supply end of 60 Hz filtering not for audio signal processing . \$\endgroup\$
    – Old_Fossil
    Oct 13, 2016 at 6:04
  • \$\begingroup\$ Thank you some much you have saved me a lot of time and probably money! \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 6:06
  • \$\begingroup\$ disc ceramics you can buy for a few cents a piece. Try shopping at electronic surplus or amateur radio flea markets (hamfests) where they sell tubes and vintage electronics. \$\endgroup\$
    – Old_Fossil
    Oct 13, 2016 at 6:12
  • \$\begingroup\$ I've actually never heard of a hamfest before now, that sounds amazing. \$\endgroup\$
    – DerivVr
    Oct 13, 2016 at 6:19
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They are, as others say, film capacitors as opposed to electrolytic, and the first one (0.015 uF) has several hundred volts of DC across it.

Different film types have different properties (search "dielectric relaxation" for one) which will affect the sound of the amplifier to a certain extent - in the UK in the 50s and 60s, "Visconol-X" was a common brand but this is obviously an American circuit, from the style of the tube symbol and the 12A?7 series as opposed to ECC81/83. So I won't recommend any, though paper-in-oil would be period-appropriate, and from hearsay (citation needed!) I'll suggest you avoid mylar film.

I think the tone (treble) and output side arrangements are pretty self explanatory, but it might not be obvious what's going on ahead of the volume control.

With 180V on the anode, and a 500 kilohm volume control, any leakage current at all in the coupling capacitor will develop some DC across the potentiometer, and that leads to loud thumps and crackling noises as you change the volume.

Acceptable from an old radio at home, but excruciatingly embarrassing on stage at Woodstock, or anywhere you mic the amp (or DI from it to the mixer) and play it though a house-sized PA stack.

And film caps, especially of that age, were prone to developing leakage. So 2 stages of AC coupling are employed, and you can easily test the first by measuring the voltage across that 470K resistor. It's a nice touch in a (presumably) professional amplifier.

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