What do Oil Caps do and how do they effect sound?

Oil caps are one of the original types of capacitors - usually using paper wound with foil, bathed in an oil dielectric. Often in a rectangular, oval, or round can with two terminals on one end. There are modern day equivalents, Jensen makes some nice ones (copper foil in oil, etc.), as does others (for even more money, e.g. Audio Note silver foil in oil....). Usually vintage caps are referred to as PIO. (paper in oil or sometimes poly in oil). They work great and are often used as filter caps in a tube amp power supply (probably their best application). Also good in crossovers, and sometimes as coupling caps between (tube) amp stages. Fairly easy to find the old ones, commonly they are available from 200v - 1000v ratings, sometimes higher. Usually values range from 1uf to 20uf but sometimes as high as 40 or 100uf for more modern ones. Modern applications include motor run caps (for DC electric motors).

-Ed

>>Whereas tubes "open up" and fail, a transistor can blow a >>few junctions and still keep operating. It might not work >>as well as it used to under extreme conditions, but it is >>still working.

This is TOTALLY wrong!! A transistor will never work even if A junction is blown!! In fact, many times transistors do not work even when a junction is not blown owing to materials impurities or if they are not thermally annealed correctly. This is TREMENDOUS amount of misinformation! Please do not do this dis-service to other members of this forum by plying them with wrong info! You have answered many, many other threads with very good info but this content in your post is far from the truth & you appear to be quite mis-informed.
Au contrare, a vacuum tube has a higher possiblity of working if it fails vs. a silicon transistor. It is a well-known fact that tubes "die" gracefully whereas silicon transistors are a 0/1 deal. In Engineering school we are taught this in Electronics 101.

>>As such, tubes are typically more fragile and >>either "work" or "don't work" acceptably.

This again a MYTH! Totally incorrect. Vacuum tubes are very hardy devices & can withstand very high voltages (as clearly mentioned by Albert Porter in his post). For eg. I was listening to an ASL 50W/ch mono block @ a dealer's store & this amp used an 845 tube. This is a radio broadcasting type tube & the grid is biased at 1500V! There is no transistor I know used in audio applications that gets biasesd this way. Tubes can take quite a beating & still reproduce sound thru them. They might not perform to their best ability (highs rolled off &/or wooly bass) but they will pass sound thru them. A silicon transistor would be long dead (smoked!) by this time. Only a catastrophic failure in the tube such as a disconnected terminal or blown grids/cathode/anode would cause a tube to fail completely.
We audiophiles are so nit-picky about our tubed gear that we have assumed that the vacuum tube is a fragile component. The vacuum tube pins are delicate & care needs to be taken while inserting the tube in its socket & yes, you can break it if you drop it on a hard surface & you could squish it if you apply too much pressure while inserting it but that does not make a vacuum tube fragile! These days not much is taught about tubes in Engineering school & so I think a lot of people are ignorant about it & assume it to be fragile.

Sorry to be so hard on you but I hope that you'll see my point & take it in the right spirit.

I agree with the oil caps explanation & why ss power amp gear is usually rated @ higher wattage for the same size chassis & relative cost. To expand this pt. further: miniaturization of silicon devices (better silicon process created by the fabs) has allowed us to put higher wattage BJTs & MOSFETs into smaller & smaller packages. Also, packaging of silicon devices has come a long, long way in the last 20 yrs thus these packages are very good at dissipating heat generated in the ss output stages. Thus, we can pack it in i.e. insert more & more of these high current carrying devices into reasonably sized chassis. The result is very high wattage ss power amps.

As hinted by Albert Porter, vacuum tubes are voltage output devices i.e. they are very good at providing a voltage swing but provide a very poor amount of current. Also, to get a vacuum tube to work with low impedances (such as a loudspeaker) one needs an impedance transformation i.e. an output transformer. You'll notice that the output xformer is huge. This is not only to provide an impedance xlation from but also to ensure that the current flow is un-impeded. There is a physical limitation of how large this xformer can become. Thus, output wattage of vacuum tube power amps gets limited. There is 1 practical way around this - the OTL amp. However, to get reasonable Watts out of it, one needs a large # of tubes (eg. Atma-sphere OTL amps) & they become space heaters (as a 2ndary function).
Hope that this helps some.

Although Sean's post raised my eyebrows too, I think it's pretty academic to focus on how a device might theoretically continue to operate to some degree when it's damaged or broken, when obviously if it doesn't sound right we're going to want to repair or replace it ASAP.

Though I'm certainly no engineer, I think many of us realize that yes, vacuum tubes can be more robust than transistors under certain unusual conditions having little-to-nothing to do with home audio (some kinds of radiation exposure, thermal stress, higher voltages), and yes, they can be made to last a pretty long time under relatively harsh conditions where transistors are at a disadvantage (in radio transmitters, microwave magnetrons).

But when it comes to home audio, I think all we tube lovers can admit that our vacuum devices will be much more likely to fail, and anyway need a hell of a lot sooner replacement, than silicon semiconductors. In fact, tubes will audibly degrade over their useful lifetime to a much greater degree than transistors. And the ways they usually fail prematurely, such as losing their vacuum, blowing like an incandescent light bulb, or getting intolerably noisy in one way or another, are going to cause us to replace them, not marvel at how they might still be able to function a little bit. (But I would think the same goes for a failing transistor - I've just never had one go bad yet). In addition, tubes are generally more sensitive to some stuff that does find its way into the audio environment, like vibration and accidental impact.

Anyway, I've never thought "graceful" when one of my tubes died, except to the extent that it didn't take out anything around it, knock on wood (and once it did). But regardless, this whole debate is irrelevent to the question asked, and I'm not quite sure why Sean brought it up in the first place, unless he meant to imply that amps using large numbers of output tubes to develop higher powers would entail discouragingly high retubing costs...

Bombaywalla: I have pulled apart dozens of high current RF transistors and found blown junctions in them. All of these were still working albeit at reduced output due to the loss of internal conductors.

Many transistors use several junctions in parallel to handle the current levels needed, so blowing a few simply reduces the gain and max current potential. In effect, they have compacted several transistors into one case and they are all working together to share the load. Blowing one of the internal junctions inside of a transistor is no different than running a dozen external transistors in parallel. One, two or three might fail, but the rest of the circuit will continue to work at reduced capacity. That is, so long as the transistors that failed blew open and not shorted. If the transistors are running in matched push-pull pairs, noticeable distortion may occur, but the circuit can be made to work again by simply deleting the the mate to the transistors that went south. Once again, it will not be as powerful, but it will work and should maintain a reasonable semblance of linearity.

As far as tubes go, they are in a continual state of decline once they are fired up. While most experience a very slow and gradual death, it is more likely for a tube to "pop" at random than for a transistor just to up and let loose. On top of this, it is not uncommon for a piece of equipment to experience major damage when a tube lets go. If you doubt this, try searching the archives here pertaining to tube failure and start reading about the fires that have resulted.

Other than that, i agree with everything that Albert had to say. High rail voltages are a must if you want realism. Then again, you have to have enough current to maintain that level of voltage when under load or the rails will sag. As such, unless you have a speaker that is of noticeably higher than average impedance across the band, at least a reasonable amount of current will be required from the output section in order to maintain linearity, dynamic headroom and a solid soundstage. As the voltage sags under load, so does the impact and spatial characteristics. Sean
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Both solid state and tube amps produce a voltage. Current is what results when voltage is applied to a load, such as a loudspeaker. An amplifier might be capable of high voltage output under no load condition, but if it has little current delivery capability this voltage cannot be maintained into a load. Your preamp typically can output a signal of several volts, enough to play a speaker quite loudly, however if you hook it up to a speaker I doubt you would hear anything. The preamp can only deliver enough current to maintain the voltage into a load of several tens of thousands of ohms, not 4 or 8 ohms.

Tube amps have high "rail" voltage (plus and minus 400 volts or more) so that the output tubes can apply a high voltage to the primary winding of the output transformer. The output tubes cannot deliver a great deal of current. However, the output transformer is wound with a step-down turns ratio, so that the secondary output voltage is down to a handfull of volts, as appropriate for the speaker, but the necessary current capability is there. You might think of the output transformer as similar to the gearbox of a sports car, which permits a small engine reved up to 7000rpm, to accelerate the car from a standing start.