Vintage Capacitor Advertising
Capacitors seem to the favorite ‘secret ingredient’ to the tone recipe of vintage tube guitar amplifiers. This Sprague advertisement entices the prospective jobber by extolling each capacitor type’s virtues; ‘tone’ conspicuous by its absence. We all know that tube Televisions get hot inside (or you should; if you’re as old as I am), so an important selling point is temperature stability. Low leakage or inductance also seems to affect a capacitor’s operation. Other ‘advantages’ are pointed out above, each capacitor type having its own unique feature(s).
I‘ll repeat this sentiment often, because it bears repeating. The advertisement above is for Mallory, and again espouses the benefits of using one of their ‘time and temperature’ stable polystyrene capacitors when called for (timing circuits, horizontal/vertical TV circuits, etc.) and the option of cheaper ceramic disc or PVC types when stability is not crucial. The charts in the advertisement above left compare temperature stability for ceramic disc (above left/top), polystyrene (STYROCAPTM, above left/middle), and PVC (Orange DropTM, above left/bottom) type capacitors. Note the change in capacitance value or ‘drift’ for ceramic disc and PVC. The ceramic disc has many variants, identifying temperature coefficiency with markings N-750, N-330, N-150, and NPO (top to bottom in the ceramic disc graph). The capacitance value of an inexpensive N-750 type would drift all over the place under changes in operating temperatures, while the more expensive NPO type would remain stable. Guess which type is prevalent in your old Fender amplifier?
The early 1950’s era advertisement from Sangamo capacitors (right) tells us that their capacitors will not derate or fail under conditions of extreme humidity; up to 95% RH! Now you can take that underwater gig you’ve been turning down because you were afraid your regular ceramic disc capacitors wouldn’t hold out.
As poor as this scan is, you may be able to make out the fact that the FP (Formed Plate) capacitor was touted as having a few advantages over regular types. The first advantage was size, as you could have a triple or a quad unit taking up no more than 1.125″ of space on the chassis. Next, the formed plate construction promised to prevent … ‘loss of capacitance‘ or derating with age. They were also touted as being the best choice for a hot, cramped chassis. The biggest advantage not regularly advertised was their ability to bypass the AC component much better. Regular capacitors are ‘wound’ layers, and at very high capacitance values, these windings can act like an inductor at very high AF frequencies. Thus, high frequencies are not filtered as well, and can actually modulate the power supply. This is most likely the perceived difference in ‘sound’ between different filter capacitors. Stereo receivers would eliminate ‘crosstalk’ (or the LEFT channel’s AC signal modulation showing up in the RIGHT cannel and vice versa at the output) by placing a .1uF capacitor across the high value filter to fully bypass the AC component. If you ever place an oscilloscope on the power supply, watch the fluctuation as the audio signal modulates the B+. Many guitar amplifier power supply mods include this added bypass capacitor, regardless of filter value. Whether or not you need it depends on how good your ears are, what value filter capacitors you have, and if the simple fact that no vintage guitar amplifier used this technique (they typically used 20uF filters) matters to you. If you like the vintage sound (or why even bother with tubes?), try using vintage design criterion. Guitar amplifiers don’t need to worry about ‘crosstalk’, or the .1uF capacitor. It’s just recently been ‘rediscovered’ by over-active wannabe engineers.
Some capacitor advertisements, like this one for Tobe, extol their high insulation resistance and the fact that they are non-inductive. This comes in very handy for RF applications. Some capacitors were advertised as having ‘low derating’ (their capacitance value wouldn’t drift all over the place) under higher operating temperatures. This is important in a TV horizontal oscillator circuit from the old tube B&W days. At guitar frequencies, capacitors are pretty much all alike. As a side note, I have never seen one advertisement for one capacitor aimed at the audio industry stating… “You get more detail and true-to-life tone reproduction from using Brand X capacitors!” I wonder why. And, finally, just in case some neophyte Television repairman of yesteryear was just as baffled by capacitor hyperbole as today’s wannabe tube guru, Mallory and Sprague (to name only two that I could find examples of) actually put out manuals advising him on the ‘best’ capacitor choice.
Of course, we should know that as long as capacitance values and voltage ratings are similar, so too are any replacements we choose to use. In defense of these manuals, I will admit that capacitors of yesteryear had a propensity of coming in all shapes and dissimilar sizes; old-timers remember when ‘bath tub’ capacitors reigned and commonly measured 2″ square! The biggest capacitor I have measures 4 1/2″ X 1 1/2″ X 1″.
Enough of these in your amplifier would make a simple ‘Deluxe’ style chassis about the size of a dormitory beer fridge. Today we don’t care about chassis space because 100uF/500VDC filter capacitors are about 1/3rd the size they once were. Even coupling capacitors are much smaller; I have a very old capacitor decade box with Mallory units that resemble medium to large sized fire crackers (the .2uF/600VDC is 3/4″ round X 2 1/2″ long while the .4uF/600VDC clocks in at almost 1″ round X 2 3/4″ long).
Lately, eBay has caused the price of old, leaky paper capacitors to soar beyond unheard of prices. To be fair; just as flies do not create garbage, sane and rational people do not create opportunistic ‘crooks’ who will sell some neophyte a beat up, used capacitor for a 10,000% mark up. The mania is not exclusive to guitar players, either. The Hi-Fi industry is full of hype and wives-tales that tests the patience and common sense of many. Below is a snapshot of typical ‘used’ paper capacitors offered for auction.
Many an auction ‘features’ capacitors that have been ‘pulled’ from various equipment; oscilloscopes being very common. There is a good reason these capacitors are no longer in the original equipment; they are almost always very leaky. These capacitors are then offered to the Hi-Fi or guitar crowd as a panacea for your amplifier that lacks in ‘tone’. The capacitors above recently fetched $140US via auction, and is proof that there is indeed ‘one born every minute’. Below we see another capacitor offered for your ‘tone-gourmet’ consumption.
p style=”text-align: left;”>This capacitor (shown above) was actually advertised as being used in early Fender Telecaster guitars. You may see that one lead is very short, also mentioned in the auction. The final price? How about $26US? I’m sure P.T. Barnum is rolling over in his grave as we speak. The topic of eBay driving up most prices for otherwise ‘junk’ to ridiculous levels is discussed on a few BBS websites I visit. Not aimed at guitar players, these are mostly for vintage radio buffs and other old-time electronic technicians. Below is a comment posted by one old-timer, in response to the view that selling a leaky capacitor was on par with robbery, as a leaky capacitor isn’t good for too much. Or is it?
“A .022uF was a common midrange-cut capacitor used by Fender and other guitar amplifier companies. In that circuit position, a leaky capacitor shifts the position of the midrange cut, thus altering the midrange dramatically. Lots of older (unmessed with) amps have this natural tone shift, so now the ‘tone gods’ have decided they need this specific age/construction capacitor to get ‘that sound’. Electrolytic cathode bypass caps that drift down in value have the same ‘mysterious’ ability to boost midrange. Rest assured that a lot of these ‘tone gods’ can only hear midrange frequencies. Anything that stands out is desirable to them. You need to remember it’s possible to have a capacitor in a passive tone circuit with no DC voltage across it. Any leakage then just messes with the tone shaping. Of course all of this is IMHO.”
It may be just one man’s opinion, but it does make a lot of sense. Whether it is correct or not in theory does bare further investigation.