filter cap question

[bob p]

Does a DC leaking cap matter that much in a guitar tone control where it is only shorting AC to ground? I am sure it doesn't matter as much in that application as it would in a tube amp.

Greg, you've figured out one of the well kept secrets that the vintage guitar guys have known for a long time -- that bumblebee type caps that may leak when blocking high DC voltage as coupling caps will normally work just fine shunting millivolts of AC to ground. In spite of the theoretical concerns, you can do it. I know several people in the vintage restoration field who recover leaky bumblebees from old tape decks and put them into vintage guitars. Doing this spares them from paying huge sums of money for vintage caps that don't leak at high voltage, or for paying stupid money for the reproductions from Fender and Gibson.

Gibson Historic Bumblebee Capacitors 2-pack | Musician's Friend

[Leo_Gnardo]

paying stupid money for the reproductions from Fender and Gibson.

Somewhere on the interwebs, can't find it right now, is a photo someone took after putting a hammer to one of the Gibson repro bumblebee. Under the black plastic was a standard film cap, the kind you could buy for 75 cents. Might have been on The Amp Garage, or in Audio Asylum.

[nosaj]

Somewhere on the interwebs, can't find it right now, is a photo someone took after putting a hammer to one of the Gibson repro bumblebee. Under the black plastic was a standard film cap, the kind you could buy for 75 cents. Might have been on The Amp Garage, or in Audio Asylum.

Here it might be
https://www.google.com/search?q=smas...EIMzAE#imgrc=_

nosaj

[Leo_Gnardo]

Here it might be - nosaj

Thanks for the link nosaj. I had a look at the first two hundred photos or so, there's a couple pics of authentic old bumblebees - like the one with a Wesco cap in it - and others with wax paper caps - but none were the photo I remember. It had a generic or IC brand cap in it. Not a bad cap as things go, but still a helluva markup for a 75 cent part. Which reminds me, I oughta sell some of my real old bumblebees too, rattle up some cash from the true believers. Good call too bob p, even a cap that's a little leaky at high voltage will work perfectly fine in a guitar tone control.

[SoulFetish]

The main modern use for high voltage caps I can think of is in SMPS. And even there we usually see a pair of 200v caps.

Au contraire, Enzo. Well, not totally contrary. You're right about SMPS being the main use for high voltage caps, but it turns out that we get to rid the coat tails of innovation for them. One of the truly great advancements over the last decade or so has been production of high performance/high voltage "DC Link" film caps which have been developed. It's almost serendipitous for us as the values fall right in our wheelhouse with products from 20µF-250µF at 500V-1000V+ ratings available. Plus, they have ridiculously low ESR ratings, which, as we all know is the MOST important criteria when it comes to tone. (By the way, any other techs end up loathing the word "tone"?)
Anyway, so ESR may not be as critical for our needs as it is in high frequency switching supplies. But now we can finally be emancipated from having no other option but to use electrolytic caps. Which suck. Lets be honest. I know you can find the rare example of them still hanging onto life after 40 years. But a 40 years ago a 60µF Film capacitor was the size of a Volkswagen Beetle, and cost twice as much. They are radial packages, so maybe that's why you don't see a lot of builders using them. But, I for one, will design around them. I ain't never going back.

[Enzo]

Just as long as we realize that you and I are not nearly even on their radar.

[Mick Bailey]

New build/new design is one thing, but repairmen (and women) are faced with different challenges. We now have to use little adapter boards to get SMD versions of chips to fit DIL pads, or sometimes make up little hybrid circuits to replace long-obsolete ICs. But getting DC-link film caps to fit an old amp (or even more modern one) has its own challenges. I guess for a PCB amp the cap would be bonded to the board on its back and wired to the original terminal pads. An older amp may need the cap to have leads attached or even mounting on a small piece of board and heat-shrinking to form an axial-lead component.

I really like film caps in amps - I've been building 'no electrolytic' hi-fi amps for a long time using motor caps, but the capacitance is restricted and they're to large for most repair work.

[bob p]

No proper High End HiFi amp is so small that it can't fit motor caps. We all know that real High End amps use separate left and right chassis, as well as a separate PSU.

[bob p]

On a more serious note, the capacitance limit of motor caps can be a real problem if you're building an obscene HiFi tube amp, the kind that you'd be willing to publish the PSU's joule rating for everyone else to critique. The size of the motor cap array in an amp like that would be huge... as in fill up a push-cart cart huge.

[soundmasterg]

Greg, you've figured out one of the well kept secrets that the vintage guitar guys have known for a long time -- that bumblebee type caps that may leak when blocking high DC voltage as coupling caps will normally work just fine shunting millivolts of AC to ground. In spite of the theoretical concerns, you can do it. I know several people in the vintage restoration field who recover leaky bumblebees from old tape decks and put them into vintage guitars. Doing this spares them from paying huge sums of money for vintage caps that don't leak at high voltage, or for paying stupid money for the reproductions from Fender and Gibson.

Gibson Historic Bumblebee Capacitors 2-pack | Musician's Friend

Hi Bob P,

Yeah I've known about the use of otherwise leaky caps in guitar tone controls for a long time. As I was suggesting, you can see lots of guys, probably amp techs, selling old caps on ebay for use in guitar tone controls, at a premium of course. I also am aware that caps that leak DC also likely have high ESR, but it doesn't matter so much in a guitar application as it would when used in a tube amp. Incidentally, those reissue bumblebee caps are some other modern cap inside the bumblebee shell...or as a friend who makes guitar pickups and is into all the mojo says, "they're fake."

Greg

[Mike Sulzer]

Hi Bob P,

Yeah I've known about the use of otherwise leaky caps in guitar tone controls for a long time. As I was suggesting, you can see lots of guys, probably amp techs, selling old caps on ebay for use in guitar tone controls, at a premium of course. I also am aware that caps that leak DC also likely have high ESR, but it doesn't matter so much in a guitar application as it would when used in a tube amp. Incidentally, those reissue bumblebee caps are some other modern cap inside the bumblebee shell...or as a friend who makes guitar pickups and is into all the mojo says, "they're fake."

Greg

The circuit of an electric guitar is resonant, specifically a resonant, low Q, low pass filter. The major effect of the tone pot, as it is turned from 10 downwards is to change the loading on the resonant circuit, lowering the Q, reducing the emphasis at the higher frequencies where the resonant frequency is located. As zero is approached, the resistance becomes small enough so that the cap matters. Its effect is to lower the resonant frequency way down, giving a bassy sound. A leaky capacitor is like having a resistor across it. But the series combination of the coil resistance and inductance reactance at these frequencies is low enough so that a somewhat leaky capacitor will not make much difference.

This is perfect situation for minor, stupid fraud: there are no real differences in the sound of different capacitor types since the circuit is so forgiving of flaws, not that types much even in critical applications. (Here the capacitor is nearly irrelevant for most settings of the tone pot.) Therefore the seller is free to lie, and the buyer can imagine anything he wants with the helpful hints from the seller.

[Randall]

Well, as fate would have it, while I am busy building my DR, an Alessandro clone arrives! Burnt screen resistor, bias jacked up to 32/35 mA @ 420 volts! Bad speaker jack, and broken wire in footswitch plug. Customer sends me a thread with Mr. A where he tells customer to bias his 6V6's and indeed "all 6V6" to 30 mA. The construction seemed a little flimsy and the lead dress will give me nightmares. Customer says it came biased the way I found it from the mod service. All very strange.

And Hammond will now be carrying this PT. They asked to borrow mine to clone and add to their line. I get the first one free!

[bob p]

So Hammond is going to be cloning your DRRI PT and not an original BFDR PT?

I have to wonder why they want your transformer when they can get one from Fender parts. Unless they're planning on selling theirs for less than the price from Fender parts, I can't see sense in buying the Hammond.

[bob p]

... an Alessandro clone arrives! Burnt screen resistor, bias jacked up to 32/35 mA @ 420 volts! Bad speaker jack, and broken wire in footswitch plug. Customer sends me a thread with Mr. A where he tells customer to bias his 6V6's and indeed "all 6V6" to 30 mA. The construction seemed a little flimsy and the lead dress will give me nightmares. Customer says it came biased the way I found it from the mod service. All very strange.

That is indeed very strange -- so strange that I'm having a hard time believing what your customer told you. George Alessandro's description of this rebuild services states that he rebuilds the classic amps to their original specifications using only premium grade parts, Switchcraft jacks, teflon coated wire and silver solder. His products look to be exceptionally well built; He uses custom fabricated boards that don't use eyelets. His layouts are impeccably orderly, his lead dress is very clean and his soldering is exceptionally clean. I can't imagine that you'd have any headaches with his layout unless someone else has been inside of the amp and changed things.

As an example of his work, here is the inside of one of his Deluxe Reverb rebuilds:



Considering the huge premium that people pay for his amps, most of his customers insist that the amps go back to him for service, unless they know of someone locally who uses teflon wire and silver solder and stocks similar premium grade parts. Most of the high-end amp people wouldn't want to pay for telfon wire, silver solder and low noise resistors and then have their amp fixed with anything less.

Regarding that comment about biasing to 30mA at 420 V -- it sounds like he's biasing the amps into Class A. That's not entirely unreasonable, but you can expect for the tubes to have shorter lives in Class A than in Class AB. It's no surprise that when one eventually fails it would take a screen resistor with it.

The bad jack and broken wires would make me wonder if the owner isn't hard on his gear. I would think that the average guy who buys an Alessandro amp wouldn't ride it hard and put it up wet.

Very strange. What does the logo on the front of the amp look like?

[Randall]

It looks pretty much like those photos, maybe not as neat. Personal preference I suppose, I don't mean to put down his amps, but my initial reaction was underwhelmed. Silver solder and telfon wire don't really move me for a tried and true imperfect circuit as much as solid construction, but that is me I suppose. There are thousands of great Fenders out there without magic parts. I suspect this amp is prone to oscillation based on when I was checking the speaker jack I plugged in a dummy input jack just to create some sound, and I found the volume of the noise floor changed drastically depending where my hands were, as in DECREASING quite a bit. as well as squealing when I placed them in other areas. I didn't probe for it though. It is interesting though that he builds with the stock iron from the reissues.