Old Fender Amp Dangers!

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

There are some dangers in using really old amps, even though they seem to work. This is especially true of the old tube amps, because of the high voltage involved. There were some old Fender amps made long before Peavey ever made a single amplifier. Yes, these old tube Fender amps work, but they have capacitors that are dried out and leak.
This means "anything" can happen. The problem is made even worse because most of these old amps have had the grounds removed from the power cords. This means that anything from 120 to 600 volts can come down your guitar cord. Lots of times this will happen on power up or power down. The old amp may keep working, but these high voltages can cause problems for other equipment in line, like effect units. I would strongly suggest having the electrolyic capacitors changed out on any amp over 15 years old. I would make sure there is a ground plug on the power cord. Make sure the case is grounded to the power cord. You can do this by checking continuity between the metal on the case and the ground on the power cord.

jim milewski · From: stowe, vermont

Kieth, I remember reading that, just damaged the switch on my LTD, what an array of wiring to it, what does the cap do, do i really need it, as the leg broke off, I had to stuff tin foil in it at a gig sat and the cap was busted off and it seemed fine

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Glad someone agrees with me on this one Keith!

If ever an amp comes in my shop that does NOT have a grounded power cable, it leaves with one. If the customer doesnt want that, then I do no work on the amp. Period. Granted there has been urban legend about the 'ground cap' switch on old fenders that had 2 wire cords beign called a 'death cap' is just that-- I've never heard of a single case there.

NOW as for having electrolytic caps replaced every 15 years or so, I HIGHLY recommend that. I've seen caps that are 40 years old that are still working, but why chance it. When the cap fails (notice I'm saying WHEN, and not IF), you could easily take out an EXPENSIVE power or output transformer, or both, and if the amp happens not to be grounded properly, well.... not good. A tune up on an old fender amp is well under $100 -- and that is replacing ALL electrolytic caps, any faling film caps, any noisy/drifted out of spec resistors (yes, the old carbon comp resistors in old fenders DO drift value over time, and become NOISY-- THEY are what make the old Fenders sound like a bowl of Rice Crispies with the popping, snapping and hissssss), rebias the tubes, clean the sockets etc. This is only an hour and a few minutes bench time for most techs, and WELL worth it.

One other note that should be mentioned, is that the filter caps are worked the hardest with the more low end you are pushing, so in other words, those that use old fenders for PSG, you REALLY are working those caps out.

... it's funny though, a guitar player/steel player would never consider playing a big gig with strings that are a year old (esp. the .011 G# *laugh*), yet they'll use that 30 year old fender with stock filter caps.

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Yes Gino,the ground cap switch on the old Fenders are called "Death Caps" for a good reason! Death, as in electrocution! Caps, as in old leaking capacitors! Even if you don't windup grave yard dead, huge voltages and currents coming down a guitar cord will cause damage--guaranteed. Try hooking up 600 volts at 20 amps to the output of your expensive effect unit and see what happens. Anything from the guitar along the connecting cord to the amp is in danger. Even an old pot pedal can't take 600 volts at 20 amps. Maybe that is why a lot of old pot pedals pop, crack, and sound like they have burned places on them. Besides safety issues,the added benefit of maintenance is a better sounding amp.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

I think a lot of guys are afraid of maintenance on an old amp becuase it may 'lower the value' or 'ruin the tone' etc.
However, I say this:
1) new filter caps will NOT lower the price of an amp, a replaced transformer WILL (which bad caps CAN fry a transformer). In fact, a 'vintage amp' that is playable would be more desireable than a vintage amp that is only good for a doorstop.
2) Humming, buzzing, hissing, popping and ghost noting amp is NOT good tone. The filter caps are NOT in the signal path, nor is a grounded cable, however these will keep the tone they way it was when the amp was built, and more importantly, as Keith is saying, your tone will be MUCH better if you are still alive to play the amp, than if you get fried.

Ken Fox · From: Nashville GA USA

A lot of people do not realize that caps are not supposed to pass DC voltage. As they get older and dry up they become a resistive path for DC. This loads down the power transformer. Eventually they short directly out and take a lot of expensive stuff with them! Also older Fender amps can have the bias current out of range. It is adjustable by a variable pot on the Blackface amps. On older Fender Tweeds, Browns, and Silverface amps a bias resistor may have to be changed out to get correct power tube bias. The electrolytic caps for cathode bypass drift upward in value, making the amp woofy sounding. They need to be changed out, too. Why is this important? Weak bias results in bad sound but a high bias current can overheat and eventually damage an output transformer and power tubes!

All said and done, tube amps need a lot of TLC to perform at their best. Get a grounded cord on that old amp and do a little preventative maintenance. Caps and resistors are like tires on a car, they wear out. How many 1955 Chevrolets have the original tires, does it hurt its value to change them out?

Those same rules for filter caps also apply to your solid state amps! After a while they shart humming from old filters and eventually can be damaged from shorted power supply caps.

You can spend upwards to $300.00 or more for a complete cap job, new tubes, bypass caps, pot and jack cleaning, a three wire power cord, tube socket/pot cleaning, tube socket retensioning and power tube bias adjustment. Sometimes that old amp is worth it, don't you think?

Rick Barber · From: Morgan Hill, Calif. USA

Obviously tube amps are here to stay. I am working on making use of control technology that will keep the currents regulated and balanced in push pull tube amps using opamps and DC feedback loops. Even as tubes age, crossover distortion will not degrade do to unequal bias currents through the output tube pairs. With these controls I think we should be able to go to toroidal transformers which are much lighter and with these control circuits we can avoid saturating them. We'll see.

As for caps I agree , they will get tired, so you will just have to do preventive maintainence. I wouldnt wait for a failure.
When replacing we have to make sure the voltage rating on the caps is much higher than what we need and when installing we need to watch the polarity of the caps (electrolytics have a + and - marking ) and so you have to install them correctly. This must be done by an authorized repair center for your amp even more so because of the deadly voltages in the amps. Also those caps will hold charge even when off so the high voltage will still be there and that can also give you a dangerous pulse even if the amp is unplugged. Some circuits have bleeder resistors which eventually help drain off the charge on the caps but dont assume anything.

Ground the chassis. If it doesnt have it or if the third ground prong is cut off or you are using a ground lift cheater , dont use it. For tube amps safety is not optional it is a must to avoid electrocution.

Best Regards,
Rick Barber

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: