Advice on old tube amp please?

Michael Miller · From: Virginia

I just got an old Dickerson lap steel/tube amp combo. The amp is pretty basic. No controls whatsoever. Plug it in and it comes on. Problem is it is very weak. Speaker cone looks fine, tubes all light up but you can barely hear it. I know these are low wattage but I know it should put out more volume than this. Where do I start to troubleshoot? BTW, this is my first tube amp and I know nothing so any advice is greatly appreciated. Thanks.

C Dixon · From: Duluth, GA USA

Michael,

If changing the tubes does not bring back the volume, and you are not familiar with testing electronic components, it may prove difficult to solve the problem yourself.

You are correct in stating it should be louder than what you describe that it is. Even cheap amps were more than loud enough until Elvis came along and R & R tried to deafen everyone by playing horridly loud.

If you are at all adept at electronics, I will share with you the following.

In my years of servicing amps, the most commmon cause of amps decreasing in volume, was resistors increasing in their rated value. So if you have an ohmeter, simply measure them (if you know how to read color code), and see if one or more has increased well beyond their rating. Make sure unit is unplugged. Ignore small changes from their rated value, since many resistors were rated + or - 20%.

There is a reason for these resistors going bad. When electronics first came into existance, resistors AND capacitors (the mainstay of electronic parts) were made of materials that invited failure. And that failure was caused by moisture.

Today, that is rarely the case, since caps are coated with Mylar or other materials impervious to moisture intrusion. And resistors are coated and often made of a plastic material that just does not let moisture in. Incidently Allen Bradley was the first company to make a resistor that did not go bad (assuming it was not overheated).

So it is rare for them to go bad nowadays. But on older amps, this was not the case. As such, resistors were prone to increase in value as moisture permeated them over time. The most severe moisture occured as the amp cooled down AFTER being on for a while where condensation was a real evil.

So again, if you do have an ohmeter and know color code, I would simply measure each one you find. If you find one that is a great deal higher than its rating, replace it.

While capacitors can cause low volume in certain instances, usually they don't. They usually cause hum if they go bad, unless they are "cathode bypass" caps, then they can cause a substantial fall off in volume. It is very hard to find a cap that is shorted or "leaky" (partial short), without being an electronic's tech. Because of "parallel" circuits, most have to have one lead disconnected to measure them accurately, and this can get very involved. NOT recommended for anyone who is not an electronic's tech.

A good way to see if a cap has opened up and causing the probelm; is to simply parallel it with a good cap while the unit is on. If the original cap is open, you will know it instantly when you parallel it with a good one.

CAUTION: While lethal voltages are used in AC powered amps, it is very rare that anyone would actually be killed if they touched such voltages.

BUT, a powerful shock CAN be had by working on amps with the power on. So please use extreme care if you do work on them with it plugged in. If you have ever gotten across a 400 volt B+ line, you will wish you were dead for a few moments

Been there done that more than once, I can garontee ya!!

Good luck, and may Jesus bless you in your quests,

carl

Jay Ganz · From: Out Behind The Barn

If you take it to any decent amp technician
he should be able to check the voltage
readings on the pins of each tube to
point him toward the problem (which
could be one of the tubes itself).
If not, then possibly one of the components
hooked to a tube (as Carl mentioned).
If all the voltages are in the ballpark
& you're sure the speaker is good, then it might very well be a partial short in the
output transformer. That's what feeds
all the sound to the speaker.

Michael Miller · From: Virginia

I guess I will hunt down a technician. It has a constant hum as well, so it may be a capicator. Thanks for the input gents!

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Ahhh! Constant hum...low output. Probably a bad filter cap.

By the way, here is one of my "tricks" that I will share, with the warning that only those with some electronic experience use it. Leaky coupling capacitors will usually heat up...not a lot, but enough that you can feel the warmth by touching the body of the capacitor with the back of your finger. Just remember not to be touching the chassis, the capacitor's leads (or anything else conductive) when you do this!

Bill Hatcher · From: Atlanta Ga. USA

Carl is probably right on about the resistor values out of spec.

Look for a large load dropping resistor that is discolored or the color codes appear dark. Probably WAY out of spec, overheating and could be dropping voltage too much to the rest of the amp. Some of these old small amps use these.

Best thing to do to any old amp is to change the filter/electrolytic caps before you do anything.

Mike Bieber · From: New York, N.Y.

Remember, also, that a leaky or worn-out filter cap is capable of giving you a lethal shock. Even playing though an amp with bad filter caps will kill ya. Be careful.

CrowBear Schmitt · From: Ariege, - PairO'knees, - France

Trade it in Michael !

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