View Full Version : Amplifier Problems - Help!

05-27-2006, 08:19 AM
I have been beset lately with noises, and I suspect capacitors. Tubes may be another possibility. Line surges are somewhat unlikely but not impossible, as I have a Square D whole house unit upstream of the individual protectors the plugs go into. One of these could be bad. My signal path is so complex I need to narrow this down a little and any help is appreciated. The noises I describe are channel specific, but different channels!

The sounds are mostly cracks and pops, very similar to vinyl noises but much sharper and louder (nice definition, the 4345's reproduce them with unfortunately terrific fidelity.:D Go JBL!) There are some thumps, too. These all come and go. These are in the right channel.

Another class of sounds was so horrible and destructive sounding that I pulled the likely amp out of the path. These beauties were hissing building from low level to a really loud "sssssss" and ending with a very loud BANG!. Then normal operation until the next incident. So bad I feared damage to the speaker, but I got lucky. This may have been a 6260 as I haven't heard it since switching out the amp; so no tubes there. Still, I could not tell if it was on some or all transducers, so the 5235 is suspect too. This was a left channel problem.

I am switching components like mad to isolate the source, but I would still love to know what exactly to look for when I have the culprit component in hand. Jolida JD100 CD player (tube output), some non-electronic source switching, 5235 xover, 6290 low and Grommes PHI-26 (SET) high. All to biamped 4345's with stock networks, which I also suspect for no particular reason.

I am of course going to look for any visible arcs, but this takes time and I'm not there yet. Sparks make great signal sources, I've heard them elsewhere.

Thanks, Clark in Peoria

05-27-2006, 09:20 AM
Hey Clark, I can only speak for tube guitar amps, but other than capacitors making loud pops I have also run into bad plate resistors. Also loose sleeves in the tube sockets that need re-crimping/cleaning are a prime suspect.

I start pulling tubes one at a time beginning with the input side until the noise disappears, this will hopefully indicator a suspicious area to start the interrogation. :D

05-27-2006, 10:08 AM
These tubes are all new or in the prime of lifespan, but I will pull them and have them tested today. Hope I don't have to track down all those possible solid state components. Ever seen the inside of a 5235? Blue caps everywhere. Good lord, and that is the simplest piece in the system. Plate resistors, here come the schematics...

What makes this really difficult is the highly intermittent nature of the problem.

I should ask Zilch about his vast 6260 knowledge.

Thanks, Clark

05-27-2006, 10:25 AM
I've had a 6260 exhibit similar behaviour. In fact, my 6260 got an extensive rebuild, due to "woofer walk", pops and crackles, and other transient no-nos. Turned out to be fried resistors and bad caps, the usual suspects.

For whatever reason, 6260s seem to "cook" some of their resistors and caps, if left on long periods of time over a long term. Once they're replaced, everything should be fine...


05-27-2006, 10:35 AM
Kind of sounds like what my subwoofer amp has been doing. In my case, it's the protection relay that keeps tripping back and forth making pops and clicks.

05-27-2006, 10:58 AM
Check the +/- 15V supplies in the 6260 on the o'scope or measure AC ripple on them. More than 10 mV ripple, replace the 10 uF electrolytic caps on the output of the 15V regulators (7815, 7915), which are cooking nicely right next to them there.

Like Todd, I suspect the protection circuitry, which runs off them.

Got my replacements at Radio Shack. :D

Ken Pachkowsky
05-27-2006, 11:34 AM
I ran into a similar problem yesterday on my brothers new system. All the drivers were snapping and popping, sounding like they were blown. It was the fact they all sounded blown that convinced me the problem lay elsewhere.

The problem was the balanced input circuit on the Perreaux SM6. I tried three sets of xlr cables with the same results. Switching to single ended RCA's solved the problem. Obviously there is a problem with the balanced input circuit.

As many here have taught me, strip the system down to its simplest form and isolate the problem.

Good Luck.


05-27-2006, 03:44 PM
Well, let's see...

First, you guys are like having a whole bunch of fairy godmothers to help a guy out of a jam. Thank you all so much!:bouncy:

Secondly, while I hate to see anything go wrong for anyone, misery must love company because I don't feel so picked on by fate anymore. I'm not the only one with gear that usually works wonderfully but craps all over me when it doesn't.

Thirdly, I see I am not the only victim of a signal path inspired by the Gordian Knot. This hi-fi quest lacks the virtue of simplicity sometimes.

Lastly, my audio shop is closed for the holiday weekend so the tube testing will have to wait. When I find out what ails my rig, I'll be sure to share the prognosis. Feel free to start a pool or something.:D

Keep the wisdom rolling in, I'll bet this topic has a wider interest than just my personal need. We newer fellows haven't faced the problems you veterans take for granted.

You are a great bunch for sure.:)


05-27-2006, 04:34 PM
Secondly, while I hate to see anything go wrong for anyone, misery must love company because I don't feel so picked on by fate anymore.
You're not alone, I've been pulling my hair out all day on a '78 Marshall....but I think I found something...I think. :blink:

05-27-2006, 08:04 PM
You're not alone, I've been pulling my hair out all day on a '78 Marshall....but I think I found something...I think. :blink:

Wow, lots of pretty colors. The inside of my late 70's Mesa Boogie is dark, crowded and dank by comparison. Two meters, I'm impressed.

Good Luck Yourself, Clark

05-28-2006, 02:08 AM
This is definitely a tube amp-related problem. Sounds like screen resistor(s) to me. Had this happen on one of my tube guitar amps. Eventually the plate on one tube started to glow bright red... luckily I noticed before there was any damage done to the output transformer.

This does not sound like a bad cap because usually a bad cap will cause a persistant hum/hiss. Checking the B+ supply w/ a scope will show any problems w/ caps.

To narrow things down a bit try pulling all the preamp tubes. (Including phase splitter) If the noises persist then you know it has to be in the output stage, and it's most likely the screen resistor or one of your power tubes. If any of the power tubes begin to glow bright red SHUT THE AMP OFF RIGHT AWAY!

If the noises have stopped, start replacing the preamp tubes one by one, begining with the phase inverter, until the noises re-appear. (You can do this safely with the power on) When the noises return, it's pretty safe to assume that it's the preamp tube you've just replaced that's faulty. I'd wait a few minutes between each tube as they take a while to warm up to operating temperature and because many problems are related to heat.

Hope this helps!

06-01-2006, 07:46 PM
I am waiting for some test equipment to arrive to diagnose the 6260. Not necessary given the leads provided here, but I plan to have fun learning anyway.:)

The tube poping is on hiatus, and the reason will have some here saying "Of course, I mentioned that!" Last weekend I pulled the two tubes from the cd player and took the Grommes tube amp, whole, to test the tubes. As reported, the place was closed for the holidays. Went home and put everything back together. Not a bad sound since.

Either messing with connections or jostling the tubes did the trick - for now - and I am not going to move anything until something goes wrong again. The rig has never sounded better.

To be continued...;)

Clark in Peoria

06-09-2006, 12:52 PM
The hissing and the ramp up are indicative of a device charging and then a dielectric breakdown resulting in the large thump. The ramp up is identifiable with the function of a capacitor to store charge or an integrating circuit to accumulate power.

Typically, this type of symptom results in output power transistors having to be replaced in solid state equipment, because the input signal was too transient and in an uncontrolled peak/spike.

I would recommend not using your power amp to trouble shoot.

Then perhaps plug in a cheap pair of headphones,..the ones junior had with his walkman clone that nobody ever uses.

Begin trouble shooting by component and by channel.

Might see if it happens on different input devices first to isolate it to the control/preamp and or power amp.

The hissing might be a leaking capacitor, indicative of the material being physically damaged or cracked. As the capacitor continues to store charge, the dielectric material surrounding it no longer has the impedance required for it to perform as the circuit was designed.

When the Capacitor reaches a point where the dielectric breaksdown, the capacitor discharges.

If this occurs in a circuit in the preamp, where filtering isn't designed at that level to handle such transients, then the signal will pass through the circuits and the power amp will properly attempt to amplify the spike. Unfortunately, such spikes might also cause other unintended damage along the way.

There might also be some corrosion on internal swithcing elements that allows spurious noise, but generally if that happens the entire switch is faulty and needs to be replaced anyways.

About all one can say for sure from the symptoms is that by isolating your major components, and reducing risk of the fault being amplified to other components, you lessen the chance of corollary incidental damages from a cheap 50cent part being out of whack.