Tube amplifier bad caps

@hitman_hifi 

Refer to your description of fault symptoms:

Hmm...now I'm second guessing. A lot of uncertainty as I've been told it's definitely the filter caps. However, the amp is not even 10 years old and I've been suspicious of this assessment. I've seen what cap leakage looks like and they definitely look fine.

I'll try and explain the problem in more detail. Upon startup, the "bacon frying" sound gradually builds and becomes constant once the tubes are lit. The power tubes flicker with every pop and crack. This symptom used to be only in the right channel and now it's in both. As indicated, I've swapped all of the tubes with new ones. I'm afraid I've probably ruined the new tubes because of this.

I purchased this amp used from a local hifi company that is out of business now. I live in a small city so finding a tech that specializes in tube amps will be difficult. Are there any fellow Canucks out there that know of a tech near Regina? As a last resort, I may end up contacting the UK manufacturer and seeing what can be done.

Explanation to similar fault referenced above from bradf:
I had very similar experience with my integrated amp.  Trouble shooting revealed resistor in circuit to -B rail voltage had failed. The failure was gradual and was a result of one of the leads becoming loose in the resistor to the point that the body of the resistor could be rotated while the lead remained stationary.  There are two rail voltage fuses close to failed resistor. I suspect while removing the fuse cover(s) the resistor may have been contacted and damaged mechanically.  I am the 2nd owner of this amp so not sure what else may have taken place to cause this resistor to fail; the resistor was found bent over to one side, not perpendicular to the board.
If your having or ever experience similar problem, I would start by checking the rail voltage.  After complete failure of said resistor, there were 0 volts at the rail supply.
Hope the above info may help someone else down the line that may experience the same problem 


Brad

If any other member may know what was the fault to the above referenced fault and corrective action, would appreciate you posting it here


thnx
Brad

hitman_hifi

Wanted to know what you (or Tech) found to be the problem that was causing the above referenced  failure generating crackling sound.

Thnx
Brad

Atmasphere-> thx for the assessment. I'm leaning towards a bad contact somewhere. It turns out I've found the local company (formerly the company that went out of business that I purchased the unit from) that is the importer for CR Developments products. I'll make sure to get their assessment and post it here for reference purposes.

Thanks again guys for your help!

I would pay attention to Ralph's (atmasphere) comments.

Atmasphere I like your style ..no BS

Man, this one sure sounds like a bad power switch to me! It **does not** sound like bad filter caps! It *could* be a bad tube, but the flicker really suggests that the contacts on the power switch are arcing and permanently damaged. I would fire the tech that gave you the bad filter cap assessment, and get the power switch replaced.

Just a quick observation that bad caps don't necessarily have a visible external leak. I've seen plenty of way out-of-spec ones that appear physically fine.

Do you have a voltage meter? Do you have a schematic that indicates voltages at various test points? (Keep in mind that voltages in a tube amp are often in the 400 to 500 volt range, or even more.) The key is to start a logical testing process to run down the problem.

Cleaning switches is a good idea (and won't hurt anything) but there are a number of possible causes for the noise. A bad solder joint, a resistor damaged from overheating, a mechanical foil trace failure, a smaller signal cap - any of these or others could be the culprit.

You have probably NOT ruined the tubes because of this, unless one of the conditions below occurs. If your tubes are driven into oscillation over an extended period, then I would worry. Also, if your tube bias circuit goes bad somewhere and the tubes glow red-hot for an extended period, I would worry then, too. I am not sure if this is comprehensive, perhaps someone else may add to this. As far as my experience and my father's advice to me on a problem with my amp goes, this about covers it. He is a retired EE professor by the way.
Are your tubes glowing red-hot, or noticeably brighter than usual? If so, and you let them run this way for a while, get them tested. Otherwise, see how they sound after you have fixed the crackling problem. They should be fine.
Good luck with your amp!

Hmm...now I'm second guessing. A lot of uncertainty as I've been told it's definitely the filter caps. However, the amp is not even 10 years old and I've been suspicious of this assessment. I've seen what cap leakage looks like and they definitely look fine.

I'll try and explain the problem in more detail. Upon startup, the "bacon frying" sound gradually builds and becomes constant once the tubes are lit. The power tubes flicker with every pop and crack. This symptom used to be only in the right channel and now it's in both. As indicated, I've swapped all of the tubes with new ones. I'm afraid I've probably ruined the new tubes because of this.

I purchased this amp used from a local hifi company that is out of business now. I live in a small city so finding a tech that specializes in tube amps will be difficult. Are there any fellow Canucks out there that know of a tech near Regina? As a last resort, I may end up contacting the UK manufacturer and seeing what can be done.

Thanks guys for your help.

My guess is that the caps are fine. One of the switches on the front is the likely culprit. Move the switches back and forth and see if the sound crackles. If you find the one, use a cleaner like Rat Shack zero residue cleaner or a Caig product, or CRC. This should do it. Caps do not go bad often. If they go bad, they usually leak a yellow hard goo on the board. Replace quickly. This is an acid and will ruiin the board over time.

my experience has been that when filter caps get old you will hear more 60hz hum or buzz from the speaker. the problem you describe does not sound like a bad cap. It does sound like an issue with the power supply however. I agree with the others. Get an estimate with a qualified tech.

I think your better off finding a qualified tech and have him troubleshoot the problem. All of the caps? that seems unlikely as the unit looks to be fairly new. Even at 40 to 50 dollars an hour I doubt the bill would be more than $200-300.

If you are convinced it is caps, the larger ones are the more likely candidates. However, you are going to have to remove the circuit board to get to them as there is likely a solder connection on the other side of the board.

That means moving and/or disconnecting other wires. That photo also shows pretty tight construction quarters so you will need to be very careful with your soldering. You wouldn't want to short a trace or heat damage a sensistive component.

There are also other factors that can cause problems. You could end up replacing caps and not fix the problem. The best advice is to get a service manual and trace through the circuitry with the appropriate test equipment and precisely locate the problem.

Professional shops may not work for free, but they should know what they are doing. Good luck if you decide to undertake this yourself.