Premier Guitar

Diagnosing a Distressed Tube Amp

December 16, 2008

Hi there,
I have a Fender Hot Rod Deluxe that is just six months old and well looked after. It has recently developed a popping, crackling sound when warmed up—even when there are no guitars or cables plugged in. What is wrong? Could the valves be on the way out? Is it something more serious?
Please help!
Ken Morton

Hi Ken,
Thanks for your question. The Fender Hot Rod series of amps is a very popular one, possibly their most popular. More than a few have crossed my bench over the years, some coming in for a “crackling” issue. There are many types of failures that cause this symptom. I’ll go through most of the typical ones here and tell you which ones I suspect might be the cause of the failure in your amp.

1. Input, footswitch and effects loop jack connections.
Jacks are probably the most physically used and abused parts in the amp. They are soldered directly to the circuit board, but are also attached to the front panel via the jack nut. This is usually not an issue unless the jack nut loosens due to vibration. Once the nut loosens slightly, the weight and motion of the guitar cable is supported solely by the circuit board solder connections. Over time, these connections can be compromised and cause crackling and an intermittent signal.

2. Broken solder joints on large components.
Another vibration-caused failure, the solder connections of larger-sized components, such as large 5-watt resistors, can eventually be compromised from the constant vibration of playing, or simply a rough ride in your trunk or the band truck.

3. Simple tube failure.
Although a tube may look good from the outside, there’s no way to tell for certain if it’s good or bad just by looking at it. Tubes can fail in many different ways. Output tubes will generally short internally and cause a fuse to blow, or they can become mechanically noisy and exhibit a “rumble” sound through the speaker when vibrated. Preamp tubes, on the other hand, will generally either become microphonic and make high pitched ringing or squealing noises, or will emit noises such as crackling and popping during idling. If the level of the noise is controllable in any way by the setting of the volume or tone controls, then there’s a good chance that the symptom is caused by a preamp tube and not an output tube.

4. Stressed component failure.
Some components become stressed over time when the typical operating parameters of the circuit are exceeded. This can easily happen with the failure of an associated component. Take for example an output tube. While a typical 2-watt resistor (used by many companies as an output tube screen grid resistor) will function fine under most normal operating conditions, that resistor may be pushed past its capabilities by constantly playing the amp at very high volumes or by an output tube shorting. This may cause excessive screen grid current to flow through the resistor, stressing it or causing complete failure. A stressed screen grid resistor can definitely cause crackling noises to occur. (As a side note, this is one reason to overbuild amps: to ensure the least possible chance of failures—using a 1-watt resistor instead of a half-watt resistor throughout the amp, for example, or a 5-watt in place of a 2-watt, etc.)

5. General component failure.
Any electrical component can fail at any time, causing anything from compromised operation to a complete shut down. I did a bit of research on this question to see if there are any known failures of this particular amp that I had not personally encountered, and it turns out that there may well be. According to the research, there may have been a bad batch of resistors used as plate resistors in the phase inverter circuit of the amplifier that were responsible for crackling noises in some amplifiers— a very strong possibility for the cause of your symptoms.

Having described the most likely candidates for the cause of your amp problem, I would say that based on your description of the amp needing to warm up first, and no guitar needing be connected, the most probable in your case is either 3 or 5. The easiest way for you to do some basic troubleshooting would be to first purchase one replacement 12AX7 tube. Then, one by one, replace each preamp tube and see if that alleviates the problem. If you still have the symptom, you should next replace the output tubes.

For the purposes of troubleshooting, you can install a new set without re-biasing the amp, but if they are indeed the cause of the noise then you should definitely take the amp in for biasing. If neither of the replacement tubes cure the problem, the amp will need to be taken in to your local experienced tech for further troubleshooting— but at least you’ll have yourself a couple of spare tubes for the amp, which no self-respecting tube-amp-playing guitarist should be without! ‘Til next time…

Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
©2008 Jeff Bober