Ruby 6L6 valves in my Fender Hot Rod Deluxe
Ruby 6L6 valves in my Fender Hot Rod Deluxe

My Fender Hot Rod Deluxe III recently developed a glassy rattle. It was only evident at certain frequencies – prevalent when I strummed low chords or struck low notes. At first the rattle was faint and only occurred occasionally. But over a number of weeks the rattle grew in prominence and occurred almost every time I played a low chord or note.

I spent a few hours sitting at the back of the amplifier, strumming chords trying to identify where the rattle was coming from. It seemed to centre around the power valves (also known as tubes) that hang behind the steel cage at the back of the amp. My Hot Rod is 18 months old and only gets an outing a few times a month, but if they were the culprit I wasn’t that surprised. Valves are mercurial beasts. You have no way of knowing how long they’ll survive. They can last a couple of weeks or a couple of years. Power valves are more prone to failure than those in the pre-amp stage. The more you play and the louder you play, the greater likelihood your valves will wear out sooner rather than later. Combo amps, like my Hot Rod, are particularly cruel to valves. Unlike a head and cabinet partnership, the valves in a combo live in a highly violent, reverberating environment directly behind the speaker. Ouch!

Unless they stop working altogether, power valves begin to fail in a number of ways. Tell tale signs are:

  • your bottom end becomes loose and flabby
  • fluctations in power or power loss
  • loss of some top end
  • inconsistent sound
  • loss of punch
  • humming
  • rattling
  • miscrophonic noises.

Pre-amp tubes exhibit the following symptoms when they are on their way out:

  • hissing
  • humming
  • loss of gain in the gain channel
  • loss of dynamics and sensitivity.

If valves do rattle, the rattle will normally only be heard at low volumes. A cranked amp will drown the sound of filament rattle.

I used a quick test for tube rattle. I let the amp cool down and unplugged it. I unscrewed and removed the back panel to expose the power tubes. I gave the one on the right a few gentle taps with my fingernail, nothing. Then I tapped the left power valve. Ahh…it rattled like a broken light bulb. Bingo! I’d found the cause: valve filament rattle.

I was a bit lucky with this test method. Often, tapping very gently on a cold tube won’t tell you anything. Tube internals expand when they are hot and contract when they are cold. So a tube may rattle when it’s hot and not when it’s cold, and vice versa. We are really only concerned if the tube rattles when the amp is on and the tubes are hot. To test for rattle when the amp is on you could very gently tap the power valves with something non-conductive, like a tooth pick. But be careful. Valves break easily. And never attempt to poke around inside an amp with the back cover removed and the power on. You are likely to kill yourself or at least blow up your amp.

As standard, Fender fit the Hot Rod Deluxe III with a pair of Groove Tubes GT-6L6 C(HP) valves for the power stage and three Groove Tubes GT-12AX7-R valves for the pre-amp stage. The GT-6L6 C(HP) tubes produce a tone and response that is warm in the mids with good harmonics, has a decent top and bottom end, and has some natural compression. I love the tone I get from my Hot Rod, but I fancied experimenting with a bit of a valve upgrade. So I did a bit of research and spoke to my local amp dealer. After much consideration I purchased a matched pair of Ruby Tubes 6L6GCM-STR valves. It’s important to get a pair that are matched for the amount of current they draw.

The Ruby 6L6s were packaged in a Peavey branded box and sold as Peavey Super 6 Power tubes. It is more than likely that they are available under a number of brand names. They can also be purchased as straight Ruby Tubes 6L6GCM-STR valves from a wide range of online suppliers.

The pair of Ruby valves cost less than £40 – outstanding value for money. After fitting the Ruby tubes – a delicate operation – my  Hot Rod has the same clarity, shimmer and chime it always had, but now has a bit more punch overall with a slight boost in the upper mids and a clean, tight bass response. Some Hot Rod owners argue that the Ruby tubes out perform the factory-installed Groove Tubes. They certainly offer maximum clean power and headroom. If you want to upgrade and stick with Fender’s stock variety, Groove Tubes sell a number of premium 6L6 valves. Many valve enthusiasts argue that the Ruby tubes closely match the tone and response of the much loved classic Philips-style STR tube. I can’t fault them.

When sapping out power tubes, remember to bias your amp or have a professional do it for you: