Hughes and Kettner 6L6 Guitar Valve Amp- Oscillation/ Rattle/ Reverb Issue

[Cowbell]

Hey,
I joined this forum today and I'm a hobbyist guitar maker from Australia.

I recently purchased a 2nd hand H&K 6L6 for $850 to use in my guitar making shop for testing guitars as they came off the bench and for some gigging (they retail in AUS for between $1900 and 2400 not including shipping/ postage)
When I tested the amp before I bought it I didn't hear any audible issues with the amp but when I got it home and plugged in a different guitar I was getting this odd metalic oscillation/ rattle not dissimilar from a spring or filament hitting glass that was audible when playing individual notes that had more bass in them (String A an D frets 6-10 were worst). So I instantly thought tubes or reverb tank (leak/ crack or the tank was cactus) were gone. I found out after calling around that the reverb tank would apparently cost around $70 to replace the part and roughly $80 per hour to fit it.

I then plugged in the guitar which I tested the amp with when I purchased it and got the same rattle. Crap!!!
I had the same issue with my H&K 20 watt (which was rectified under warranty and still works fine after 5-6 years and I adore the tone of which is why I bought this one as the price was right).

I replaced the preamp tubes (there was no audible difference) same issue...Then I tested the 6l6 tubes by lightly tapping them with a piece of plastic whilst playing the notes causing the oscillation (nothing audibly different occurred here either). Putting my ear closer to the electronics enclosure I realised it sounded as if it was coming from in there (where I would imagine the reverb tank is also housed as it is not housed on the bottom of the cabinet like some Fender Style Amps).

So the next step would be to take it to a technician and spend probably between $80 and $100 to diagnose the problem and suggest a solution. Which I will be doing. But before I do that I thought I'd chuck it on a forum and get another opinion...

So the question I pose is:
If this is a design flaw with combo amps/ the specific type of reverb tank/ tubes being so close to the dual speakers in higher wattage combo amps (60 Watt) would it be a better idea (or even possible) to MOD the amp by separating the head and the speakers into two separate cabinets. I.e. removing the valve head/ electronics and making a box for the valve head, reverb and then using the factory cabinet with the speakers provided as the speaker cab. Thus potentially avoiding future reverb issues down the track...Could you hypothetically ever just remove the reverb tank and use a pedal instead? Or is the reverb a crucial part of the circuit? I have practically 0 knowledge of valve amplifiers/ their electronic make up.

Any suggestions are much appreciated,

Stuart

[Mick Bailey]

Combos are always a compromised design and will vibrate components more than a similar head/cab setup.

Having said that, you need to locate the source of the noise before considering anything as drastic as splitting the combo into two. You have to assume that the amp worked ok when it was new. The simple way to elimitate the reverb is to turn the reverb control to zero. You can also remove the recovery tube (if it's tube based reverb) or unplug the tray.

If your amp has an FX loop try plugging your guitar staight into the 'return' - this should eliminate the preamp. You may need a booster or preamp pedal to get a loud enough signal - depends on the amp. If the problem goes away, then you know the problem is before the power amp section. If you still get the rattle it will most likely be within the power amp.

Rattles can be caused by loose elements within the tubes and this isn't confined to preamp tubes; your 6L6s may be ok when tapped, but the characteristics of vibration through the cabinet and chassis will be different and there could be mechanical resonance or sympathetic vibration that only manifests itself at certain frequencies.

A bad solder joint can also generate strange symptoms, but first try to narrow down the fault to a particular section of the amp.

[Cowbell]

Thanks for the advice Mick. The main problem it seems with these specific combo amps is that the reverb tank is apparently in the chassis and not fixed to the side/ bottom of the cab. The position of the reverb puts it within 4 inches of the back of the driver...

I think it is transformer driven reverb. I'll test some more to see if I can narrow it down

Stu

[Cowbell]

Have tried as you suggested (through FX return) on both the gain and clean channel. With Reverb knob wound all the way Down & also reverb wound all the way up. At a mid to high volume there is a rattle within the cabinet/tubes (cannot discern) on certain notes. Turning up the Reverb knob doesn't appear to exacerbate the severity of the rattle but merely adds colour to the rattle. Cheers

Stu

[Gtr_tech]

Combos will eat up tubes faster than a head/cab configuration. This is just the way it is. All the vibration from the speaker will rattle the tubes mechanically and loosen the parts inside, then they will rattle freely with vibration and cause the noise you are hearing. output tubes are more likely to cause you problems because they are larger and have more to them internal parts wise. They also get mechanical stress under normal operation from the expansion when in use/contraction when cooled down.

So, replace the output tubes and I'll bet ya lunch the noise will be gone.

[Gtr_tech]

As a side note...you may have cracked solder on the tube sockets which can cause similar racket. Check that first...and not just on the output tube sockets...*all* of them.