Help with diagnosis...

[agdrago]

I have a Roland Blues Cube 60. The amp works perfectly when it is first turned on, but then as if by a function of how hard it is driven and how long it has been on it develops a ticking sound reminiscent of a playing card against the spokes of a bicycle wheel. The sound of the guitar can still be clearly heard as well as the ticking noise.

The cadence of this tick changes based up on the attack of notes played. It also occurs in both preamp channels as well as if the guitar is fed just to the power amp via the effects return.

The guaranteed way to cause the ticking is to hit a 6th string 5th fret or a 5th string 7th fret harmonic. As the note decays the ticking slows and then abruptly disappears.

Any and all help is appreciated.

Thanks,

- Grady

[J M Fahey]

It *might* be a mechanical/connection problem, specifically one of the  flexible leads joining the speaker's solder terminals to the voice coil itself , don't know the English name for that, but not very likely.
The strongest suspect is a very high frequency oscillation, not continuous, caused by a marginally stable amplifier,  triggered by the amp driving a complex load such as a real-world speaker at certain frequencies.
Unfortunately I don't know how to "remote-control" help you, since it's an elusive problem and involves trial and error.
*If* you have access to an oscilloscope and signal generator, drive the power amp (as you correctly guessed the problem is there) sweeping the frequency, say, between 70 and 300 Hz; at certain points you'll see the clean sinewave "growing a mole", a small oscillation superimposed on the large one, or a sudden "jump", much smaller than the clean signal itself, that's heard as the "tick".
Below a certain level or a other frequencies it does not appear.
I´ll dig the schematic to see if I can offer some practical suggestion.

[agdrago]

Thanks for the response.

I checked that the speaker was not the issue by playing through a remote cabinet. Unfortunately the same behavior was noted playing through known good speakers.

Performing the test mentioned above helped me to identify that a component of the ticking sound is not coming through the speaker but instead from a component in the amp. I'll remove the amp from the cabinet and see if I can identify what component that sound is coming from.

I do not have access at this time to an oscope or a signal generator (other than a guitar/bass/keyboard).

I have ordered a service manual which will be here later this week. I will post the schematic for the power section if I can provide additional info on the state of the amp.

Thanks again.

[teemuk]

http://www.blueguitar.org/new/schem/misc_amp/roland_blues_cube30_60.pdf

[agdrago]

Thank you! I Googled an Yahoo'ed and found no schematics on the web so I cracked and ordered one from Roland.

If this is the same service manual as I ordered (probably is) they've got some nerve given what they charge for it.