| Author | Message | ||
| jeff in s.f. |
Before I start messing with repair shops and tube replacements I'm curious if this symptom has an obvious source. My amp is a '74-75 410-130 with a 12AX7 and EL44s. The tubes are 20 year old Rubys but the amp was in storage for the last 15 years. The were tested and were still strong actually putting out slightly over 140 watts. My amp settings are: High power, Vol 4, Treb 9, Mid 3, Bass 6, Rev 2, Mstr 6. I'm using a '67 335. On a few notes, especially 12th fret B on the B string there is a distinct humming undertone while the note rings. Other than that all seems fine. I'm certain its an amp symptom because this doesn't happen when I play the same guitar through other tube amps. Is this old tubes, caps or something else? Much thanks in advance for your opinions. | ||
| jeff in s.f. |
I mean EL34s. Sorry. | ||
| Larry |
one way to find out would be if you had some EL34's and a phazer tube to test the theroy. Could be one tube, you could pull every other power tube and test them on each remaing pair. Yes 2 power tubes work fine in these amps. 12ax7 tubes are cheap, you could grab one for around $10 bucks to test it, if it hasnt been capped, you may need to cap it and also test bias voltages for any problems. | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
This problem could related to microphonics (vibrations causing unwanted electrical signals). Pulling back to two tubes (on on each side) is a great way to isolate a tube if that is the problem. Resistors, solder joints and capacitors can also become microphonic. Sometimes you have to feed in a signal that will cause the problem, then go through the amp (taken out of the chassis first) tapping, pressing and tugging on components and wiring until you can identify the source. Tired old speakers can also cause this. Speakers left to sag in the upright position can produce "ghost tones" when excited by the appropriate frequency signal. Steve |
Tue, 03/25/2014 - 09:27
#1