Working on a AA270 circuit for a 1971 Vibrolux Reverb.
The customer complains of a harshness that gets introduced when the amp is loud enough to start breaking up...which is pretty loud, so I can't really hear the problem. In fact there may not even be a problem! Just not well enough acquainted with this amp to know what it is capable of.
Also the vibrato circuit intermittently produces a pronounced ticking noise. I haven't been able to reproduce the issue, but the customer has noticed it before.
Checked under the hood, the HT power supply caps have been replaced but everything else is stock...bias caps, cathode bypass caps (original white body Mallory), etc. Probably will replace all of these.
any clues?
Thanks!
adam
The customer complains of a harshness that gets introduced when the amp is loud enough to start breaking up...which is pretty loud, so I can't really hear the problem. In fact there may not even be a problem! Just not well enough acquainted with this amp to know what it is capable of.
Also the vibrato circuit intermittently produces a pronounced ticking noise. I haven't been able to reproduce the issue, but the customer has noticed it before.
Checked under the hood, the HT power supply caps have been replaced but everything else is stock...bias caps, cathode bypass caps (original white body Mallory), etc. Probably will replace all of these.
any clues?
Thanks!
adam
Hi adam,
It sounds like a common oscillation problem but if you're not able to reproduce the ticking, maybe it's only audible with very high control settings.
Without any guitar plugged in, does the ticking show up when volume, intensity and speed are cranked all the way up?
If so, sometimes changing a little the lead dress of that section of the amp gets rid of the oscillation. Is the amp board dusty or dirty? Sometimes that makes the oscillation signal leak into the audio signal.
Hope it helps!!
Fran
It sounds like a common oscillation problem but if you're not able to reproduce the ticking, maybe it's only audible with very high control settings.
Without any guitar plugged in, does the ticking show up when volume, intensity and speed are cranked all the way up?
If so, sometimes changing a little the lead dress of that section of the amp gets rid of the oscillation. Is the amp board dusty or dirty? Sometimes that makes the oscillation signal leak into the audio signal.
Hope it helps!!
Fran
If one or both 6L6's are low, because the bias to the grids are modulated with the vibrato, they may run into grid current, causing a ticking noise. Check the bias voltage and ensure both valves are balanced. If C36 is low capacitance, the bias voltage will be too low. Make sure the valves are 6L6GC.
Thanks guys!
The board is an eyelet board, typical CBS era wiring...I've come across dirty circuit boards causing issues, but where can leaks occur in this type of setup? Cranking the volume and vibrato controls did not induce the ticking noise.
I was also suspecting oscillation, and was thinking that replacing the 3 vibrato ceramics (.01, .01, .02) with all .02 (added effect of slowing the vibrato a hair), plus replacing the cathode bypass caps would cure this issue...but now I'm rethinking that approach.
Measured the tubes..both 6L6's in place, one was pulling 46mA and the other 60mA. A quick turn of the balance pot brought both to 54mA (@ 395v plate voltage). Perhaps this unbalanced draw could have caused the harsh distortion?
The board is an eyelet board, typical CBS era wiring...I've come across dirty circuit boards causing issues, but where can leaks occur in this type of setup? Cranking the volume and vibrato controls did not induce the ticking noise.
I was also suspecting oscillation, and was thinking that replacing the 3 vibrato ceramics (.01, .01, .02) with all .02 (added effect of slowing the vibrato a hair), plus replacing the cathode bypass caps would cure this issue...but now I'm rethinking that approach.
Measured the tubes..both 6L6's in place, one was pulling 46mA and the other 60mA. A quick turn of the balance pot brought both to 54mA (@ 395v plate voltage). Perhaps this unbalanced draw could have caused the harsh distortion?
Not related to the ticking exactly, I've noticed 40 year old white case Mallory electrolytic caps are ****. I've got dozens of them in two organs, and they make them sound like kazoos until replaced. They throw the tone shaping circuits way out of balance when the values are off, and the tone changes with temperature radically. I use polyprophylene film caps or polyester if I can't afford or fit PP,
I once had a similar "ghost" problem in a deluxe reverb, late issue, early eighties ( I was told). The guy said the amp would just cut out, occasionally, for a couple seconds. I played that amp half volume for almost 45 minutes and it never did it. So I changed the speaker output cable/plug because it was a piece of telephone wire (yes, plenum, 24guage!) with a straight plug. I replaced it with a proper set of 18AWG wire with a right angle plug. I thought that the original (I should say the poor "replacement" cable/plug) plug might have been hitting the wall on which the amplifier was leaning against. Later I decided to measure all the voltages inside the amplifier and while doing so noticed that there wasn't any voltage after the first series resistor (after the first HV node). I ended up taking the capacitor cover off and tried to trace the voltages on the cap board. There was 400 something on one side of the resistor and nothing on the other side! It turned out that the pin of this 2W resistor had become "floating" in the eyelet thereby making an intermittent contact (probably changing with heat, as the amp never did it cold). The amp had also fallen down a couple of times, so I am suspecting a poor initial soldering had become eaten up by flux (lots of yellowing on the solder joints) and loosened.
Anyway, my whole point is you might be in presence of such a problem... And I know it's hard to find in tube equipment because you can't just move things around with your hand without touching lethal voltages!
Anyway, my whole point is you might be in presence of such a problem... And I know it's hard to find in tube equipment because you can't just move things around with your hand without touching lethal voltages!
If you can't play it loud enough to hear the distortion, use a dummy load and scope it. You may find a soft filter cap or two is allowing too much ripple on peaks. Or you may find some intermediate stage is clipping.
Ticking trem. Look. DO you have a 10 MEG resistor from the plate of the trem tube? That is a tick suppressor. it is probably there though. I find the main reason for ticking trem is lead dress. With the trem running, try moving the wires from the trem tube socket and see if you can find some ticking. Then move them as far from that position as you can.
Also, on this and many other fender amps, you need the footswitch to turn the trem ON. On the bench I usually just grab a clip wire instead of having a footswitch and cable in the way. But we do need to check it with the footswitch to be thorough. The footswitch also controls the reverb by shorting the reverb pan return. The shielded reverb lead and the unshielded trem wire are in the footswitch cable. The reverb line shield is also the return for the trem wire. It is possible some trem noise is coupling into the reverb line. Turn the reverb up and see if the ticking emerges.
Ticking trem. Look. DO you have a 10 MEG resistor from the plate of the trem tube? That is a tick suppressor. it is probably there though. I find the main reason for ticking trem is lead dress. With the trem running, try moving the wires from the trem tube socket and see if you can find some ticking. Then move them as far from that position as you can.
Also, on this and many other fender amps, you need the footswitch to turn the trem ON. On the bench I usually just grab a clip wire instead of having a footswitch and cable in the way. But we do need to check it with the footswitch to be thorough. The footswitch also controls the reverb by shorting the reverb pan return. The shielded reverb lead and the unshielded trem wire are in the footswitch cable. The reverb line shield is also the return for the trem wire. It is possible some trem noise is coupling into the reverb line. Turn the reverb up and see if the ticking emerges.
Thanks Enzo,
I finally got some new parts for this amp (cathode bypass caps, replaced all ceramic discs, bias supply caps, etc) and I've got it back on the bench. Cranking the reverb and moving the lead dress around did not cause the ticking noise.
Do you have it in you to elaborate on the above procedure?
Signal, 1K ok? What input voltage? where should I look within the stages and what am I looking for? I'd love to get this process down but can't seem to find decent instruction.
Thanks!
I finally got some new parts for this amp (cathode bypass caps, replaced all ceramic discs, bias supply caps, etc) and I've got it back on the bench. Cranking the reverb and moving the lead dress around did not cause the ticking noise.
Do you have it in you to elaborate on the above procedure?
Signal, 1K ok? What input voltage? where should I look within the stages and what am I looking for? I'd love to get this process down but can't seem to find decent instruction.
Thanks!
To add, here are some new voltage readings:
B+ 396
Plates 391
Screens 390
Cathode current 56.5mA
Driver plates 257
Input buffer plate 216
Preamp plates 205,210,210,220
Vibrato plates 383,335
So the output section appears to be starved a bit, according to the schematic anyway (about 50V low on the plates and screens. Funny, by the time it was printed the input power supply voltage was already 120vac.
B+ 396
Plates 391
Screens 390
Cathode current 56.5mA
Driver plates 257
Input buffer plate 216
Preamp plates 205,210,210,220
Vibrato plates 383,335
So the output section appears to be starved a bit, according to the schematic anyway (about 50V low on the plates and screens. Funny, by the time it was printed the input power supply voltage was already 120vac.
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