Vibro King Issues - Round 3

[mikeboone]

In a previous thread I mentioned that there was a strange oscillation or pulsing and occasional squeal when the amp was turned up at or near 10. I replaced V2 with a brand new 12AX7 and the squeal along with some other artifacts are gone. I also found that the amp was under-biased by a significant margin. The voltage was at -62V where the schem suggests a reference on -46V. I set the voltage to -46. The amp sounds very good. I will use my bias rig to tweak it. The pulsing problem at high volume remained.

After further listening, the pulsing is very much like the vibe circuit but inconsistent in frequency. I had orginally ruled out the vibe circiut as is was switched off.

Observation: I got curious and cut the lights, when I strike a chord at high volume levels the vibe lamp is indeed pulsing thus explaining the pulsing effect.

Note: The problem is not present when I plug the guitar into the Effects Return.

Note: I do recall that you guys mentioned that C34 & 35 are suspects. The new caps will be in Monday. I just thought I'd mention the vibe lamp detail in case I am missing something.

Is there some voodoo in wire dressing or some other common cause for this? I have tried moving some wires around to no avail.

I am getting very close to having this amp in good working condition...

[TubeGuy]

Fender vibratos often cause ticking noises - whether the vibrato is turned on or not. Its a common problem and its the first thing I do to any Fender amp I open up. There is a Fender service bulletin on it, and I have yet to see following everything in there to fail to cure the issue. If you google it you should find it.

[Alex R]

...but the vib optocoupler lamp (looks orange in colour, inside the 'roach') is meant to run for the whole time anyway. If the pulse you hear is not in time with the lamp then the lamp is not the cause.

Sounds to me rather like 'motorboating' (low frequency internal feedback) - in which case, yes, C34 and 35 should be changed as a first step, as the cause is an unwanted coupling back between stages in the amp, and part of the function of filter caps is to suppress this by passing signal to ground and thus preventing it from travelling through the power supply and down onto another grid and feeding back around and around.

I'm not aware of lead dress problems inside this amp; it's a pcb with not too many long leads - that's something you're alleged to get in silverface amps though I've seen very little of it personally. But the vk has a gainy little preamp and it is prone to internal feedback/oscillation problems, especially with so many preamp stages using the same power supply node ('X' on the schematic). I know this because I've tried to handwire one, and I most certainly had feedback problems with it for a while.

Microphonic preamp tubes generally cause squeals not motorboats, which is perhaps why the squeals went away when you changed the tube.

I'd say that at -67v your amp was over- not under-biased; running a bit too cold. Just a terminology thing. If plate voltage x current per tube = less than about 20 you're ok. Less than 15, it's too cold/overbiased. But you knew that.

[mikeboone]

Thanks... I will return, hopefully victorious, to this thread after the caps come in.

Mike

[WholeToneMusic]

Ticking

The ticking is usually caused by improper lead dress. Try dressing the leads to the Vibrato, Speed and Intensity controls away from the Filter leads and Tone Control leads. Also bunch together the component lead wires near the fiberboard that go to the vibrato tube (12ax7). This should fix it, If not you can install a .01uF 600 volt mylar cap from the 10 Meg and optocoupler are joined to the negative side of the 22 uF cap located to the left of the Optocoupler; if your looking from the rear of the chassis.

Time should be the only thing ticking away