Crate Voodoo 60 combo Blue

[bsco]

Good day ladies and gentlemen....anybody here have a schematic for this model of Crate......this one uses the 12AU7 tube....has 5 pre-amp tubes in total....with two output tubes.......any help greatly appreciated.....
Cheers

[Jazz P Bass]

Here you go.

Crate BV 60.zip

[Enzo]

Or this version:

[bsco]

Thanks guys....This is the correct one.....the one I had must be for a different one....now let's see how much trouble I ca get myself into here.....Thanks for the correct schematics....
Cheers

[bsco]

Ok. I had a chance to check this out....I found a resistor opened up in the power supply....I think it was something like 630 ohms at 3 Watts....it was open so i replaced that...the amp fired up and plays good but when the volume of the clean channel is turned down to minimum, there is a low level noise there like bacon frying or a fire crackling...which usually points to noisy plate load resistors and or noisy pre amp tubes....So I replaced all the plate load resistors in the first 3 stages....the sound was still there...I subed out the tubes with new ones...same thing....the over drive channel is fine....so I must be missing something....I even cleaned the sockets but same thing....if I turn up the volume slightly, this noise will get masked by the electronic noise associated with turning up the volume with nothing plugged into the input....If I pull V3 the noise is gone.....can't find any components out of spec.....not yet anyway.......I tried to scope this noise as well but nothing shows up on the scope screen.....and I had it set at 50mV/ div....anybody interested in tossing around some ideas???
Cheers

[Enzo]

50mv/div? Is your probe on X10? WHich means .5v/div.

Noise signal need not be large to hear, so knock that down to 5mv, see what you see.

Instead of just replacing a bunch of parts willy nilly, isolate the problem. You say turning the volume up maks this noise by increasing other noise. Great, so turning the volume to zero eliminates that noise leaving the bacon frying. That means the bacon is after that control.

Out of spec parts don't cause noise, noisy parts cause noise. A 100k resistor can measure precisely 100k and still be noisy as hell.

Pulling V3 kills the bacon? OK, so it is either at or before V3. Does pulling V4 kill it? How about V2? The master only works in drive channel, yes? meaning in clean channel volume control is way back in V1 stage.

Plug a guitar into the FX return and zero its volume control. Noise gone? That will point to or eliminate half of V3.

[bsco]

Thanks Enzo.....I will try what you have suggested....I had run into times where I had noisy plate load resistors and even with the volumes turned all the way down, you would still hear this crackling sound........anyway, the scope probe was on X1...If I pull V1 and or V2, the noise is still there....if I pull V3 it goes away.....yes, the master is only for the drive channel....I'll check out that return jack with a guitar and let you know what I find....
Cheers

[bsco]

Ok Enzo....When I plug the guitar into the return jack...it plays fine......and that noise goes away.....unplug the cord and it returns...so it is in the pre-amp section somewhere......I printed off the schematics and enlarged them to 11x17.....to make it easier to read.....that never quite worked out as that part of the schematic is very light and doesn't lend itself well to enlargements and darker print......I have to sit down with the schematic and copy on the computer and mark in the component designations.......
Cheers
P.S. I double checked the setting on the scope that I was using......it was set to 5 mV/div.....couldn't see any noise at all......I ran a cable from the send to another small guitar amp and the noise is there.....pulling V3 kills the noise.....it is not the tubes...it is something in the associated components somewhere......

[g1]

What happens when you pull V4?

[cjenrick]

bacon frying can come from tubes also, easy to check,


no spare tubes? use the ones you got, plug v3 tube into v1 etc

live in the rain? maybe a lil deoxit in the pin holes,

oh boy, J176 FETS, could be trouble, why do people use FETS in hi voltage gear?

can anybody testify?

[Mick Bailey]

The gear may be high voltage, but the switching FETs usually operate within their design limitations. Unless there's a fault elsewhere that causes excessive voltage they're happy switching away for year after year. Think of all the Fenders that have them and how infrequently they fail.

[Enzo]

He swapped out tubes in post #5.

oh boy, J176 FETS, could be trouble, why do people use FETS in hi voltage gear?

can anybody testify?

"High voltage gear" does not consist of only high voltages. The JFETs are always in low voltage circuits. Q15 for example is the reverb mute. It is a shunt to the signal path there, and is at line level, since the reverb out goes to a cathode follower to drive the FX send. On the preamp page, Q1-6 are all also on the signal path. Q1,Q2 enable cathode bypass caps, and so have only a couple volts across them ever. Q3 puts a 15k in parallel with the grid return of V2a and never sees much. Q5,Q6 are in the ground legs of the tone stack and the volume control, enabling them when needed. They are also at line level for signal, as those controls feed into the same cathode follower as the reverb.

So none of those JFETs ever sees more than a few volts. As Mick pointed out, there could be some heinous short circuit, but then that could happen to any part in an amp, not just JFETs. They are effective and reliable.

[cjenrick]

well i had an amp where the gates were not protected, kept getting spiked by relay coil flyback,

plus if people don't use esd precautions, you can get failures regardless of the voltages involved, why not use dependable 2N parts?

[Enzo]

Don't blame parts for poor design. I don't usually see relay coils wired to JFET gates.

JFETs act as a low resistance bidirectional path for current/signal. Bipolars put rectifying junctions in the path, not the best for an audio switch.

[g1]

why not use dependable 2N parts?

You mean for Fet's? Things may have changed, but of the hundreds of Fet failures I saw, usually in muting applications, the most common was 2N5638.
I don't consider fet switching the most reliable method, but I think the cost wins out for the manufacturers, even after they consider warranty costs of fet failures.