Fender Blonde Vibrato question

[bal704]

I'm building a Fender 6G8-A type amp. My voltages are higher than shown on the schematic, with B+ around 480 VDC.

http://www.prowessamplifiers.com/schema ... matic.html

I'm getting sound out of the amp, but the vibrato is not 'vibrato-ing'. Turning the intensity and speed knobs does nothing. I've never built a 2.5 tube trem, so I have a couple of questions...


1) the circuit has a vibrato jack, with one wire coming out of the circuit to the jack. I assume this wire gets connected to ground. Does grounding the wire turn the vibrato on or off? I've tried grounding the wire and letting the wire hang out in space. It doesn't 'vibrato' either way.

2) I'm assuming there should be a voltage changing proportional to the intensity and speed knobs. Where would be a good place in the circuit to see the voltage changing?

I have measured the speed and intensity pots, and they measure out at 3M and 10M, so I think they're good.

[sluckey]

Grounding the footswitch jack will kill the oscillator. If the oscillator is working you can see a changing voltage on the plate of the oscillator tube. An analog meter will show the needle moving up/down at a 3 to 8HZ rate. You can also see this sine wave on a scope. I can even measure the RMS voltage and frequency at the top of the INT pot with a Fluke 87 DMM.

I have a description of circuit operation for that circuit that may help. Look at page 12 of this pdf...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[Leo_Gnardo]

Quick things that may cause the oscillator to not oscillate:

A: poor connections to 12AX7 oscillator tube. Clean corrosion off the pins.

B: insufficient gain in 12AX7 osc. tube.

C: failed cathode bypass cap on oscillator triode.

D: failed or poor quality caps in oscillator circuit, the two .01 and .02 nearest the control jack.

These usually take care of a failed oscillator in an old amp. Wouldn't expect them to be a problem in a new amp with new parts. If you're using disc caps for the .02 and two .01's try good film caps instead, orange drop or Mallory 150. Fancy expensive caps not necessary, just good competent ones.

[bal704]

I'm assuming the left most tube in the schematic (nearest the vibrato peda ljack) is the oscillator tube.

Thanks for the tips. I will try them this weekend and report back.

[pdf64]

Yes, V3 of the original layout http://bmamps.com/Schematics/fender/twin_6g8a_schem.pdf when using the normal nomenclature.

[Phil_S]

B: insufficient gain in 12AX7 osc. tube.

Along with this, you need a good strong 12AX7 and not some other tube. The Ra factor is a big piece of the puzzle, which is why other tubes don't work. Of the 12a?7 family of tubes, the X has the highest Ra spec.

[bal704]

I have Vibrato!!! I had a dodgy solder joint on the ground of the trem tube. I now have something to work with.

This is a one input amp, and I added a PPIMV. I have a lead coming out of the preamp tonestack, and I can clip it into the circuit using an alligator clip. This way I can get a feel for how the normal and vibrato channels sound.

I noticed the Vibrato channel is much louder than the Normal channel. Is this normal?

[Reeltarded]

I have Vibrato!!!

I noticed the Vibrato channel is much louder than the Normal channel. Is this normal?

Nope. It's vibrato

[bal704]

But seriously.....I caught that after I typed it, hoping nobody would go there...Oh well, I gambled, and lost.

Is the large volume difference between channels to be expected?

[bal704]

I'm making some progress on this amp. The Vibrato/PI/Power section is working. This is going to have 2 channels (a channel switcher), clean with 6SL7 and crunch.

The crunch channel is working, the clean channel is struggling with low, low volume. I've been swapping tubes, clipping in resistors and capacitors etc, but to no avail.

1) I'm going to have to learn to read tube data sheets apparently. The data sheets have curves for plate voltage, but I'm not sure how to interpret them. I'm trying to optimize the voltages to get the most of the 6SL7.

2) It appears I'm getting no current flow through 1/2 of the 6SL7, which I assume is leading to the low volume. However, I have swapped parts, checked solder joints etc, and can't figure out why.

3) I've tried adding cathode by-pass caps on both cathode resistors (one at a time), and it resulted in a farty tone that died off into NO tone. Totally dead. I'm pretty sure I didn't kill the tube, but what could cause this?

Attached is a schematic, with voltages. Any tips or suggestions?

[brewdude]

Are the coupling caps are on the wrong side of the anode resistor?

[bal704]

Only on my 'back of the napkin' sketch.

Hopefully my final mistake of the day.....

[sluckey]

Put a 1Meg resistor from pin 4 to ground.

[bal704]

I put a 1M resistor on pin 4 to ground (with clip leads). It increased the volume into the workable area. However, when I unclipped the resistor, the volume was still increased, so I'm not sure if it worked or not.

It's not clear to me why I need this resistor. The other grid in the tube has a ground reference, and I've seen designs with 12ax7's that only have one ground reference.

Is there something about the 6SL7 tube that makes this necessary?

[tictac]

you need a ground reference to the grid unless it's direct coupled to the previous stage. Your circuit is not direct coupled....

Someone else can explain why better than I can but you need it, just do it...

TT