Stumped on a 5f11

[jukelemon]

Hi all.

I built an amp for my Dad a long time ago. It is probably one of  the best sounding small amps I have ever heard. Perfect circuit.

It has developed an issue ONLY when the Trem is activated.  Clean is working perfect.

When the Trem is activated and the Depth turned up, I get a nasty distortion with each each pulse. It is not over the whole note. almost like there is 2 signals in one amp and one signal is so distorted and the other is fine.

Not sure how else to explain it.  Definitely more noticeable/pronounced on the lower notes.

Tried new tubes. Nothing changed.

Like I said, the clean sounds amazing.

http://www.thevintagesound.com/ffg/schem/vibrolux_5f11_layout.gif

Any ideas?

[HotBluePlates]

When the Trem is activated and the Depth turned up, I get a nasty distortion with each each pulse.

How high does the Depth have to be up for it to happen? Near max depth?

[PRR]

Voltages. Particularly the "-31V" bias and the "+260V" B+ to the wobbulator.

If the wobble (after processing) is bigger than the bias it WILL distort on trem peaks. If Depth is reduced it should be OK. If you get ample trem at part-Depth, you just have too much wobble signal for your bias. This may be weak bias, hot wobbles, or a bad part in the trem wobbulator.

[stratele52]

Schematic here;

http://www.el34world.com/charts/Schematics/files/fender/vibrolux_5f11_schem.pdf

Photos of inside the amp will help , see if any capacitance between some wires .....Some lead dress to check and clean ?

We like to see this good sounding amp

[jukelemon]

When the Trem is activated and the Depth turned up, I get a nasty distortion with each each pulse.

How high does the Depth have to be up for it to happen? Near max depth?

Hi.

No, it starts right at ON i.e. ~2 on the Depth dial is when the Trem kicks in and then continues to the end.

In other words, I cannot dial it out. It definitely is lessened by the Depth control but still there at the very onset of the effect.

Jason

[jukelemon]

Voltages. Particularly the "-31V" bias and the "+260V" B+ to the wobbulator.

If the wobble (after processing) is bigger than the bias it WILL distort on trem peaks. If Depth is reduced it should be OK. If you get ample trem at part-Depth, you just have too much wobble signal for your bias. This may be weak bias, hot wobbles, or a bad part in the trem wobbulator.

The distortion at Trem peaks is a very good way of describing I think.  That distortion goes right along with the Effect like it is hitting once then not, then once, then not like you mentioned.

At ~5-6 on the Depth dial, it is for sure a strong Trem effect.  From 6-10 doesn't really do much more.

The bias is set at 22ma's on each tube. I would not consider that weak bias. Agree?

What is a "hot wobble"?

Thanks.

Jason

[HotBluePlates]

The bias is set at 22ma's on each tube. I would not consider that weak bias. Agree?

22mA is pretty normal for 6V6's in this circuit and with this B+.

PRR means "weak bias" as is "small bias voltage" (which equals high tube current). A strong trem signal then overpowers the smallish ("weak") bias voltage, and distorts on the peaks.

The distortion at Trem peaks is a very good way of describing I think.  That distortion goes right along with the Effect like it is hitting once then not, then once, then not like you mentioned.

At ~5-6 on the Depth dial, it is for sure a strong Trem effect.  From 6-10 doesn't really do much more.

This is what I was asking, but I didn't want to "lead the witness." So you need a weaker trem signal to fix it.

You could cripple the oscillator gain, but then it might not start up. You might try a different tube, one with lesser gain (especially since this problem didn't always exist; could be a hot tube).

Alternatively, you could try inserting resistance between the oscillator and depth pot. Try maybe 220kΩ between the 0.1uF cap and the end of the depth pot not connected to the bias supply.

[PRR]

> You could cripple the oscillator gain, but then it might not start up.

Oscillator gain does not set amplitude. Except "zero" and "full". If it starts, it self-excites itself until it hits its limits. (Fancy oscillators have AGC, but that's slow to stabilize; Fender just slams B+.)

> try inserting resistance between the oscillator and depth pot.

+1

[jukelemon]

Thanks all.

I put in a 12at7 in v1 and that changed things quite a bit. I actually prefer that as opposed to the ax7.

I can now control the issue with the depth i.e. it now hits at about 9 as opposed to earlier.

I am going to try some resistance as you all suggested.

Thanks again!

[stratele52]

What about some photos ?

[jukelemon]

What about some photos ?

Photos of ?

[stratele52]

What about some photos ?

Photos of ?

   The amps , circuits, wiring