strange brown concert vibrato circuit help please

[Randall]

I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

http://ampwares.com/schematics/vibrasonic_5g13.pdf

[nosaj]

I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

http://ampwares.com/schematics/vibrasonic_5g13.pdf

This may help you identify it better and possible use the 6g12 schematic.

https://www.premierguitar.com/articl...cert_4x10_Amps
nosaj

[Randall]

"This may help you identify it better and possible use the 6g12 schematic."

Yes, I've already been down that road. That's where I learned that the two schematics in 1960 were very close.

[olddawg]

What do you mean by “recap”? ..... Power supply and bias circuits only? Imho 90% of Trem circuit problems are do to old caps.

[xtian]

Wow, that's complex. Is that a harmonic tremolo?

[nosaj]

I have a 1960 Fender 4x10 Concert Amp 5G12 in for an overhaul. I recapped it, put in a 3 prong AC cord, adjusted the bias resistors to get it up over 50%, replaced the wrong value screen resistors (they were 470K). Now she is running well except for the vibrato, which very weak indeed. I have never seen this vibrato circuit before, and to be honest, am not really grasping how it works. I have oscillation that follows the speed knob on the top plate of the tube, but not on the bottom plate. Should it be there? I also get oscillation at the intensity pot, but it only swings about a couple of volts.

Any help I could get with this would be greatly appreciated. Oh, and I am working with the 5G13 Vibrosonic schematic, as I believe it is practically identical to the near impossible to find 5G12 Concert.

http://ampwares.com/schematics/vibrasonic_5g13.pdf

How about posting some tube voltages to see if they are conducting?

nosaj

[Randall]

"What do you mean by “recap”? ..... Power supply and bias circuits only? Imho 90% of Trem circuit problems are do to old electrolytic caps."

All electrolytics, PS, bias and cathodes. AFAIK there are none in the trem circuit. There are only five on the main board, all cathodes.

[Randall]

OK jason, I'll play. With speed and Intensity at zero on the trem tube:

1: 132v, fluxuating slightly
2: 0.03v fluxuating slightly
3: 1.7v

6: 180v
7: 0.1v
8: 1.7v

[Justin Thomas]

Are there 5 preamp tubes? Or only 4? You should have 2 trem tubes. And unfortunately, ANY cap in the trem circuit could be bad... Which IIRC, there's a crap-ton of them...

Justin

[nosaj]

OK jason, I'll play. With speed and Intensity at zero on the trem tube:

1: 132v, fluxuating slightly
2: 0.03v fluxuating slightly
3: 1.7v

6: 180v
7: 0.1v
8: 1.7v

Is this V3 or V4? Can you post for V4 also?

Thanks,
nosaj

[Randall]

Yes, there are a LOT of Astron caps in this one. Five preamp tubes. The schematic doesn't distinguish which tube is V3 or V4, but if we go with typical right to left, what I posted would be V3 and here is V4:

1. 334v
2. 0.09v
3. 3.8v

4. 330v
5. 0.3v
6. 3.8v

[g1]

Suggest changing all non-electrolytic caps in trem circuit. Like olddawg said above, most trem issues are the caps.

Also, even though it is not an issue in this circuit, something to remember is that warming the bias on a bias type trem can reduce the effect.

[Randall]

"Suggest changing all non-electrolytic caps in trem circuit. Like olddawg said above, most trem issues are the caps"

Well, I have measured them, and most of the Astrons are drifted out of spec, some by 50% or more. I wonder how fussy I need to be about the actual values used here, like 0.25uF and .03uF and .05uF?

[g1]

.22, .033, & .047 would be the standard value replacements.

[Helmholtz]

Well, I have measured them, and most of the Astrons are drifted out of spec, some by 50% or more. I wonder how fussy I need to be about the actual values used here, like 0.25uF and .03uF and .05uF?

If foil caps show increased capacitance, this typically indicates that humidity has entered the interior via micro-cracks. Consequently it's likely to find significant leakage.