| Author |
Message |
Gareth Magennis
Guest
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 Posted:
Thu Oct 20, 2005 11:07 am Post subject:
Fender vibrato |
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Hi,
I have a Fender Super Reverb which has the typical Neon/LDR arrangement.
The lowest speed you can get is about 6 hz. As you increase the speed the
Neon's percieved brightness drops and eventually is just glowing (with
slight modulation) rather than oscillating betwen dark and bright. (I've
fitted a new neon, no difference). The VCA follows this behaviour, meaning
that it doesn't much tremolo at anything higher than half speed. Is this
normal? In the absence of any reference I have no way of telling.
If this is normal, I'd like to play with modifying the circuit for more
maximum intensity - I've seen alternative circuitry on the net but really I
want to learn a bit more about this one. The existing LDR is about 0.5
MOhms in the dark and 2K with a desk lamp at half a metre. Modern LDR's
with a quoted resistance of 0.4MOhm in the dark are quoted 9K at 10 lux so
it sounds very roughly in the same ballpark. So that leaves modifying the
VCA circuit, something I really know nothing about.
https://schematicheavencom.secure.powweb.com/fenderamps/super_reverb_aa763_schem.pdf
What values could I change in the above circuit to change the amount of VCA,
and more importantly, what should I leave alone?
Thanks,
Gareth.
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Gareth Magennis
Guest
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| Posted:
Thu Oct 20, 2005 11:29 am Post subject:
Re: Fender vibrato |
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| I realise now it's not a VCA, the LDR is just shunting the signal to ground. |
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Gareth Magennis
Guest
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| Posted:
Thu Oct 20, 2005 12:35 pm Post subject:
Re: Fender vibrato |
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"Phil Allison" <philallison@tpg.com.au> wrote in message
news:3rpeb7Fl0s1aU1@individual.net...
| Quote: |
"Gareth Magennis"
I have a Fender Super Reverb which has the typical Neon/LDR arrangement.
The lowest speed you can get is about 6 hz.
** It should be lower - check the speed pot is *actually* a 3 Megohm
one.
|
Still the original 3Meg. The caps (.01/.01/.02) in the circuit all read
low. I'll sort this speed thing out first, maybe the rest will follow.
| Quote: | As you increase the speed the
Neon's percieved brightness drops and eventually is just glowing (with
slight modulation) rather than oscillating betwen dark and bright. (I've
fitted a new neon, no difference).
** Basically an optical illusion - due to persistence of vision.
The VCA follows this behaviour, meaning that it doesn't much tremolo at
anything higher than half speed. Is this normal?
** You must check "Vibrato" amplitude modulation with a scope and sign
gen.
At low speeds, modulation depth should be circa 50% - diminishing to
30% at max.
|
Nowhere near!
Cheers Phil. |
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Gareth Magennis
Guest
|
| Posted:
Thu Oct 20, 2005 1:29 pm Post subject:
Re: Fender vibrato |
|
|
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:3rpigfFkj87sU1@individual.net...
| Quote: |
"Gareth Magennis"
"Phil Allison"
** It should be lower - check the speed pot is *actually* a 3 Megohm
one.
Still the original 3Meg. The caps (.01/.01/.02) in the circuit all read
low. I'll sort this speed thing out first, maybe the rest will follow.
** You must check "Vibrato" amplitude modulation with a scope and sign
gen.
At low speeds, modulation depth should be circa 50% - diminishing to
30% at max.
Nowhere near!
** Replace the CdS cell - with age its sensitivity to light diminishes
so modulation depth is lost.
BTW
The neon /CdS cell combo should be inside a black plastic tube or piece of
black shrink tubing as any ambient light will spoil operation.
......... Phil
|
Thanks Phil.
These disk caps in the oscillator, 10nF and 20nF, which types should I
use/avoid? 20nF is at the extreme range or unavailable in Farnells Ceramic
Disc types.
Cheers,
Gareth. |
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Phil Allison
Guest
|
| Posted:
Thu Oct 20, 2005 4:41 pm Post subject:
Re: Fender vibrato |
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"Gareth Magennis"
| Quote: |
I realise now it's not a VCA, the LDR is just shunting the signal to
ground.
|
** Unless VCA = voltage controlled attenuator.
............ Phil |
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Phil Allison
Guest
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| Posted:
Thu Oct 20, 2005 4:45 pm Post subject:
Re: Fender vibrato |
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|
"Gareth Magennis"
| Quote: |
I have a Fender Super Reverb which has the typical Neon/LDR arrangement.
The lowest speed you can get is about 6 hz.
|
** It should be lower - check the speed pot is *actually* a 3 Megohm one.
As you increase the speed the
| Quote: | Neon's percieved brightness drops and eventually is just glowing (with
slight modulation) rather than oscillating betwen dark and bright. (I've
fitted a new neon, no difference).
|
** Basically an optical illusion - due to persistence of vision.
| Quote: | The VCA follows this behaviour, meaning that it doesn't much tremolo at
anything higher than half speed. Is this normal?
|
** You must check "Vibrato" amplitude modulation with a scope and sign gen.
At low speeds, modulation depth should be circa 50% - diminishing to 30%
at max.
.......... Phil |
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| Back to top |
|
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Phil Allison
Guest
|
| Posted:
Thu Oct 20, 2005 5:56 pm Post subject:
Re: Fender vibrato |
|
|
"Gareth Magennis"
| Quote: | "Phil Allison"
** It should be lower - check the speed pot is *actually* a 3 Megohm
one.
Still the original 3Meg. The caps (.01/.01/.02) in the circuit all read
low. I'll sort this speed thing out first, maybe the rest will follow.
** You must check "Vibrato" amplitude modulation with a scope and sign
gen.
At low speeds, modulation depth should be circa 50% - diminishing to
30% at max.
Nowhere near!
|
** Replace the CdS cell - with age its sensitivity to light diminishes
so modulation depth is lost.
BTW
The neon /CdS cell combo should be inside a black plastic tube or piece of
black shrink tubing as any ambient light will spoil operation.
.......... Phil |
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| Back to top |
|
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Phil Allison
Guest
|
| Posted:
Thu Oct 20, 2005 6:43 pm Post subject:
Re: Fender vibrato |
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|
"Gareth Magennis"
| Quote: |
These disk caps in the oscillator, 10nF and 20nF, which types should I
use/avoid?
|
** Normal 400 volt DC polyester film ( PETP ) caps should be fine.
......... Phil |
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