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Sunn Amp Serial Number Lookup Engine
I had a client bring me his 1995 Fender Prosonic (in red Lizardskin!) with an unusual complaint - that in the studio, with reverb 'off', he could still hear a slight reverb slap-back at low volumes.
The Prosonic was designed by Bruce Zinky of the Custom Shop, but none of these amps were actually built there, nor were they sold as Custom Shop amps. The “LO” prefix to the serial number indicates that the amp was made at the old Sunn Amp factory in Lake Oswego, OR, and labeled “Fender Custom Shop”. Later, production was moved to Corona, and those amps are indicated by the prefix “CR”, and were not labeled 'Custom Shop'. The Lake Oswego amps are supposed to have better-quality components than the later models.
This particular Prosonic was produced in 1995 or early 1996, based on the Serial Number and the transformer codes. They only made 300 of the red “Lizard Skin” amps, this was #5 / 300.
The Prosonic was a complete departure for Fender. It was unlike any Fender amp ever made. It is probably closer in design to a Mesa or Marshall. The Class A setting is very reminiscent of a Vox AC30.
The amp features cascading gain stages with Master volume and an interesting 3-position rectifier selector for 1: Class A, tube rectifier/cathode biased, 2: Class A/B, tube rectifier/fixed bias, and 3: Class A/B, solid-state rectifier, fixed bias.
The Prosonic is a rare and highly sought-after amp – hard to find because the people who have them tend to hang on to them.
Anyhow,
I did notice a faint high-frequency reverb slap on striking damped strings with the reverb control “off”. The footswitch was cleaned and checked for continuity to ground. The condition persisted with the reverb turned off with the footswitch.
The Fender Prosonic schematic (for the combo amp, the head did not have reverb), had several errors in the reverb section. The footswitch schematic is not correct – it’s actually backwards on the schematic. In addition, the signal pull-down resistor, R304, is shown as 10K but the actual resistor is 20K. This is pretty typical of Fender schematic/production differences – not too alarming.
The Reverb footswitch acts by taking the reverb signal to ground, essentially turning it off. The amp was performing as designed in this regard; the reverb would turn off when the footswitch was pressed, except for that tiny signal that was leaking through somehow.
So back to the schematic (see partial schematic, Figure 1., below).
The footswitch grounds the signal at the wiper of the Reverb pot, before R42, the “blend” resistor. But wait – why is C24, a little 220 pF ceramic cap, sitting parallel on top of R42? After R42 the signal goes directly to shake hands with the Phase Inverter. It seems to me that this is the source of the leaky reverb – grounding the left side of C24 would not prevent some signal from scooting by R42 and joining the dry signal at the PI.
If we assume that C24 is acting as a high-pass filter, then any reverb signal above about 1.54kHz could, in automotive parlance, “blow-by”.
After some trepidation, I lifted one end of C24 and played the amp again. With the reverb on, I could not tell any difference with C24 out of the circuit. It is a really good sounding reverb (in the interest of full disclosure, I am deaf in my left ear).
With the reverb off, I could not detect the faint echo (slap-back, really) that was present before. I repeated this several times, because I wasn’t sure I could trust what I was hearing (or not hearing).
The fact is that this appears to be a design flaw, albeit one that very few people would detect; the effect is very, very small. I could not trust my ears after a while. So, oscilloscope.
The oscilloscope could readily detect delayed ringing when the reverb was on, even at a low reverb setting. However with the reverb off, The signal collapses more or less instantly. not even minor ringing was detected – so either the ringing isn’t there, or the oscilloscope cannot detect it (my scope is a 100 MHz DSO).
Thoughts?
Best Regards,
Don
The Prosonic was designed by Bruce Zinky of the Custom Shop, but none of these amps were actually built there, nor were they sold as Custom Shop amps. The “LO” prefix to the serial number indicates that the amp was made at the old Sunn Amp factory in Lake Oswego, OR, and labeled “Fender Custom Shop”. Later, production was moved to Corona, and those amps are indicated by the prefix “CR”, and were not labeled 'Custom Shop'. The Lake Oswego amps are supposed to have better-quality components than the later models.
This particular Prosonic was produced in 1995 or early 1996, based on the Serial Number and the transformer codes. They only made 300 of the red “Lizard Skin” amps, this was #5 / 300.
The Prosonic was a complete departure for Fender. It was unlike any Fender amp ever made. It is probably closer in design to a Mesa or Marshall. The Class A setting is very reminiscent of a Vox AC30.
The amp features cascading gain stages with Master volume and an interesting 3-position rectifier selector for 1: Class A, tube rectifier/cathode biased, 2: Class A/B, tube rectifier/fixed bias, and 3: Class A/B, solid-state rectifier, fixed bias.
The Prosonic is a rare and highly sought-after amp – hard to find because the people who have them tend to hang on to them.
Anyhow,
I did notice a faint high-frequency reverb slap on striking damped strings with the reverb control “off”. The footswitch was cleaned and checked for continuity to ground. The condition persisted with the reverb turned off with the footswitch.
The Fender Prosonic schematic (for the combo amp, the head did not have reverb), had several errors in the reverb section. The footswitch schematic is not correct – it’s actually backwards on the schematic. In addition, the signal pull-down resistor, R304, is shown as 10K but the actual resistor is 20K. This is pretty typical of Fender schematic/production differences – not too alarming.
The Reverb footswitch acts by taking the reverb signal to ground, essentially turning it off. The amp was performing as designed in this regard; the reverb would turn off when the footswitch was pressed, except for that tiny signal that was leaking through somehow.
So back to the schematic (see partial schematic, Figure 1., below).
The footswitch grounds the signal at the wiper of the Reverb pot, before R42, the “blend” resistor. But wait – why is C24, a little 220 pF ceramic cap, sitting parallel on top of R42? After R42 the signal goes directly to shake hands with the Phase Inverter. It seems to me that this is the source of the leaky reverb – grounding the left side of C24 would not prevent some signal from scooting by R42 and joining the dry signal at the PI.
If we assume that C24 is acting as a high-pass filter, then any reverb signal above about 1.54kHz could, in automotive parlance, “blow-by”.
After some trepidation, I lifted one end of C24 and played the amp again. With the reverb on, I could not tell any difference with C24 out of the circuit. It is a really good sounding reverb (in the interest of full disclosure, I am deaf in my left ear).
With the reverb off, I could not detect the faint echo (slap-back, really) that was present before. I repeated this several times, because I wasn’t sure I could trust what I was hearing (or not hearing).
The fact is that this appears to be a design flaw, albeit one that very few people would detect; the effect is very, very small. I could not trust my ears after a while. So, oscilloscope.
The oscilloscope could readily detect delayed ringing when the reverb was on, even at a low reverb setting. However with the reverb off, The signal collapses more or less instantly. not even minor ringing was detected – so either the ringing isn’t there, or the oscilloscope cannot detect it (my scope is a 100 MHz DSO).
Thoughts?
Best Regards,
Don
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Sunn Amps History
- International group in which owners of Sunn amplifiers can show their amps and discuss Sunn amps. Selling in this group: YES, but keep in mind: - State your location (and willingness to ship) - A price or ONO (or near offer) price MUST be mentioned - NO commercial/business advertising/sales It's pretty simple, so keep it respectful, nice.
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- Sunn amp serial number lookup. A Note on Hamer Serial Numbers. Waves vocal rider cracked. In 1974 Hamer used a four digit number stamped into the wood startingwith.
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