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2005 Vox AC30 , AC30CC2X
snap almost
Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly soldered pcb wire links (lead free solder again, note only 2 or 3 years olkd) . But there is a low level throbbing rumble noise obviously associated with the tremolo oscillator on the effects board, as it cycles up and down with varying tremolo pitch and depth. The owner lives with this but is there a simple cure, assuming its a grounding/screening/decoupling/smoothing problem. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.ne |
2005 Vox AC30 , AC30CC2X
N Cook wrote in message
... snap almost Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly soldered pcb wire links (lead free solder again, note only 2 or 3 years olkd) . But there is a low level throbbing rumble noise obviously associated with the tremolo oscillator on the effects board, as it cycles up and down with varying tremolo pitch and depth. The owner lives with this but is there a simple cure, assuming its a grounding/screening/decoupling/smoothing problem. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.ne Can anyone say for what mucic reason there is a valve/tube rectifier in this amp rather than 1 or 4 silicon diodes ? Loads of op-amps so obviously not overall faithfull to 60s technology -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
The message
from "N Cook" contains these words: N Cook wrote in message ... snap almost Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly soldered pcb wire links (lead free solder again, note only 2 or 3 years olkd) . But there is a low level throbbing rumble noise obviously associated with the tremolo oscillator on the effects board, as it cycles up and down with varying tremolo pitch and depth. The owner lives with this but is there a simple cure, assuming its a grounding/screening/decoupling/smoothing problem. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.ne Can anyone say for what mucic reason there is a valve/tube rectifier in this amp rather than 1 or 4 silicon diodes ? Loads of op-amps so obviously not overall faithfull to 60s technology -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ A valve rectifier will start providing H.T at the same time as the O/P valves draw current. If a diode bridge is used the H.T may rise above the working voltage of the smoothing caps until the O/P valves start to draw current. -- Jocelyn Tetigisti acu. (Titus Maccius Plautus 254 - 184BC) |
2005 Vox AC30 , AC30CC2X
J M Goodey wrote in message
... The message from "N Cook" contains these words: N Cook wrote in message ... snap almost Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly soldered pcb wire links (lead free solder again, note only 2 or 3 years olkd) . But there is a low level throbbing rumble noise obviously associated with the tremolo oscillator on the effects board, as it cycles up and down with varying tremolo pitch and depth. The owner lives with this but is there a simple cure, assuming its a grounding/screening/decoupling/smoothing problem. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.ne Can anyone say for what mucic reason there is a valve/tube rectifier in this amp rather than 1 or 4 silicon diodes ? Loads of op-amps so obviously not overall faithfull to 60s technology -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ A valve rectifier will start providing H.T at the same time as the O/P valves draw current. If a diode bridge is used the H.T may rise above the working voltage of the smoothing caps until the O/P valves start to draw current. -- Jocelyn Tetigisti acu. (Titus Maccius Plautus 254 - 184BC) I'd not considered that, but a thermistor plus Si diode/s would avoid that potential problem. o/p valves for the soft limiting in overdrive but a thermionic rectifier seems unjustified/unnecessary instead of diodes plus thermistor. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
N Cook wrote:
J M Goodey wrote in message ... The message from "N Cook" contains these words: N Cook wrote in message ... snap almost Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly soldered pcb wire links (lead free solder again, note only 2 or 3 years olkd) . But there is a low level throbbing rumble noise obviously associated with the tremolo oscillator on the effects board, as it cycles up and down with varying tremolo pitch and depth. The owner lives with this but is there a simple cure, assuming its a grounding/screening/decoupling/smoothing problem. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.ne Can anyone say for what mucic reason there is a valve/tube rectifier in this amp rather than 1 or 4 silicon diodes ? Loads of op-amps so obviously not overall faithfull to 60s technology -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ A valve rectifier will start providing H.T at the same time as the O/P valves draw current. If a diode bridge is used the H.T may rise above the working voltage of the smoothing caps until the O/P valves start to draw current. -- Jocelyn Tetigisti acu. (Titus Maccius Plautus 254 - 184BC) I'd not considered that, but a thermistor plus Si diode/s would avoid that potential problem. o/p valves for the soft limiting in overdrive but a thermionic rectifier seems unjustified/unnecessary instead of diodes plus thermistor. Maybe it`s the monetary and snob value of having the words vacuum tube rectifier in the blurb. Some amps have both ss and valve rectification, switching from one to the other does produce a variation in the sound. Ron(UK) |
2005 Vox AC30 , AC30CC2X
In article ,
Ron(UK) wrote: Maybe it`s the monetary and snob value of having the words vacuum tube rectifier in the blurb. Some amps have both ss and valve rectification, switching from one to the other does produce a variation in the sound. If the PS is producing a variation in the sound it's poorly designed. -- *Why doesn't Tarzan have a beard? * Dave Plowman London SW To e-mail, change noise into sound. |
2005 Vox AC30 , AC30CC2X
Dave Plowman (News) wrote:
In article , Ron(UK) wrote: Maybe it`s the monetary and snob value of having the words vacuum tube rectifier in the blurb. Some amps have both ss and valve rectification, switching from one to the other does produce a variation in the sound. If the PS is producing a variation in the sound it's poorly designed. No, it`s intended that way, an amp with a valve rectifier has a certain amount of sag in the ht voltage. On ss amps, there is often a resistor in the feed to reproduce the same sag. Ron(UK) |
2005 Vox AC30 , AC30CC2X
Ron(UK) wrote:
Dave Plowman (News) wrote: In article , Ron(UK) wrote: Maybe it`s the monetary and snob value of having the words vacuum tube rectifier in the blurb. Some amps have both ss and valve rectification, switching from one to the other does produce a variation in the sound. If the PS is producing a variation in the sound it's poorly designed. No, it`s intended that way, an amp with a valve rectifier has a certain amount of sag in the ht voltage. On ss amps, there is often a resistor in the feed to reproduce the same sag. Ron(UK) We are talking guitar amplifiers here btw, not hi fi. Ron(UK) |
2005 Vox AC30 , AC30CC2X
This throb noise is due to loading of the HT which goes up and down with
tremolo speed and depth. With EL84s in and all 12AX7 pulled the throb is still there, no signal throughput of course, even makes the standyby LED dim in sympathy when at the deepest depth. Only stops if I disconnect the ribbon that goes to the tremolo pots to kill the tremolo oscillator Leaving, likely ,just a JRC 2147D high voltage dual op-amp (+/- 28V not 300V) and a 2SC2910 associated with the tremolo area and something marked SiLN D25D or SiLN D250 a 3 pin TO92 device, the S being the Nazi SS lightning flash S. Anyone recognise the Logo ? Whatever this is thyristor ? is connected via 220K to the HT line. I thought tremolo just bent the frequencies in a cyclic fashion, not modulating the HT. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
N Cook wrote in message
... This throb noise is due to loading of the HT which goes up and down with tremolo speed and depth. With EL84s in and all 12AX7 pulled the throb is still there, no signal throughput of course, even makes the standyby LED dim in sympathy when at the deepest depth. Only stops if I disconnect the ribbon that goes to the tremolo pots to kill the tremolo oscillator Leaving, likely ,just a JRC 2147D high voltage dual op-amp (+/- 28V not 300V) and a 2SC2910 associated with the tremolo area and something marked SiLN D25D or SiLN D250 a 3 pin TO92 device, the S being the Nazi SS lightning flash S. Anyone recognise the Logo ? Whatever this is thyristor ? is connected via 220K to the HT line. I thought tremolo just bent the frequencies in a cyclic fashion, not modulating the HT. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ Angled/Jagged Si logo is probably Supertex, so from their site its a LND250 500V, 3mA ,1K RDS on, n ch MOSFET , even as a SOT23 package -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
"Dave Plowman (News)" wrote:
*Why doesn't Tarzan have a beard? * More importantly, Why did he leave all those monkeys here in Florida? Those movies were filmed at Silver Springs, a few miles from Ocala. A few years after they stopped making the Tarzan movies there was a monkey epidemic in Ocala. The nasty little boogers were all over the place stealing food, and flinging crap at people. In fact, it kind of reminds me of some of the critters I encounter on USENET. :( -- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida |
2005 Vox AC30 , AC30CC2X
The LND250 had failed, didn't pass the DVM diode, lead swap, test.
Erroneously replaced witha non-depletion type n channel Mosfet but at least the rumbling had gone. Owner always ran it with minimum tremoplo speed and depth which gave minimum throbbing. Still no tremolo function. Now to find a 400V to 500V depletion mode mosfet from somewhere. BSS126 and BSP135 are a couple near equivalent device types found -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
What I like to do. If something is as rare as hen's teeth then bodge up a
work-around with standard parts. This high voltage , but depletion type Mosfet, is an active part of the tremolo circuit , there is no separate oscillator. So re-biased the mosfet up a few volts. Using a TO220 size standard IRF740 enhancement type, fudged pinning, bled off the 28V line with a 2.7V zener, couple of resistors and a preset pot, to the gate, now have a fully working work around this tremolo problem. Full range of .2 to 20 Hz or so and 0 to excessive depth. Its a bit too critical on preset setting but the owner had not used the tremolo for ages. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
2005 Vox AC30 , AC30CC2X
So as research is not wasted
For the archives , some high voltage, depletion type , n channel mosfet type numbers LND250 LND150 C633 , Teledyne BSP135 BSS126 DN3135 DN3145 DN3545 DN2540 BSR58 , BSR56 ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
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