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.

 

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

 

The message <ev4rjj$qac$>

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.

 

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)

 

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)

 

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.

 

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

 

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

 

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

 

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.

 

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 ?