Vox AD30VT small combo, 2007, PbF
N_Cook
powering up with me. Now permanent no output except background, but phones
output ok and its diverter sw ok. Output hybrid ok with feed to its input or
either pin2 or pin 6, IC2, JRC 2082DD further back. No springing back to
life with prodding to either board
I suspect a control signal is not getting to DG211 an sw from the SM laden
top board. Anyone more insight ? to avoid a load of signal and control line
tracing. Probably end up bridging phones across to o/p permanent and no
bypass of output
N_Cook
socket of some pinning like ECC82 or 83, to give the faux valve signature
delay-on via the heating of heaters and then produce a gating signal or
something. Otherwise any valve effects are produced in the digital stuff of
course, the valve just being sham, or anyone know any different? The phones
output is direct it would seem. Bad contact between some pins and socket ,
compounded by low V and low I presumably, would seem to have been the
problem. I was testing without the valve in place as previously meeting
these I thought the valve was for show only, but a marginally bit more than
that.
N_Cook
pcb-type pins of the valveholder. After redoing solder could simulate the
reported previous intermittant situation of half power and no power by
hovering a variously angled hot valve over the socket pins. The 2 cathodes
are tied to -15V , beyond that quite complex, traces to njm2082 dual FET
opamp and the DG211 an. sw. etc, certainly no proper anode Vs
Meat Plow
Last one I worked on had some bad solder joints. Can't remember much more
detail.
--
Live Fast, Die Young and Leave a Pretty Corpse
N_Cook
news:pan.2010.10...@lmao.lol.lol...
The previous 2 I looked at needed the o/p hybrid replacing.
The an. sw. IC is functional at switch on presumably for muting but not with
absent valve, tripled up control line goes to 3.3V for a second then 0V
after that
Arfa Daily
"N_Cook" <div...@tcp.co.uk> wrote in message
news:i9s9fu$jr$1...@news.eternal-september.org...
Do you have a schematic for it ? This Vox is pretty similar
http://www.blueguitar.org/new/schem/misc_amp/vox_vr30reverb.pdf
and also uses a 12AX7 with an anode supply of just 15v, and it does appear
to be configured as a conventional genuine amplifier stage. I seem to recall
Elektor magazine doing some stuff with ECC / 12AX series twin triodes.
operating with low anode voltages.
Arfa
Arfa Daily
"Arfa Daily" <arfa....@ntlworld.com> wrote in message
news:VCqwo.67452$no2....@newsfe28.ams2...
Ah. Just found a copy of the actual schematic for the AD30 at
http://www.valvetronix.net/docs/AD15_30Servicemanual.pdf
At a first glance, the circuit around the 12AX7 does look a bit odd, but
actually, it is still a pretty 'standard' common cathode stage, albeit
operating with a low (effective) anode voltage of 15v, much like that VR30.
The anodes are, as you say, without supply as such, but the cathodes are
at -15v, so that's just the same as the anodes being at +15v. With AC
coupling, doesn't matter how you arrive at your voltages, as long as the
anode is above the grid and cathode.
Arfa
N_Cook
I've never seen the like, the -15V cathodes , next to nothing anodes and the
numerous weird-looking electrolytic caps are the same as this AD30VT . The
11 way selector for the amp types is on the digital board but some control
line could go down to the valve area. The real test would be twin beam
scoping of phones o/p and speaker o/p for differences. But I imagine musos
would smell a rat if the "sound" through the phones was different to the
speaker. Phones o/p is there whether valve is present or not, immediate o/p
as no heaters must make them a bit suspicious surely.
Next time I power up my valve tester I will try the lowest anode setting
possible
N_Cook
(250V measured ) CH/R of 3 megs.
There is 6V ac across the heaters
cathodes are -15V wrt to ground
grids are -14.6V
anodes are -.04V and -.1V
I'd not noticed before than my valve tester goes down to 20V anode setting,
only ever use 250 to 350V.
I wonder what gain you get with a good ECC83 running at 20 anode volts and
zero bias , i doubt negative negative bias is settable on it. I will have to
try progressively lowering from 250V and -2V to 20V, 0V
N_Cook
one triode of a good ECC83 on an Avo 160
250V an, -2V g, gain 2.7
200,-1.5, 2.7
150, -1, 2.5
60, -.3, 2.3
40, -.1, 2.1
40,0, 1.7
Without putting an micro-ametter in there I cannot measure the anode current
with any confidence at those low levels
Meat Plow
Don't they call that a starved cathode configuration?
Arfa Daily
Arfa
Meat Plow
Hmmm makes sense but there is another mode they run these effects units
in that utilize a 12AX7 called starved cathode. Unless I'm completely
mistaken.
Arfa Daily
"Meat Plow" <mhy...@yahoo.com> wrote in message
news:pan.2010.10...@lmao.lol.lol...
I have seen a number of effects pedals that use a 12AX7 with a very low
anode voltage applied, to make deliberate (effects) use of the non linearity
of the anode characteristic that this produces. However, what this is called
as a technique, I've really no idea.
I just looked up the term "starved cathode" and it does appear to refer to
the same non linear effect achieved with very low anode voltages, so maybe
it's just a case of which angle you look at it from, as to whether to call
it starved anode, or cathode. Prior to this discussion, I had only ever
heard it referred to as anode.
Arfa
Meat Plow
LOL I've only heard it referred to as starved cathode :) My Peavey Tube
FEX uses two 12AX7's for gain and tube effects. I'm sure they run in a
starved mode. Worked on a Mesa Boogie tube pedal with a 12AX7 in it that
ran off a 12 volt wart. If you want some tube sound and gain the Mesa is
the way to go although you'll pay a pretty penny for a vintage model.