Vox AD30VT small combo, 2007, PbF

Although only -15V on the valve it is necessary to have something in the 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.

In the end it was duff PbF solder meeting 50 yearold technology, the 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

Last one I worked on had some bad solder joints. Can't remember much more detail.

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Live Fast, Die Young and Leave a Pretty Corpse

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

Do you have a schematic for it ? This Vox is pretty similar

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

Ah. Just found a copy of the actual schematic for the AD30 at

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

the

cathodes

recall

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

With this vox in running order whether good ECC83 in there or bad ECC82 with (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

I did not expect that 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

Don't they call that a starved cathode configuration?

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Live Fast, Die Young and Leave a Pretty Corpse

Starved anode, I think.

Arfa

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.

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Live Fast, Die Young and Leave a Pretty Corpse

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

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.

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Live Fast, Die Young and Leave a Pretty Corpse