Baffled by 12AX7 circuit in Fender Performer

[Stratfordade]

I’m fixing up a Fender Performer that has one or more poorly op-amps in the pre-amp. Almost all my experience is with solid state but even so this valve newbie is baffled by the double triode included in the ‘drive’ circuit.

Several things puzzle me: The anode of V1A is directly connected to the cathode of V1B; the signal from the op amp output on left is routed to the V1A anode not the grid; the grid of V1A is connected to the anode of V1A; there doesn’t seem to be any high voltage supplied to the anodes of either triode indeed the test voltage for the second anode is -2.0V.

Please can anyone explain what is the theory behind what looks like nothing I’ve ever seen in valve theory articles! Thanks in advance.

[g1]

Looks to me like they are just using the tube as 2 back-to-back diodes for clipping, like you would often see with regular diodes or LED's for clipping.
Seems a waste of a perfectly good tube to me, but maybe I'm missing something.

Sort of like this, but with more 'bells & whistles' :

[Helmholtz]

Yes, that's a stupid excuse-tube circuit (> marketing!).
The triodes have their grids connected to the plates, so are operated as anti-parallel diodes to provide some soft clipping.

[Stratfordade]

Oh I see! Thanks both for enlightenment. Yes it would have been motivated by marketing, but maybe the clipping is slightly more soft in a tube vs semiconductor (now I sound like Marketing). I’m still baffled by the power supply or lack of to the plates though!

[loudthud]

The odd thing about that circuit is that they wasted three 5W resistors and a couple of zeners to get the DC heater Voltage.

Want to hear what is does ? Just pull the tube.

Take a look at this thread: https://music-electronics-forum.com/...ent-of-a-12ax7

[Stratfordade]

The odd thing about that circuit is that they wasted three 5W resistors and a couple of zeners to get the DC heater Voltage.

Want to hear what is does ? Just pull the tube.

Take a look at this thread: https://music-electronics-forum.com/...ent-of-a-12ax7

Yes and the three 5W resistors had cold solder joints, making me scratch my head over why the heaters were not glowing!

[Helmholtz]

I don't expect the grid to cathode diode to perform like normal when grid and plate are connected and plate voltage is very low or even negative as it seems to be here.

[glebert]

Here's a thread I started talking about that circuit. Still love those amps. Wayyyyy too much gain in the lead channel from the op amps, though. The clipping from the tube is definitely softer than a silicon diode.

https://music-electronics-forum.com/...-tweaks/49225-

[Chuck H]

It also sets the stage for more symmetrical clipping in following circuits. Though you could also do that with diodes. This tends to have a harder sound (if that's not too subjective a term?) So, softer clipping, harder sound. Probably a good crunchy tone. The tubes would be more "elastic" than diodes to some degree with the flattened part of the wave form so there may even be some dynamic differences with pick attack. Though in my experience diodes don't really clip that hard unless you hammer them. The tube will still clip a little softer but I wonder if the average player (including myself) would really hear a significant difference between a tube or a pair of diodes in this circuit.

[Stratfordade]

Thanks all for the extra info/comments on the diode arrangement. That thread on the Roc Pro/Performer 1000 with same bodge is fascinating too.

[Greg Robinson]

How much of the soft clipping is due to the fact a tube is used, and how much is due to the 100k (plus 220Ω in series, negligible {EDIT: would actually be the parallel combination of 220Ω and 1k5 for ~192Ω, even more negligible}) resistor feeding the clipping stage? I'll bet you wouldn't hear any difference substituting silicon diodes with a 2V forward voltage...

[Helmholtz]

How much of the soft clipping is due to the fact a tube is used, and how much is due to the 100k (plus 220Ω in series, ...

Agree that there are many ways to shape the waveform with a diode clipper.
But would like to add that a large current limiting resistor actually makes the clipping harder.
Voltage drive would result in softer clipping than current drive as used here.

[Greg Robinson]

Agree that there are many ways to shape the waveform with a diode clipper.
But would like to add that a large current limiting resistor actually makes the clipping harder.
Voltage drive would result in softer clipping than current drive as used here.

Quite right, I was thinking about the circuit incorrectly. I was thinking about it like a diode with resistance added rather than feeding it.
I still doubt there'd be any real difference between silicon and tube clipper.