What do you make of this?

[WGTP]

http://aronnelson.com/gallery/WGTP/SolidStateStage?full=1

This is 1 of 5 stages used in a very popular tubelike solid state amp.  The resistor and cap values change from stage to stage, so this does not represent any one stage, but an average of the values.  The transistors appear to be in a Darlington arrangement and are medium to high gain.

The entire preamp has 2 channels and uses a variety of additional filters, tonestacks, negative feedback, and 1 set of in-line diodes.  Unless the diode shown gets involved in clipping, there are no other diodes to ground or in feedback loops creating distortion.  I thought it was interesting and welcome any comments or explanations.   

[KerryF]

Can you show the full schematic maybe?  What amp are you talking about?

[WGTP]

I only have the schematic on paper.  TransTube.   

[WGTP]

There has been a distinct lack of responses to this which means:

1.  Everyone else already understands it and thinks someone else will explain it to me
2.  I have committed a crime by posting it, I'll be happy to delete if so
3.  No one else understands it (I doubt that)
4.  It's too boring

Can someone explain the diode in parallel with the resistor?  I sort of get the rest of it.  Thanks.   

[KerryF]

I dont think this would be "illegal" to post.  Its only an amp schematic (only a part actually), so its fine.  all I can think of with this diode->resistor is that maybe it is acting to prevent to much voltage from getting to the diode, like a LED and resistor.

[pjwhite]

The diode will clip the signal when it gets above about +2.2 volts (0.7V diode drop + 1.5V power supply).  The 470K resistor provides a 1.5V bias to the first transistor base.  The signal will clip when it goes negative (below ground).  The first transistor is an emitter-follower buffer, which has no voltage gain.  The second stage adds gain, with the emitter circuit of resistors and capacitors providing some tone control.

[dano12]

The idea of good sounding solid state amplification is something of a focus for me lately. I am working on my Tube Cricket schematic and am still struggling with some of the simpler concepts, like the correct values for my coupling capacitors

The goal of this circuit is to use a 12AU7/12AT7 as both a preamp and tone-stack gain recovering stage into a LM368 chip (much like the Smokey/Ruby amps). So far it is sounding good, but not yet great. This is how it stands, any comments or suggestions would be appreciated!

[George Giblet]

The main contribution is the diode at the input.  This acts like the grid conduction of a tube.

The resistor provides biasing.

There is a patent (5647004?) for this circuit and all is explained there.  They explain a JFET version then indicate the darlington transistors can do the same job.

I'd be interested in seeing the schematic too.

[jrc4558]

Diode is "biased" by a resistor to...
Dang, someone else has answered!

[WGTP]

Wow, thanks for the responses.  Check with your PV service person, that is where I got the paper copy.  It looks like the TransTube amps use the same pre-amp.  The rest of it is switching and pretty much like a typical multiple gain stage guitar pre-amp with Fender/Marshal type tone controls.  I thought the by-pass resistor cap circuit was interesting, I haven't seen one that complex before.  With 1.5v I can start using AA batteries.   

[WGTP]

The resistors in parallel with the diode range in value from 82K to 2.2M!   

[lovric]

Constantin, please finish the sentence 

At first I thought it looked like Vulcan...

What happens with the tone when grid starts to conduct? If in PV it comes to play only when signal tops 2.2V then this isn't the first stage?

Is it of any importance that the collectors are tied together regarding negative feedback?

Could (finally and excuse my 'walters' barrage) the first collector be just connected directly to 30V supply

[George Giblet]

>The resistors in parallel with the diode range in value from 82K to 2.2M!

Do they keep the input cap at 100nF in all cases?

[WGTP]

No, the stages vary quit a lot from each other in terms of cap and resistance values. 

I was told by the repair guy that PV would send these schematics upon request, but it may just be to repair guys.  He also spoke well of the tech support.

Is this at all like the Vulcan stages?

[George Giblet]

No.  With the vulcan the diode is in the signal path all the time, it changes the non-linear behaviour; the diode is not a clipper.  In the PV case the diode only conducts when the signal is large, when it does conduct the diode is like half a clipper.  When the signal is small the diode remains reversed biased and has no effect.

PS: Thanks for the info on the caps.

[WGTP]

So by using 2 stages you clip both sides of the signal.  Using an odd number creates asymmetrical distortion.   I'm going to have to go listen to one. 

[George Giblet]

Something worth noting:   The reason the different stages have different base resistors is because the some stages have different emitter resistors.  In order to get the correct collector bias point the base resistors must be tuned/adjusted.