Funky distortion circuit discussion - fender 'super twin' amp

[tele_guitarist]

http://www.indyguitarist.com/schemat...super-twin.pdf

I'd be interested in hearing your guys' opinions on this circuit...
I'm actually a bit stumped too. I'm not familiar with what the distortion control is doing to achieve distortion, the tube stage does not have a plate resistor, cathode cap and resistor not going to ground... funky!!

thoughts?

I'm feeling like a serious newb on this one.

The signal appears to come out of the anode @ v3a into the .047uf cap and then into both v3b AND the hi pass filter going to the gain control which is a .0047uf and a 68k. The opposite lug on the gain control is going to the cathode cap and resistor on v3b (?) which are connected to the inductor and a 2.2k resistor to ground.

What function does this inductor do here?

On v3a there is a 3.9k to ground before the grid there... I wouldn't think the signal would be THAT strong after the eq so as to need this resistor that small... thoughts? No cathode cap or resistor there either, and a 47k plate resistor (?)

I'm still not seeing any sort of of plate resistor for v3b - is this a different type of tube circuit that I'm just not familiar with?

Thanks again for the "schooling"!

bw

[Enzo]

V3B is called a "cathode follower" stage. Look it up. It is common enough.

I am not sure just what the distortion circuit is doing, frankly.

V3A? We are used to seeing a cathode resistor (unless it is a "grid leak" stage, look that up too.). The purpose of such a resistor is to bias the tube. The current through the tube causes a voltage drop across the resistor. That leaves the cathode a couple volts more positive than the grid. That bias is needed to allow room for the signal.

But the idea of bias is about the relationship between the grid and cathode, not absolute voltages per se. That 3.9k is part of a voltage divider off the -60v rail. It establishes that -2v at the grid. -2v grid and zero volts cathode (grounded) us exactly the same as zero volts grid and +2v cathode as far as the tube is concerned.

That cathode circuit with the inductor is not speaking to me, but it has been a long night. Might later. Hopefully someone else has the insight. The negative rail switches levels when distortion is on, but I don't quite see why.

[tele_guitarist]

Originally Posted by Enzo

V3B is called a "cathode follower" stage. Look it up. It is common enough.

I am not sure just what the distortion circuit is doing, frankly.

V3A? We are used to seeing a cathode resistor (unless it is a "grid leak" stage, look that up too.). The purpose of such a resistor is to bias the tube. The current through the tube causes a voltage drop across the resistor. That leaves the cathode a couple volts more positive than the grid. That bias is needed to allow room for the signal.

But the idea of bias is about the relationship between the grid and cathode, not absolute voltages per se. That 3.9k is part of a voltage divider off the -60v rail. It establishes that -2v at the grid. -2v grid and zero volts cathode (grounded) us exactly the same as zero volts grid and +2v cathode as far as the tube is concerned.

That cathode circuit with the inductor is not speaking to me, but it has been a long night. Might later. Hopefully someone else has the insight. The negative rail switches levels when distortion is on, but I don't quite see why.

Thanks Enzo, I thought it may be a cf of some sort, but the inductor had me stumped. clay jones wrote this about the circuit though, may be of interest for the archives:

It's a follower circuit with an over designed bias network built around the negative supply biasing the power tubes. I say over designed only because it seems like a lot of work to get a distortion capability which by all accounts seems to be pretty crappy sounding.

Building on what JHS mentioned...
The plate takes a 300 vdc charge and sits at AC ground through it's PS node filter cap. Like any follower there won't be any gain and a slight attenuation. Based on shifting VB3's negative grid voltage it'll distort though. With "distortion off" the diode is reverse biased which sends a large enough negative voltage on to the grid so that a full signal swing is well under the cathode's upper limit. When this voltage gets raised to a certain level it looks like it works to oppose the tie in (through the 470k R) to the + rail so that the diode gets a forward biasing voltage and starts to conduct. The current flow stablizes the voltage at whatever the diode conduction limit is and this raises the grid bias up closer to where the cathode sits. Much closer. The static K voltage without distortion is + 3.5V. This would shift with the new bias conditons (to what, I don't know - it's hard to say) but not so much that the new "distortion on" grid bias with input signal couldn't reach it. It looks like it's meant to pit the rail voltages against each other until a point is reached where the diode kicks in and raises the grid bias up to a level where doinking around with the variable resistor starts to impact the grid/cathode interaction creating more or less distortion at the input. This signal gets "followed" out of the cathode circuit to the volume pot slightly smaller but with drive potential.

The stage before this doesn't need a huge amount of gain. Actually it can get by with very little because even a small signal voltage would cause an ugly (even uglier?) distortion had they not added the ground lift network "under" V3B starting at the coil. The grid of V3B is connected to the bottom of the Cathode RC network, through the 100k distortion pot. The signal goes from there into one end of the Volume pot and then divided down. This whole network sits on top of another network which looks like it's meant to allow for the clean headroom needed for "distortion off" settings along with the bias shift adjustment needs for "distortion on" setting which I mentioned above.

The coil looks like a two way street. Input one way, feedback the other. Filtering high frequency content either way; just keeping frequency crud off the grid and letting the bypass cap at the cathode do it's usual thing but with the higher frequencies cut out.

A word of warning... I haven't built an amp in probably 5 years. I haven't l even looked inside one and very rarely even think about them anymore so these are just "at a glance" observations and I am rusty to say the least.

very interesting

[Steve Conner]

I'm not too sure how it works either. Maybe it's similar to the crappy sounding distortion circuit that Ampeg put in some of their models: I seem to remember that used some kind of feedback too.

[tele_guitarist]

Originally Posted by Steve Conner

I'm not too sure how it works either. Maybe it's similar to the crappy sounding distortion circuit that Ampeg put in some of their models: I seem to remember that used some kind of feedback too.

http://www.indyguitarist.com/soundcl...ndclip-mp3.mp3