Author Topic: Le Craquiement Thermonique (SP?) debugging  (Read 5115 times)

PRR

Re: Le Craquiement Thermonique (SP?) debugging
« Reply #20 on: June 03, 2010, 04:18:59 PM »
> we do not cover cathode ray tubes

CRTs??? Nobody uses them any more. (Yeah, you can find a CRT TV or PC monitor here or there, and oscilloscopes still are made with CRTs, but LCDs with CPUs are taking over.)

Is this a sharp left turn? Or are you thinking the tube in this pedal is a CRT?

> Does the gate correspond to any part of the CRT?

An amplifier vacuum tube is very much like a J-FET. Grid is gate, cathode is source, plate is drain. If they taught basic simple J-FET circuits, you have seen the gate tied to ground via a large resistor (current is near zero so a large resistor gives no voltage drop and negligible loading of the signal input). The source is stood-up on a few-K resistor. The drain-source path passes current, so this resistor makes a 0.5V-3V voltage drop, which puts the JFET gate-source port somewhere between OFF and ON, where small inputs make larger outputs. The drain is pulled up by a battery (etc).

Differences:

Vacuum is an insulator. You have to get some free electrons in there, typically with a hot cathode.

Silicon conducts better than empty vacuum. Vacuum tubes typically run higher voltage for similar or lower currents than JFETs.

The simplest vacuum tube amplifier, the Triode, has significant internal plate-to-grid negative feedback, giving lower gain and output impedance. A Pentode eliminates this for high voltage gain and high output impedance, similar to a JFET except the pentode needs another connection (Screen, G2).

jbord39

Re: Le Craquiement Thermonique (SP?) debugging
« Reply #21 on: June 03, 2010, 08:36:29 PM »
I was confusing CRT's and the tube amplifier.  We did cover JFET's.

  

Thanks a bunch for the reply, it is very helpful.  What are all the extra pins for?

-John
« Last Edit: June 03, 2010, 09:50:47 PM by jbord39 »

jbord39

Re: Le Craquiement Thermonique (SP?) debugging
« Reply #22 on: June 12, 2010, 04:45:14 PM »
I just finished putting the circuit in its housing.  There is a very bad, low frequency buzzing (if the tone knob is not turned all the way up it is very loud).  All of the guitar effects still work, but the buzzing is worse with triode switched on, and also without superheated.

Any ideas?

Thanks,

John

jbord39

Re: Le Craquiement Thermonique (SP?) debugging
« Reply #23 on: June 12, 2010, 05:15:06 PM »
Problem was the wall wart is unregulated. 

Thanks,

John

(By the way, the pedal sounds great!!!)