Bluejack a écrit :
Sinon, qui peut me dire les fonctions de V1 V2(reverb je crois?) V3 V4?
Le bias doit-il etre aussi regler pour les lampes de preamp ?
Il me semble que seules les lampes de puissance doivent etres biaser, mais suis pas sur.
Une 12ax7 EH en V1 semblerait etre un bon choix.
Les 12ax7 EH coutent 2,5 fois moins cher que les 12ax7 groove tubes, et pourtant sont-elles de meilleures qualités ?
Apparement on peut mettre une 5751 en V2, quel serait l'interet ?
Pas de réglage bias pour les preamp
V1 : gain stage
V2 : (12AT7) sert de reverb driver
1/2 de V3 sert de sortie pour la reverb.
l'autre 1/2 de V3 nouvel étage de gain après reverb.
1/2 de V4 sert pour le tremolo, 1/2 de V4 est l'inverseur de phase pour les tubes de puissance.
C'est en V1 que tu peux mettre la 5751 sur un Princeton pas en V2
(V2 c'est sur les autres Fender qui ont canal normal et canal reverb)
Enfin, à mon avis, toutes les marques de tubes se valent tant qu'on arrive à en trouver qui soient aux specifications.
Un peu de littérature sur le fonctionnement du circuit du Princeton reverb :
"I'll get it started. Plug into input one. The signal goes through two 68k resistors in parallel (34K)
and there is a 1meg resistor from signal to ground.
The series resistance reduces rF interference and the 1meg resistor sets the impedance.
The signal hits the grid of V1A.
The gain of this stage is set by the 100k plate resistor and the 1.5k cathode resistor,
which is bypassed by a 22uF electrolytic cap which increases gain across the audio spectrum of the guitar.
The signal leaves the plate and hits the tone stack.
The tone stack is typical blackface Fender that creates an inherent mid cut which is the classic blackface Fender tone.
In this amp there is bass and treble, but no mid control.
The midrange is fixed with a 6.8k resistor.
In a three band eq Fender this resistor is replaced with a 10k pot.
The 6.8k resistor is approximately the setting of the mid control 2/3 of the way up.
The volume control is between the tone stack and the next gain stage.
The tone stack creates a lot of signal loss.
A gain stage immediately follows the lossy tone stack, after the volume control.
The stage is identical to the first stage.
Because of this arrangement...gain stage/lossy tone network/gain stage, it is harder for overdrive to build up in the gain stages.
In many amps, the gain stages precede the tone stack (59 Bassman/Marshall) and these amps are easier to break up.
Next the signal hits the reverb resistor, a 3meg resistor with a 10pF cap in parallel.
This resistor/cap will keep the input and output signal to and from the reverb circuit isolated.
The signal also splits off to the reverb drive amp...
a 12AT7 with both halves paralleled with a small output transformer driving the input of an Accutronics reverb tank.
The output of the reverb tank goes to 1/2 of V3.
This is a gain stage identical to the preamp gain stage. It amplifies the small reverb signal.
This signal goes through a reverb pot, and 470k resistor which brings the signal to the other end of the 3meg reverb resistor.
This mixes the reverb with the straight signal.
This 3meg resistor causes a lot of signal loss, so the second half of V3 is another gain stage bringing the signal back up.
That's the preamp. Gain stage/tone stack/gainstage/reverb/gainstage.
Next is the Phase inverter.
Half of this tube is the inverter, which splits the signal into two halves which drive the power tubes.
In most blackface Fenders a "long tailed pair" PI is used, which is comprised of both halves of a tube...
usually a 12AT7. This type of PI increases the gain.
Not so in the PRRI. There is a slight bit of loss in this PI so the power tubes aren't driven as hard as say a DRRI...
a very similar amp.
The power tubes are fixed bias.
They utilize a 1 ohm resistor from the cathode to ground on each power tube...
R20 and R32 I believe. If you measure across either of these resistors, you will get a reading in millivolts.
This translates to the milliamp current draw of that tube. This measurement is what you use when you adjust bias.
You only need a volt/ohm meter to measure this.
The bias adjust is R22, a 10k pot.
The schematic shows to adjust the bias pot for a 23mV signal across R20 or R32.
If one reads higher than the other, use the higher of the two.
The amp is on when you measure this, so all due care is needed.
The other half of the PI tube is used as the oscillator of the vibrato, which is actually mis-named.
It's actually a tremelo. (Conversely, a guitar tremelo is actually a vibrato.)
In most bf Fenders, this oscillator modulates a light.
A photo resistor next to the modulating light is fixed between the preamp signal and ground.
As the light modulates, the resistance goes up and down, and modulates the volume of the preamp signal.
Voila...tremelo.
In this amp the tremelo modulates the bias voltage.
When you adjust the bias you are adjusting the negative voltage applied to the grids of the power tubes.
The higher the negative voltage, the quieter the tubes become.
By modulating this voltage, the volume of the power tubes is turned up and down.
The bias setting fixes how loud the tubes can get.
This sounds different than the photo resistor/lamp method, and many far prefer this type of tremelo.
The power tubes drive the output transformer, which drives the speaker.
This amp is very similar to the Deluxe Reverb, except for the tremelo, the phase inverter, there is no normal channel,
there is no 47pF bright cap on the volume control, and the output transformer is smaller."
"Yesterday today was tomorrow and tomorrow today will be yesterday"
