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FBLEAGH 18th November 2004 11:11 PM

tube preamp distortion
hi guys

I am currently building a mono preamp kit ( from jaycar in australia ).
Its mainly designed for guitar ( which is what im gonna use it for ).

I have 2 questions.

1) how would i modify this circuit to improve the warmth ? would i reduce the neg feedback ? how and which components should i "adjust"?

2) how would i get this amp to go into overdrive/clipping ? increase the gain ? could i add a pot to adjust this and where would i put it/ what value ?

anyway here the schematic and the board layout

thanks in advance for your help and time guys.

SY 19th November 2004 02:23 AM

My first crack at fattening the tone a bit would be to drop the 100K plate resistor on the first stage by 10-20% or so.

Sch3mat1c 19th November 2004 03:14 AM

If you want crunch, just overdrive it (more input signal than either stage can handle, it should start clipping when the first stage overdrives the second at around 1-2V, which is maybe 20mV input) and tack on an output volume control so you don't overdrive what's following it.


SHiFTY 19th November 2004 03:23 AM

2 Attachment(s)
To massively increase gain, delete the 220nF 630V and the 2 33K resistors coming from the anode of the second triode (pin6). They dont need to be there IMO!! If you get hum try running the filaments on DC.

To make the overdrive adjustable, try this: buy a 500K pot, and replace the 1M resistor from pin 7 to ground. The wiper goes to the grid as per the attached drawing. you could use a 1M pot but i can never find them new. Note I have removed the filaments on schematics for clarity... They still need to be attached :)

Also I would replace the 100n input cap with a 68K resistor as in all old Fender designs!

Keep in mind the output will be a bit hotter so you might need to put a pot on the output as well (shown), however this might not be necessary... Try it without first.

Also try and get a NOS 12AX7 they sound better! :D

gingertube 19th November 2004 03:48 AM

The whole hog
G'day from Adelaide,

If you want the whole hog - I suggest the following

BIG HINT: Draw the circuit changes on paper before you start so you can see what you will be doing.

1) remove the 33K + 33k + 220nF feedback network between V1b pin 6 and V1a pin 1

2) put a 47uF/16V electrolytic across the 1K from V1a pin 3 to GND (+ve to pin 3)

3) cut the track between the 1M resistor and the 100nF input capacitor and pin 2 of V1a and and then bridge the cut with a 68K resistor (ie put a 68K resistor in series with the feed to pin 2)

4) change the 1K and 220uF from pin 8 of V1b to GND to 3K3 and 1uF - this runs the 2nd stage at low current. The voltage at the anode (pin 6) will move from +160V to probably about +220V which will introduce a fair amount of distortion and if driven hard enough give you clipping on positive going side of the signal.

5) change the 100nF at pin 1 of V1a to 22nF to limit the bass a bit

This will "voice" the preamp for guitar and give you a Marshall like CRUNCH sound.

Optional Gain Control:
lift the end of the 100nF (changed to 22nF above) which feeds
pin 7 (the grid of V1b). From this capacitor "free end" (the end which used to go to pin 7) wire a 470K resistor with a parallel 470pF cap to the top of a 1 Meg log "Gain" potentiometer. wire the other side of the gain pot to GND. Wire the wiper of the pot to pin 7. and last add a 470pF from the wiper of the pot to the top (where the 470K and 470pf connect) to give a bit more brightness at lower gain settings.
You could also change the 1M resistor from pin 7 to GND to say 4M7 or simply leave it out.

Good Luck,

FBLEAGH 20th November 2004 02:39 AM

that lat post reminds me - is there anything i should do different if it a bass guitar ?

PS. thats for all the ideas guys :)

I'll give them a go and post my results later :)

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