where does distortion come from??

[liwei]

i built a deluxe ab763 with only normal channel(no vibrato no reverb).i cut off the vibrato circuit and vibrato channel preamp circuit. i use 1.5k cathode resistor at 12ax7.less tubes,less current,so the plate voltage of preamp get a bit higher.after trying different value dropping resistors now i get pretty much the same plate voltage as the original schem.
problem is there is no distortion (overdrive?) at any volume, only clean. and the tone is dry and hard and bright,bad sound. i tried to bias the 6v6 plate current even more than 30ma, no significant change for the tone.
is it just because i delete a channel??
i use hammond ot, jensen c12n,chinese 6v6,should i change some parts?
and take ab763 for an example, where does the distortion come from??preamp or pi or poweramp?
thanks

[MWJB]

A good start would be to post the voltages that you actually have at tube plates, screens & cathodes.

In reality you might find many Deluxes run over 200vdc on the 12AX7 plates, so how high are your 12AX7 plate voltages?

Non reverb Deluxes are not really reknowned for their distorted tone, the additional gain of the reverb is quite desirable for most.

You could try a 12AX7 in the PI tube instead of the 12AT7, 820ohm cathode resistor at pin 8 of V1, a larger value NFB resistor (820ohms stock)?

[tubeswell]

Further to what MWJB said, another way of getting distortion (besides from the pre-amp) is by lowering the output stage voltages (to around 340 -350 Plate and 290-300 screens) (try a 5V4 or even a 5Y3GT rectifier) in conjunction with going to cathode bias and biasing your 6V6s to run at max dissipation, and then increase the output of your PI signal. (You should still be able to get some good distortion this way with a 6k6 reflected load). Although you still might have to tweek the pre-amp to get pleasant distortion with all those stages in there

[liwei]

thank you MWJB

Non reverb Deluxes are not really reknowned for their distorted tone, the additional gain of the reverb is quite desirable for most.

you mean reverb make the amp easier to get distortion??



now i get
B+:400V
6v6 plate current:24.6ma 26.3ma
PI 12at7 plate:177V
voltage on 470ohm resistor which connected to 12at7 cathode:1.68V
preamp 12ax7 plate:165V
12ax7 cathode resistor voltage:1.22V

actually i have tried 2 power transformers,the previous one produce too high voltage.i changed it.so i have experienced 12ax7 plate voltage from 165v to about 200V,no big change.

yes 820ohm nfb resistor

another thing, when i powered the amp,at begining, the plate current of 6v6s are almost the same,they're matched.but several minutes later,one of the tubes get about 7% higher current than the other one.i want to buy a pair of tung sol 6v6 to try.maybe it's all about the tubes??

and if i use 12ax7 for PI,what else parts should i change?

[liwei]

Further to what MWJB said, another way of getting distortion (besides from the pre-amp) is by lowering the output stage voltages (to around 340 -350 Plate and 290-300 screens) (try a 5V4 or even a 5Y3GT rectifier) in conjunction with going to cathode bias and biasing your 6V6s to run at max dissipation, and then increase the output of your PI signal. (You should still be able to get some good distortion this way with a 6k6 reflected load). Although you still might have to tweek the pre-amp to get pleasant distortion with all those stages in there

thanks
it sounds like a tweed deluxe.maybe i will try it later.

[MWJB]

The additional gain stage following the reverb makes it easier for the amp to distort.

1.68v at the 12AT7 cathode is wrong, you might get that between the cathode and the PI tail resistor (22K), but you should have a lot more voltage than that at pins 3 & 8 of the 12AT7?

+/- 5mA is deemed to be a "match", your 7% deviation is what, under 2mA? If so don't worry about it.

You don't NEED to change any parts other than the tube itself, when subbing a 12AT7 PI for 12AX7. Though, if you are looking for more drive you could reducethe value of the 22K tail resistor to 10K.

[hasserl]

In that amp you'll loose a lot of gain thru the tone stack. You could try increasing the value of the mid resistor to reduce the loss of gain in the stack and keep a stronger signal. Try values in the 28k range, maybe work with a 50k pot wired as a variable resistor and play the amp while you make adjustments to the pot. If you find a good setting measure the resistance and replace the pot with a mid resistor of that value. Some folks even lift the ground from the mid resistor which will completely bypass the tone stack. You'll pick up a lot of gain that way, but it may be a little too much.

You also might look at reducing the amount of nfb, try a 5k pot in series with the 820ohm nfb resistor and dial in some resistance, that should get you some distortion.

And you might try replacing the 12AT7 pi with a 12AX7 to increase the signal level to the power tubes and push them into distortion.

[liwei]

The additional gain stage following the reverb makes it easier for the amp to distort.

1.68v at the 12AT7 cathode is wrong, you might get that between the cathode and the PI tail resistor (22K), but you should have a lot more voltage than that at pins 3 & 8 of the 12AT7?

+/- 5mA is deemed to be a "match", your 7% deviation is what, under 2mA? If so don't worry about it.

You don't NEED to change any parts other than the tube itself, when subbing a 12AT7 PI for 12AX7. Though, if you are looking for more drive you could reducethe value of the 22K tail resistor to 10K.

i mean 1.68v on the 470ohm resistor,i think it is grid-cathode bias of 12at7.

[liwei]

In that amp you'll loose a lot of gain thru the tone stack. You could try increasing the value of the mid resistor to reduce the loss of gain in the stack and keep a stronger signal. Try values in the 28k range, maybe work with a 50k pot wired as a variable resistor and play the amp while you make adjustments to the pot. If you find a good setting measure the resistance and replace the pot with a mid resistor of that value. Some folks even lift the ground from the mid resistor which will completely bypass the tone stack. You'll pick up a lot of gain that way, but it may be a little too much.

You also might look at reducing the amount of nfb, try a 5k pot in series with the 820ohm nfb resistor and dial in some resistance, that should get you some distortion.

And you might try replacing the 12AT7 pi with a 12AX7 to increase the signal level to the power tubes and push them into distortion.

do you mean the 6.8k resistor?i have tried the mod on schematicheaven.com,20k pot replace the 6.8k resistor.yes it changed the sound,but not what i expect.for the nfb,i will tried it.thanks
i will make some record for it later,maybe you guys can find something out.

i have read some thread about the distortion thing on this forum,but i can't understand thoroughly.for the blackface of ab763 schem,does the distortion comes from preamp 12ax7? it doesn't come from PI or power tube,does it?
actually i love the clean tone of blackface amp, but my built sounds too clean.it sounds solid state...

[MWJB]

"i have read some thread about the distortion thing on this forum,but i can't understand thoroughly.for the blackface of ab763 schem,does the distortion comes from preamp 12ax7? it doesn't come from PI or power tube,does it?" In most successful designs distortion does not come from on, single part in the circuit, preamp and power tubes compliment each other. Most folk sare talking about the reverb channel when discussing AB763.

Even so, most guys who want an overdriven sound from an AB763 might stick a tubescreamer in front of the amp.

"but my built sounds too clean.it sounds solid state..." This doesn't sound right, did you check out the voltage at the PI cathode?

[txstrat]

i have read some thread about the distortion thing on this forum,but i can't understand thoroughly.for the blackface of ab763 schem,does the distortion comes from preamp 12ax7? it doesn't come from PI or power tube,does it?
actually i love the clean tone of blackface amp, but my built sounds too clean.it sounds solid state...

In a typical BF amp the distortion (I prefer the term drive) is made up in the power stage including PI. Try inserting a master volume before the PI, turn the master down and crank the preamp vlume. What you get is a clean tone at a lower volume only.
For a drive tone you need to push the power tubes.

Even so, most guys who want an overdriven sound from an AB763 might stick a tubescreamer in front of the amp.

I agree.
Other way to overdrive the preamp is to add one more gain stage such as in a early 80s Champ II.

The 820 ohms nfb resistor might be to small since the amount of signal fed back from the OT lowers the strength of the signal and therefore cleans up the amp.

[MWJB]

Pins 3 & 8 of your PI tube are linked by a jumper wire - what is the voltage at these tube pins? It should be much, much higher than 1.68volts. If it is 1.68volts something is miswired.

[liwei]

In a typical BF amp the distortion (I prefer the term drive) is made up in the power stage including PI. Try inserting a master volume before the PI, turn the master down and crank the preamp vlume. What you get is a clean tone at a lower volume only.
For a drive tone you need to push the power tubes.

thank you Matt
i tried adding a 1m master volume pot as you said,and removed the 220k mixing resistor.i still haven't get the distortion with the pre stage vol cranking up, and the tone get worse.but when i turn up the master vol to max with the pre stage vol at 3 or 4, i get the tone!! nice tone!! i think it was the 220k mixing resistor who eat the tone....
now i love the sound,although i still haven't get the distortion.but i guess,as MWJB said,a tubescreamer can solve this problem.

voltage now
b+ 399V
plate current of 6v6 24ma 25.7ma
12at7 plate 169.7v 169.6v
cathode 77.4V the other side of the 470ohm resistor 75.7
12ax7 plate 172.8v 173v
cathode 1.278v 1.276v

thank you guys!!

[txstrat]

I think I didn't make myself clear enough. The master volume is not necessary to get the tone you have now (full open it acts like a plain wire in the circuit).
If you're looking for more distortion you might like the 6g3 circuit.
Heres a thread where you can find some sound clips:
http://www.music-electronics-forum.c...ead.php?t=9928

[liwei]

I think I didn't make myself clear enough. The master volume is not necessary to get the tone you have now (full open it acts like a plain wire in the circuit).
If you're looking for more distortion you might like the 6g3 circuit.
Heres a thread where you can find some sound clips:
http://www.music-electronics-forum.c...ead.php?t=9928

i have read your post,nice work and nice tone.i did'nt know fender history well,so i always thought blonde or brown deluxe was aa763 too...turns out it was early than 1963....i will try a 12ax7 for PI as 6g3 schem shows,it seems really worth a shot..

the max master vol means .047uf cap directly connect to .001uf cap with a 1m resistor to ground.so the 220k resistor between the caps (original ab763 schem) was removed.i don't exactly know why,but it did change the tone to some degree.and the plate voltage adjustment might help a bit.