5F6A won't break up

[greekie]

6,1VAC is very much within spec (+/- 10%), and is probably due to tolerances in the transformer, and perhaps in the measurement device as well - e.g. my Fluke measures a bit different than some of my cheaper meters. Also, -41 is perfectly fine. I've had 6L6's that needed as low as -36 to give 20-25W a side.

[MWJB]

-41v does not tell us how much current the tubes are drawing, this will vary with tubes of different brands & even between tubes of the same brand. I have seen tubes that run from 7mA to 40mA at the same negative voltage, in the same amp! In a fixed bias amp it is not advisable to bias to 20-25W per side, maybe 20W per side max (usually 15W or so), if you have genuine 30W tubes (many current production 6L6 type tubes are not). 5F6A clearly states "-48v" at 432v on the plates, you can scale up & down the negative voltage in relation to plate voltage to keep to "factory spec", but always check actual plate current & adjust accordingly.

[greekie]

That was my point exactly - that thinking that there needs to be a fixed voltage there, for use with all tubes, is not exactly how it should be. Randall Smith of Mesa/Boogie has his oppinions about this, but as changing the bias is a breeze on all my amps, I don't listen to him much. I've seen a load of Mesa/Boogie amps with non-Mesa brand tubes, giving out 10-40W in a 100W head.

Kind regards, Jake

[el_fela]

There is a large voltage drop on B+ if I make the grid voltage less negative. And the 5f6a was greatly gounded! maybe its the guitar...

[el_fela]

well, what do yout think about the sound clip?

Today I mixed a el34 and a 6l6. It clipped a lot more musically than ever before!

I just dont know how to isolate the problem.

About the grid wires, i removed the cable and soldered a 68k resistor right to V1 pin2, with a 1 inch long wire and a jack. I plugged in the guitar right there. no results. Then tried something similar with V2 pin 2, bypassing the wires and the pot. No good.

[loudthud]

I listened to the sound clip. That is really ugly strange distortion. In the original 5F6A there is a 27K resistor that connects between the speaker connection back to the Presence control. This is the feedback resistor around the output stage. As an experiment, try disconnecting one end of that resistor to see how it affects the distortion.

[el_fela]

nope, it just sounds a litter louder.
I noticed that when i plug the guiitar in the normal channel, with the normal por all the way down, and the bright pot all the way up, the guitar sound goes trough, but with a loud hum. It sounds crunchy, almost without bass, much better than before. ¿is that normal?
Here is a sound clip. First, the situation i described. The last part (the bassy one) is the guitar plugged in the normal channel with notmal vol at 7.
5f6a

[loudthud]

I noticed that when i plug the guiitar in the normal channel, with the normal por all the way down, and the bright pot all the way up, the guitar sound goes trough, but with a loud hum. It sounds crunchy, almost without bass, much better than before. ¿is that normal?

If you have followed the 5F6A schematic exactly, this usually indicates a bad capacitor on the first tube cathodes. Most builders avoid this as Marshall did and separate the cathodes with different voicing capacitors on each. This could be what is causing the other problems so it is strongly advised.

[el_fela]

wow, ill try that! thanks!!

[el_fela]

wel, its been a few months now. i recently bought a 30W OT for new a deluxe reverb build, and i wonder... ¿can replace the bassman OT with this new one?
¿just to see what happens? ¿could anything break?

[loudthud]

Without knowing the specifics of your transformer, you are probably safe as long as you don't turn all the knobs up to 12 and grind the guitar like you are trying to break all the strings.

[el_fela]

Well, after a long time with no improvements, I decided to "listen" to the signal at several points, in order to locate the source of all problems.

I used cap and a 2M resistor to conect several points of the signal path to a line input.

Red circle: Fizzy sound and ugly distortion present
Green circle: Clean sound




I guess i can forget about power stage problems, but
¿can the OT be responsible for this? ¿or the power supply?
¿Or should I focus un V2 and the related part of the circuitry?

I learned a lot, but i lost faith. Help pleasE!!!

[black_labb]

seems that valve 2 is breaking up. check the voltages on each of the pins and get back to us.

edit: sorry, had it in thread mode and didnt see the rest of the thread, but still check these voltages.

[el_fela]

Thanks black_labb, I'll check those voltages once more, but i tell you, they were exactly as per fender schematic last time i checked.

Now I'd like to make sure the problem is not related to the board, so, I was wondering:

If i disconect V2 pin8 (cathode) from the board and then connect it to pin 2 on V3 using a .1uF cap, that would bypass the whole tonestack, right?
I would need the 100k resistor (cahtode to ground) which I assume is the load resistor, correct?

Would be te right way to do it?

[el_fela]

Well I checked that and:

B+ = 405 VDC
V2:
pin1:170
pin3: 1.18
pin6: 315
pin8: 170

it seems just a little bit lower than standard. I dont know if its cause im using much lower value caps on the tonestack.

Bypassing the tonestack helped clean up the sound, but still ain't sweet...