Ax84 6l6 se p1 weak and distorted sound

[Skev]

Hi everybody.
I m building my third amp.
A 6l6 single ended "Ax84 6l6 p1" type amp.
The amp switch on, no problem.
Without guitar plug in , all the command ( treble-medium-bass-volume) work normally. but when i plug the guitar there a weak volume which goes lower and lower and distorted. I checked all voltages and all are normal from my ez81 to the 12ax7 and 6l6 ( except pin 5 grid which is at -50v ?!)
My pt is a 275-0275v , i m in 5kohm ar primary ot.
No cold solder. I think there is a problem on input guitar or preamp.
But i can t find what it is.

[martin manning]

There is surely a problem with pin 5 on the 6L6 at -50V... that should be zero, and I don't understand how you are generating a negative voltage. What are the voltages on pins 3, 4, and 8?

[Skev]

Thanks for the answer!
Pin 3 is 403v
Pin4 is 400v
Pin 8 is 23v
Pin5 is -50v
12ax7 and ez81 are brand new. Not totally sure about the 6l6.

[martin manning]

23V on pin 8 through a 330 ohm cathode resistor says there is 70mA there, but that's about half of what the 6L6 data sheet says should be flowing. The -50V on pin 5 doesn't make sense.

[Skev]

Ok, strange indeed
I use a 220ohm resistor at the cathode as the schematic notice it ( but they note 314v on the plate instead of my 403v!).
I measured the -50v between pin5 and pin 8 .
I use a 6l6gc

[Structo]

Are you using a single ended output transformer?

I'm not sure about SE but for push/pull cathode bias you usually set the bias to 100% dissipation.

P / V = I

30/ 403 = .074 or 74 mA.

[Skev]

Thanks for your help
Yes i m using a se output transformer.
What is 30 in the p=vi ? Is it 30w?
If it is, how did you find that number, please?

[Skev]

30w is the plate dissipation sorry for my question

[rock_mumbles]

It seems to me that 403V on the 6L6 plate is high for a 275v-0v-275V PT ???

[martin manning]

Post a link to the schematic you are using.

[Skev]

Thanks for help
I use an ez81 for dc voltage.
But it seems a bit high , can t explain why, but the 403v is real.

I really wonder if my bias is not wrong, really strange i m trying to calculate ...

[Skev]

What about 400ohm cathode bias resitor so that i could low down my bias at about 45-49ma ? Instead of about 100ma!!! I didn t even notice that!
I don t know if it causes my weak and distorted sound, but a bias at about 100ma is a stupid mistake!! Shame on me! Lol

[Stevem]

A P1 build calls for a PT of 275/ 275 with diodes which gives Ya 392 on the plate, how are you ending up with 403 thru a recto tube!
What is your B+ AC voltage into the recto tube?
Is your output tube socket wired right?
Do you have 6.3 volts for the heaters?
Accutally what is the PT you are using and how much current is the B+ winding rated at?

[passfan]

If signal is driving your output tube grid negative do you have a spare preamp tube to try swapping out?

[Skev]

It works!! The problems were the bad cathode bias resistor and a bad solder on a ground.

A big thanks to you guys! 😉
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