2nd amp project is Finished! :D A few questions...

[tribi9]

Never mind, the amp works fine and spunds awesome. The output on the guitar i was testing was fucked. Worked on the first try with a different guitar woohoo

Cheers and thx to all who helped me.

[tribi9]

It's quiet too. No humming or weird noises. Im really happy, i had a really shitty weekend and this just made my day.

[martin manning]

Glad you got it figured out and congrats! One suggestion, it's safest to bring a new build up without tubes installed, maybe with some kind of current limiter too. Keep your eyes, ears and nose open, and then check the B+ and bias voltage and range if it's a fixed-bias power section. That can save you some greif and fried components if something isn't right.

[tribi9]

Thx Martin, I was going to test it with a Variac at about 1/10th of the voltage but my variac hook-up fell through.

A couple of observations...

Im using a 5Z4 rectifier instead of a GZ34.

The Voltage on my plates is 370 V. I get a read of 30 V on the Cathode. I didn't measure cathode to plate, I will in the morning. Homework and eating caught up to me.

I would have thought my 355-0-355 PT would put something a bit higher than that.

So far 6V6's sound better to my ears than 6L6's. I havent taken any voltage measurements with 6V6's yet.

And last but not least....

People that build amps need to eventually evolve a third arm.

[tribi9]

Question being that is cathode biased.

I haven't measured plate to cathode voltage but I did measure plate and cathode to ground. (Will do tomorrow) Will it be the same as reading plate to ground and then subtracting the cathode to ground voltage reading?

[martin manning]

I haven't measured plate to cathode voltage but I did measure plate and cathode to ground. (Will do tomorrow) Will it be the same as reading plate to ground and then subtracting the cathode to ground voltage reading?

It has to be the same... It's the law! Kirchoff's Voltge Law, that is ;^)

[tribi9]

So, Im finding that I like the sound better with 6V6s than 6L6's. The only thing Im worried about is that I think I might be running the tubes way too hot. Im running 6V6's at 381 V on the plates and only 23 on the Cathode and according to the Weber calculator that puts the plate current at 40 mA.

If I were to up the Cathode resistor a bit, say for go from a 270R to a 330R, would I be able to leave the same cathode bypass cap? I have a 25 uf @ 50V on the amp right now.


Cheers

[Structo]

The cap is fine.

So if you are running 381v at 40ma that is 15.25 watts (per tube), including screen current.

You can also measure the voltage drop across the resistor and divide that by the resistance of the cathode bias resistor.
That will give you the current on the bias.

You can also do that on the screen resistor to see what current the screens are taking.

On cathode biased amps you can go close to 100%, but you may like the tone of the amp better a little lower, you just have to experiment.

Some Marshall 18 w amps use a large cathode cap like a 50uF to 100uF 50v.

[tribi9]

Edit, nevermind. I figured it out.

These are my readings with the 270R cathode resistor.

6L6's
plate 419
Screen 378
Cathode 32

6V6's
Plate 436
Screen 391
Cathode 25

[tribi9]

I got something that is troubling me. I have looked for the answer and I honestly cant find why this is...

The tip on both my speaker jacks both have a reference to ground. With or without a speaker cable plugged in. Shouldn't only the sleeve reference to ground?

Is this normal, is this how it should be?

[Structo]

I can't tell from the pictures how you have them wired.

EDIT, if you are measuring continuity, then yes you will see it as a short to ground because the are on the same coil and very low resistance on the coil.

[tribi9]

EDIT, if you are measuring continuity, then yes you will see it as a short to ground because the are on the same coil and very low resistance on the coil.

Yes, ok. Thx that's the answer I was looking for.