Finished Express but have buzz

[Legin]

I re-did the grounding to Pauls spec and it has removed the buzz (thanks Paul) I still need to tweak the lead dress to fine tune but the amp is singing now! I have found my bias pot 25K is not enough to get my bias correct so I am going to have to mess with that area.
Nigel

[slajeune]

Score another one for Paul.

Paul, you are amazing! Your posts here on buzzing has solved MANY hum / buzzing in the last few days.

Thanks!
Stephane.

[UR12]

Nigel

Instead of changing the bias pot try changing the 220K to a 150k or 180k to bring the bias in range on the pot.

[Legin]

Nigel

Instead of changing the bias pot try changing the 220K to a 150k or 180k to bring the bias in range on the pot.

Thanks, that was my intention I think 180K will do the trick, I am only 3 or 4 mA out.
Nigel

[UR12]

I ran into this problem too. I switched to a 150k but could only get a range down to about -33v. My amp works fine there but I would like to change it to a 180k and see if I cant bring it up to about -30v. with the 220k I could only get the pot to go up to -28v. Is this what you are seeing?

[Legin]

Exactly!

[MooreCowbelle]

Nigel,
pics of the new build?

[Legin]

Has anyone experimented with any other values in the volume control, I am finding it jumps into loud fairly fast and thought maybe a 2M A pot would control the volume uptake a bit more.
Nigel

EDIT: Just been reminded I need to go down in value to reduce volume

[Legin]

Nigel,
pics of the new build?

Here is a record of what I have done so far apart from the new grounding scheme.
http://kirkby.com.au/g2/main.php?g2_vie ... itemId=528
Nigel

[Legin]

Sorted the bias issue out by replacing the 220K with a 180K, now perfect bias for my EL34s is midway through my pot however it did jump around a bit and I think I might replace the single turn trimmer with a ten turn or such.

I am a bit perplexed about the volume pot though, it really does jump from nothing to V loud and I would like to fix that if anyone has a clue, I bumped up another post which did not come to a conclusion in hope.
Nigel

[MooreCowbelle]

Have you checked to make sure the volume pot is good?

[paulruby]

Well... A wreck has no MV and the volume pot is really a gain pot. So, a wreck does tend to go to full volume very early on the volume pot and then just controls the amount of dirt and feedbacm/sustain.

FYI, this is somethin I tweak on mine by either reducing the plate resistor of the 3rd stage or making a split-load plate resistor on the 3rd stage. Simply reducing the plate resistor works very well. There s a slight difference in tone using a split load since that allows the tube plate voltage to run full scale (so you get the 2nd order harmonics) while the output signal is reduced.

In short, you need to hand-tweak the "MV" that is hard-wired into the the plate resistor of the 3rd stage.

[MooreCowbelle]

More or less like the Fender Deluxe. Full volume at 2, just more dirt after that.

Paul, can you describe the split-load arrangement a little bit for us mere mortals? Or, is it on your site?

[paulruby]

To reduce gain in a stage you can simply reduce the plate resistor value. Cut that resistor's value in half and you'll only get half voltage gain from the stage.

But, the idle plate voltage will but much closer to B+ and will not swing as far toward 0V. As such, the the transfer characteristic will be more linear, will not approach saturation. So, loses the tube characteristic that we want in the tone.

The solution: use two plate resistors in series. For example, use two 51K resistors in series to replace the 100K plate resistor. Since the tube still sees 100K plate resistance, the plate voltage still has full swing and the tube will saturate the same and you get all the 2nd order harmonics. But, you take the signal from the junction of the two resistors, not the tube plate. You only get half the voltage signal but that signal is of the same shape and character as it was with the full 100k plate resistor.

Another way to think of it is to have a 100K pot as your plate resistor and take the output from the wiper. This will act like a volume control and you can have full output to zero output. (But, I wouldn't actually use a pot since it is carrying DC current and exposes the pot to full B+. Not safe to do.)

It's just a hard-wired MV that you can tweak to balance preamp and power amp gain to where you like.

[MooreCowbelle]

OMG! I actually understand that!

Paul, thanks.