Issues regarding hum balance

[fritzthecat]

Hi there

I just finished my first Kaiju build yesterday - attached my testspeakers (Quadral Merlin - nothing special - not very efficient and not very good sounding as well) and even with these super low efficiency crap speakers the Kaiju was humming like a fat bumblebee before starting the hum balance adjustment. Then I started adjusting the hum balance and got (almost) rid of it.

But actually this was very difficult as I had the following problem: The pot used for hum balance adjustments within the Kaiju is a 10 Ohm wirewound type - correct me if I am wrong - so you feel the slider inside jumping from one wire to the next while turning the pot - and exactly here seems to lie the problem (at least that's my conclusion - but most possibly I am wrong - helpful advise would be very much apreciated):

Here we go: At position 1 (of the hum balancing pot) I read 12 mv - at position 2 (the next wire the slider jumps to) I read 7 mV - position 3 (third wire it jumps to) I read 14 mV. Trying in both directions from position 2 onwards it only gets worse means I get higher mV readings. 7 mV is the best I am able to get on my left channel and 5 mV the best I am able t o get on the right channel (which at least is what the manuals says).

So my question: In other kits (transitor ones and on the CS4 board in the Kaiju) I built, multiturn trimpots are used to adjust values - is it possible to replace the hum balancing potentiometer through something that makes finer adjustments possible or is there any way to maybe attach another finer potentiometer to make very fine adjustments possible???

[Paul Joppa]

The issue with wirewound pot is true. In earlier amps we used a 50 ohm pot; now we have the 10 ohm pot with an added 22 ohm wirewound resistor on each side, which gives a 54 ohm total value with 5 times better resolution.

Multiturn precision wirewound pots do exist, though they are quite expensive.

I have stayed with the wirewound types because they are generally quiet with direct current flowing through them. Carbon is not. There are cermet pots which might be quiet, and there are bulk-metal foil trimmers which seem likely to be quiet. Not all types are available with solder lugs; trimmers are almost always made for printed circuit mounting. Be sure that whatever pot you use is rated for at least 1 watt.

Hope that helps - good luck!

[Paul Birkeland]

For the price of the multiturn pots, you're not going to be that far off of adding the DC supply.

The behavior of your wirewound pot is a little odd in that the hum null is so close to one end of the rotation.  Does this behavior exist with both tubes in the same channel, or does it follow a particular 300B?

[fritzthecat]

For the price of the multiturn pots, you're not going to be that far off of adding the DC supply.

The behavior of your wirewound pot is a little odd in that the hum null is so close to one end of the rotation.  Does this behavior exist with both tubes in the same channel, or does it follow a particular 300B?

Good morning Paul

I'll check this!

Am I getting you right: I'll swap the 300bs channelwise measure again what happens during hum balance adjustments and if the problem swaps as well with the tubes to the other channel it must be the tube and if it stays I'll get back to you for advice. EH standard types by the way is what I'm using right now like BH recommends.

I have the possibility to check against paired couples of EH Gold or PSVane 300b-N as well - let's see.

I'm not sure if I got it right over from Swiss German to English (English is not my mother tongue) so let me try to make it a bit clearer: It is not that the best position is close to one or the other end of the pot's rotation way - actually I think it is quite in the center of the whole way the pot can be turned - it is more like there only is one position of lowest hum (5mv on one channel (which is what the manual says we should try to achieve here) 7mv on the other side) and no matter if you turn clock- or counterclockwise there is no position on the whole pots way where I can get any lower values on both channels.

The problem is, that the values,on the 7mV side are jumping in 2 to 3 mV steps as the slider inside jumps from one of the pots wire winding to the next and it actually does on both sides but I'll recheck that to be on the safe side and to be sure it is on both sides (jumping effect).

Maybe it's my perfectionism again that just makes it more complicated than it actually is - I installed the DC Filament upgrade yesterday evening - I realized, that the red solid core wires coming from 14u are not in a straight line and I had to correct it - no wonder my wife left me years ago, I guess I'm a pain in everybodies a..s - sometimes - sorry for being such a crazy f.... .

Even with the DC Filament upgrade installed I still have to adjust for the lowest possible values regarding hum balance - right?

[Paul Joppa]

The DC filaments produce practically no hum; the hum balance in Kaiju should have no effect on hum.

However, many observers feel that regulators for filament power produce some subtle distortion products. The hum balance pot will allow these to be mostly cancelled. We have not yet tried to determine the audibility of this effect ourselves.

[fritzthecat]

The DC filaments produce practically no hum; the hum balance in Kaiju should have no effect on hum.

However, many observers feel that regulators for filament power produce some subtle distortion products. The hum balance pot will allow these to be mostly cancelled. We have not yet tried to determine the audibility of this effect ourselves.

Thank you - DC and hum - actually this was a stupid question. I gonna find out tonite - always need some time until I got the heart to find out if the smoke stays inside the circuit afrer changes.