Using a pot instead of 2 resistors for a heater center tap

[CitizenCain]

The Heathkit amp that I'm converting has a hum balance system that essentially is a pot taking the place of the 2 100R resistors you'd normally see used as a heater artificial center tap.

The pot is a 2k pot. I like the idea of keeping the hum balance control. Is there any problem using that 2k pot instead of the two resistors?

The reason I ask is because Heathkit runs the output of the pot to a voltage divider that's tapped from one of the bigger filter caps so it's raised about 17 volts above ground. The junction of the resistors that form the voltage divider has its own 40 mfd filter cap, too. I had planned to strip it down and just run the center lug of the pot to ground.

Anyone see if there'd be any issues with that?

[hasserl]

I'd leave it alone. Floating the ground at 17 volts will help reduce hum from the filaments, as will the balance control. I see no advantage at all to ripping it out and going to a lesser system of two resistors directly to ground.

[Don Moose]

It's a fairly common noise-reducing trick to elevate the heater winding's CT a bit.

Since they spent money on the parts to do that, I expect they found it necessary.

Go ahead and unhook it, but try not to be too destructive about it - you may be putting it back.

Hope this helps!

[bluefoxicy]

It sounds like your amp uses DC filaments....... I tend to just twist the AC filament leads together, YMMV. This sounds interesting though.

Also, because the filaments tend to run on 6.3V, I've taken to hitting DC filaments with a 4700uF 16V cap, which costs like $4. YMMV.

[CitizenCain]

Nope, plain old AC filaments. No rectifier anywhere in the circuit. I think you guys are right. I tried the stripped down version last night. No matter what I did with wire routing, making sure all the circuits had only one path to ground , etc, massive hum was the result.

Gonna put the voltage divider back and hook it up to the filter cap again to see if it quiets back down. Hoping I didn't mess something else up whilst tinkering

[bluefoxicy]

Nope, plain old AC filaments. No rectifier anywhere in the circuit. I think you guys are right. I tried the stripped down version last night. No matter what I did with wire routing, making sure all the circuits had only one path to ground , etc, massive hum was the result.

Gonna put the voltage divider back and hook it up to the filter cap again to see if it quiets back down. Hoping I didn't mess something else up whilst tinkering

What the hell does a filter cap do in an AC power circuit? It's not carrying signal and there's no way in hell you're going to filter the ripple out without rectifying it. Sounds like your amp builder was brain damaged.

[CitizenCain]

You know, I never thought about that. It does seem kind of screwy, doesn't it? The designer was some dudes at Heathkit about 40 years ago, the amp is a 1966, judging from the pot codes.

Maybe the cap smoothes out the reference voltage (even more, it's already tapped off a filter cap)?

Here's what the schematic looks like.

http://www.visi.com/~sstolle/heath/aa-23-mod-002.JPG

[Paul P]

It's to smooth out the dc offset voltage to the heaters, which comes off
the first filter and may not be all that smooth.

Paul P

[bluefoxicy]

.... what the fuck?

Okay, you have straight AC going across the filaments of several tubes (recto tube gets the 5V so ignore that one).

That hum balance is your high-voltage tube driver secondary being plugged into your filament circuit. WTF? The only caps I see are the filter caps that should be in that DC circuit, other than that that pot should act like a straight resistor of its own value, and what the hell brain damage is this shit?

This may be a dumb suggestion, but have you tried disconnecting that pot entirely? See if the designers actually stumbled upon something or if this is as retarded as it looks.

[Paul P]

C24 in the schematic is the dc offset voltage filter cap.

Paul P

[CitizenCain]

It's to smooth out the dc offset voltage to the heaters, which comes off
the first filter and may not be all that smooth.

Paul P

OK, that's what I thought. Thanks

bluefoxicy, the designers were some dudes from back in the day when tubes were common. I'm thinking they probably had a clue or two


One thing I should mention, I actually have two of these amps that a buddy sent me. He wanted me to put one in a conventional amp chassis and my pay was that I get to keep the other one. So, I'm trying stuff on mine, that I want to incorporate into his during the changeover.

That's where this question came from. I thought as long as I'm rebuilding it basically as a fresh-wired guitar circuit, I could do away with the DC ref voltage for the hum balance in his chassis. I'm using a slightly modified Princeton Reverb chassis from Marsh for this.

Here's as far as I am with his...



And here's what mine looks like

[CitizenCain]

And the guts of mine

[Bruce / Mission Amps]

.... what the fuck?

Okay, you have straight AC going across the filaments of several tubes (recto tube gets the 5V so ignore that one).

That hum balance is your high-voltage tube driver secondary being plugged into your filament circuit. WTF? The only caps I see are the filter caps that should be in that DC circuit, other than that that pot should act like a straight resistor of its own value, and what the hell brain damage is this shit?

This may be a dumb suggestion, but have you tried disconnecting that pot entirely? See if the designers actually stumbled upon something or if this is as retarded as it looks.

Jeeze, calm down!!
Your absurdly borish language is not needed nor do I believe respected here... participate but don't look like a fool or you will end up being the one appearing to be retarded or to have brain damage.

The 6.3 volt AC filament string is just being biased through either a virtual or real center tap in the winding with a small DC source from the B+ rail. It still held up from ground be a very high DC resistance....
If you'd read up a little more about hum reduction methods before going off on this guy you'd learn that this is a VERY common and well respected way to help offset filament hum pickup in preamp tubes and has been in use since I can remember and it goes back 50-60 years ago.

[6267]

I find that when converting something which had been expensive (a commercial PA system, an organ, a hifi) to something cheap (a guitar amp that cost probably $25 at the time), I will leave some aspects of the circuit on the chassis alone. Heaters, some PS components, etc. may be better designed than the equivalents on the guitar amp, because higher performance was expected. That being said, sometimes the sound of old guitar amps comes from terrible design in the power supply etc., but I never heard of a guitar amp famous for the contribution of its heater hum to tone (maybe some punk music... ). I would carefully examine the schematics and see if the heathkit kids did something cool worth keeping before gutting. I have often ended up just needing to change a few cap values and install a jack to get an amp that sounds better than whatever "reissue"s are floating around right now.

[Steve Conner]

I've heard of that hum reduction circuit too, even though that Heathkit is old enough to be my dad.

The designers of these things were neither "brain damaged" nor "retarded". They were actually pretty smart, and as a professional engineer myself, I have a good deal of respect for them. This was back in the day when you actually had to understand what every single component was doing, and the interactions between them, rather than just sticking integrated circuit building blocks together, like I mostly do in my day job. You had to do a lot with a little.

So there are plenty of subtle, clever things going on and I suggest you keep studying the circuits till you get them. Think of them like the kata in martial arts. You may never use that exact combination of moves, but you'll feel the benefits in other ways.

You probably want to get a copy of The Art Of Electronics, too. It's not cheap, but it's awesome. And maybe RDH4 too, if you like the online version:

http://geek.scorpiorising.ca/RDH4.html