Any advantage using DC for filament?

[Alan0354]

Is there advantages using DC voltage for tube heater filament? Any noise advantage?

[soundmasterg]

Is there advantages using DC voltage for tube heater filament? Any noise advantage?

The quietest amp I've ever made used DC filaments for the first preamp tube for each channel. The DC is regulated and out of the transformer it is around 13V and regulates to 6.3V. If you do DC filaments wrong, it can be even noisier than AC. There is also no need to do it for the whole amp....usually the input tube for each channel or the first couple tubes in the preamp is fine.

Greg

[soundguruman]

Quite often AC used for filaments can enter the audio path, and is amplified. You hear buzz or hum.

If the amplifier is lower gain, like a twin reverb, the problem is small...
If the amp is high gain, example mesa boogie, then it really becomes annoying. It's really loud and irritating.

Also for high gain stereo hi fi, like a turntable phono preamp, it's really critical...
anytime you are amplifying a very small signal.

So, YES - it's a HUGE advantage. The difference is like night and day.

Layout is critical, to avoid the ingress of hum entering your audio path.
An amp design which is properly laid out and positioned will greatly decrease the need for DC filaments...but in some designs it's unavoidable.
High quality parts, used in the circuit, will also decrease the ingress of noise, which is then amplified.

It was the HI FI guys, like Jim McIntosh, who paved the way for hi gain low noise tube amplification.
His tube hi fi preamps used DC filaments.

The best example I can think of, is Vox AC 30 /6 TB.

Because of the layout, this amp has the worst heater hum of just about any amp I have ever seen.
The AC filament voltage runs right down the middle of the preamp, parallel to the audio path.
It is the prime example of the noisiest layout possible. It probably could not be worse.
It also has the worst grounding plan imaginable...

However, hook the first 3 preamp tubes to DC filament voltage, separate the preamp ground from power supply ground,
and it's dead quiet. The hum is completely gone.
DC filament changes it from "UN-usable," to functional.

Who is using DC filaments?
Bogner, Peavey, Mesa Boogie, Carvin, Bugera, and a bunch of others...

[silverfox]

Hum solutions

One challenge is curing an existing hum problem or working with a transformer that doesn't have the higher voltage required to provide rectified 6.3 V. Question, do you need 6.3 volts DC or can it be 5 volts. The Carvin MTS 3200 employed 5 volts DC. But then I didn't really like that amp. Sounded weak to me.

One cure to hum problems is to use ground lift and a hum balance pot. That seems to reduce the noise factor to a very acceptable level. I've seen AC heater feeds placed on a PCB and the previously mentioned solutions use with the result being a very quiet high gain amp.

If someone could explain the hum balance pot as well as how lifting the ground reduces AC hum that would be great. Is it like putting two loud talkers above floor level in a room so there noise is over head?

Silverfox.

[Alan0354]

Thanks for all the useful info.

I tested a Morgan amp. It is amazing the amp is so much quieter with strat single coil pup. I had two amp side by side and one hum like anything and the Morgan is much quieter. Of cause it's not humbucker quiet, but close to Lace sensor. What did they do as I cannot get their schematic?

[tedmich]

with really quiet designs its NEVER any one thing that's responsible, there are MANY issues such as lead dress/routing, using shielded cables, ground plane organization, elevated voltage on the heaters etc etc.

People who think DC heaters or regulated DC heaters or anything else is the total solution for a "noisy amp" often don't even differentiate between high end hiss in the signal and more typical 60-120Hz hum from the AC supply. Its everything together and one of the most painstaking parts of designing a high gain tube amp. And the pickups, guitar shielding and cable, and source electricity can have the biggest impact IME.

Most if not all high end amp manufacturers do not provide ready access to their schematics, often even to their official service reps! Or provide schematics with purposeful errors to negate "cloning" (Mesa-cough!). Typically you want to pay attention to the input signal and the 1st gain stage, but AC heaters, well done and at specific stages can be the most quiet in some instances.

I've seen people bend over backwards putting in voltage doublers to feed LDO regulators or entirely separate power supplies/ PTs to give DC heaters to preamp or even PA tubes and they are usually underwhelmed with the results, caveat emptor!

[big_teee]

Since I have been doing the Valve Wizard method of tight twist AC heaters, and improved grounding?
I've not had any more problems with hum or excessive noise.
T

[Alan0354]

I have worked on a few Fenders and Marshalls. I never have hum problem. Yes, there is hissing problem ( I don't know whether it's a problem, it's there), but that is a TOTALLY different topic involving thermal, shot and 1/f noise. No shielding and layout can help that. If you guys refer to the hum of the amp, not hum due to single coil pup, I see no advantage of having DC filament as I have not seen one amp that sit there and hum when cranking up. Of cause if you drip the untwisted filament wire over the preamp, you are asking for it!!!!

Could it be you need DC filament for pcb design as you cannot tightly twisted the trace. Only way to do is using multi layers board and design the trace like coupled lines one on top of each other with say 5 mil dielectric in between to get very tight coupling and sandwich between two ground plane like strip line.

If you are talking of lowering the hum if a single coil guitar plugged in, then we can talk more. I started a thread about Morgan amp has lower hum with the single coil.

To be honest, the hum of the amp is the least of all the problems. You have a single coil guitar, it dominates everything. Even a humbucker hums.

Even the hissing is usually not a problem compare to the volume of the guitar sound. Use MF resistor on the first stage is about all you can do. Lowering the circuit impedance to lower the thermal noise does not work very well as a lot of circuit need to be high impedance. Other thing is lower the current to lower the shot noise.

[Mick Bailey]

I build plenty of Hi-Fi amps and my most recent is back to AC to the heaters. Even with my Mordaunt Short 103 db speakers there is absolutely no hum even with my ear pressed against the driver. The only hum is mechanical hum from the PT, which I need to work on. The breakthrough for me was to elevate the preamp heaters to a high enough voltage. My recent design uses a voltage divider off the b+ to reference the heaters to 120v.

So, the design is an important consideration. I fully understand the differences between hi-fi and guitar amps, but it's possible to build even high-gain amps without hum so long as the design is well-considered and a lot of attention given to layout.

I suspect that DC heaters have become more prevalent because a) many modern preamp tubes are inherently noisy, and b) it's an easy way to apply a cheap fix to mass-produced amps that otherwise have a less-than-ideal layout.

A Soldano SLO 100 'clone' I worked on a while back had no hum whatsoever and AC on the heaters - it was a pretty high gain amp, so it can be done.

[big_teee]

I have worked on a few Fenders and Marshalls. I never have hum problem. Yes, there is hissing problem ( I don't know whether it's a problem, it's there), but that is a TOTALLY different topic involving thermal, shot and 1/f noise. No shielding and layout can help that. If you guys refer to the hum of the amp, not hum due to single coil pup, I see no advantage of having DC filament as I have not seen one amp that sit there and hum when cranking up. Of cause if you drip the untwisted filament wire over the preamp, you are asking for it!!!!

Could it be you need DC filament for pcb design as you cannot tightly twisted the trace. Only way to do is using multi layers board and design the trace like coupled lines one on top of each other with say 5 mil dielectric in between to get very tight coupling and sandwich between two ground plane like strip line.

If you are talking of lowering the hum if a single coil guitar plugged in, then we can talk more. I started a thread about Morgan amp has lower hum with the single coil.

To be honest, the hum of the amp is the least of all the problems. You have a single coil guitar, it dominates everything. Even a humbucker hums.

Even the hissing is usually not a problem compare to the volume of the guitar sound. Use MF resistor on the first stage is about all you can do. Lowering the circuit impedance to lower the thermal noise does not work very well as a lot of circuit need to be high impedance. Other thing is lower the current to lower the shot noise.

Incorrect, pickups don't hum!
They only pickup and recieve the hum that is present.
Pickups are like Antennas.
T

[R.G.]

As an observation on the general state of things, humans have always been willing to believe in magic, and to believe in packaging magic solutions up into a nice, tidy, simplistic package. Modern humans still believe in magic, but they frequently conceive vaguely of this as "science" or "computers" or some such.

Sadly, the real world does not work that way.

Anyone with a desire to really understand what's going on and not just instant gratification would do well to take note of that post up there from TedMich. It's not one thing. There is no magic bullet, only many, many details and they all interact to matter. To get really quiet amps, you have to do everything right. And the onion effect is alive and well. If you fix something that's a minor contributor to hum/noise, you may well think that the fix was a failure, as it's being covered up by other, bigger contributors to the problem.

It is true that magic exists - but it exists only inside the human brain. Sorcerers achieve their effects by manipulating what other people see and hear to create the illusion of magic inside the observers' heads. Sorcerers are by and large ordinary humans who are willing to do the long, exhausting research, learning, and practice to do these manipulations successfully.

There is no "THE SOLUTION" to hum. There is only careful, exhaustive work to remove the sources as much as possible from the amp, shield and reject the general hum-miasma that modern AC power gives us. You remove one layer from the onion, see the next layer in front of you, and generally shed tears.

[Chuck H]

You remove one layer from the onion, see the next layer in front of you, and generally shed tears.

That's a little fluffy for you! Did you have a good day or something?

[R.G.]

It's the cold weather and the Scotch.

What a piece of work is man, how infinite in reason...

[Alan0354]

I build plenty of Hi-Fi amps and my most recent is back to AC to the heaters. Even with my Mordaunt Short 103 db speakers there is absolutely no hum even with my ear pressed against the driver. The only hum is mechanical hum from the PT, which I need to work on. The breakthrough for me was to elevate the preamp heaters to a high enough voltage. My recent design uses a voltage divider off the b+ to reference the heaters to 120v.

What is the theory behind raising the bias of the filament to 120V DC?

[Alan0354]

Incorrect, pickups don't hum!
They only pickup and recieve the hum that is present.
Pickups are like Antennas.
T

Of cause I assume we all know that!!! It pick up the EM from the surrounding.

But I can tell you, in my room, I know the hum is from the power lines. The contractor made a mistake when I renovate the house and put in new power lines. Long story short, the hot and return wires are not closely coupled together and form a magnetic dipole loop. I can control the amount of noise by switching lights of the upstairs bath room light and fan!!! I can assure you the Morgan amp is quieter compare to the Fender, a lot quieter. I don't believe it's the shielding inside the Morgan amp that help because the noise source is from my house wire for sure, not from the power transformer of the Morgan.