Any advantage using DC for filament?

[Alan0354]

Maybe you said already?
How many preamp tube stages?
How much gain?
I have 2 2204 Amps, with cascade preamps, and I have a good amount of gain.
No excessive hum with AC heaters.
T

That's information that is useful for me and others.

[Alan0354]

That's fine SGM. But that would require a transformer with an appropriate winding (that 99% of guitar amps or readily available transformers do not have) or an auxiliary transformer. Both of which get us even further into the "more trouble than it's worth" category. If you're a hobbyist and tweaking out that sort of thing for little or no advantage is what gets your nut, go ahead. If you're a designer for a guitar amp manufacturer that supplies you a budget and they want you to design and spec parts for their new high gain model, go ahead. Neither of these things apply to me.

That's the reason I started this thread, so people can tell us their experience. That's what I think so too. I won't think twice using a bridge rectifier and a cap as everything is there already. Doing a regulated DC, I need to listen to how people justifying it. I am not for or against anything, it's all about practicality.

[soundguruman]

Borrow a couple volts from somewhere else in the amp. It works out.
You are correct, highly impractical....unless you want the lowest noise possible.
Then becomes necessity.

Noise in high gain amp drives guitar player crazy.

[Leo_Gnardo]

As for mic amp, are they supposed to be much higher gain than guitar amp, like the cascade gain amp? I read the Fender PA100 schematic, the gain is in the ball park of a Twin which is low gain. So is there a compelling reason for a regulated DC filament?

Don't be fooled by the PA100 lo gain - similar to Twin - mic pre circuit. PA100 was intended to take high-impedance mics which have outputs similar to guitars, and spin 'em up to PA volume for use in small clubs. Low-impedance mic outputs are typically passed thru a transformer on the way into a professional console preamp, and that transformer multiplies the mic voltage by quite a bit say 20 to 30 dB before it gets to the first pre stage. In the era the PA100 was built common stage mics were Shure Unisphere PE585 and similar units, not SM58 or 57 which started to dominate all stages by the 80's. Early users of balanced lo-impedance mics frequently used outboard mic/line transformers to get sufficient gain when plugging them into hi-impedance unbalanced-input mixers.

[Alan0354]

Borrow a couple volts from somewhere else in the amp. It works out.
You are correct, highly impractical....unless you want the lowest noise possible.
Then becomes necessity.

Noise in high gain amp drives guitar player crazy.

I think the noise from the single coil guitar drives people crazy. Even humbuckers are not that quiet. I don't think any hum from the amp alone can compare to the hum even from a humbucker.

My experience of high gain amp is the loud hiss that can be bad, not the hum from the power line. Those has nothing to do with the filaments, those are thermal, 1/f and shot noise.

But of cause that does not stop people from sticking their ears into the speaker, turn up the amp and say, "I hear hum!!!" I so wish someone strum the guitar while they are complaining!!!!

[uneumann]

JM2C about this dead horse...

Alan wants to know if there is a compelling reason to use DC filaments in a guitar amp. I don't think we've poo poo'd DC so much as to mention that most long standing alternatives work fine without issues if implemented properly. I never have and may not ever make an amp with DC filaments. BUT, there are two scenarios "I" can think of where I wouldn't hesitate. And I think it's these same two reasons that DC filaments seem to be getting more popular with manufacturers and getting more press on forums like this one.

1) Uber gain guitar amps. Some of these amps multiply the sensitivity to such extremes that smart design means every single noise reduction method possible should be employed.

2) PCB construction. With modern PCB construction the filament circuit is usually on the board. It's much easier to layout the board if the filaments are DC because their actual trace path can almost be ignored in lieu of other, more critical considerations.

Since many modern manufactured amps are both very high gain AND of PCB construction with tube sockets on the board I think DC filaments make a lot of good sense. Even in these scenarios I don't think regulation would be necessary.

A typical vintage design Marshall or Fender type amp that is hand wired or uses a PCB without board mounted sockets probably can't be significantly improved by using DC filaments and I would never bother.

Two good points - especially #1. I'll add a third...

3) When you have a high impedance cathode circuit in an early preamp stage. Most early stages use bypassed cathodes so what little hum current (microamps) is induced onto the cathode by the AC heater is basically bypassed to ground anyway - next to no voltage is induced on the cathode so there is minimal impact on the tube output signal. However, if you don't fully bypass the cathode, those induced microamps turn into millivolts and cathode millivolts are no different than grid millivolts - something to be amplified by the tube. Here is a case (admittedly rare) where there was no choice but to use DC for the filaments. See: https://sites.google.com/site/string...attredirects=0This preamp uses a cascode circuit and the upper tube of the cascode pair sees a very high impedance on it's cathode. There is no way to around DC for filaments in this case.

Last point I'll make - there is no need for separate transformer windings. The usual 6.3v AC winding works fine with a bridge rectifier and a filter cap. If the filtered DC voltage is slightly too high (likely if you're only powering 1 filament) use a couple of 1 ohm dropping resistors to trim it to nail the 6.3v spec. The design referenced above used two DC supplies (why not - they're cheap). One runs off the standard transformer - and one uses a separate transformer to elevate just those tubes that needed elevated filament voltage references.

[soundguruman]

I think the noise from the single coil guitar drives people crazy. Even humbuckers are not that quiet. I don't think any hum from the amp alone can compare to the hum even from a humbucker.

My experience of high gain amp is the loud hiss that can be bad, not the hum from the power line. Those has nothing to do with the filaments, those are thermal, 1/f and shot noise.

But of cause that does not stop people from sticking their ears into the speaker, turn up the amp and say, "I hear hum!!!" I so wish someone strum the guitar while they are complaining!!!!

single coil noise drives player crazy -- you right!

A good example is Mesa Boogie--
the first ones had AC filaments, the hum was very loud, almost distracting. It was very bad.

Later: Mesa then used DC filaments, and continues to do so...
That's why. People complained about the noise a lot.

The reason you only hear hiss:
Manufacturers became serious about making low noise amps.
Before this, the noise was VERY bad.

The more we complain, the quieter the amps are made.
If we don't complain about noise, they will just keep making noisy amplifiers.

And Yes, I do hear HUM. I want it GONE.

AND: I will take credit for complaining THE MOST.
Because of my whining, amp manufacturers made quieter amps. It's about time.

SOMEDAY, YOU GUYS
guitar amps will have the same specs as hi fi equipment. There will be high gain, and NO noise.
That fine day is coming...

[Alan0354]

I assume all the interference is gone, only on the hiss( white noise).

When comes to hiss, there's only so much you can do. You use metal film at the input stage to get rid of the 1/f noise. You lower the current to lower the shot noise. You try to lower the resistance to lower the thermal noise. You need to do a lot of calculation on the noise. The 68K in series with the input like in the Fender need to be look at.

then you have to find a low noise tube for the first tube. But after that, you reach the end of the road.

Then the next step will be lower the band width as noise is proportion to square root of BW. But that starts to impede on the sound quality. That's the reason you see all the low pass filters along the signal chain. Mesa put the pole so low to prevent oscillation and lower the noise that if you crank the gain channel high and back off the volume of the guitar, you hear all muffled.

You want anything lower, you need to use liquid nitrogen, that lower the thermal noise a lot!!!. I did a lot of ultra low noise designs that measure nV and pA. There only so much you can do. And yes, we use nitrogen.

[soundmasterg]

The amp that I used a regulated DC filament in started life as a Sovtek MIG 100U. I modified it into a two channel amp, the first channel being a stock Vox AC100 setup, and the second channel being my own higher gain channel using a pentode among other things. The Sovtek used DC filaments on the first couple preamp tubes, so I decided to keep that in my build. I kept the Sovtek power transformer, which happened to have about a 13V rail that they used for this purpose. I chose to use it on the input tube for each channel, and along with my layout being good and grounding being good, that amp has a very quiet noise floor. Would I use regulated DC filaments in every amp now? No, not at all...but it can be a benefit in some designs. It like everything else depends on everything else you are doing with your design and how you build it and lay it out.

Greg

[soundguruman]

"then you have to find a low noise tube for the first tube. But after that, you reach the end of the road."

Not exactly.
You can install negative feedback to limit the high frequencies. This kills the HISS.
(the negative feedback in a Mesa Boogie is not shown on the schematic)

A low noise tube is also a low gain tube.
Not exactly what a guitar player is looking for.

So there is a compromise between too much noise, and too little gain.

[Chuck H]

The usual 6.3v AC winding works fine with a bridge rectifier and a filter cap. If the filtered DC voltage is slightly too high (likely if you're only powering 1 filament) use a couple of 1 ohm dropping resistors to trim it to nail the 6.3v spec.

Or you could use a zener circuit to pseudo regulate and satisfy some other viewpoints here.

Excellent point regarding an un-bypassed cathode circuit on the first gain stage. This rarely comes up since most designers are trying to get as much amplification as possible right there. But it does happen. In fact I'm working on a design right now that uses the first stage cathode for tone shaping and leaves the LF largely un-bypassed. So this may be just the thing! Bitchin'.

[Chuck H]

I just want to note that Mesa, for a long time, was at the forefront of increased gain in guitar amps AND that they also put their filament traces on the board... Hmmmm!?! That would certainly explain some of the objectionable hum in some of their designs. It's frightfully hard to twist board mounted traces

[soundguruman]

I just want to note that Mesa, for a long time, was at the forefront of increased gain in guitar amps AND that they also put their filament traces on the board... Hmmmm!?! That would certainly explain some of the objectionable hum in some of their designs. It's frightfully hard to twist board mounted traces

SOME of their designs?

[soundguruman]

"The usual 6.3v AC winding works fine with a bridge rectifier and a filter cap."

If you don't mind all the ripple that remains.........AND you don't mind using 20,000 uF capacitor to kill the ripple.

[Alan0354]

"then you have to find a low noise tube for the first tube. But after that, you reach the end of the road."

Not exactly.
You can install negative feedback to limit the high frequencies. This kills the HISS.
(the negative feedback in a Mesa Boogie is not shown on the schematic)

A low noise tube is also a low gain tube.
Not exactly what a guitar player is looking for.

So there is a compromise between too much noise, and too little gain.

Negative feedback absolutely DO NOT reduce noise. There is detail study and even in text books proving in calculations already. Putting low pass in negative feedback resistor is no different from just a simple cap to ground like Mesa or a lot of other amps do.

I did not calculate the thermal noise contribution of the series resistor at the input that people like to use to limit the frequency using the miller cap of the tube. That resistor is suspicious.

There are really not too many ways to lower noise other than what I said in the last post. Nitrogen is the way to lower thermal noise. It is common to use nitrogen in scientific instruments. Can you imagine you pour nitrogen before a performance and it'll give you some smoke effect??!!