Any advantage using DC for filament?

[Alan0354]

So is the consensuses are DC filament is not necessary? I still interested in why raising the filament to 120V help.

Again, I did two high gain front end, I never notice the hum stood out at all. Hiss, yes, hum, no.

Again, could it be DC is more important for pcb where you can't twist the two filament wires.

[nashvillebill]

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.

Although it wasn't a high-gain amp, a while back I refurbished an old RCA power amp ( a console pull). All AC filaments; no hum balance pot; filaments weren't tied to cathode; filament wiring wasn't even twisted and just laid in the chassis willy-nilly; grounding was individually going to a big buss wire going the length of the chassis. Completely against what is preached. I completely recapped it and replaced all resistors, then fired 'er up to see how noisy it was. Except...it wasn't noisy. In fact I couldn't even tell it was on: no hum whatsoever, no hiss. Did I screw up? I connected an audio source and she sounded great!

Granted, it wasn't a high-gain amp, but I was impressed. Maybe RCA knew a thing or two about making a simple, perfectly functioning amp? (6BQ5's in PP, which incidentally were being run at plate voltages considerably higher than their very own RCA handbook ratings.)

[Chuck H]

There are some tricks that get used. One is to run a dummy lead from the AC filaments near the grid lead of a preamp stage that is out of phase with the most hummy stage OR the dummy lead can run near that stages grid as long as it's out of phase with it. The point is phase cancellation of course. I've seen it done in a couple of high gainers before DC filaments were all the rage. It could be that the RCA amp takes advantage of this principal with it's layout. It could even be that the people who built those amps actually knew to move leads around to quiet the amp.

[nashvillebill]

Didn't look like they paid any attention to lead layout, and no dummy leads.

Could be the tubes also: 6FQ7's which may be internally shielded (depending on which internet source you read)

[big_teee]

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.

So Mick, You do a filtered voltage divider to elevate the Heater AC, is this correct?
Any elaboration will be great!
T

[Tom Phillips]

...the Valve Wizard method of tight twist AC heaters, and improved grounding...

Merlin Blencowe explains those approaches very well in his books and articles. However, as I think you know, he didn’t invent the concepts. His publications are very good and he maintains high integrity. I just want future readers of this thread to realize that the basic concepts are ancient. Ancient in the electronics time line anyway. I can tell that Merlin Blencowe realizes this by the tone of his writing and things like the fact that he includes an extensive bibliography in his books. These items that are conspicuously lacking from the publications of some self proclaimed guitar amp gurus. The writings of those other guys imply they think they discovered the electron.

[big_teee]

Merlin Blencowe explains those approaches very well in his books and articles. However, as I think you know, he didn’t invent the concepts. His publications are very good and he maintains high integrity. I just want future readers of this thread to realize that the basic concepts are ancient. Ancient in the electronics time line anyway. I can tell that Merlin Blencowe realizes this by the tone of his writing and things like the fact that he includes an extensive bibliography in his books. These items that are conspicuously lacking from the publications of some self proclaimed guitar amp gurus. The writings of those other guys imply they think they discovered the electron.

I know that all of this is old and basic, I just like how the wizard shares so much.
Some of his web stuff are teasers to buy his books.
I have his preamp book and rumage through it pretty regular.
The Wiz does have some good info, and some good ideas for non Amp professional guys like me!
T

[R.G.]

There are some tricks that get used. One is to run a dummy lead from the AC filaments near the grid lead of a preamp stage that is out of phase with the most hummy stage OR the dummy lead can run near that stages grid as long as it's out of phase with it. The point is phase cancellation of course. I've seen it done in a couple of high gainers before DC filaments were all the rage. It could be that the RCA amp takes advantage of this principal with it's layout. It could even be that the people who built those amps actually knew to move leads around to quiet the amp.

I saw a more universal and more thoughtful application of this.

The idea was to take AC from the mains, distort it, filter the result to mains frequency again, also second, third, fourth, etc. harmonics with active filters, then run the resulting batch of several signals through phase shift stages, and sum them again in a variable-gain mixer, then feed this into a spare input on the amp.

With a little care, you can null out the mains frequency, second, third, etc harmonics to any degree you like.

The only big problem is that it's location specific, in that disturbances from outside the box will vary, as might the exact amount of each component. But it's one better than the old hum balancers and such. Neat idea. I built one once. Worked. Just not as general as you'd like, and needs re-touching to keep hum down.

[Chuck H]

Indeed!?! Neat! This is a much simpler circuit that just involves several inches of lead attached to one side of the filament circuit (usually at the preamp end) then you move it around until it quiets things down. Simple and effective IMHO. I haven't tried it yet. My amps haven't been so objectionably hummy as to try a new trick, but I logged it in my 'bag o tricks' when I saw it.

[Leo_Gnardo]

take AC from the mains, distort it, filter the result to mains frequency again, also second, third, fourth, etc. harmonics with active filters, then run the resulting batch of several signals through phase shift stages, and sum them again in a variable-gain mixer, then feed this into a spare input on the amp.

With a little care, you can null out the mains frequency, second, third, etc harmonics to any degree you like.

The only big problem is that it's location specific, in that disturbances from outside the box will vary, as might the exact amount of each component. But it's one better than the old hum balancers and such. Neat idea. I built one once. Worked. Just not as general as you'd like, and needs re-touching to keep hum down.

Popular Electronics @ 1969. I think I clipped the article & it's somewhere way back in my paper files . Seems a long way to go, to panel-beat a hum or buzz to death.

[Alan0354]

I saw a more universal and more thoughtful application of this.

The idea was to take AC from the mains, distort it, filter the result to mains frequency again, also second, third, fourth, etc. harmonics with active filters, then run the resulting batch of several signals through phase shift stages, and sum them again in a variable-gain mixer, then feed this into a spare input on the amp.

With a little care, you can null out the mains frequency, second, third, etc harmonics to any degree you like.

The only big problem is that it's location specific, in that disturbances from outside the box will vary, as might the exact amount of each component. But it's one better than the old hum balancers and such. Neat idea. I built one once. Worked. Just not as general as you'd like, and needs re-touching to keep hum down.

I used this technique to cancel the noise of an electrometer amplifier that read pA. Because the amp had to float on top of 100V adjustable supply, I use this technique to be able to put a switching supply in the same box as the amp using cancellation technique.

BUT the real question is whether it's necessary. As some people hear said they have no problem with AC filament, I never had issue with AC filament for cascade gain amp.

These cancellation technique is not simple, you need to compensate the amplitude and phase of all the frequency components. Sounds like people are making a mountain out of a mold hill.

[Mick Bailey]

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

It was part accident, part design. It has a stage on each channel where there's an elevated heater-cathode voltage and some newer tubes would eventually break down. I decided to split the heater supplies and keep the output stage as ground-referenced AC and wire in an additional heater transformer for the preamp. I then referenced this to an experimental voltage divider between B+ and ground. My intention wasn't to reduce hum (which wasn't noticeable at normal listening distance form the speakers), but to reduce the heater-cathode voltage to within specs.

I noticed two things with the amp; there was a spot around 20v where hum almost disappeared, but this didn't elevate the heaters enough for what I needed. Secondly, when the voltage was gradually increased the hum totally disappeared at 120v and gave me the safe operating voltage i needed.

The two are probably linked.

The main thing with voltage-referencing is to ensure the heater is more positive than the cathode. With a cathode-biased SE amp you'd tie the preamp to the cathode resistor of the output tube. For Fixed-bias or PP you'd use a divider.

It's also a trick to use in amps that eat CF tubes - it can open up a broader choice of tubes for that position.

[Alan0354]

It was part accident, part design. It has a stage on each channel where there's an elevated heater-cathode voltage and some newer tubes would eventually break down. I decided to split the heater supplies and keep the output stage as ground-referenced AC and wire in an additional heater transformer for the preamp. I then referenced this to an experimental voltage divider between B+ and ground. My intention wasn't to reduce hum (which wasn't noticeable at normal listening distance form the speakers), but to reduce the heater-cathode voltage to within specs.

I noticed two things with the amp; there was a spot around 20v where hum almost disappeared, but this didn't elevate the heaters enough for what I needed. Secondly, when the voltage was gradually increased the hum totally disappeared at 120v and gave me the safe operating voltage i needed.

The two are probably linked.

The main thing with voltage-referencing is to ensure the heater is more positive than the cathode. With a cathode-biased SE amp you'd tie the preamp to the cathode resistor of the output tube. For Fixed-bias or PP you'd use a divider.

It's also a trick to use in amps that eat CF tubes - it can open up a broader choice of tubes for that position.

1) You are talking about preamp tubes?
2) If you tie the filament to the cathode resistor of the output tube, then the filament will be bouncing up and down with the signal, how does that help?
3) What isCF tubes.

You think the hum is due to the emission of electrons from the filament to cathode. By biasing to higher voltage, the electrons are repelled by the more negative cathode?
Thanks

[Dave H]

What isCF tubes.

Cathode Follower, e.g the tone stack driver in a Marshall and 5F6A.

[Mick Bailey]

So Mick, You do a filtered voltage divider to elevate the Heater AC, is this correct?
Any elaboration will be great!
T

I use two resistors to get the correct voltage - just a simple voltage divider. I usually connect it to the 1st cap node. If you place it too far down the filtered B+ nodes it can introduce noise of its own into the preamp. A cap can be added to the lower resistor to stabilise the DC reference - 22uf is fine. When choosing resistor values, bear in mind the dissipation. I use the lowest values possible so they're not running hot but give the correct ratio.

So, you've then got the DC reference. Just connect it to a 100 Ohm resistor each side of the Heater AC supply the same as you would if you were using resistors to ground.

20v seems a sensible level to me for a regular amp. If you took the node up to (say) 80v than this would reduce the heater-cathode voltage difference on a CF by that amount.

EDIT: Aim for between 220k and 470k for the overall divider resistance. If you have a bleeder across the 1st cap this replaces it.