Drop DC heater voltage by 1 volt, what resistor

[daz]

I'm gonna try this again tonite. I found some small values1 R and under. Is 5 watt ok?

[R.G.]

AC heaters.

That's a toughie. My circuit-design mind went into design space on this, and came up with some things that would work, but are sure to not please the crowd that's for only resistors, caps, pots and tubes.

Heaters are a problem. They are an unfortunate necessity in tubes, and eliminating heaters is one of those grail quests that only ended with field-emission-array cathodes. We want the heat, not the side effects of getting it. DC heaters solve most of the problems, excepting the ones related to uneven heater filament wear. Heaters are wires carrying volts of AC mains power that are as close as we (well, we tube-designers, or humans in general) can get them to the cathodes which are supposed to react well to microvolts from the grid. Keeping that genie in the bottle is hard. One way is balance; if your heater for *each cathode* could be center tapped and fed with a balanced AC voltage, then induced hum would reduce down to the imbalance between half-sections and the cathode. I don't think any tube does that. 12A?7s use dual heaters, one 6V heater for each triode, and wound so that they're kinda hum balancing if you wire them up right and the two half sections are in the right places in the larger circuit.

Not ground referencing the heaters lets their voltages be impressed by capacitive coupling to the cathodes and grids, with bad results as we know.

Hum balancing pots are the tangible recognition that some hum is gonna get through, perhaps not even from the cathodes/heaters, and a mechanism to offset the cathodes above/below ground to introduce a cancelling hum. This works, but has to be tuned for each amp and each new tube. Wanting on top of that to vary the AC voltage to the heaters is getting tricky.

To convert a CT heater with hum balancer to variable voltage without going to DC, the thing to do is to leave the total heater circuit plus humdinger balanced to the CT, and add balanced resistance in series with the heaters on both ends at the same time.

The simple solution is a dual, low-ohms power pot which can insert a 0-N ohms in both sides of the actual heaters balanced at the same time. A good pot like this probably costs more than the amp.

A side-step is to go to a duty-cycled, balanced, variable resistance. The way I'd approach this is to put a fixed resistor in series with both ends of the heater string that gets the heaters below the voltage you want them, and then to use a MOSFET pair on both sides to "short out" the extra resistor in a symmetrical PWM way. The difference in voltage between MOSFETs on and MOSFETs off will be a volt or so, and can be done at a frequency which doesn't interfere with the audio.

Of course, it may be simpler to use a DC power supply. Certainly a simpler design, if more intrusive on the "no-modernness" and "retroness" of the amp.

[Chuck H]

I'm sure I'll just drop the voltage, no regulation. I need something that is easy to implement and inexpensive. There are six amps (other than my own) that I'll need to do this to and I'm not taking time to build a thirty dollar board for each and possibly rewire all the filaments.

Did I interpret correctly that it's best to use TWO resistors (or two diode pairs), one on each end of the winding to minimize the possibility of hum?

As to the affect on tone that lower emissions might have... Consider that Eddie Van Halen ran his Marshall from a Variac at 97VAC!!! Assuming the stock amp put 6.3V on the filaments with 115V mains, 97V mains would run the filaments at 5.3V!!! I think most rockers liked his tone!?!

[daz]

Did you try the 2 diodes like in the Valve Wizard Diagram?
It will drop your voltage by 0.7 Volts.

No, i don't have any of those.

[g1]

Sorry to go sideways a bit Chuck, but how about the other voltages? If everything is a bit high, including B+, how about a bucking transformer (R.G.'s vintage voltage adapter)?
I do agree with where this thread seems to be leaning, that heater voltage is VERY overlooked. I try to make a habit now of always checking it right away, give a better idea of what kind of incoming AC the amp may have been designed for.
I now often find high heater volts is accompanied by B+ voltages that are too close to the filter cap ratings.

[daz]

Tried a .22 or .47, forget which. But it was a 5w and it got very hot very quick. Thats the non tech way to determine minimum wattage.

[R.G.]

Did I interpret correctly that it's best to use TWO resistors (or two diode pairs), one on each end of the winding to minimize the possibility of hum?

Yes. If the CT voltage is grounded and (kind of) balanced for lowest hum, you want to preserve what you can of balance. Splitting the resistance/diodes into two at each end of the heaters avoids unbalancing by the amount of voltage dropped.

Remember that Schottky is about half the voltage of a silicon diode, and germanium about 1/3 if two silicon drops is too big.

As to the affect on tone that lower emissions might have... Consider that Eddie Van Halen ran his Marshall from a Variac at 97VAC!!! Assuming the stock amp put 6.3V on the filaments with 115V mains, 97V mains would run the filaments at 5.3V!!! I think most rockers liked his tone!?!

Not a good model to follow. He was fine if his tubes lasted one show.

Sorry to go sideways a bit Chuck, but how about the other voltages? If everything is a bit high, including B+, how about a bucking transformer (R.G.'s vintage voltage adapter)?
I do agree with where this thread seems to be leaning, that heater voltage is VERY overlooked. I try to make a habit now of always checking it right away, give a better idea of what kind of incoming AC the amp may have been designed for.
I now often find high heater volts is accompanied by B+ voltages that are too close to the filter cap ratings.

Good one, g-one, and very appropriate.

[Chuck H]

Sorry to go sideways a bit Chuck, but how about the other voltages? If everything is a bit high, including B+, how about a bucking transformer (R.G.'s vintage voltage adapter)?
I do agree with where this thread seems to be leaning, that heater voltage is VERY overlooked. I try to make a habit now of always checking it right away, give a better idea of what kind of incoming AC the amp may have been designed for.
I now often find high heater volts is accompanied by B+ voltages that are too close to the filter cap ratings.

That's an excellent observation. Using the Hammond iron I learned that one a long time ago. I do order the iron that will produce the voltages I want. Up to now I've neglected the filament stringencies though. Bias voltages are adjustable and my HV is always what I expect (within 10V +/- anyhow). So I really just need to stop burning up the cathodes!

[Chuck H]

Yes. If the CT voltage is grounded and (kind of) balanced for lowest hum, you want to preserve what you can of balance. Splitting the resistance/diodes into two at each end of the heaters avoids unbalancing by the amount of voltage dropped.

Makes sense. Got it.

Remember that Schottky is about half the voltage of a silicon diode, and germanium about 1/3 if two silicon drops is too big.

Yup. Figured that one out fast. I found some PIV 40V 5W Schottky's with .5Vf. An inverse pair on either end should change my 7.0V-7.2V to 6.0V-6.2V rather handily. The only issue is that I have two custom units out there with elevated heaters to allow for high cathode voltages in the design (and I can't remember how high the elevation is!?!) With only 40V PIV I may need to get more creative with those. Maybe use parallel 1Vf silicone with a higher PIV (four on each end). No problem.

Not a good model to follow. He was fine if his tubes lasted one show.

I did not know that! But was it due to low filament voltage?

[g1]

Seems a lot of transformer makers still call their iron "120V" or even 115. I haven't checked enough new built ones to see if the heater windings are according to that, but I would expect them to be in accordance with the primary rating.
So where I am we are fairly leading edge of hydro-electric power (those DC HV lines RG mentioned elsewhere, which they call "bi-pole").
Our AC is 125V. I've only seen one transformer (a webber) that had a 125V primary tap. As far as I'm concerned, all PT's should have that.
From what I understand, the entire North American grid will be 125V, which probably means as high as 127 or 128V.

[Chuck H]

I've only seen one transformer (a webber) that had a 125V primary tap. As far as I'm concerned, all PT's should have that.
From what I understand, the entire North American grid will be 125V, which probably means as high as 127 or 128V.

Where I lived in Ca. the mains typically ran at 127V off peak hours and drifted between 120V to 125V when the grid was at peak use. This was about eight years ago. My little Mesa amp at the time went through EL84's like a car uses oil!!! I guess the higher emissions combined with the high Vp (420V-444V) just chewed them up! They would go microphonic and weak within four months with only moderate use! Good thing I had scored a couple dozen for free through a paint job client!!! I just used the last of those tubes in 2010! They were old Sovtek's. Good sounding and durable as hell in everything but that Mesa!

[R.G.]

The only issue is that I have two custom units out there with elevated heaters to allow for high cathode voltages in the design (and I can't remember how high the elevation is!?!) With only 40V PIV I may need to get more creative with those. Maybe use parallel 1Vf silicone with a higher PIV (four on each end). No problem.

Actually, it's not a problem. With a back to back pair, the highest reverse voltage either one ever sees is the forward voltage of the other diode. You could use 10V diodes and not have a problem.

[Wombaticus]

Given the evidently high incidence of higher-than-nominal voltages in the US (though I reliably get 120-121V myself), perhaps this is a good opportunity to ask R.G. what he thinks of the practice of putting an otherwise unused 5V rectifier heater tap in series with the primary in order to effectively add a few extra turns to a PT primary winding and thereby effectively alter a 115V transformer to one optimized for 120VA

EDIT: changed typo 129V in last sentence to 120V.

Seems a lot of transformer makers still call their iron "120V" or even 115. I haven't checked enough new built ones to see if the heater windings are according to that, but I would expect them to be in accordance with the primary rating.
So where I am we are fairly leading edge of hydro-electric power (those DC HV lines RG mentioned elsewhere, which they call "bi-pole").
Our AC is 125V. I've only seen one transformer (a webber) that had a 125V primary tap. As far as I'm concerned, all PT's should have that.
From what I understand, the entire North American grid will be 125V, which probably means as high as 127 or 128V.

[Chuck H]

Of course! Not sure how I missed that. Beer? Nah, only two this evening. Must be stupidity! Sometimes I wish I were intoxicated if only for the excuse

[loudthud]

Since August 2009 Hammond has included a 125V primary tap on the 200 series power transformers. These are the most common types used in the US where a universal primary is not needed. Most are rated at 60Hz only but about 10% are rated at 50/60 Hz.