Voltage divider resistor choice for DC elevated heaters

[abbey road d enfer]

Afaik, it is the parasitic resistance, not capacitance, between heater and cathode that can be varied by dc elevation - the capacitance remains constant.

Although the conductance variation vs. voltage is the dominant factor, the dielectric permittivity also varies, as is the case with most ceramics.
It also varies with frequency, which is well documented.
Since capacitance effects at 60 (or 50) Hz are generally negligible, the voltage coefficient is not well documented, just mentioned in passing.

 

[abbey road d enfer]

How do you make a high-quality anode choke? What do you need to consider and what are the pitfalls?

IMO a choke is nearly as difficult to DIY as a good transformer.
Of course, you don't have the same constraints regarding DCR, coupling and insulation, but the damning factor is parasitic capacitance, which requires chambering and specific winding methods (honeycomb, pilgrim step).

 

[rock soderstrom]

IMO a choke is nearly as difficult to DIY as a good transformer.
Of course, you don't have the same constraints regarding DCR, coupling and insulation, but the damning factor is parasitic capacitance, which requires chambering and specific winding methods (honeycomb, pilgrim step).

I feared it; there is definitely a reason why high-quality chokes are so expensive. I also looked at a thread by CJ about the anode choke of the V72, and the courage to do it myself as a first project diminishes immediately.

 

[trobbins]

I haven't come across any anode chokes that identify their impedance response with frequency, such that resonance behaviour is identifiable. That may be a way of distinguishing how well a choke has been made when compared to others (similar to output transformer construction that aims to push resonances out as high in frequency as possible, and to dampen resonant behaviour).

Of the various chokes I've measured for impedance, they have all had their first self resonance at no more than a few kHz, except for flouro light chokes that have their SRF extending out to about 10kHz.

 

[ruffrecords]

Afaik, it is the parasitic resistance, not capacitance, between heater and cathode that can be varied by dc elevation - the capacitance remains constant.

I always ASSumed that with heaters grounded, the cathode was usually positive with respect to the heaters so it could act as an anode and heater noise could enter the signal chain that way. Elevating the heaters makes the cathode negative with respect to the heaters so this can no longer happen

Cheers

Ian

 

[abbey road d enfer]

I always ASSumed that with heaters grounded, the cathode was usually positive with respect to the heaters so it could act as an anode and heater noise could enter the signal chain that way. Elevating the heaters makes the cathode negative with respect to the heaters so this can no longer happen

Cheers

Ian

The dominant effect at play is the non-linearity of the heater insulation, typically an alumina based ceramic. Emission from the filament is much smaller.
All litterature agrees to say that the polarity of the elevating voltage is almost irrelevant, but it is also clear that making it positive ticks all the boxes.

 

[trobbins]

For many preamp circuits the cathode is just a few volts positive, and the heater would swing to 4.5V positive, but yes would on average be more negative. Although for indirectly heated filaments, the filament is typically folded back on itself within the cathode, so a bit more complex.

However for the common filament encased in alumina, and loose fitted inside a cathode cylinder, leakage was initially related to impurities within the alumina, which they then refined away over time. But it is more complicated than that, as measurements typically show a lower resistance when heater and cathode are about the same voltage - which has been interpreted as a diode characteristic, but again measurements show both diode polarities, as well as saturation style responses, including metal whisker/diffusion growth over time.

Whatever the mechanisms, the outcome appears to be vary between valve samples, and leakage is reduced when the heater and cathode have a significant voltage differential (either positive or negative).

 

[abbey road d enfer]

For many preamp circuits the cathode is just a few volts positive, and the heater would swing to 4.5V positive, but yes would on average be more negative.

In most guitar amps, it is much less, close to 1V, sometimes less. That may be part of the "mojo".
I don't recall seeing a guitar amp with elevated heaters in the preamp section.
Maybe would sound "sterile"?

 

[trobbins]

The effect/hum is commonly imperceptible, and even more so with a bypassed cathode stage, which is what most guitar amps use.

Modern guitar amps that incorporate lots and lots of preamp gain are more likely to use it.

 

[Gus]

trobbins

"including metal whisker/diffusion growth over time"
I did not know about metal whiskers part this do you have a good book(s) or link to read about this?

 

[rock soderstrom]

This thread has an overall very high information density! It's really great to learn the physical foundations that correlate with my practical experiences.

I have been able to achieve the best SNR results with AC heater using the Humdinger variant + elevated heater voltage.



almost done!
Humdinger is available from the outside

Now I need a frontplate for the gain pot and NFB "EQ"

 

[abbey road d enfer]

The effect/hum is commonly imperceptible, and even more so with a bypassed cathode stage, which is what most guitar amps use.

Modern guitar amps that incorporate lots and lots of preamp gain are more likely to use it.

My comment was a little OT, since it pertained to distortion induced by the non-linearity of the heater-to-cath impedance. It's bound to be more measurable (probably not so much audible) when the cathode is bypassed. When it is not bypassed, the c-to-h impedance is significantly bootstrapped.

 

[Rusan]

his thread has an overall very high information density! It's really great to learn the physical foundations that correlate with my practical experiences.

I have been able to achieve the best SNR results with AC heater using the Humdinger variant + elevated heater voltage.

That's my favorite recipe as well, that I used a lot back in the days of my single ended hi-fi and guitar amp hobby; more than quiet enough for high gain preamps. I used a 50 ohm pot, with a 10 ohm resistor from each end to the horseshoe to the wiper, and nominally about 25V of elevation. The pot's ends were then connected to the heater winding through the requisite pair of 100 ohm resistors.

My reasoning was that with only 14 ohms of total adjustment range, you have much finer, easier control to dial in the precise null point. As a side benefit, in the highly unlikely event the pot failed there's still a hardwired 110 ohms from each end of the 6.3V winding.

On the topic of anode chokes, here's a handy Excel calculator that does the math for you.

 

[soapfoot]

In most guitar amps, it is much less, close to 1V, sometimes less. That may be part of the "mojo".
I don't recall seeing a guitar amp with elevated heaters in the preamp section.
Maybe would sound "sterile"?

I think I’ve seen this in some Magnatone and Ampeg

 

[soapfoot]

There are two motivations for elevating heater voltage.
One, as Ian mentioned is making sure the cath-to-heater voltage doesn't exceed a limit. For example, a cathodyne PI (aka concertina) has its cathode standing at about 50-70V, so elevating the heater voltage to 30-50V makes sense.
The other reason is reducing hum and distortion. Due to the non-linear characteristics of the heater insulating material, the capacitance and conductance between cath and heater is variable, with a maximum when the voltage is low, so in order to reduce capacitive coupling between the heater AC and the cathode, a bias is beneficial.
I have never seen that mentioned in tube manufacturer's datasheets, but it is mentioned briefly in RDH and a few disseminate articles (e.g. heater-Cathode Insulation performannce by Klemperer)

Interesting! Would you please help me understand the mechanism by which elevated heaters could change the distortion profile/performance? Does the heater elevation impact the behavior of the cathode in some way?

 

[rock soderstrom]

That's my favorite recipe as well, that I used a lot back in the days of my single ended hi-fi and guitar amp hobby; more than quiet enough for high gain preamps. I used a 50 ohm pot, with a 10 ohm resistor from each end to the horseshoe to the wiper, and nominally about 25V of elevation. The pot's ends were then connected to the heater winding through the requisite pair of 100 ohm resistors.

My reasoning was that with only 14 ohms of total adjustment range, you have much finer, easier control to dial in the precise null point. As a side benefit, in the highly unlikely event the pot failed there's still a hardwired 110 ohms from each end of the 6.3V winding.

Sounds clever, I'll take a closer look at that.

On the topic of anode chokes, here's a handy Excel calculator that does the math for you.

Wow, thank you!

 

[abbey road d enfer]

I think I’ve seen this in some Magnatone and Ampeg

Correct! Interestingly Magnatone did it on the 180 but not on the 480. Ampeg do it on many models, sometimes positive, sometimes negative. Not on SVT, though, which would make sense, since the input stages have unbypassed cathodes...

 

[Matador]

The effect/hum is commonly imperceptible, and even more so with a bypassed cathode stage, which is what most guitar amps use.

Elevating heaters past 20V in one of my (mid-gain) guitar amps reduced 60Hz hum signature by over 6dB compared to 0V: even with the first stage bypassed. So it was definitely perceptible. I did not recall hearing any tonal shift as the elevated voltage was changed, but maybe I need to listen closer.

I've quipped about this before, but elevating to 40V and tightly twisting the wires in a drill is nearly indistinguishable (background noise wise) from DC heaters in my testing.

 

[abbey road d enfer]

Interesting! Would you please help me understand the mechanism by which elevated heaters could change the distortion profile/performance? Does the heater elevation impact the behavior of the cathode in some way?

Forget about it! I deleted my post. The effects on the cathode's impedance to ground are utterly negligible.

 

[soapfoot]

Correct! Interestingly Magnatone did it on the 180 but not on the 480. Ampeg do it on many models, sometimes positive, sometimes negative. Not on SVT, though, which would make sense, since the input stages have unbypassed cathodes...

If I remember correctly, some revisions of B15N have elevated heaters and some don’t. All perform acceptably but the ones with heater elevation can be *dead* quiet

They also all(?) have hum balance pots (aka “humdinger”) on the filaments, so someone really cared about avoiding heater noise

I rebuilt a totally-wrecked B15 last year and was very careful about heater lead dress, with very tight twists and careful routing near sockets. I employed the elevation scheme from a later revision and of course the hum balance control

I should also concede that I did the grounds in a non-stock fashion… all jacks and pots were isolated from chassis and the entire amp grounds to chassis at only a single point (added .01uF capacitors shunt RF to ground at the input jacks)

In its quiescent state it is so impressively quiet that the first time I powered it up I didn’t think it was working—it sounded like it was in bypass.

There’s really so, so much performance you can get with just careful implementation