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mikemac 20th June 2011 04:05 AM

Floating Dc heaters, capacitor to ground
I have been looking for this answer for a far while. Please help me. Some stuff just makes immediate sense to me (no examples spring to mind!), but impedance and relative grounding dont.
I am building an akido pre amp on a glass ware board.
1. Can i just ground my DC heaters, coming from a different board, with a capacitor?
2. Will this negate the requirement for elevated heaters to solve the cathode to heater voltage rating?
3. It seems that the PCB is designed for the "end" of the heater chain to be grounded, both + and -, like a centre tap, but with capacitors not resistors. I have seen another commercial schematic like this....
If it was my car or my sons toy i would go with my instinct, but its not. Its big scary voltages. Your help greatly appreciated by me and those that follow

mikemac 20th June 2011 04:08 AM
sorry i am not sure how to post the actual image. Like the heaters in this

Rod Coleman 20th June 2011 07:45 AM

The reason for raising heaters above ground is to properly reverse-bias the thermionic diode formed between the heater and the cathode. 30 to 60V will do fine. If you don't do it, there will be some leakage, at dc, at mains frequency, and of noise. It varies greatly - with the quality of the insulation of the heater.

I many circumstances, you can get away with just connecting the heater to ground. The nearer you are to the output stage, and the higher the quality of the valve, the more likely it is that this will be OK.

But with line level and lower, the sound will probably suffer, and you may get hum.

A pair of resistors to divide B+ [HT] and a capacitor [I use 0,68uF 250V stacked MKT] to ground, is sufficient. With high B+ [300V+] make sure you don't violate the resistor spec for VOLTAGE. Use 2W film types to get 500V durability, or 2 series-connected 1W. Don't use old junk box resistors here, they should be FLAMEPROOF rated, if you don't like the risk of setting the house alight.

A capacitor on its own will give unpredictable results.

Rod Coleman 20th June 2011 07:52 AM

As for the vintage schematic, where 10n capacitors decouple the heaters to ground - I note that 7025 and 7189 tubes are specified. These are very high quality parts, and you may have been able to assume effectively zero leakage.

But to work with modern production, or even regular NOS, I would say that better care is needed.

trobbins 20th June 2011 08:40 AM

Imho the 10nF capacitor humdinger would not provide any (significant) hum current flow to ground. As such, the heaters would sit at some uncertain floating voltage wrt ground, or any particular cathode bias voltage. I suggest the design as shown was not the best option that could have been deployed.

An elevated DC humdinger works by pushing the emission conduction between heater and cathode in to a voltage saturation region, where the effective resistance to hum currents between heater and cathode is at its highest - which then minimises hum voltage developed across the cathode bias voltage (and hence amplified to the anode).

It seems you are worried about heater-cathode voltage rating - " to solve the cathode to heater voltage rating" - is that an issue in your setup? If you have a main amp ground extending to a separate preamp board then there is no issue wrt just using the humdinger on the main amp. Using a pot to 'tune' the humdinger may well give you a significant improvement.

Cia, Tim

mikemac 21st June 2011 04:20 AM

"It seems you are worried about heater-cathode voltage rating - " to solve the cathode to heater voltage rating" - is that an issue in your setup? "

It is an Akido amplifier and am led to understand that as the two cathodes are "stacked" atop one another, they have to be elevated so the difference in voltages are close enough not to exceed the ratings (i think?). I will use a voltage divider. Mostly i was trying to understand the idea of "floating" vs "Biasing". Also learning more about the idea of a capacitor to ground as an AC earth. Thank you all V much

trobbins 21st June 2011 09:32 AM

My apologies for akido naivity - yes if you set the heater DC elevation for midway between the cathode idle levels, then each heater-cathode will operate at voltage saturation, which should minimise whatever heater hum current flows.

Minimising peak heater cathode withstand voltage would probably indicate a higher heater DC voltage I guess, up to a level where the bottom tube is still fine, and the top tube heater-cathode is still in voltage saturation in idle (ie. circa 20-30V difference). Don't forget to ac bypass the divider tap.

DavesNotHere 21st June 2011 04:28 PM

about 25 years ago it was told to me there is always two types of grounds in a electronic circuit. AC ground and DC ground. the first one taught in school was the dc ground this is where the dc operation exists as setting up the required voltages to allow current to flow.

AC ground is the signal's ground as any source or anti-soure of electrons. DC ground and DC voltage supply source are the AC ground in the circuit.

ac null is the null potential of the ac signal this is different from the ac grounds as this is the midway point of the ac signal from negative peak to positive peak.

the caps on the ends on the vintage heater create a voltage divider (in AC) and they are grounding the ac null to dc ground. if you need to elevate heater voltage, you would use a simple resistor voltage divider and place this AC null at your voltage divider node. some people put a low value potentiometer there and attach the wiper to DC ground (hum balance) instead of using the caps.

the akido cap in series voltage divider is actually coupling any and all noise that is on the dc positive rail, the resistor network is really an attenuator that injects a percentage of that noise to counter react noise and cancel its presence. ( active noise reduction circuit )

velvetsunrise 21st June 2011 07:25 PM

Hi, The Curcio Daniel mk 1 preamp used just a capacitor to ground for the heater supply, i dont pretend to understand how it finds its own reference voltage via leakage currents, but i have used this method lots of times with no sparks or hum.

trobbins 23rd June 2011 12:06 PM

I was just looking at the McIntosh C-8 preamp circuit - it shows a humdinger pot with wiper taken to 0V within the main amplifier chassis, but with 10nF caps bypassing each heater arm to chassis within the remote preamp module. This would appear to maintain a low impedance to preamp chassis ground for audio frequencies (ie. not a hum issue per se) that leak through the cathode-heater resistance.

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