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Old 12th September 2012, 05:56 PM   #1
tomchr is offline tomchr  United States
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Default Heater-cathode voltage spec interpretation

Folks,

This has been in the back of my mind for a while:

Indirectly heated tubes have a heater-to-cathode voltage spec, Vhk. If this is violated, arc-over between the cathode sleeve and the heater is likely.

My question is: Is this spec a DC limit or a transient limit?
I.e. if the idle DC operating point of the circuit does not violate the Vhk spec, but the spec could be violated at the peaks of the signal swing when the circuit is driven hard, would this cause issues with breakdown of the heater insulation?

This is a bit of a concern of mine for cathode followers with a large drive signal.

Thanks,

~Tom
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Old 12th September 2012, 06:09 PM   #2
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As I can see in some tube manufacturer´s datasheets, it is a DC or AC RMS voltage. Sometimes, they add a resistance value that can't be violated. In example, EL34 specification says no more then 20K. I don't know why.
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Old 12th September 2012, 06:14 PM   #3
DF96 is offline DF96  England
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Insulation breakdown rather than arc-over is more likely. The insulation is usually a metal oxide, which can act as a semiconductor.

The resistance limit is usually to stop the heater circuit from floating. This spec is usually relaxed for cathode followers. I'm not sure why.
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Old 12th September 2012, 08:07 PM   #4
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I think a lot depends on the data sheet and manufacturer. Some specify just a voltage and others specify a dc component and a peak (dc + signal). To be safe I always assume the given figure is to be interpreted as dc + peak signal.

Cheers

Ian
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Old 12th September 2012, 09:16 PM   #5
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In 1972 Philips was very specific and confident with respect to their products:

<< The published limiting values of Vkf apply to the positive and negative DC component of the voltage between the cathode and any of the heater terminals.
The limiting peak value is TWICE the rated D.C. value with a maximum of 315 volts. >>

See paragraph 9 on page 23 of this pdf:
http://www.tubebbs.com/tubedata/shee...s1972Part4.pdf
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Old 12th September 2012, 10:04 PM   #6
tomchr is offline tomchr  United States
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Quote:
Originally Posted by payloadde View Post
See paragraph 9 on page 23 of this pdf:
http://www.tubebbs.com/tubedata/shee...s1972Part4.pdf
Nice find!! Thanks.

~Tom
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Old 12th September 2012, 10:46 PM   #7
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Quote:
Originally Posted by payloadde View Post
In 1972 Philips was very specific and confident with respect to their products:]
And then goes on to modify that statement in the case of the PCC88.

Cheers

Ian
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Old 12th September 2012, 11:21 PM   #8
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by ruffrecords View Post
To be safe I always assume the given figure is to be interpreted as dc + peak signal.
Me too.

In addition, I have seen that some valves (some rectifiers) specify significantly different limits for heater-cathode voltage depending on whether the heater is positive or negative with respect to the cathode. Always worth checking the datasheets.
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Old 13th September 2012, 03:51 PM   #9
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Hi Guys

Tube ratings in general are "average" either ACrms or DC.

RCA suggested that the noise suppression of a DC-standoff heater is better for positive than for negative standoff. Most tubes have about a 90V rating, so typially you see 40-70Vdc used. This provides about 30db of suppression of heater noise.

A faux-CT should be made of 1W metal-oxide Rs and tied to the DC standoff. The standoff should be derived from a quiet supply node but not near the input, more like a middle node. The faux-CT should not be near the input tubes either.

Overall this is simpler to implement and has the same noise reduction as DCing the heaters.

Have fun
Kevin O'Connor
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Old 13th September 2012, 04:26 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by payloadde View Post
In 1972 Philips was very specific and confident with respect to their products:
<snip>
And this should be taken as applying only to the Philips product line of the time, other manufacturers may have different limits, and the recommendations in the data sheet should be adhered to.

ruffrecords interpretation of the specification is conservative and I think very reasonable - I'd try to adhere to that recommendation where possible.

Modern production tubes are of course a crap-shoot because in most cases the manufacturers do not seem to publish very accurate or comprehensive data sheets for their current production versions, so I would err on the side of caution here. QC in the old days was generally better than is the case today IMO, I wouldn't assume that a vintage Mullard data sheet applies verbatim to a modern Expo Pul "Mullard" for example..

YMMV..
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