Testing direct heated tube with AC or DC on filaments gives different results

[kagliostro]

I was just talking to a friend right now and a question came out to which we have not been able to give a certain answer

He was testing some old (1920) direct heated tubes and noticed difference if the tube is tested using AC or DC on heaters

if the tube is tested with DC he shows lower performance

Which can be the reason for this difference ?

Many Thanks

Franco

[trobbins]

What is the test setup and 'performance' parameter?
Is the heater centre tapped to ground?

[kagliostro]

I'll ask to my friend the exact parameters and post It


Thanks


Franco

[thetragichero]

something like this came up on the marshall forum a few weeks ago, as a fella was adamant that dc heaters sound worse than ac heaters. in that case, it turned out to be the result of the dc heater supply being a bit lower than the ac heater supply. just throwing that out there

[PRR]

Direct heat from 1920 was almost sure to be designed for battery.

AC heat works better with an indirect cathode to smooth the temperature pulsations, although large tube can be tolerable on AC.

[kagliostro]

Thanks for replies friends

Here are the data

tube on test is an O1A



https://frank.pocnet.net/sheets/021/0/00A.pdf

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/01-A.PDF

filament is performed with toriated catode

Tested with

Plate @ 135 V

G1 @ -9 V

Filament 5 V AC

results: Ia 4.9mA  --  Transconductance 0.9mA/V

Tested with

Plate @ 135 V

G1 @ -9 V

Filament 5 V DC

results: Ia 3.8mA  --  Transconductance 0.7mA/V

Tubes in test are two buth both gives the same results on testing

--

My friend say that if he test indirected heated filament tubes using AC or DC there is no difference

Many Thanks for any intervention

Franco

[66Strat]

As this tube has a directly heated cathode, wouldn't the 5 VDC make the grid more negative with respect to cathode, thus increasing the negative bias?

[PRR]

...wouldn't the 5 VDC make the grid more negative with respect to cathode, thus increasing the negative bias?

You got it.

This is also why the filament pins have polarity marks "F+" "F-". (Not all sheets show this.)

-9V of applied bias may be -11.5V or -6.5V of effective bias.

You should also know that it was common to put a rheostat in the heater circuit to control volume (by change of gain at lower heat).

[kagliostro]

Many Thanks Friends

and this too, with your help, has been cleared up


Thanks Again


Franco