Playing with screen resistors

[rdf]

Last night I lashed up a 6c45-KT100 UL SE and started playing around with some demo measurement software. My mind drifted to the question of the screen resistor's effect on distortion spectrum. Attached is a zip file containing two images. The details of the circuit aren't important and sure to change before Monday, it's the first cut at a new design. Perhaps the one relevant factor is the use of cathode shunt feedback which amounted to barely 2-3 dB in total.

Both display the harmonic spectrum for the amp at 4 watts in to 8 ohms at 600 Hz. One uses a 100 ohm screen protection resistor, the other 12,000 ohms. The KT100's DC bias changed maybe 10%. The effect on upper harmonics was surprising. Examined closely the 12K condition has very slightly lower 2nd and which an unweighted THD meter would probably display lower distortion.

Got bored, thought the results might be of interest to others. For those wondering, the difference between 100 ohms and no resistor was miniscule with no clear winner. If I'm still bored later I'll try runs with the more popular 1000 ohm and 1500 ohm values.

[richwalters]

Johan# With the distortion arguement one gets the suspicion that the screen grid within the tube isn't physically optimised vs. tap to anode...... Looking at the Mullard 20W design, that configuration using a 1K mentions "improves the linearity". Yes it does. A 1960's NOS RCA circuit using 6550A's uses no screen grid resistor at all. Either those early tubes had properly optimised internals or the new tubes of today i.e SV6550C's aren't quite optimised. I guess it only takes a fraction of a mm to create such differences. By chance I plugged in a KT90 set and as expected the thd shot up more than double.

The screen current varies with frequency and power, but as only a few mA is used one senses that proper current sourcing would be better but I will settle for a 680 ohms but try on another o/p transformer with different characteristics and see what results I get. Typ on a 100W design the 1K thd is around 0.15% with 220R and with 680R drops to 0.05%. That is signifigant.

richj

[nafunga]

Hi Rich,
I recently tried lowering the screen volts of a SE KT66 amp which runs at around 70mA (300v HT. 2.5k primary)to see the effects . I tried various screen resistor values and even an OD3 at 150v. This effectively limited the Ia to 50mA but had a (subjective benificial effect) on sound quality. Have you been down this road? Any observations please?

Nafunga.