Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Playing with screen resistors
Playing with screen resistors
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 8th April 2006, 08:28 PM   #1
rdf is offline rdf  Canada
diyAudio Member
rdf's Avatar
Join Date: Jun 2004
Location: big smoke
Default Playing with screen resistors

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.
Attached Files
File Type: zip screen_resistors.zip (15.3 KB, 138 views)
Ears aren't microphones.
  Reply With Quote
Old 2nd May 2007, 02:56 PM   #2
richwalters is offline richwalters
diyAudio Member
richwalters's Avatar
Join Date: Sep 2003
Location: Schwartzwald
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.

  Reply With Quote
Old 6th May 2007, 07:48 AM   #3
nafunga is offline nafunga  United Kingdom
diyAudio Member
Join Date: Jul 2005
Location: Whitstable , Kent. UK
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?

  Reply With Quote


Playing with screen resistorsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Screen grid resistors stoo Tubes / Valves 66 20th October 2015 07:35 PM
FS: Resistors, LM3886, speaker terminal, variable resistors, switches and rectifiers chris-man Swap Meet 1 28th May 2009 11:47 AM
what's better, a mid playing high, or a tweet playing low??? cody6766 Multi-Way 10 20th February 2005 09:40 PM

New To Site? Need Help?

All times are GMT. The time now is 01:04 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio