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

SE Pentode, regulated screen voltage?
SE Pentode, regulated screen voltage?
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
Reply
 
Thread Tools Search this Thread
Old 7th November 2017, 06:50 AM   #1
glina is offline glina  Poland
diyAudio Member
 
Join Date: Oct 2003
Location: Warsaw
Default SE Pentode, regulated screen voltage?

My amp is a parallel single ended class A pentode based o 2x 6L6GC per channel. Cathode bias based on resistor plus bypass capacitor for 60mA bias current per tube. B+ is 380V, screen 280V, cathode 18V. This is the typical point of operation as outlined in the 6L6GC datasheet.

I've read numerous threads on importance of screen voltage in P-P pentodes. My screen supply is a separate transformer tap with a CRC filter (100uf-1k-100uf).

According to the 6L6GC datasheet for my point of operation, the screen currents are:

Zero signal screen current 7mA
Maximum signal screen current 2.5mA

According to PSUD simulation of my screen supply this results in a voltage swing of up to +20V on the screen.

It is my understanding that Class A draws 100% current with no signal, and minimum of 75% with peak signal (is that correct?). This peak signal condition, according to PSUD, would generate up to +10V on the plate voltage.

My questions are
1. How important is the screen voltage in SE operation?

2. Which is the condition for lowest distortion, best linearity:
a) (Vplate-Vcathode)-(Vscreen-Vcathode)=constant
b) (Vplate-Vcathode)/(Vscreen-Vcathode)=constant
c) other?

3. I'm considering building a Mosfet regulated supply for the screens. Would a regulated screen supply be beneficial?

4. Is there a way to correlate the sag of relevant voltages for minimum distortion? Perhaps a regulated screen supply + additional resistor for sag? How to calculate?

Thanks
  Reply With Quote
Old 7th November 2017, 08:25 AM   #2
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
The maximum signal screen current is more likely to be 25mA,
not the 2.5mA you wrote.
  Reply With Quote
Old 7th November 2017, 08:34 AM   #3
glina is offline glina  Poland
diyAudio Member
 
Join Date: Oct 2003
Location: Warsaw
Thanks. Seems I mixed up.
Zero signal current is 2.5mA, maximum signal current is 7mA.
http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf

This would imply that plate and screen voltages swing in opposite directions as signal increases.
  Reply With Quote
Old 7th November 2017, 10:12 AM   #4
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by glina
It is my understanding that Class A draws 100% current with no signal, and minimum of 75% with peak signal (is that correct?).
No. Class A draws the same average current whatever the signal level.

Quote:
1. How important is the screen voltage in SE operation?
The issue is not SE operation, but Class A pentode operation. For minimum distortion the screen voltage should not vary with signal, which means a low impedance screen supply. However, this primarily means low impedance at audio frequencies. It is less important that the screen supply has low impedance at DC, although this still matters.

Quote:
2. Which is the condition for lowest distortion, best linearity:
a) (Vplate-Vcathode)-(Vscreen-Vcathode)=constant
b) (Vplate-Vcathode)/(Vscreen-Vcathode)=constant
c) other?
You need (Vscreen-Vcathode)=constant, and Vcathode=constant. If you cannot achieve that, then try for (Vscreen-Vcathode)/(Vcathode)=constant.
  Reply With Quote
Old 7th November 2017, 11:31 AM   #5
glina is offline glina  Poland
diyAudio Member
 
Join Date: Oct 2003
Location: Warsaw
Quote:
Originally Posted by DF96 View Post
No. Class A draws the same average current whatever the signal level.
The datasheet linked above quotes
Zero signal plate current 54mA
Maximum signal plate current 66mA.
For Class A operation.

How should I interpret that for power supply load?

Quote:
Originally Posted by DF96 View Post
The issue is not SE operation, but Class A pentode operation. For minimum distortion the screen voltage should not vary with signal, which means a low impedance screen supply. However, this primarily means low impedance at audio frequencies. It is less important that the screen supply has low impedance at DC, although this still matters.
I intend to build a regulator based on the circuit from Pete Millet:
DCPP Amp

Does a large enough output capacitor satisfy the low AC impedance requirement?
  Reply With Quote
Old 7th November 2017, 11:47 AM   #6
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by glina
The datasheet linked above quotes
Zero signal plate current 54mA
Maximum signal plate current 66mA.
For Class A operation.
That is because of second-order distortion. This generate harmonics and intermodulation; it also causes a DC shift.

Quote:
Does a large enough output capacitor satisfy the low AC impedance requirement?
Only if the regulator is designed to drive a big cap. Many regulators have an inductive output impedance so adding a cap can increase output impedance.
  Reply With Quote
Old 7th November 2017, 06:12 PM   #7
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Quote:
This would imply that plate and screen voltages swing in opposite directions as signal increases.

No, the above is not true.


Lets look at Pentodes and Beam Power Tubes

Pentode Connected Mode:

As the control grid voltage goes less negative with respect to the cathode voltage,
then both the plate current increases and the screen current increases.

Suppose the plate is driving a 5K output transformer.
And suppose the screen voltage is fed by a series resistor coming from a regulated voltage supply.
Then both the plate voltage and the screen voltage will decrease (voltage drop = I * R).
The plate voltage and screen voltage move in the same direction.

Caution, when the screen voltage is connected to a very well regulated stiff supply voltage:
If the plate voltage swings much lower than the screen voltage, then the screen current will increase to a very large value (sometimes not a safe value).
Look at pentode curves that show the plate voltage, screen voltage, and screen current all on one graph.

As the plate voltage drops far below the screen voltage, the electron cloud only “sees” the screen, because the plate is physically on the other side of the screen, but the plate voltage is “closer” to the cathode voltage.

The plate impedance is very high in pentode mode, the damping factor without negative feedback is very low.

Ultra Linear Connected mode:

Ultra Linear mode causes the screen voltage to decrease at the same time the plate voltage is decreasing, often at a 40% rate (i.e., 100V plate swing, 40V screen swing). Ultra Linear is a form of local negative feedback (not global, that would require additional circuitry).

The plate impedance is high to medium in ultra linear mode, the damping factor without even using (global) negative feedback is low to medium.

Triode Wired Mode:

The plate impedance is lower in triode wired mode, the damping factor without even using negative feedback is medium to high. Triode wired mode is a form of local negative feedback
(not global, that would require additional circuitry).
  Reply With Quote
Old 7th November 2017, 11:37 PM   #8
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
G2 current does change. However 90+% of hi-fi plays under 10% maximum power.

Peaks are a few milliSeconds, rarely ever 100mS.

So do you have a big cap on the G2 supply? A few mA change at 280V, with a 40uFd cap, will take *2.5 seconds* (2500mS) to shift. Much longer than hi-fi peaks last.

Don't listen to me. Don't listen to PSUD. Put voltmeters on the amplifier (carefully!!) and play it. Most listening there will be "no change". I've done this for hours (testing repairs and new builds). If you are not grossly clipping, the voltage dips are very-very small.

Small change of Vg2 will not upset the amplifier.

In ideal class A, the Plate current is constant, yes. In real class A audio amplifiers, it depends. If the load impedance is correct for the tube idle operating point the current stays the same up-to and past clipping. But the makers' data-sheet often cheats the load down, so at clipping the current rises, because this gives a higher "Max" power output without exceeding the idle dissipation limit. I would guess that the majority of DIY amps don't have an exact-right load on test bench. And when driving a speaker, the load is "wrong" and high over most of the audio band. Driven into clipping, current will tend to fall. Of course this is far past the point of clean "quality".

That's for hi-fi which may touch but never stays in clipping. Guitar amplifiers driven to CLIP!! are a further study (not simple) but not on-topic here.
  Reply With Quote
Old 8th November 2017, 02:00 AM   #9
audiowize is offline audiowize  United States
diyAudio Member
 
Join Date: Aug 2005
Location: Seattle
Send a message via AIM to audiowize
Quote:
Originally Posted by PRR View Post
Don't listen to me. Don't listen to PSUD. Put voltmeters on the amplifier (carefully!!) and play it. Most listening there will be "no change". I've done this for hours (testing repairs and new builds). If you are not grossly clipping, the voltage dips are very-very small.

Small change of Vg2 will not upset the amplifier.
I made this same observations some years ago with a single ended EL84 amp. It was quite surprising.
  Reply With Quote
Old 8th November 2017, 02:50 AM   #10
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
GoatGuy's Avatar
 
Join Date: Dec 2012
Location: SF Bay Area
Quote:
Originally Posted by DF96 View Post
That is because of second-order distortion. This generate harmonics and intermodulation; it also causes a DC shift.


Only if the regulator is designed to drive a big cap. Many regulators have an inductive output impedance so adding a cap can increase output impedance.
I tip my hat to you, ol' debating partner. You're THE man.
__________________
John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
  Reply With Quote

Reply


SE Pentode, regulated screen voltage?Hide 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
Importance of Screen Voltage of Pentode??? howe0168 Tubes / Valves 30 23rd November 2017 11:18 PM
Pentode's screen voltage/current Osvaldo de Banfield Tubes / Valves 22 20th January 2014 12:13 PM
Preamp Pentode Screen Voltage and Gain LATUBEGEEK Tubes / Valves 11 30th November 2013 12:28 AM
Pentode cathode follower with regulated screen voltage? G Tubes / Valves 14 18th November 2013 11:01 PM
screen voltage for triode strapped pentode Bigun Tubes / Valves 6 11th April 2012 04:21 AM


New To Site? Need Help?

All times are GMT. The time now is 10:56 AM.


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
Wiki