LS50/GU50 Both Grid Drive - Page 5 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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 17th September 2008, 05:57 PM   #41
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by smoking-amp
"vague chicken scratchings on the back of a pizza coupon"

You mean like this?
I couldn't find a crayon handy though.

Don

OOPs, I just noticed that the GU50 g1 drive signals need to be crossed over between tubes due to the Mosfet's inversion at its drain. Hmmm, so the non-linear screen current only effects the "OFF" tube's g1 drive. Hmmmm, but then the Mosfet driving the "ON" tube's g1 has no current thru it's drain resistor. I think some fixup is called for here.

Thanks!

Yes, control grid signals have to be swapped, also 100K grid leak resistors are needed connected to 100K pots to set idle currents, and -60V source is needed for pots.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 17th September 2008, 06:19 PM   #42
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
hey-Hey!!!,
The simple solution is to drive it Class A...2 of course.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 17th September 2008, 06:21 PM   #43
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
The simple solution is to drive it Class A...2 of course.
cheers,
Douglas

Yes, I assumed class A.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 4th October 2008, 04:51 AM   #44
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
After presenting Zobsky with the schematic, he seemed enthusiastic
to actually built it. There will be some minor changes to accommodate
a few things I didn't realize (for example: his Karlson K15 is loaded
with a vintage 16ohm coaxial, not 8 ohm as I had assumed.)

And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit.
I am still intending to abuse only positive drive(s), no caps, and stay
entirely in A2 at all times.
  Reply With Quote
Old 4th October 2008, 05:34 AM   #45
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by smoking-amp
"vague chicken scratchings on the back of a pizza coupon"

You mean like this?
I couldn't find a crayon handy though.

Don

OOPs, I just noticed that the GU50 g1 drive signals need to be crossed over between tubes due to the Mosfet's inversion at its drain. Hmmm, so the non-linear screen current only effects the "OFF" tube's g1 drive. Hmmmm, but then the Mosfet driving the "ON" tube's g1 has no current thru it's drain resistor. I think some fixup is called for here.

By the way, I like your drawing: it has the name of the topology stamped on it already: "Free Cookies"!
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 4th October 2008, 01:31 PM   #46
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
When it comes to legible renderings, Don is the one true master!
  Reply With Quote
Old 4th October 2008, 11:04 PM   #47
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"it has the name of the topology stamped on it already: "Free Cookies"! "

Yes, I like the name too! The Free Cookie! (seems there is a Free Lunch around also.) Can we use it to refer to gm scaled g1/g2 drives in general, or do you want to reserve it for your specific implementation?

"When it comes to legible renderings, Don is the one true master!"

Good thing I had some junk mail around to write on, I would have had to use TP otherwise. Some of those sale tubes came wrapped in it.

Don

"And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit."

If you scale the drives by 1/gm, you can just use the standard curves with the gm doubled (effectively halving the drive voltage from g2 or g1 only drive).
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 19th October 2008, 07:27 PM   #48
zobsky is offline zobsky  India
diyAudio Member
 
zobsky's Avatar
 
Join Date: Mar 2002
Location: Dallas, Tx, USA
Since it's my particular GU-50 in question and I am pea-brained, I had to ask:

"is it safe to run the tube and draw grid current in A2 CONTINUOUSLY ?" I understand the driver limits the current that the GU-50 grids see but I always considered A2 as something that tubes only operate in intermittently.

Thanks
__________________
"Any fool can know. The point is to understand" - Albert Einstein
  Reply With Quote
Old 19th October 2008, 08:52 PM   #49
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Watts is Watts. Voltage times current is Watts. Voltage is low
here, and those grids look pretty darn heavy under the XRay.
I wasn't around when they wrote the spec sheet for G1=G2.
Who knows what they had in mind?

I'm not sure its the smartest thing I've ever suggested, to have
a positive G1 focus the flow of electrons into its shadow (where
G2 normally hides)... G2 either melts down in a huge industrial
accident when it gets too hot, or it doesn't.

The drive circuit was specifically designed to limit the wattage
that could be sucked from the grids. 1 combined watt or less.
Forget how I computed that figure now??? And anything extra
that strikes, only serves to lower the voltage. Its a theory...

------------------------------------------------------------------------------

If you don't care for positive drives, Telefunken's G1=G2 curve,
or something closer to g1=0.5*g2. You are going to need a lot
higher B+ with conventional negative g1 drive. Maybe around
800V would do it, not counting AC added by the plate swing.

I thought you wanted this to be safe, cheap, and simple?
At least safe for you in the same room, if not for parts...

I got a lamp dimmer and some incandescent bulbs to fake a
poor idiot's variac. We could bring up B+ slowly, in the dark,
while watching for any unusual fireworks... Would two fuses
in series be a ridiculous suggestion?

And as its entirely DC coupled, might add a subsonic filter.
  Reply With Quote
Old 19th October 2008, 11:31 PM   #50
zobsky is offline zobsky  India
diyAudio Member
 
zobsky's Avatar
 
Join Date: Mar 2002
Location: Dallas, Tx, USA
Fine. I still need to get that GU-50 back from you. I'll harass you for it in the near future.

I found a 400V + 12.6V transformer in the parts bin that should work fine for this project once the B+ is adjusted.
__________________
"Any fool can know. The point is to understand" - Albert Einstein
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
How do I design "SCREEN GRID DRIVE" ANALOG GUY Tubes / Valves 27 26th December 2013 02:35 AM
Output (beam) pentodes / tetrodes suitable for screen grid drive ilimzn Tubes / Valves 19 15th June 2013 06:23 AM
Gu50 Set ? pwan Tubes / Valves 3 22nd April 2007 04:15 PM
Control grid Voltage / grid stoppers... jon_010101 Tubes / Valves 2 4th May 2006 01:59 AM


New To Site? Need Help?

All times are GMT. The time now is 08:39 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2