G1=G2/mu Scaled Drive Strawman Design - Page 3 - 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 29th March 2010, 01:33 PM   #21
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
I'll be listening for the News: "Fireball envelops South Florida"
Not today. I was hoping for some rain yesterday so I would have an excuse to play with some electricity (safely inside the house). About 5AM this morning I was awakened by some serious thunder and lightning. When I left the house this morning the streets were flooded over and hail was popping off the winshield. I guess we went from winter to summer overnight!

Quote:
If you want some cheap 6BQ6GAs, I've got a box of them (40 or 50) from the last tube sale
I think that I have caught my limit on those guys. I got two batches from AES when they were 98 cents each. Then vacuumtubes.net had them by the box full for 65 cents each so I bought a box. Some of the AES tubes are obviously bad, so they may become firebottles.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 29th March 2010, 08:53 PM   #22
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by Michael Koster View Post
I have never understood the popularity of ultralinear. By the time you add enough NFB to get a decent output resistance you can consider a number of other options like local Schade feedback *instead*. I suppose I need to read H&K again ... or not ;-)
It looks different, and audiophools like that sort of thing, and even come up with considerable "folk wisdom" as to its supposedly "magical" properties. UL is just another form of local NFB. Since all my work has been with types like the 807 and the horizontal deflection finals that like to see low screen voltages, UL just won't work. For 807s, I used the usual form of parallel local NFB: from plates back to the grid drivers (what's since come to be called "Schade feedback" here). The 6BQ6GTBs didn't require the extra help to clean 'em up, and a modest amount of gNFB was all that this type required to sound really good.

Doesn't mean that I would never consider it, it's just that it hasn't come up as a design possibility just yet.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 29th March 2010, 09:59 PM   #23
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by Michael Koster View Post

@Anatoliy: I have never understood the popularity of ultralinear. By the time you add enough NFB to get a decent output resistance you can consider a number of other options like local Schade feedback *instead*. I suppose I need to read H&K again ... or not ;-)
I share your opinion, Michael!

I'm going to try a positive FB on the 3'rd grid of GU-50 (in addition to a negative feedback to the 1'st grid)
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 30th March 2010, 12:22 AM   #24
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
UL? UseLess?

Couple of things that bother me about UL mode. The most obvious problem is that the tube's current capability drops off just when you need it most. Then there is screen current distortion in spades when plate V drops below screen V. A separate screen winding or Fet driver at lowered DC voltage could fix the second problem.

But I suspect that the compressive screen current distortion is being played off against normal expansive 3/2 power distortion in the usual tapped xfmr designs. Even then, the 3/2 power distortion is plate current related, while screen current dist. is plate voltage related. Maybe this is OK if it's tuned up for a resistive load, but a reactive load (speaker) makes them go out of phase with each other.

Then the usual xfmr tapping arrangement won't even work readily for a "sweep" tube. (sound of Axe falling)
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 30th March 2010, 03:08 AM   #25
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
I'll be listening for the News: "Fireball envelops South Florida"
Well, one of the local TV stations has been running teaser commercials to get you to watch the 11PM news. They have been showing a rather large fireball that somebody taped while driving down I-95. Not me though, I'm 10 miles west of I-95. The fireball was related to a tornado and some power lines.

On a much smaller scale, I got a wild idea, wired 4 $1 tubes together with no coupling caps. It looked cool, until I turned it on. Then one tube turned red, and a resistor fried in half. Game over for tonight. May have time tomorrow night.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 30th March 2010, 03:55 AM   #26
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Default 6bq6gtb

I just realized 6BQ6s are basically a dollar. Where have I been? How can I resist...? It would make a nice 30W per channel amp for next to nothing in tubes. Scaled drive would be in the same 60-80V range as with the 6DQ6
Attached Images
File Type: jpg 6bq6-loadline.jpg (89.8 KB, 268 views)
  Reply With Quote
Old 30th March 2010, 06:22 AM   #27
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by Michael Koster View Post
I just realized 6BQ6s are basically a dollar. Where have I been? How can I resist...? It would make a nice 30W per channel amp for next to nothing in tubes. Scaled drive would be in the same 60-80V range as with the 6DQ6
Been there; done that. I'm getting better than 37W from a PP pair of Class AB1 6BQ6s (both the slim 6BQ6GTB and the fatter 6BQ6GA). I also discovered that these sound really good as well. They're like a 6V6 on steroids. I run mine with a bias current of 50mA / phase (17.5W of Pd -- 12W rated). That gets them more towards Class A operation, and definitely improves the sonics. Since these were intended for the more demanding horizontal deflection duty, busting the Pd spec doesn't result in red plates, nor does it have much impact on service life in the much less demanding audio final use.

I went ahead and got a 50 pack myself before these get expensive because we keep talking 'em up as audio VTs.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 30th March 2010, 01:59 PM   #28
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
I just realized 6BQ6s are basically a dollar. Where have I been?
These were the tubes that I learned on. In the late 60's you could find these and 6DQ6's by the handfuls at the local trash dump, inside all the discarded black and white TV sets. Transformers, octal sockets, and all the other parts were free too. 6L6 types were very rare in the dump but 6V6's could be found if you knew the magic word, "Magnavox". I came to the conclusion that the 6V6 wasn't much fun because they died too easy at the hands of a teenage amp maker that didn't understand the meanings of things like "bias" "load line" or "impedance".

Now I have over a hundred 6BQ6's all purchased at under a buck. When I first got some I did some tests. Pictures are in the "tube sale at AES" thread, posts # 127 and 128.

Observations discovered somewhere else in that thread (or maybe another one) found the limits of screen drive. As the screen is driven positive the plate is heading toward zero. When the plate voltage goes low enough and the screen voltage is high enough the screen will draw a lot of current, enough to cause it to glow brightly. As the plate voltage begins to return to a high positive voltage, that glowing screen grid will emit electrons that will travel to that positive plate resulting in a nuclear fireball inside the tube and lots of blown components! Dual grid drive may allow for more power without such bothersome side effects.

Tube sale at AES

Note the observations I made in post #146. I found that the best distortion characteristics in screen drive could be obtained with G2 idling at +80 volts and G1 idling at -19 volts. Given these conditions it would seem that you could drive both grids with G1 remaining entirely in the negative voltage region. This could lead to a very simple directly coupled circuit.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 31st March 2010, 02:15 AM   #29
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Last night I created a furball of clip leads and tag boards in about 1 hour. I powered it up and got some glow and some smoke. I flipped the master kill switch and left the room.

Today I got home from work late and turned on the TV to get my weekly fix of Lost. During a commercial I discovered last nights blonde moment. The negative power supply was turned off....DOH. Of course things got hot.

So, tonight I tried again.....and the furball makes 40 watts at 4% distortion without any feedback or coupling caps. The only sand is a CCS in the tail of the LTP. All with 360 volts of B+. Dialing back the drive gets 0.9% at 5 watts.

All is not perfect yet since the power goes down if I crank the B+ any higher, and stability goes away over 400 volts. Still, not bad for an hours work and it proves the viability of "Dual Drive". The output tubes are 6BQ6GA's and the input tubes are 6LQ8's although just about any pin compatible that I tried worked to one degree or another.

No more time tonight. More experiments at an unknown later date, and of course I need to trace out just what I made and draw a schematic.
Attached Images
File Type: jpg DualDriveP-PAmp_A.jpg (269.9 KB, 247 views)
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 31st March 2010, 02:39 AM   #30
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
OK, here is the 5 minute simplfied schematic. This is half of the circuit since it is a push pull amp. There two of these circuits. Grid and screen stoppers (1K) are present on the input tube. There are none on the output tubes. The screens of the input tubes are tied together. There is a balance pot (200 ohm) feeding each cathode from the CCS. The grid of the input tube that is not shown is grounded through the stopper.

This can be viewed as a place to start for experimentation. It is not a "build it and it will work" circuit. At work we would call this a "proof of concept".
Attached Images
File Type: jpg HalfSchematic_A.jpg (162.5 KB, 252 views)
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  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
screen-drive (g2 driven) plate curves spacing linear! oldeurope Tubes / Valves 27 26th June 2008 06:43 AM
HK 'Deuce front end on G2 drive output stage? GordonW Tubes / Valves 0 9th June 2008 02:22 PM
6JN6 screen-drive (g2 driven) plate curves? GordonW Tubes / Valves 26 23rd April 2008 01:56 PM
Anyone know the g1-g2 capacitance of 6AU6? ray_moth Tubes / Valves 10 29th August 2004 07:19 AM


New To Site? Need Help?

All times are GMT. The time now is 07:21 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