Go Back   Home > Forums > > >

Tubelab Discussion and support of Tubelab products, prototypes and experiments

Reply
 
Thread Tools
Old 7th January 2021, 07:35 AM   #61
sunil is offline sunil  India
diyAudio Member
 
Join Date: May 2003
Location: Bangalore, INDIA
George and all - Best wishes for the New Year. Stay safe.
  Reply With Quote
Old 11th January 2021, 08:46 PM   #62
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Hi George, or anyone else willing to look at my simulation.

I tried simulating your posted circuit and get horrific distortion (5713.491356%) at 1.6Vrms out.

The .sub and .inc files won't attach so here are the lists:
****
*
* R McLean 24 June 2005
* combine best features of Duncan Amps and Korens version
* 6AU6 pentode
* parameter extraction, from GE tube sn001, 24 June 2005
****
*
.SUBCKT 6AU6 A S G K
+ PARAMS: Ex=1.29250482718994 Kg1=1066.05313830636 Kp=233.038097126617 Kvb=6.26797667003452 Mu=39.2114621352395
+ Exd=0.927382241593347 Kd=1427.5732907893 Ks=2.80629495414095 Kg=0.00189539610643784
+ CCG=5.5P CPG1=.0035P CCP=5.0P
Eat at 0 Value={limit(0.636*ATAN(V(A,K)/KVB),0,1)} ; arctangent shaping
Eme me 0 VALUE={PWR(LIMIT(V(A,K),0,2000),EXD)/KD} ; diode line
Egs gs 0 VALUE= {V(S,K)/KP*LOG(1+EXP((1/MU+V(G,K)/V(S,K))*KP))} ; effective voltage
Egs2 gs2 0 VALUE={(PWR(V(gs),EX)+PWRS(V(gs),EX))/(KG1*0.636)} ; total space current
G1 A K VALUE={LIMIT(V(gs2)*V(at),0,V(me))} ; plate current
Escrn sc 0 VALUE={KS*V(gs2)*(1.1-V(at))} ; reverse arctan shaping for screen current
G2 S K VALUE={V(sc)*LIMIT(V(S,K),0,10)/10} ; screen current
G3 G K VALUE={PWR(LIMIT(V(G,K)+1,0,1E6),1.5)*(1.25-V(at))*KG} ; grid current
C1 G K {CCG} ; CATHODE-GRID 1
C2 A G {CPG1} ; GRID 1-PLATE
C3 A K {CCP} ; CATHODE-PLATE
.ENDS 6AU6

*Vacuum Tube Tetrode (Audio freq.)
.SUBCKT X50C5 A S G K
*
* Calculate contribution to cathode current
*
*the number at the right end determines sharpness of knee
Bat at 0 V=0.636*ATAN(V(A,K)/12)
*the URAMP(V(S,K)/# mostly determines peak plate current, grid line spacing nearly constant
*the number at the right end determines slope of grid lines
Bgs gs 0 V=URAMP(V(S,K)/6.2+V(G,K)+V(A,K)/90)
*the exponent sets the linearity of grid line spacing, and big impact on peak plate currrent
Bgs2 gs2 0 V=V(gs)**1.58
Bcath cc 0 V=V(gs2)*V(at)
*
* Calculate anode current, grid line spacing adjust and peak plate current
*
Ba A K I=1.35E-3*V(cc)
*
* Calculate screen current
*
Bscrn sc 0 V=V(gs2)*(1.1-V(at))
Bs S K I=0.6E-3*V(sc)
*
* Grid current (approximation - does not model low va/vs)
*
Bg G K I=(URAMP(V(G,K)+1)**1.5)*50E-6
*
* Capacitances
*
Cg1 G K 13p
Cak A K 8.5p
Cg1a G A 0.6p
.ENDS X50C5
Attached Files
File Type: asc 50C5Unset.asc (7.2 KB, 12 views)
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote
Old 12th January 2021, 05:35 AM   #63
dch53 is offline dch53  Australia
diyAudio Member
 
Join Date: Jun 2015
Location: Perth, Western Australia
Quote:
Originally Posted by TheGimp View Post
Hi George, or anyone else willing to look at my simulation.

I tried simulating your posted circuit and get horrific distortion (5713.491356%) at 1.6Vrms out.
Maybe the distortion to the output waveform might suggest something. Apart form the obvious glitches the positive excursion is greater than the negative excursion.

I tried disconnecting the feedback to the screen of U1 while duplicating it's biasing but that didn't help.
Attached Images
File Type: png Output 1.png (60.6 KB, 429 views)
  Reply With Quote
Old 12th January 2021, 06:29 AM   #64
indra1 is offline indra1  Indonesia
diyAudio Member
 
Join Date: Nov 2010
Location: Bogor
Quote:
Originally Posted by TheGimp View Post
I tried simulating your posted circuit and get horrific distortion (5713.491356%) at 1.6Vrms out.
Reverse polarity of L3.
  Reply With Quote
Old 12th January 2021, 11:53 AM   #65
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
I did not try it yet, but L3 needs to be reversed, otherwise this is a push-push circuit, and like a tug of war, the strongest tube will win, or both lose.
__________________
Tubelab, I blow stuff up so that you don't have to.
  Reply With Quote
Old 12th January 2021, 01:31 PM   #66
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Aha, I am blind in one eye and can't see out of the other. L3 was the problem.

Thanks.
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote
Old 15th January 2021, 01:52 AM   #67
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
I managed to modify and tweak the circuit for the 6P1P-EV.

>30W at 10% THD!

The tubes are running at 85% max dissipation (12W).

IT will probably take me a while to test this, but I really look forward to it.

I had a number of issues with LTSpice reporting step size errors and had to isolate the screen supply to the input tube.

I also switched to the 6CB6 as the input tube as I have a nice supply of them.
Attached Images
File Type: png 6P1PUnset6CB6_2nd_good_.png (57.6 KB, 379 views)
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote
Old 18th January 2021, 02:11 PM   #68
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Since the grid is tied to ground, is this a grounded grid topology with a FET buffer driving the cathode?

The P-FET has no voltage gain since the source is attached to the cathode of the tube and it acts as a source follower providing a low impedance drive.
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote
Old 18th January 2021, 07:18 PM   #69
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
In the circuit you are simulating the grid is not grounded. Negative feedback via a resistor directly from the plate of the tube makes the pentode look a lot like a triode. This was discovered by Schade back around 1940, but always involved at least one capacitor in the path.

Now that you have your sim running put a probe on the cathode and control grid of an output tube. All will be moving in phase, but at different levels. The only element that's not moving is G2. So the drive is directly applied between G2 and the cathode while the drive minus the local negative feedback is applied between G1 and the cathode.

Using this arrangement has several advantages.

No negative voltage supply is needed.

The total output stage dissipation is split between the tube and the mosfet. The ratio of this split is dependent on the idle voltage on G1.

At idle the tube sees a voltage on its cathode as in cathode bias, but this voltage is not wasted in a resistor as when driven to full conduction the cathode voltage is near zero putting all the B+ to work driving the OPT.

The "triodness" of a given pentode can be adjusted by changing the amount of feedback from the plate. This can be done easily with a large can and a variable resistance from G1 to ground, also offering a low frequency rolloff. For those who prefer not to stick a big cap here, one can switch different plate to grid resistors, while also switching a resistor from G1 to B+ to result in the same voltage on G1. I have a small tube version working with a 6 position switch to go from triode like curves, to full pentode.

This concept can be used with true sweep tubes that need G2 around 150 volts. It can also be used with the common audio tubes with G2 at a higher voltage. The G2 supply is the only thing that needs to be changed.

The same concept can be used to reduce the effective Mu of a triode. I have made it work in simulation with those high Mu regulator beam triodes, but the first test in a real circuit blew up in an oscillatory fashion as those tubes are prone to do. Mu=300 Gm=65000 very hard to tame!

Disadvantage, all signals are in phase! care must be used in layout, and decoupling is likely needed on power supply lines even in a two stage amp.
__________________
Tubelab, I blow stuff up so that you don't have to.
  Reply With Quote
Old 18th January 2021, 10:31 PM   #70
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Thanks George. I should have realized this as I noted the in phase relationship of the grid and cathode. And while adjusting the plate to grid feedback noted it's impact of the spectrum of harmonics.

I have an SE version of this simulating now using a 6P41S that looks like it can hit 17W at 10% THD but third dominates above 1W. If I increase the feedback to get more triode like behavior I can still get over 10W at 10% THD.

Pretty amazing circuit.
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote

Reply


UNSET is coming?Hide this!Advertise here!
Thread Tools

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
New F6 diy coming soon ? Nounours18200 Pass Labs 37 31st August 2012 04:35 PM
AES coming to NYC Oct 7-10 jackinnj Everything Else 0 14th August 2005 06:00 PM
Zen V7 coming up... Fuling Pass Labs 6 21st October 2004 08:39 PM


New To Site? Need Help?

All times are GMT. The time now is 02:12 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki