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Old 29th February 2016, 07:28 AM   #1
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Default A2 p-p with Millett driver

Finally got enough other (non-tube, non-electronics) projects out of the way such that SWMBO is allowing expenditure and effort in smoke making.

I got a bunch of VT61A HF triodes a while back - very old WWII surplus. These are a cute-as Mickey Mouse tube used as finals in the radio sets in Spitfire aircraft. Who could resist? Closest data I could get was RK34 which is electrically identical, different base.

So the plan is for a 'sl7 LTP buffered by Pete's clever little circuit and swinging the required 100v p-p into the VT61A driving it as close to AB2 as I can get without sounding horrific.

OPTs are probably going to be Edcor 15w open frame jobs with power coming from Edcor as well. Optional OPTs are some nice vintage NOS 8k:8 units built to go into high end local EL84 pp designs.

The design goals are stringently non-audiophile. Sure, it has to sound acceptable but the main goal is something looking cool (shallow Hal is me) in a steam-punkish sort of way, not the standard path EL84, with an interesting mix of old and new-ish tech.

A very standard design kwila wood frame topped with an aluminium plate is the chassis.

Speakers are a very simple early '70s vented box design with paired 16 ohm 8 inch drivers in parallel in each box. Not at all peaky or hard to drive, but also not SOTA.

Source is a Sumsung HDTV, processing (badly) DVD/CD, Spotify from a no-name Android tablet via HDMI, and a mix of mp3-esque players. I am going to be all rash and assume standard 2v source output. Eventually a pre-amp (likely a simple versions of SY's Heretical) will do front end duties and liberate me from the TV as preamp.

We'll forget about the turntable atm since that just complicates things further.

Comments and advice encouraged as we go - this may take some time so sit back and don't hold your breath...
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Last edited by aardvarkash10; 29th February 2016 at 07:31 AM.
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Old 29th February 2016, 08:31 AM   #2
Koonw is offline Koonw  Malaysia
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vt61a datasheet

CF/CCS, can it be made simpler?
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Old 29th February 2016, 01:55 PM   #3
Koonw is offline Koonw  Malaysia
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Here LTSpice model for VT61A I painted, not terribly accurate, but doable, grid current included.
Attached Images
File Type: png VT61A paint-1.png (444.6 KB, 510 views)
File Type: png VT61A plot-1.png (281.4 KB, 490 views)
Attached Files
File Type: asc VT61A plot-2.asc (2.2 KB, 11 views)

Last edited by Koonw; 29th February 2016 at 02:01 PM.
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Old 29th February 2016, 02:30 PM   #4
Koonw is offline Koonw  Malaysia
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Because grid current is not on the same plot, I have to readjust it here is more accurate model.

**** VT61A ** Advanced Grid Current **********************************
* Created on 02/29/2016 22:27 using paint_kit.jar 3.0
* Model Paint Tools: Trace Tube Parameters over Plate Curves, Interactively
* Plate Curves image file: vt61a.png
* Data source link:
*----------------------------------------------------------------------------------
.SUBCKT VT61A 1 2 3 ; P G K
+ PARAMS: CCG=6P CGP=2.7P CCP=2.1P
+ MU=14 KG1=1575 KP=206 KVB=4609.4 VCT=0 EX=1.22
+ VGOFF=-0.291 IGA=0 IGB=1 IGC=23.2 IGEX=1.26
* Vp_MAX=350 Ip_MAX=150 Vg_step=10 Vg_start=60 Vg_count=9
* Rp=2840 Vg_ac=1.41 P_max=7 Vg_qui=-63.4 Vp_qui=204
* X_MIN=131 Y_MIN=124 X_SIZE=825 Y_SIZE=719 FSZ_X=1605 FSZ_Y=1003 XYGrid=true
* showLoadLine=n showIp=y isDHT=n isPP=n isAsymPP=n showDissipLimit=n
* showIg1=y gridLevel2=y isInputSnapped=n
* XYProjections=n harmonicPlot=n harmonics=y
*----------------------------------------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G ; TO AVOID FLOATING NODES
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
RE2 2 0 1G
EGC 8 0 VALUE={V(2,3)-VGOFF} ; POSITIVE GRID THRESHOLD
GG 2 3 VALUE={(IGA+IGB/(IGC+V(1,3)))*(MU/KG1)*(PWR(V(8),IGEX)+PWRS(V(8),IGEX))}
.ENDS
*$
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Old 29th February 2016, 09:21 PM   #5
exeric is offline exeric  United States
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Am I reading you right that you are using 2 voltage gain stages in your proposed amp? It doesn't seem necessary to me if one of the voltage gain stages is a 6sl7 and you are only driving el84s. Maximum P to P voltage for full output for an el84 is about 22 to 30 volts probably before feedback. Double that to give 6 db global feedback potential. More than that and you have the potential to decrease the spaciousness of the sound presentation.

The mu of a 6sl7 is 70 so typical amplification with an LTP pair will be 25 to 32 or thereabouts. A 2 volt rms input from the usual line sources is 5.6v p-p. Multiply the nominal 25 gain times that input and you have 140v P-P. Unless you're using some very strange topology that requires huge amounts of feedback it seems like the vt61a is redundant. But if you really like it then maybe you can just frame it and put it on the wall by itself.
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Old 29th February 2016, 09:51 PM   #6
exeric is offline exeric  United States
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Ok, I reread your first post and I guess you're using 1 voltage gain stage and using the spitfire tubes as the outputs. That makes more sense. Never mind.
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Old 29th February 2016, 10:42 PM   #7
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Quote:
Originally Posted by exeric View Post
Ok, I reread your first post and I guess you're using 1 voltage gain stage and using the spitfire tubes as the outputs. That makes more sense. Never mind.
you got it!
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Old 1st March 2016, 03:22 AM   #8
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I'll be following with interest. My experience with circuits such as you're planning is that significant 'crossover' distortion occurs as the grids go positive, apparently due to non-zero driver output impedance.
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Old 1st March 2016, 05:41 AM   #9
Koonw is offline Koonw  Malaysia
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I haven't yet sim a driver that drive the grid positive. It appears the input impedance is around 3K above zero, if input drive is 25V, this is over 8mA grid current. At -25V bias, with 25V input, there is no crossover distortion seen at the output only just over 2W rms.
Attached Images
File Type: png VT61A-2.png (199.9 KB, 410 views)
Attached Files
File Type: asc VT61A plot-pp-test.asc (5.3 KB, 7 views)
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Old 1st March 2016, 07:08 AM   #10
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Pete's original thread on the buffer circuit
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