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Low Rp tube for line stage - ideas?

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#101
rongon said:
If you want a line preamp with gain of less than 10X, and you don't want to use negative voltage feedback, then 6N30P and 6N6P will end up with too much gain (at least 12X, even with cathode degeneration). That's why there's interest in using power output tubes for a line stage. There are several on diyAudio including a 12B4A version, a 6V6-triode version, and several others. 4P1L-triode also fits this category, but I know andjevans has tried that and other similar things.
Click to expand...
This. Gain has to be below 10x. The 6C4C stage was for a guy whose amp has a 10K input impedance at 1V sensitivity. Points straight to the 6C4C or 2a3. The other valves I've used in preamps are 6AH4 and 6G6G, and also 2P29L which is easy to build with. They require a slightly higher input impedance like 27K or above and they all sound good.

The 6C4C sounds different from all the rest. The closest I can get is to say it's like using an amorphous core OPT.
 
#103
andyjevans said:
This. Gain has to be below 10x. The 6C4C stage was for a guy whose amp has a 10K input impedance at 1V sensitivity. Points straight to the 6C4C or 2a3. The other valves I've used in preamps are 6AH4 and 6G6G, and also 2P29L which is easy to build with. They require a slightly higher input impedance like 27K or above and they all sound good.

The 6C4C sounds different from all the rest. The closest I can get is to say it's like using an amorphous core OPT.
Click to expand...
I'm not saying any 2A3-oid is a bad idea. 🙂
I like the analogy to an amorphous core OPT. I think I know what you mean. I'd describe it as 'sparkly clean and clear' and maybe even a bit 'lean', but not 'mellow' or 'warm' for sure.

Wavebourn used to sing the praises of 6P43P-E triode strapped. That would have rp <1k and low mu (I think about 6?). I've been meaning to try a pair as a line stage with a bit of gain to it. I keep getting sidetracked with digital stuff breaking, and of course there's work to contend with, and cars.... Ugh.
 
#104
andyjevans said:
I just finished a 6C4C line stage which is very interesting - exceptionally lean, clean and detailed. Just about the complete opposite of my 26 stage which is full and lush. It's different from anything I've heard or built. Intriguing. For what it does it's quite remarkable, but it isn't full and lush at all. Very good on classical music and piano, wide precise soundstage. Dynamic and effortless - it's an output tube after all. Low gain and 800 ohm output impedance. Svetlana quality tubes. I could probably put more current through it. KBG PIO output caps.
Click to expand...
Sounds like this what I seek. Will try 6B4G(have plenty of them), but at somewhat higher current, 25mA with ca 100V Ua.
 
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#108
Hi the main "issue" about very low Ri and High transconductance tubes is - open anode.
as the S is higher the anode is more "open" reducing the plate surface...
That is affecting the sound.
And that is the same for the pentodes in triode mode.
.
It is better to use for preamp, boxed and "closed" anode shape.
As 10Y has, Also 2A3, 45, 6B4G, 300B and so.
for pentode in triode mode EL156, EK84, 6V6 etc.
.
These boxed anode tubes has a bit larger Ri but still small enough to drive higher impedance inputs as tube power amp has. 470K typicaly. But sound is abssloute the best 🙂
 
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#116
TonyTecson said:
works for me.....i reported it..
Click to expand...
Work for me too:

Screenshot 2024-06-21 131453.png
 
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#119
andyjevans said:
Here's some better pics of my Leak Varislope 3 rebuild with 6AH4 tubes. Circuit attached.

View attachment 1322482View attachment 1322483View attachment 1322484
Click to expand...
Hey Andy! Looks like a fun project! Can you explain why you’re running the 6ah4 with only 10ma? Are you perhaps limited by the Leak power supply? I run my 6ah4 linestage at 20 to 24ma. I recently switched over to some Hashimoto output transformers which reduce gain (and noise) very nicely. They’re still breaking in so it’s too soon to make any conclusions about sound quality.
Dave
 
#120
Here is the 7233 tube spice model. Note that is not so linear. 8W is the limit.
2 operating points one low voltage one higher...
Anode is w-i-d-e open...
.
7233 settings tools 01.png


7233 settings tools 02.png


Code: 
**********************************************
* Created on 06/13/2024 09:34 using paint_kit.jar 3.1
* www.dmitrynizh.com/tubeparams_image.htm
* Plate Curves image file: /Users/zoran/Desktop/Model_Paint_Tools/Graph/
* Data source link:
*----------------------------------------------------------------------------------
.SUBCKT 7233 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=7.5P  CGP=14P CCP=2.2P RGI=2000
+ MU=3.749 KG1=1050 KP=19 KVB=400 VCT=3.36 EX=1.617
* Vp_MAX=600 Ip_MAX=500 Vg_step=10 Vg_start=10 Vg_count=15
* Rp=4000 Vg_ac=3 P_max=8 Vg_qui=-40 Vp_qui=147
* X_MIN=58 Y_MIN=64 X_SIZE=897 Y_SIZE=602 FSZ_X=1754 FSZ_Y=792 XYGrid=true
* showLoadLine=y showIp=y isDHT=n isPP=n isAsymPP=n showDissipLimit=y
* showIg1=n gridLevel2=n isInputSnapped=n 
* XYProjections=y harmonicPlot=y dissipPlot=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
D3 5 3 DX ; POSITIVE GRID CURRENT
R1 2 5 {RGI} ; POSITIVE GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS 7233
*$
 
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