Sub miniature pentodes - what shall I do with them?

[Shoog]

As I see it, the oscillation of these tubes is a pure mechanical vibration of the different electrodes in the glass tube.
As far as I remember, the most annoying thing was not manifested through the loudspeakers and the music, it was a weak audible whistle, ca. 8 kHz directly from the ringing of the tubes themselves.
After a while, this drives one mad and is unlistenable.
Apart from this, the sound of this line/RIAA phono preamplifier with dual tone controls was really good.
Some of the tubes started "singing" right away after power up, the "good" "silent" ones would start singing when excited by a knock on the table or even by the music playing at moderate room volume. No rubber rings or foam could suppress this behaviour.
I searched for a solution on the internet, but I found none.

Had he same issues with 6080/6as7's. Only audible on 100db+ speakers but couldn't solve it no matter how hard I tried. Most people using the same valves never even heard the singing.

Shoog

 

[aspringv]

@needtubes don't suppose that you've that spice model for the 6418 handy? I'd appreciate a copy if that's no trouble.

And thank you for he offer of PCBs, but I'm considering doing something a little different - parallel a few of them and then use that to drive a buffer of some kind.

So it seems that this might be worth a bit of a play at the least. I might plan something and see how it goes!

 

[Bigun]

30 V anode supply

Max permissible plate voltage is 30V, did you run them hot - wondering if thermal issues could exacerbate your mechanical oscillation problem ?

 

[needtubes]

@needtubes don't suppose that you've that spice model for the 6418 handy? I'd appreciate a copy if that's no trouble.

Sure, here it is.

.SUBCKT TRIODE_6418 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=2.3P  CGP=.1P CCP=1.5P RGI=2000
+ MU=7.64 KG1=20160 KP=112 KVB=1.144E-4 VCT=3.281E-4 EX=1.93 

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

So it seems that this might be worth a bit of a play at the least.

I'd say so. It can be a very nice sounding tube in the right circuit.

Max permissible plate voltage is 30V, did you run them hot - wondering if thermal issues could exacerbate your mechanical oscillation problem ?

I wondered that as well. I run mine quite cool and the whistling always dies down over time. Other theories I've read about suggest starving the filaments a bit to keep the tube calmer.

 

[aspringv]

Current thoughts for a build

So I'm thinking that I might try the following.

Parallel a pair of the 6418's and cascode them with a MOSFET at about 30v, and load it with a sand ccs, thus getting a greater effective mu and swing to more effectively drive a sand power buffer or similar.

Cheers for the model! I'll have a play tomorrow night I'd say.

Thoughts and opinions are welcome

 

[aspringv]

And now I get to learn all about finessing a load line and how to bias a directly heated cathode. I get tubes as a rough analog for a jfet, but the direct heating bit has me confused as to how the bias is set.

I'll be back once I get my head around that...

Thanks!

 

[Eli Duttman]

And now I get to learn all about finessing a load line and how to bias a directly heated cathode. I get tubes as a rough analog for a jfet, but the direct heating bit has me confused as to how the bias is set.

I'll be back once I get my head around that...

Thanks!

You reverse bias the grid by inserting a resistor between the filament and ground (common return). The voltage drop in the resistor raises the cathode (filament) above ground potential. To prevent loss of gain, from degeneration, the cathode bias resistor is bypassed with a capacitor. It's easy enough to "float" the 1.5 VDC filament supply. An alkaline "C" cell should keep a few of those little buggers heated up and emitting for a fair amount of time.

There is no difference in how you self bias the tube from how you do it with a JFET.