Tag: 3A5

Filament Bias: a practical example with 3A5 DHT

Introduction

cartoon-july4_0006

Recently I was asked about whether I could write on my blog about how to design a filament bias stage. My immediate answer was:

  1. I don’t have much time these days am afraid to write extensive articles (and sometimes to even write-up at all)
  2. Thomas Mayer has written about it (see here). Of course, I completely forgot that Thomas never completed his intended series of posts around filament bias, so I decided to attempt explaining the practical aspects of its design in this blog.

Before you continue reading this post, I suggest you read first Thomas’ article above and get yourself acquainted with DHTs and triode amplification. I’m not going to cover any of that theory which I will give it for granted that the reader is experienced with valve circuits and in particular with the hybrid mu-follower amplification stage with gyrator load.

3A5 DHT example

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3A5 DHT (Continued)

Lars requested the paralleled triode curves. Here they are:

3A5 RCA 2 triodes SMALL And the SPICE model which is more accurate than my previous version:3A5 2 triodes spice model

 

* Created on Sat Jan 19 15:50:19 GMT 2013 using tube.model.finder.PaintKIT
* model URL: http://www.bartola.co.uk/valves/
* Created by Ale Moglia 2013 [email protected]
*————————————————–
.SUBCKT TRIODE_3A5-2 SMALL 1 2 3 ; P G K ;
+ PARAMS: CCG=.9P CGP=3.2P CCP=1.0P RGI=2000
+ MU=12.46 EX=1.3857 KG1=1965.0 KP=132.0 KVB=1.875 VCT=-1.6 ; Vp_MAX=200.0 Ip_MAX=0.032 Vg_step=2.0
*————————————————–
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
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 ; FOR GRID CURRENT
R1 2 5 {RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$

Looking at distortion, as expected the pair of paralleled triodes performs better than a single triode:

3A5 paralleled THD Vo=10Vrms

3A5 DHT

IMG_3621
The 3A5 DHT is another interesting valve to bear in mind.  Many have used them successfully even in a phono stage. Great examples are Dmitry Nizh’s DHT phono stage and Sheldon’s. I will probably use them as well at some point.

Can you hear the microphonics SMALLWell, many DHT have been dismissed over the years due to their microphonic sensitivity. True, they are more microphonic, but this is manageable. Look at the 4P1L for example. I built a great preamp a couple of years ago and played around with the mechanics of the preamp to quiet it down. And I managed to, succesfully (as many have as well). The 4P1L sounds fantastic, despite so many turning it down because of its natural singing skills 🙂 Now, 4P1L is looked after, many are building DHT preamps and even full DHT amplifiers.

Looking at the specifications of the 3A5, we can see that the heater is attractive to be used with rechargeable batteries. Well, at least 220mA @ 1.4V is manageable. Key specs are:

  • Mu of 15
  • Anode resistance (ra) is 8,300Ω.
  • Anode dissipation (Pa) is 500mW per valve.
  • Vf=1.4V / If =220mA (paralleled filaments)
  • Transconductance (gm):  1.8mA/V
  • Maximum anode voltage: 135V

How linear is this DHT? Let’s look at the curves first:

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