3A5 DHT

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.

Well, 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:

I Tested a set of military 3A5 RCA NOS in my curve tracer. They do look very linear to me.

So, let’s see how this little chap performs at the suggested operating point:

Very well, but not very impressive for a DHT. How linear is then this valve when swinging larger voltages (e.g. 10Vrms):

THD of 0.09% is very good and is much better than many valves used out there.

But let’s look now at a different point in which the distortion is minimised for small signal. I looked at pushing the 3A5 to its Pa limits. This is what I found to work well:

A very nice result I think. THD of 0.03% is outstanding for small signal indeed. Look at how close H3 is to H2, interesting to see that.  But then, how good is driving larger signals at this operating point?

Not great, keep this one for small signal amplification then 🙂

Finally, for those who love testing their designs with SPICE, here is my version of the spice model for this triode.

I had some challenges in matching the model on the right top corner from gate voltages of 6V to 12V. A minor compromise as I preferred exact matching in the areas this triode is likely to be used. Anyone who can match this better, please let me know.

* Created on Sat Jan 19 11:13:31 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 1 2 3 ; P G K ;
+ PARAMS: CCG=.9P CGP=3.2P CCP=1.0P RGI=2000
+ MU=12.75 EX=1.36 KG1=2970.0 KP=102.0 KVB=0.145 VCT=0.595 ; Vp_MAX=200.0 Ip_MAX=0.016 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
*$

I’m sure I will give this one a try. DC filaments on this DHT with rechargeable batteries is very tempting, or if not using the great Coleman heater regulators. I’m sure that all precautions regarding DHTs should be taken if planning to use them in a phono stage like guarding them inside the chassis (don’t worry they don’t glow at all), use shielded sockets, rubber standoffs, etc.

It’s now out there, not very frequently here in London. So I will go out and have some fun, enjoy!

Ale