Phono Preamplifier with 3a5 Directly Heated Triode

Author: Dmitry Nizhegorodov ( My other projects and articles

1.   Intro

3A5 is a small, 7-pin double directly heated triode (DHT) tube with medium gain and filaments suitable for battery operation as a radio receiver tube. Its Gm=1.8 is far from impressive but a DHT enthusiast can find its Ra=8.3k, Mu=15 more than adequate for various audio amplification projects. The standard datasheets for 3a5 [1] emphasize its battery operation and show plots intended for operation with positive grid which hides the fact that the tube is extremely linear in class A [2]. Note: Manfred Huber's site is down and here I show 3a5's plate curves from his site. My own measurements will be posted soon.

2.   Design

For 3a5 phono preamp a 3-stage design with split-load RIAA and no feedback was chosen:


This is a moving-magnet cartridge preamp. The amplifier provides gain of 42.1dB at 1kHz.

The first stage is grounded-cathode amplifier with RIAA correction network fed with the signal from the plate. This

network is RC low-pass filter with frequency of 2122Hz (75us). It also provides shelving at some high frequency - 47kHz - to compensate for loss of emphasize at ultra-high-frequency [3]. The second stage is grounded-cathode, with the second RIAA-correction network, taking care of the 500.5Hz (31.8uS) corner and the 50.05Hz (3180uS) corner.

The output impedance of a feeding stage is a part of each RC network. The resistors R3 and R8 need therefore be adjusted for the most linear response curve. When these are dialed correctly it is possible to achieve an extremely small RIAA response error, 0.05dB or less, see the plot on the right. The insert on this picture shows the inverse RIAA network used to fine-tune the design. The output of the network feeds the input of the preamp. See [3] for detailed description of this inverse network.

The fact that the first network takes care of the high frequency roll-off while the second one takes care of the 50/500Hz shelving - and not vise versa - arguably improves signal/noise ratio.

Rambler filtering below 10Hz is achieved by means of 3 interstage capacitors.

The signal/noise performance of the first stage is further improved by parallel connection of both triodes of the first 3a5 tube.

The most unique feature of this design is biasing of the tubes with the string of two 1.4V voltage supplies. These can be filtered or regulated DC supplies or batteries. Since 3a5 was orginally designed for dry cell operation and as the result can operate with starved filaments, it is very temptive to consider rotating banks of NiCd or NiMh batteries with each bank lasting for several hours while the other charges. With filament consumption of 0.2A per tube (two triodes), the stereo amp can run on 8 AA NiMh batteries for several hours, enough time for another 8 AA batteries to recharge.

This idea was inspired by the way filaments connected in old battery radios that use DHTs. Note that there is zero crosstalk between the stages because there is no signal across the batteries due to low impedance of them and the filaments.

Biasing of the stages is chosen to optimize each stage's performance. For the first stage the biasing is very low. In fact one side of the triode surface is not biased at all. This maximises Gm while reducing noise. The other two stages are deeper biased (one side is 1.4V and another side is 2.8V) since the first stage's filament DC is in the chain. This stack of DC supplies gives just enough headroom for the second and the third stage.

Overall, this design provides very low distortion and very good overloading performance. The plot on the right shows that the amp delivers less than 0.1% of distortion, all practically being 2nd harmonic in nature, at 1KHz at max rated input level which corresponds to 0.6-0.7VRMS out. With + 20dB of overload the distortion is still under 0.6%, most of it is the 2nd harmonic. The plot draws distortion in % and output voltage in in Volts, and the harmonics: 2nd (red), 3rd (blue) and so on. The insert is exactly the same data but drawn in logarithmic scale.

The output impedance is somewhat high and one need be careful with the load. The 100k resistor shown here as the ouside load is actually a volume pot of a DHT tube amplifier. To drive wide range of loads including SS amps a triode CF buffer can be used to compliment this design.

3.   SPICE model

A 4-pin SPICE model for 3a5 used in this design is:

** 3A5 Dual Composite
* Created on Fri Mar 12 18:53:28 PST 2004 using tube.model.finder.PaintKIT
* tube data:
.SUBCKT TRIODE_3A5_COMPOSITE      1 2 3 4 ; P G K1 K2
+ PARAMS: CCG=.9P  CGP=3.2P CCP=1.0P RGI=2000  RFILQ=1.6
+ MU=15.7 EX=1.4 KG1=2130 KP=94.5 KVB=24.0 VCT=-0.52 ; Vp_MAX=150.0 Ip_MAX=0.0080 Vg_step=1.0
RFIL_MID1  31  35 {RFILQ}
RFIL_MID2  35  41 {RFILQ}
E11 32 0  VALUE={V(1,31)/KP*LOG(1+EXP(KP*(1/MU+V(2,31)/SQRT(KVB+V(1,31)*V(1,31)))))} 
E12 42 0  VALUE={V(1,41)/KP*LOG(1+EXP(KP*(1/MU+V(2,41)/SQRT(KVB+V(1,41)*V(1,41)))))} 
RE11 32 0 2G 
RE12 42 0 2G 
G11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))/(2*KG1)} 
G12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))/(2*KG1)} 
RCP1 1 3 2G 
RCP2 1 4 2G 
C2 2 1  {CGP} ; GRID=PLATE 

You can put this model in any *.lib file and use suitable 4-pin symbol for it. Otherwise, download my library of DHT models, see the last sections of [4] for details. Shortcut to the zip file containing the library is here: ./

4.   Construction

The tubes and the circuit of a tube preamp must be properly shielded. It is not a very good idea to expose the glowing tubes to the world; it's best to reserve this to power amps which have completely different (very modest) requirements for noise. In any design, quality ceramic or Teflon/PTFE tube sockets with shields must be used. Proper, instrumental-style grounding is a must, too. The input sockets should be used as hubs for star-grounding. The B+ power supply is best to be regulated otherwise very heavy filtering is required. The ideal filament supply is a pack of rechargeable batteries with relay-controlled flip.

5.   References


[2] Manfred Huber DHT plots for 3a5, [alas, his web site is gone]


[4] ./composite-triode-models.htm

Author: Dmitry Nizhegorodov ( My other projects and articles