Pencil tube phono preamp

 

 

Phono preamplifier for moving magnet cartridges with pencil tubes in SRPP configuration

Used in the Attic

 

Associated equipment:                                                                                      Last updated: May 28, 2012

 

Homemade multimedia speakers (Monacor)

Homemade line-level preamp (Pass Bride of Zen)

Homemade power amp (Pass Zen Revisited)

Homemade FM tuner (based on Ambit modules)

Pioneer PL-12D turntable with Audio Technica AT66 cartridge

 

 

 

Pencil tubes have always fascinated me. They were the last thermionic devices to be developed, together with nuvistors, and were built to rigid, military specifications. In my own circle of relatives and friends, I never encountered a commercial audio product using subminiature valves, but I have always wondered what a preamp with such devices would sound like. Now I know. A pencil tube phono preamp has been added to the audio chain in my attic.

In the winter of 2001 I bought some 6021W pencil tubes which were on sale at Antique Electronic Supply for $ 0.50. I was tempted to use these for a RIAA phono preamp after I spotted several turntables on a local flea market. I bought a Pioneer PL-12D with Audio Technica AT-66 cartridge for 4 US dollars which appeared to be in mint condition. However, at home I discovered that it had three problems: (i) the belt  was loose; (ii) the bearing of the platter was not OK as the platter couldn't move, and (iii) the needle of the cartridge was worn out. Fortunately, (i) I could buy a new belt in a local store for another 4 US dollars; (ii) the platter could be moved again after I had applied a generous amount of oil and waited a few days (I subsequently removed the excess of oil); and (iii) a student at the Delft University of Technology which has its own internet store sold me a new Audio Technica needle for only 10 US dollars including postage (he boasts that he is the cheapest source of replacement needles on the WWW, which may be true).

So, for just 18 dollars I have a working turntable in my attic. Definitely not high-end, but above average. The sound is nice, but in heavily modulated passages it becomes somewhat congested. Rumble, wow and flutter are inaudible. Not bad for 18 dollars! The cartridge is the weakest part. In the late 1960s and early 1970s it was a very common audio component, costing about 30 dollars at that time, and it was considered a “best-buy with” an excellent price/performance ratio.

John Broskie's Tubecad program indicated that a SRPP gain block (symmetrical totem pole amplifier) would show a gain of 28. I supposed that two of such gain blocks combined with a passive RIAA network would make a decent phono preamplifier.

Using strips of unetched PC board glued to small pieces of Pertinax (5 x 4 cm) for ugly-style wiring, I made four gain blocks which showed the following specifications:

B+ (Volts)

310

310

310

312

i (mA)

4.3

4.6

4.3

4.1

Gain (x)

27.1

25.0

25.0

26.7



Stability and square wave response of the gain blocks is excellent, even a 100 kHz square wave is well reproduced!

I used a RIAA network as published by J.P.Güls in the German edition of Elektor. The circuit of my preamp is shown below:


The two prototype channels measured as follows (gain values in dB unless otherwise indicated):

Frequency (Hz)

Channel 1

Channel 2

RIAA/IEC

Curve

deviation

Curve

deviation

20

16.8

16.8

16.3

+0.5

+0.5

50

16.1

16.2

16.3

-0.2

-0.1

100

12.8

12.8

12.9

-0.1

-0.1

200

8.0

8.2

8.2

-0.2

-0.0

500

2.7

2.7

2.6

+0.1

+0.1

1000

0

0

0

0

0

2000

-3.1

-2.9

-2.6

-0.5

-0.3

5000

-9.2

-8.9

-8.2

-1.0

-0.7

10000

-15.0

-14.4

-13.7

-1.3

-0.7

20000

-20.9

-20.6

-19.6

-1.3

-1.0

Gain at 1kHz (x)

51

53

 

 

 



As you can see, both channels are virtually identical. Although there is slight roll-off at HF, the RIAA-corrected frequency response is +0.1, -1.3 dB from 50 Hz to 20 kHz.

Subjectively, the breadboarded preamp sounds quite good. The reproduction of dynamics (from pianissimo to fortissimo) seems better than a JFET/bipolar transistor preamp which I used for a comparison; imaging is also very good. It is great fun listening to all my old records (I have a collection of about 800 LPs and inherited another 500 from my dad). The sound is lively, involving and threedimensional, whereas on most cheap turntables it is twodimensional at best.

Even in the breadboard stage without any shielding, the pencil tube preamp is VERY quiet. I used DC heating and the minus pole of the heater power supply is connected to the signal ground. With one ear very close to a speaker and the volume turned up, a slight buzz is heard. Noise is completely inaudible.

Power supply

 

Several people have asked me which power supply I use for the pencil tube phono preamp. Here is the answer:

1. I use a transformer cannibalized from an old Dual record player. The transformer has two secondaries: 230 V / 100 mA, and 6.3 V / 2 A.
2. The heater power supply was copied from: Denzil Danner, "A Low Dropout Voltage Regulator", Glass Audio 4(4):14-16, 1992. Denzil’s ingenious circuit can produce +6.3V, 1.2A from the 6.3 VAC secondary of the Dual transformer. If you can’t find Glass Audio in your local library, drop me a line and I will send you a scan by e-mail. Mr.Danner has also presented the layout for a PCB.
3. Plate voltage (280-290V DC) is supplied by a very simple circuit which Richard J.Murdey has posted here . Hum is very well suppressed.

Happy listening!


Questions, comments: aren.van.waarde@hetnet.nl

Tip: Use only my initials if you want an answer to your e-mail

 

Post Script

Although this is not indicated in the schematic shown above, it may be a good idea to wire grid stoppers (resistors of a few hundred Ohms) close to the signal grids of V1b and V2b (particularly V1b).

The breadboarded gain blocks were quite stable but stability may be compromised in real life. Grid stoppers will suppress oscillations at VHF frequencies.

 

After I had built my pencil tube preamp, I found out that many other DIYers have used the 6021 tube. Here are some examples:

All American 5 Submini Tube Radio: WA2ISE in the United States.

Balanced Line Stage: John Broskie also in the US.

Balanced Phono Stage: John Broskie once more.

Full-function preamplifier: Alexander Kriegel in Germany.

Guitar amplifier: Marc Lavelle in the United States.

 

There used to be more circuits with the 6021 on the web, but unfortunately these have disappeared.

(I can send you scans in case you are interested)

 

A Parafeed Line Stage was published in Valve Magazine by John Berlin.

And an elaborate Subminiature Preamp was described by Manfred Huber.

 

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