18dB Gain CX-301A preamplifier to feed the First Watt F4

This is my implementation inspired by Ale Moglia’s 01A preamp Gen2 (http://www.bartola.co.uk/valves/dht-pre-amplifier/01a-preamp-gen2/). The plate gyrator is Ale’s design and it works like a charm. I want to thank Ale for his great dedication to bring this gyrator to high performance levels. I am using a Dynamicap 0.22uF in place of C1 and a 0.47uF Mundorf Supreme for C2. I also tried the FT3 0.22uF and like it better than the Supreme…I made the boards for smaller caps and couldn’t fit it in there.

Here is the schematics I used:


I chose the filament bias resistor of 30 ohms in order to raise the headroom to 8 dB so the preamp can be used from a standard 2Vrms source. I am using the extremely good, latest version  of Coleman regulators adjusted to 0.25A. The reason I am not using starved filaments is first to lower the output impedance and second that I could not notice a big improvement in microphony from 250mA to starved at 200mA.

The power supply is overdesigned by added SSHV just in case I want to add a line output transformer and make it switchable from Gyrator mode ( 18dB gain and about 1K output Z) to LL2745/PP 5.6:1 mode ( 3dB gain and lower Zout about 400 ohms).

I designed the layout as always  preferred ( when there is no issues with complete screened enclosure) : one top plate ( Aluminum 1/8″) that has all the components attached to, and one wooden enclosure. In this case there are no heat management issues so I had to drill less holes on the top plate. Usually I like to use Front Panel Express to order the top plate ( for a more professional look and easy to assembly)..this time I just draw it in CAD and printed out to use for exact drilling locations. The top plate is standard 12″x12″( McMasterCarr)  and consequently the wooden case is 13″x13″x3.5″.

The 01A sockets are attached to the top plate using neoprene vibration damping sandwich mount ( type 9241K41 at McMasterCarr).

CX-301A preamp measurements March 14th, 2015

I am happy with the results of today’s measurements. The tables below show results for THD and FR.

Channel Input voltage 1kHz (Vrms) Output voltage (Vrms) THD(%)
Right 0.5 4.1 0.015
1 8.4 0.033
1.93 16.1 0.064
Left 0.5 4.1 0.008
1.04 8.8 0.016
1.93 16.6 0.034
Channel Frequency (Hz) Level (dB V/V) Roll off (dB)
Right 8.8 16.45 0.53
20 16.05 0.13
1,000 15.92 0
20,000 16.3 0.38
30,000 16.45 0.53
Left 8.8 14.4 0.55
20 13.98 0.13
1,000 13.85 0
20,000 14.26 0.41
30,000 14.51 0.66


I can conclude that the preamp has a maximum gain of about 8.5 for a THD of 0.033% where the second harmonic is dominant at about 70dB below the fundamental and more than 20db above the following harmonics. These results are expected and the distribution of the harmonics shows that the sound of this preamp should be extremely pleasant. Indeed the sound of the CX-301A has some magical touch specific DHT’s and even beyond expectations.

I am using it right now to supply to a mini F4 recently built. The reason I have built this preamp is because of the F4. It has no problem driving the F4. It also can be used to feed a low gain power amp if the input capacitance is in the lower hundreds of pF.

I am extremely impressed with the sound of 301A and definitely this will be a keeper. Again some pictures:

Some early development:

close up 01A Early breadboard test with Gyrator Ales designbreadboard2 breadboard1

Layout design: designstage

overview inside inside sockets inside psu2 inside psu inside 1 top no tubes 2 top no tubes top 2 straight top front case bottom case

2 thoughts on “18dB Gain CX-301A preamplifier to feed the First Watt F4

  1. Radu: great job, it looks really nice. I’m glad you enjoy the 01a. It’s a superb DHT and it sounds is unique.
    Cheers, Ale

  2. Thanks Ale,
    It is indeed a great valve. I measured the Zout and it gives me a 400 ohm value which is not supposed to be the right value. I used a high value load (1M) and low value load (47K) Vrms measurements and all looked correct. Did you measure yours?

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