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

6H30 preamp and mini F4 integrated

All started after I built  and listened to the First Watt F4 and liked it very much. The original F4 has 6 power mosfets per channel, draws about 160W and weights about 35 lbs, delivering 25 current buffered W per channel at 8ohms (2.58Ax2.58Ax8 ohms/2= 26.6 W). Then I built the Mark Audio 7.3A speakers and thought of pairing them with an amp that will deliver enough power to listen up to 90 dB levels. From my previous experience with the 4P1L – F4 integrated, I learned that a 16-20 dB gain it is plenty to drive the F4. The output impedance of the 6H30’s is even lower than the 4P1L’s, so no issues of driving capability nor gain. Actually I needed a little more gain as my small FLAC player has a lower output compared to a CD player. I chose 6H30 for multiple reasons: very linear, low Rplate, good gain and overall low PSU demands. PSU uses Antek 160V/50VA main transformer, FRED rectifiers, choke 157G and a 555 based 30 seconds HV time delay circuit. A hand made PCB holds the noval socket, the RC cathode bias, the grid resistors and the coupling caps. Separate PCBs are used for each anode load DN2540 based cascode CCS. 6H30 runs at about 125V/20mA.

I used a standard aluminum enclosure from Ebay and modified the top plate to fit all the pre-amplifier components. The (so called) mini F4 has only 4 power mosfets per channel and each runs at 300mA…this provides a 1.2×1.2×8/2=5.76 W pure class A with the heatsinks at reasonable temperature. At this size of the enclosure (wxlxh 9.5″x10″x5″) everything fits tight and weights 20 lbs. I am very happy that it fits well on my working desk.

The combination of the Mark Audio MTL Alpair 7.3A (design by Bob Brines) and the Mini F4-6H30 pre integrated sounds awesome: great dynamics specific to F4, low distorsion from the 6H30 and pleasant sound from the Alpair ( lows are excellent enough considering the size of the driver, mids are well defined and highs are awesome..this driver can go up to 30kHz…very unique).

F4 6H30 preamp side CCS F4 6H30 board F4 6h30 PSU 6h30 top F4 6h30 psuF4 F4-6h30 side F4 6h30 front F4 6h30 back