How six JAN (joined army navy) tubes, after some 40 years of rest,  finally make music in.... Aren's Attic.

I started this project with the idea to build a simple and cheap single-ended tube amplifier. I didn't have any experience with the construction of tube power amps so I was looking for a beginner's project. Such a project should be simple (for easy debugging) and it should also be cheap. Nobody likes to blow up a valve of 300 dollars by a small lapse of attention...

As far as simplicity is concerned, nothing beats the "Darling" series of amplifiers which are described on the website of the New Yorker Bob Danielak. They make use of power triodes type 1626 which can be obtained cheaply from various sources (I bought several for 3 US dollars apiece). Old readers of this page may remember that the 1626 was employed in various transmitters for the American airforce, e.g. the AN/ARC-5 series (BC-457, 458, 459 and 696). Since they were used in military airplanes, 1626s are rugged valves which can stand shocks, vibration and electrical insults. Once I had excessive power dissipation in two 1626s because of a wiring error. The plates became fiery red and blazingly hot and remained in this condition for one or two minutes before I discovered my mistake. Yet these tubes still sound and measure OK. So the Darling amplifiers make excellent projects for stupid beginners like me! As you will read below, the project started cheap but ended rather expensive. But the final result was great, so please read on.

After some soul-searching, I decided to make Jeremy Epsteins DC-coupled variant of the Darling amp with two parallelled 1626s per channel. The schematic is shown below:




Simple, isn't it? Initially, I followed Jeremy's schematic to the letter, although I used a  GZ34 rectifier in stead of a 5U4G. The Hammond 125E output transformers, 1.5 H choke plus the 8532 and 1626 tubes were from Antique Electronic Supply in Tempe, AZ. A hefty Prova power transformer supplied 2 x 345, 5 and 6.3 Volts. I acquired this power tranny and the GZ34 rectifier from Frits Meuris Electronics (Sittard, Holland). The 12.6 Volts for the 1626 heaters came from an obscure transformer from my junkbox which I bought for one Dutch guilder from Conrad Electronic.

I built the amp in the old style, hardwired on an aluminum chassis in a wooden frame with all tubes, chokes and trannies on top and all electrical wires below. Special parts were not used. I took what was in my junkbox (carbon resistors and styroflex capacitors from the former DDR,  electrolytics from Czech origin, wirewound resistors from Vitrohm in Germany). After I had corrected my wiring error, I ran some tests. All voltages were within a few percent of Jeremy's values and square waves looked OK. No trace of microphonics. So on for the listening tests!

After a day of burn-in, I listened to some live recordings from twentieth-century classical music (Schönberg Ensemble), using the cheap speakers which are described elsewhere on this website. The amp sounded wonderful, as if you were in a very good seat in the concert hall. Marvelous! I was really excited about the "SE magic". But after a while I became disappointed. The amp sounded excellent with some programme material (chamber music, small combos), but it didn't sound good at all with other types of music (large orchestra, grand piano, church organ, big drums). Then, the presentation seemed "anaemic", "whitened" or "bleached". Some piano recordings sounded terrible. A few measurements indicated what was wrong:



As you can see, the frequency response of this version of the amplifier was 75Hz...14 kHz (-3 dB). With orchestral music and grand piano, I was missing the lower two octaves. For Jeremy this didn't cause problems, since he employs his own Darling as the treble amp in a bi-amped system. But as a stand-alone amp, the results were not satisfactory.

Jeremy thought the cheap Hammond transformers were limiting the bandwidth. So I decided to try more expensive output trannies. I bought two Lundahl LL1664s from Aqua Blue in Belgium and replaced the Hammonds with these Swedish chunks of iron. The (almost fivefold) difference in price certainly resulted in improved specs (see below).



With the Lundahls, the frequency response was flat from 8 Hz...55 kHz (-3 dB). This was a major improvement! Grand piano now had weight and sounded like the real thing. Sibilants were somewhat emphasized in the Hammond 125E version, but sounded natural with the Lundahl LL1664.

So I was very happy for a while. But the bug of perfectionism hit me once again. IMHO, the amp still had a major flaw. There was a large amount of hum in the output. With the cheap speakers, the hum was not objectionable during the reproduction of music. But in silent passages, it could clearly be heard. And on high-efficiency speakers like the Jericho horns, it would be terrible. Therefore I performed additional measurements. It turned out that the signal-to-hum ratio was only -55 dB with shorted inputs!

At first I thought that I had made a grounding error, but this proved not to be the case. Then I thought: maybe I should use DC heating. However, when I hooked up the 8532 heaters to a DC power supply, the signal-to-noise ratio remained identical. I also found out that the hum had a frequency of 100 Hz rather than 50 Hz. So I concluded that it originated from the B+ supply.

After running some simulations on Ben Duncans PSU designer program, I decided to modify the B+ circuit. After the GZ34, I maintained the 50 uF electrolytic capacitor.  Both channels are hooked to this cap via individual LC filters (10 H chokes and 200 µF/500V electrolytics). This modification resulted in a 29 dB improvement of the signal-to-noise ratio. Although the load to the GZ34 seems severe, it survives. And hum and noise are now completely inaudible.

Specifications of the final incarnation:

Output power: 2 x 1.5 Watts in 8 Ohms
Voltage gain: 8.6 x
Input sensitivity: 400 mV
Frequency response: 8 Hz...55 kHz (-3 dB)
Signal-to-noise ratio (guesstimated): -84 dB (related to 1.5 W output)

This is the best amp on this website. It sounds VERY lively, agile, and natural. Makes you tap your feet and sing along with the music.

for info from aquablue email at : sales@diyparadiso.com