Published in magazine audioXpress, USA
 Autor: Dipl. Ing. (F.H.) H.Bubeck

The JJ Electronic ECC99 – the better driver tube for power triodes?

Electronics, Slowakia has introduced the double triode ECC99 especially as a driver tube for power triodes, such as the 2A3 and W300B. The ECC99 offers a high forward conductance (Gm) of 9,5mA/V, a low plate resistance (Rp) of 2,3 k Ohms and a max. power capability(W ) of 5Watts per system. This should allow to drive power triodes.(see fig.1, Datasheet values). The ECC99 fits in a Noval socket and the pin out is the same as for the 12AT7 / ECC81 family.
This article discusses the test results I obtained in comparing the ECC99 with other popular driver tubes, such as the 5687 and 7119 / E182CC. The key parameter I tested was the total harmonic distortion (THD) in relation to the anode voltage swing capability. I also tested the tubes in driving a push-pull triode power stage. Here I used the VV30 from AVVT, a tube which is similar to the W300 B. (see picture)
                        Fig.1                       Gm         Rp           v              at Ia
                        Datasheet             mA/V     k Ohm                         mA

                        ECC99                     9,5        2,3           22               18

                        5687                        8,25      2,1           17               21

                        7119/E182CC           15         1,6           24               36
 

 

Test circuit and bias

To make testing simple, I designed a test board consisting of a preamplifier and phase splitter stage (12AX7 / ECC83), the driver stage in the standard amplifier configuration and the power stage in push-pull configuration.. (see fig. 2). Obviously, a driver circuit in a cathode follower configuration may finally give better results in driving power triodes, however I just wanted to make a comparison of the tubes and not to optimize the whole amplifier.
With an 360V supply, I choose the plate resistor with 15 k Ohm and set the plate current at 12 mA per system. Consequently the plate voltage was 180V. This setup should allow for a sufficient plate voltage swing. To get these conditions for all tested tubes, the appropriate grid voltage was generated with a common cathode resistor of 220 Ohms for the ECC99, 330 Ohms for the 5687 and 250 Ohms for the 7119/E182CC.
I tested 2 tubes per part-number: TheWA5687 came from Sylvania(Syl) and Tung-Sol(Tung). The 7119/E182CC from Valvo(V) and Philips(P).

Results

In comparing the basic parameters given in figure 1 it is clear, that all three triodes are indeed very similar. The high Gm and low Rp of the 7119/E182CC is stated at a fairly high plate current and will be closer to the other tubes at an equal plate current level.
According to Fig.3, the 7119/E182CC had the highest amplification of 19 as to be expected from fig. 1. The ECC99 was very close however, while the 5687 was noticeable lower at around 14. Matching of all tube systems were better than 2%, except of the Philips 7119/E182CC.

Fig. 3
Amplification (v)  at Ia = 12mA, Ra = 15k

Tube                v 1            v 2         delta (%)
ECC99-A         17,35         17,5            0,8
ECC99-B          17,8          18,1            1,7

E182CC-V         19            18,65          1,8
E182CC-P         19,5         19,0            2,5

5687-Syl            14,5         14,7           1,4
5687-Tung          13,9         13,7           1,4

THD, driver stage

THD was measured by connecting the AC signal source (1 kHz) directly to the grids and the THD was obtained with a distortion meter at the plates. The graph (Fig. 4) shows the THD (%) in relation to the amplified peak to peak AC plate voltage. At the given bias, both of the ECC99 came in with the lowest THD, followed by the Sylvania 5687, than the Tung-Sol 5687. Both 7119 / E182CC had the highest THD. This first test indicates that the ECC99 should be a good driver tube for power triodes.

 


THD of the amplifier with AVVT power triodes VV30 in push-pull configuration
 

 

The final comparisons were made with a matched pair of AVVT’s VV30 power triodes. I biased those at Ia = 50mA with a Grid Voltage of – 80V. At 10 Watts into 4 Ohms, no neg. feedback, THD were below 1 % for all tubes except the 7119/E182CC which were slightly above. At 15Watts, THD were still below 2% for all tube configurations. With a neg. feedback of about 12dB, the THD dropped to 0,5% at 15Watts. Basically, the results from figure 4 were confirmed, with the ECC 99 offering the lowest distortion at high output levels. At lower levels (below 1 Watt), all of the tubes were very close together and differences were hardly to measure.
I also checked on the upper frequency limits and the pulse response, however there were no relevant differences between the tubes.

Conclusion
The tests showed that the ECC99 has a low total harmonic distortion at high plate output voltages coupled with high amplification. This makes this tube very suitable as the driver stage for  power triode amplifiers. Given the low cost of less than $15, the ECC99 is certainly the tube I will use for my amplifier designs.

 

Test equipment used:

Hameg Distortion Meter HM 8027

Hameg Function Generator HM 8030-5

Goldstar Oscilloscope, 2ch, 40 MHz, OS 8040D

Monacor Audio mV Meter VM 500

About the author:
Hans Bubeck,
graduated from the State College of electronic engineering and communications at Esslingen/Germany in 1967, he spent most of my professional time with Hewlett-Packard in marketing and selling electronic components in Europe.
Nowadays, he is designing hifi amplifiers and components for state of the art tube amplifiers.
He is married and have two daughters. He is a collector of historic electronic equipment and spents also his free time with fishing and playing tennis.

For further discussions on this report, please sent me an email.

 Hans Bubeck , email hans_bubeck@t-online.de