Upgrading 801A x 3 PSE amp (2011)
801Ax3 parallel 15W amp
I have been enjoying 4 sets of home-made monaural single ended amplifiers using DHT transmitting
tubes (811A, 100TH, 838 and 801Ax3) and I have presently no desire or plan to build new one at all. In the mean
time, I have attempted further touchup to those amps from time to time for enhancing sound quality under current
So far, I have made a series of renovations to the 811A SE amp and the100TH SE amp from scratch.
This time, I executed another renewal to the 801A PSE amp limited in the area of EL-circuitry, without metal-work.
A unique idea of employing a twin triode 6AS7 as for the 2 set of series regulator; one unit acting regulator for the output power stage, while the other unit is for the voltage amplifier stage independently. It was due to the fact that I have got a favorable result by adding a choke exclusively for reinforcement of decupling circuit for the voltage amplifier stage of 811A and 100TH amps.
Replacement of tube from 12BH7A (CF + resistor load) to the computer twin triode 7119(CF + CCS) to expand driving ability and lower the output impedance. The reason is to review the drive stage and concluded to employ cathode follower with constant current sink (CF + CCS) rather than 12BH7A (CF + resistor load). 7119 is the charming tube at a) GM with 15,000 (CF output impedance is of 66 ohm), and b) Amplification factor: 24, and c) Anode dissipation 4.5W.
The Output transformer is MAGNEQUEST FS-007 specially designed for the 807 SE amp use, and its primary impedance is of 6,000 ohms. With its large core volume and 50W of output power, it is hard to understand the specification regarding its maximum current be up to 60mA. Therefore I opted to increase the plate current to 70mA and have used it for while. Now I returned to the original amount 60mA, then the output power downed to 15W from 17W. RCA's data sheet indicates that the plate resistance of an 801A is suggested between 4.3 K ohms to 5.0 K ohms, and its composite resistance of 3 tubes in parallel connection would be 1.4K to 1.6K ohms mathematically. And divided by the load resistance of output transformer of 6K ohms, rerated rate is 4.4 ~ 3.1 times. This rate is adequate.
Summary and overall Impressions;
In general, this renovated 801A PSE amp sounds fairly good, however I cannot tell black and white difference between the old and new one in actual listening. As the measurements shows that frequency response became deteriorated by 0.5dB down at 10,000Hz and -3.8dB down at 30,000 Hz from the 0dB at 1,000 Hz. It is assumed that the Miller effect appeared is to be attributed by the parallel connection of three tubes. Anyhow I do not sense that there is a shortage in higher frequency energy.
Though a sort of solution to compensate would be feasible by adding a small amount of global negative feedback, however I do want to stay with no NFB for this amp.
I would say that modified 801A PSE amp has a sort of feeling like "easy" and "mellow" taste once compared with my 811A SE amp.
Noise: 0.1mV(JIS-A scale)
Total Harmonic Distortion graph
Frequency response graph(0dB=1.25W)