All about electronic valves and hi-fi
We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.
When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO) to fudge the real power of a solid state amplifier.
Finished the second channel and tested the filament supply. The filament array which is formed by two paralleled pairs of 20W 10Ω wirewound resistors gets hot as expected. The array temperature is about 110-126°C at an ambient temperature of 24°C. The anode of the 46 gets to 49°C after 20 min of use and the heatsink stays at 42°C whereas the regulator TO-220 transistors are about 45°C. There is about 30W dissipated on each array. Yes I know, a lot of power but the filament bias is hard to beat in terms of sound in my view.
Continuing with this series of blog posts around the 6C4C push-pull design. As suggested my 45 in my previous blog entry, here is the 4P1L-4P1L version:
The first 4P1L driver stage remains unchanged, as does the output stage. The addition of the 4P1L differential pair with CCS tail and LL1660/pp IT is the main change of this design. The LL1660 is configured in ALT M or 2.25+2.25:2+2. I guess that a different IT could be used instead to get a lower output impedance on the diff pair and improve the performance in A2. The amp has more gain that I need in this configuration as it delivers its maximum power (circa 8W) when input is 1.2V peak.
So how does it performs?
Very low distortion indeed. About 0.06% up to 8W. Mainly odd harmonics dominating the sound of this amp.
Interesting findings…
Some drilling and mounting work done this morning on the 4-65a SE amp breadboard:
During the afternoon, I managed to wire a filament supply for one channel 46 driver. Tested and working ok, now can move to the next one:
In the quest of refining the 6C4C push-pull design I started not long ago, it was suggested by 45 to try a different driver. The famous E180F russian equivalent with gold grid: 6J9P-E. So I went back at my design and replaced the 4P1L driver with the 6J9P-E. Instead of filament bias, the bias was provided by a pair of red LEDs. The low dynamic impedance of these avoid the use of a bypass capacitor. Setting the operating point with the existing supply is a bit tricky but I found that Va=210V / Ia=14mA provides good swing and minimum distortion. Biasing the valve at around -3.7V keeps the valve away from grid current which will increase the distortion unnecessarily. So here is the circuit:
Back to work and starting to laying out all part of the 4-65A SE amp.
Thanks to the great feedback from 45, we found out that I made a mistake in modelling the LL1682 OT in my previous post. In a nutshell, I was getting half of the power, doh!
I should have started from scratch, looking at the push-pull curves and estimating at least the A1 power from a pair of 6C4C in push-pull. So following the B.J. Thomson method plotted the curves in push-pull:
I love the sweetness of my 45 SE amplifier, but you know what? A great push-pull (PP) amp has a fantastic presence, bass and dynamic response. Whenever I listen to a good PP amp, I get to the conclusion that I need to have different amps ready to be played depending to the music I want to listen to! My last two years have been devoted to what Morgan Jones calls in his book “single-ended madness”. And yes, my 4-65a SE in class A2 is slowly coming to life and when ready so then I will be properly mad.
6C4C amps I listened so far made a great impression both in SE and in PP. Owning all components required, I embarked on refining a full DHT push-pull design and again, cap-less (excluding the power supplies of course).
I always found my FE167E full-range driver bass reflex speakers to be lacking of the fullest treble. I clearly knew it was a compromise of this full-range driver, but after 3 years of listening to them, I decided to do an upgrade to the system. The FT96H horn tweeters are a nice match for the price range I was looking for:
After playing for some time with Dmitry’s great DHT composite triode models, I looked at refining the model by matching my own set of curves of the 4P1L in triode-mode. Here is my take on it:
