After some further efforts, I managed to complete the power supply. And it works, which is always rewarding when you get it right first time:
Author: Ale Moglia
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
6Z49P-DR/6J49P-DR Pentode
A forgotten Russian pentode
There’s not much information around the 6Ж49П-ДР or 6Z49P-DR / 6J49P-DR pentode. This pentode was designed for high-frequency operation and durability. It has relatively high transconductance and high mu when is triode connected. Hence, an interesting device for a phono stage given the low capacitances in place as well. The 6Z49P is close to his brothers 6Z51P and the famous 6Z52P albeit it has different characteristics.
Flexible HT Power Supply (Part II)
Here are some picture of my slow progress on this flexible power supply. I started with the layout of the parts before any marks were made on the top plates:
Once I was happy with the layout, I submitted myself to the mercy of the most dreadful job on earth which is metalwork!
Slew Rate (Part 2)
Get thy bearings
Merry Christmas to all!
When I was still a teenager and learning saxophone I came across this wonderful version of “Get thy bearings” by King Crimson. The power of the electrified alto sax blew me out my mind. Ian McDonald clearly found a way to drive the saxophone there and bring a new sound to the 1969’s progressive rock. Surely he didn’t experience any of this slew rate thing 🙂
In my previous post, I explored the slew rate challenge of the 01a preamp:
Slew Rate in Preamps
Introduction
You may probably already asked yourself: what is this “slew rate” business? Has this guy lost his mind? Probably I have, but not due to this interesting phenomenon. The second question that should have probably popped in your brain is: why? Well, writing about this stuff came up by sheer coincidence.
We tested my friend Tony’s 01a preamp which has an older gyrator board I made for him about 3 or 4 years ago. The preamp (as well as his system) sounds extremely good in my opinion and the 01a has brought a new clarity which is what you’d expect as a result of the introduction of a DHT stage. We decided to run some frequency response tests since Tony has made some interesting mods to his Push-Pull amp. We encountered an unexpected challenge as I didn’t have the right XLR connectors for my testing gear so we run the FR tests on the power amp with the 01a.
The 01a preamp measured as expected with a flat response up until 40kHz before the sound card rolloff kicked in – I’m aware of this and this is the limited bandwidth of my current portable measurement gear. The distortion was also really low (H2 predominantly ) with less than 0.02% for 2 Vrms output signal. This test was done with Pete Millett’s interface which has an input impedance of 100kΩ: Continue reading “Slew Rate in Preamps”
WE 16a Horns!
16a horn
hi-fi speaker cables 🙂
If my wife thinks I’m insane, she should have come to Peter’s place to see this marvellous Western Electric 16a horn replica made by this man. A beautiful horn indeed. The sound is quite particular and has very attractive sound image which is closer to a mono than stereo and you need to get used to it. It’s worth listening to. Peter has an three-way active crossover. The horns are driven by a 6B4G amp. We listened to several tracks and enjoyed listening to it during the brief session before a fantastic Curry we had last night.
Gyrator PCB prototype
First sets of PCB arrived today. Thanks to Tom Browne for the great PCB work, and it worked first time round. The PCB can accommodate different FETs like 2SK170, BF862 and J310 to suit most of the requirements
If you’re interested in the final PCB let me know, production run shortly due to large number of request for this circuit for DHT preamps and drivers
Ale
Goodbye 814SE Amp, it’s been a fantastic journey
After 2 years of listening pleasure, it’s time to retire the 814SE monster amp. With its breadboard nature and high voltages around, this is not safe to have around with my daughter now being 8 months old. Sad to see this amp go and I hope to listen to it some time in the future.
Filament bias and winter heating system
814SE Amp
814 output valve
814SE Amp and Lenco Turntable
+HT supply
6D22S rectifiers
HT power supplies
What is was really painful for my back was to lift this ton of iron up to the attic. It’s so heavy and not keen to move it for some time. This amp has been a fantastic learning experience for me, as well as a huge challenge. High voltages, A2 operation, DC-coupling and the mix of sand with plenty of iron here in a 100% DHT amp was the right mix to give me several headaches along the build and testing stages. However, it’s a beauty to listen to, in particular thanks to the 46 and 814 valves which are unique in my view for this circuit and also the optimal tuning of the operating points and the iron used.
Enough for now, however I’m still working on my safer 4P1L PSE amp, so watch this space 🙂
Gyrator bias discussion
A very interesting point was raised on the 4P1L DIYAudio thread around the gyrator circuit using CCS and whether a simple resistor divider was better than the CCS due to the LND150 temperature drift.
I’ve tried both options and I’d say I prefer the CCS despite the variation with temperature for the following reasons (which people may well disagree):
- It’s true the LND150 varies a lot with temperature (see attached), however if it’s operated at low current (e.g.<500μA) the variation is small. In a cascoded pair for this circuit the drift in the output voltage is small. Simulated in Spice I get about 6.35mV/°C. The resistor divider will be better of course but you need a smaller values to reduce impact of dR/dT. This creates another problem which is the reduced PSR. With a compromise divider to balance idle current and PSR you can get 5 times less variation with temperature in the circuit under discussion – see below (e.g. 1.4mV/°C)
- For a smaller value of resistor divider the PSR is impacted and significantly lower than the CCS. If you don’t have a well filtered supply, the PSR benefits of the gyrator will be reduced due to this. For example, I did some quick comparisons by simulating my 01a preamp. I used a 235KΩ/220KΩ and a 23K5Ω/22KΩ divider options with a typical film decoupling cap of 4.7μF.
The PSR of the CCS is above 100dB whilst the PSR of the resistor divider goes from 56dB (235KΩ/220KΩ divider) down to 37dB (23K5Ω/22KΩ divider).
In practice, I implemented two different circuits as I had a shunt regulator before when I had a resistor divider and now I don’t have any shunt regulator but I use the CCS version.
Looking at the output PSR as the gyrator provides additional rejection to noise. The resistor divider PSR is about 73dB and CCS is 30dB better anyway
The voltage variance is really small with temperature and this circuit in particular isn’t affected by such small drift in my view
Flexible HT supply
The return of the Jedi
Time has come when breadboarding and HT is not longer an option. With a baby around, I was forced to remove most of the valve equipment. However, the return of the Jedi is closer than you expect. With very little time, albeit at continuous pace when I get a free 30min here or there every weekend, I keep very focused on my objective. The 814 SE monster amp is close to retirement (probably before Christmas) and although I have a fantastic LME amp already working, I need to get the valves back on the scene providing I can prove I’ve complied with “heath and safety” – as my wife well put it.
Well, I had a nice attempt of a wooden frame made out of pine wood. Yes, I’m really bad at woodworking but I’m getting better. The top plates are 4mm thick. I have added a nice set of 2 100mA meters at the front of the frame. A 300VA custom-wounded multi-tap HT transformer from JMS which gives me full flexibility from 100 to 500V in 25V steps. The supply is choke input (LL1638) and filtered with a set of nice 50uF oil caps and 20H chokes. Damper valves used with some nice SiC to form a hybrid rectifier.
Top plates are protected with paper before drilling!
This looks to be quite neat in my view!
The idea is to use this supply for my new 4P1L PSE, 300B and DHT Schade experiments with 6P21S/47 and 1624 valves. A lot planned and so little time for this unfortunately.
Cheers, Ale