Ok, title isn’t very appealing. I know, however it truly reflects what I experienced over the past few days. Let me clarify before I start: a gassy tube developed a nice fault-finding journey. You always learn a lot from all this, for sure. Luckily, due to the holiday season, I had the time to work on this. I’d rather have spent it otherwise, but my 300B amplifier was dead.
6BY5-GA damper
I had a gassy 6BY5-GA damper rectifier tube. I should have changed it, but I was lazy and I paid the price for it. When turned on the 300B HT power supply, it will flash blue for a sec or two and the gas ionisation will disappear after a minute or two when tube was warm enough and operated as expected. Problem was that my stash of these damper tubes was up in the storage, who knows where.
Firstly, my apologies to all readers for not writing much over the last year. Things have gone a bit crazy between work and family commitments. Also, modular synthesiser work has taken a lot of interest lately to me. Finally, this blog was hacked twice and have been painful to get it back up again. Needless to say, I’m not an IT expert so I do the best I can!
Selling my beloved 4P1L
All good things come to an end. In this case, my loved 4P1L PSE Amplifier. I moved listening rooms with the growing family and space is now an issue. I no longer can store this beauty and I think it would be wise for someone else to enjoy it rather than collecting dust somewhere my attic (which is mostly full).
This amplifier gave me a lot of joy and sums up a “no-compromise” approach to DHT considering the design and component selection of the power supplies. It’s a very flexible beast as the power supplies (HT and LT) can be used in many different configurations – all the way up to a 300B and more.
For the ones who remember my 814 amplifier, I reused the amazing power supplies which were feeding the 46 driver stage. The DHT filament raw supply is similar to these:
One PS per channel (2 boxes on the right in the picture above)
It’s a no-compromise design here. Best custom-made JMS transformers designed to fit this amp supplies with split-bobbins and copper screening.
The HT supply is my design for 300B and 4P1L PSE. It has a lot of capacity which can be reused and is very flexible with its design. It has a surge protection start up circuit and two current meter (one per channel).
I will dig out the circuit and share all documentation for whoever buys this amplifier.
The amp use MM OPTs, 4mm plate machined by Schaefer. Filament bias and circuit as per own my design.
All cables provided.
4 valves selected and matched to gm and Ia by me with eTracer
You will need a preamplifier / driver – which isn’t included. Anything with gain of x8 will work (preferably a DHT stage like 01a, 26,4P1L, 2P29L, etc.) – this isn’t included.
This is “the one and only” Bartola 4P1L amplifier. It has gone back and forth to ETF.18 and got into quarter finals on the competition (personally and a few others think it could have gone further but listening levels and setup were not the best).
Needless to say:
No reserves
Payment upfront
Only collection in London, UK. No deliveries
Sold as-is. You need to know what you’re doing here 🙂
I haven’t got idea of what price to ask, need to think about it.
Brief post of a very long experiment. Unfortunately I’d have to dig out a long list of notes, tests and simulations to get the full thing out there. Unfortunately due to constraints, this will be brief but I hope it’d be entertaining.
Here is a very nice build from Dan Kercher on his contraption of the 300B amplifier with auto bias. The driver is an EF86 with SiC diode biasing, the topology is a hybrid mu-follower and a source follower drives the output stage. Filaments are DC with Rod Coleman’s board:
Dan Kercher’s EF86 – 300B XLS SE Amplifier build
More details can be found below:
Hi Ale,
I wanted to thank you for all of your PCBs. I just finished a project that uses your Gyrators, SiC bias boards, and SiC Source Followers. I'm so happy with the results. Details posted on Audio Asylum.
https://www.audioasylum.com/forums/tubediy/messages/27/276814.html
Cheers,
Dan
After a few exchanges with different people, it was time to make a few tweaks to the EF37a driver. Increasing the G3 voltage helps in squaring up the pentode curves as shown by few before. By increasing G3 to +12V or up to 20-25V can be beneficial in the long voltage swing of the pentode. There are a couple of interesting threads in DIYaudio to check out if you’re interested.
I should have fired up the eTracer and play with G3 biasing before modding the driver, but couldn’t help myself from doing this mod as it was very simple. The Screen voltage circuit I use has 2 identical versions of it in the PCB as it was designed for a stereo setup in mind. As I reused a PCB I built for other tests, I had a readily spare screen voltage regulator which could be easily tweaked to supply G3 at the levels needed: Continue reading “Mono Amp: EF37a driver (part III)”
I have plenty of these German WWII pentodes. I’ve been saving them for a few projects. Quite likely for a push-pull amp given the OB speaker project I’m cooking. Either way, I have a nice set of CFB SE as well as PP OPTs from Toroidy custom made. They are very good transformers.
So, if you want a simple SE amplifier that can do 8W, here is a good contraption to go after:
It was about time to get my hands on this driver experiment. I’ve been trying to find the time for a while and could only make it due to the obliged COVID-19 isolation upon return from holidays.
The idea is simple. I wanted to use a pentode driver to swing large volts (e.g. 200Vpp) whilst retaining the triode-like characteristic from harmonic perspective and low distortion. A nice challenge and fun to work on.
Have to say that the parallel/parallel feedback (also referred to as “Schade feedback” by some in audio) when applied locally in the output stage, does sound very nice and is a very nice way of implementing high-gm pentodes used for vertical service in TV. They can produce very low distortion and sound amazing when implemented correctly. I’m not covering this now, as it has been dwelled on for some time by many good people out there.
I’ve been enjoying and carefully listening my new 300B amplifier. I have to say that I love every bit of its sound, treble detail and strong bass. The amplifier is fast and can drive very well my speakers. I only discovered that due to my low level DAC, the gain of the D3a in triode is yet not enough to get it to maximum power. So, I hooked in my beloved 01a preamp. The overall gain is too much of course so had to place the volume control at the output of the 01a stage.
I think a gain of about 130-140 should be ok. Perhaps if I get around in adding the 6SF5 stage then it may be good enough.
So this got me thinking. Of course I have on my list 2 driver tests:
Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.
More than 8 years ago, I played with this valve after Thomas Mayer wrote about it here. It seemed like a good option but despite a few bench tests, I never got around building an amplifier with it.
I’m preparing for ETF.19 and will be bringing my eTracer to the event so I built a few socket adaptors. It was the time for the 12-pin Compactron to come to life.
Through my few Compactron valves, I bumped into the 6HS5 again, so I decided to put it to the mercy of the tracer. I wanted to see the positive grid current behaviour, as haven’t seen curves around with it:
6SH5 triode with positive grid bias.
What can we say of the above curves? Well this triode could do 7-8W easily as Thomas says. It has an anode power dissipation of 30-35W. The anode slope is between 7 to 10K so local feedback will be needed. You will need to aim for 600-700V bias at least, probably higher with slightly negative bias. However, you will need a hefty source follower to drive the grid. Look at the grid current. It can be 50mA at 5V and creeps up the higher the grid voltage it goes.
Interesting valve with a 6.3V/1.5A heater, and the high-mu (80-100) can work out as a SPUD amplifier with local feedback. Driver should be able to handle the grid current and low impedance. Not an easy task though.
