Hi Ale
That's a nice design you linked to. I like the way you load the source follower. 😉
Why not attach C1 to the other side of the 10M resistor? Then you could probably get away with a smaller value I think....
Ian
Hi Ian,
That is a diagram typo! some have pointed it out before but I never got around updating the diagram!
I generally use 10Meg/100nF pair in the setup. I prefer keeping C1 small enough.
BTW: loading the source follower in DC is very useful for high ra DHTs (there are a few good ones out there). In particular if you use MOSFETs with low Crss and high Gfs to improve bootstrapping of the Cin. Frequency response is improved a lot that way and you get a very nice sounding stage 🙂
Cheers
Ale
That is a diagram typo! some have pointed it out before but I never got around updating the diagram!
I generally use 10Meg/100nF pair in the setup. I prefer keeping C1 small enough.
BTW: loading the source follower in DC is very useful for high ra DHTs (there are a few good ones out there). In particular if you use MOSFETs with low Crss and high Gfs to improve bootstrapping of the Cin. Frequency response is improved a lot that way and you get a very nice sounding stage 🙂
Cheers
Ale
Hi Rajko
Here are my preferred ones (so far):
C3M0280090D
IXFK80N60P3
FDPF5N50NZ
IXTP08N100D
AOT1N60
On all of them I look out for the same: low Crss and as high Gfs as possible. Still run them over 10 to 20mA and leave a headroom of 50V at least to make sure you minimise the leakage capacitances at play
Cheers
Ale
Sadly, he was a great inspiration! Still read his books over and over....
What I need to get back to is to implement back his super CF. I used it before as a line stage with a 6j52p (as triode) and was very pleased with the result.
Hi Ale
The super CF works! I only did it with 3x DN2540 and 1x IRFBC20.. it will be going in my most recent phono cascode.. which is also inspired by Allen... 😉
But to drive 2a3 or 300b grid, I think your implementation is probably more than good enough.
The super CF works! I only did it with 3x DN2540 and 1x IRFBC20.. it will be going in my most recent phono cascode.. which is also inspired by Allen... 😉
But to drive 2a3 or 300b grid, I think your implementation is probably more than good enough.
Monkey
Also look for and read up about Thorsten's Monkey DC coupled design.
It is "self adjusting" unlike other DC designs.
A very clever design
And I've heard reports that it sounds "just right".
I remember seeing that he even did a design with a D3a driver with a CCS (single fet) load.
best
t
Also look for and read up about Thorsten's Monkey DC coupled design.
It is "self adjusting" unlike other DC designs.
A very clever design
And I've heard reports that it sounds "just right".
I remember seeing that he even did a design with a D3a driver with a CCS (single fet) load.
best
t
Nice one Ian!
I still need to get back on my phono stage. Shunt cascode jFET input and E810F gain stage with an SF output. Don’t want to move out of the OP request
For the 2A3 drive it all depends on the gain needed. However I agree on the comments that when you drive the grid on DC with an SF it comes to life!
My prefered options are d3a, C3g, 6j49pdr, e280f, 6p15p, 6j52
If DHT is affordable, 4P1L, 2P29L with input SUT or not. All depends on the gain needed
I love DHT driving DHT!
[emoji2]
I still need to get back on my phono stage. Shunt cascode jFET input and E810F gain stage with an SF output. Don’t want to move out of the OP request
For the 2A3 drive it all depends on the gain needed. However I agree on the comments that when you drive the grid on DC with an SF it comes to life!
My prefered options are d3a, C3g, 6j49pdr, e280f, 6p15p, 6j52
If DHT is affordable, 4P1L, 2P29L with input SUT or not. All depends on the gain needed
I love DHT driving DHT!
[emoji2]
You want to create a CCS just load the plate with a decent choke ,Electra Print have plenty of good iron rather then all the solid state stuff creeping in to designs and PCB cluttering up under the chassis IMHO !
Solid state used in the right way can work extremely well. mosfets have a place as CCSs and followers, not for amplification.
You would be surprised with an AB comparison of same stage with choke loaded and gyrator load version.
We’ve done that with several stages here with audio circle members in London and results were not what you indicate
Either way, it’s a personal decision in the end
You would be surprised with an AB comparison of same stage with choke loaded and gyrator load version.
We’ve done that with several stages here with audio circle members in London and results were not what you indicate
Either way, it’s a personal decision in the end
Hi. There is nothing wrong with using an old-school inductive load (i.e. plate choke). You will get lots of swing if it is implemented with the right valve/tube! 🙂
But when you do calculations, you will see that for high mu devices you need a choke with a truly huge induction. Let's have some fun. Imagine you have a really big 300Hy plate choke. Yes, electra print only list ones up to 100Hy, but as you will see, it is not nearly enough for your 6SF5. Let's imagine that you find some old 300Hy Thordarson (don't forget you will need a pair....). You imagine that it would be nice to hear some bass notes in your music, so let's set low frequency cut-off at 10Hz.
The math is simple:
2*Pi*Freq*H = Z (choke impedance)
6.283*10*300 = 18.85 KOhm
Wow, you think. That is waaaay to low for my 6SF5. I usually use a 100K ohm resistor as a load.
ok so you think... hmmm maybe the bass notes are not so important. Maybe I don't listen to music with organ, or bass, drums, etc. No Bob Marley, Pipe Organ, Hippity-Hop, etc. Heck, let's put the low level cut-off at 50 Hz.
6.283*50*300 = 94.25 kOhm
So this looks like it will work. But what it mean? It means that if you like to hear any bass, you need a whopping huge plate choke that doesn't exist.
Or you can choose a different valve/tube than 6SF5 (probably with a much lower mu). Or maybe just stick to the 100 kOhm resistor and save yourself the hassle...
If you do want to try plate choke, then I would look at checking out the Russian 6e5p (triode strapped). Yes the mu is only around 30, but it has a really small 1 kOhm plate resistance and is nice and linear. It's still *affordable* too. You could use an Electra Print AC8037 with it its only 70Hy so you might consider AC8034 which is 100Hy (note: I never tried them). They cost $50 and $70 respectively, and you would of course need a pair. Obviously the cheapest thing is the valve/tube in this arrangement... Make sure you bias it to run it at 20mA or so at least. Your power supply will need to handle that extra current. It's just an idea.
🙂
But when you do calculations, you will see that for high mu devices you need a choke with a truly huge induction. Let's have some fun. Imagine you have a really big 300Hy plate choke. Yes, electra print only list ones up to 100Hy, but as you will see, it is not nearly enough for your 6SF5. Let's imagine that you find some old 300Hy Thordarson (don't forget you will need a pair....). You imagine that it would be nice to hear some bass notes in your music, so let's set low frequency cut-off at 10Hz.
The math is simple:
2*Pi*Freq*H = Z (choke impedance)
6.283*10*300 = 18.85 KOhm
Wow, you think. That is waaaay to low for my 6SF5. I usually use a 100K ohm resistor as a load.
ok so you think... hmmm maybe the bass notes are not so important. Maybe I don't listen to music with organ, or bass, drums, etc. No Bob Marley, Pipe Organ, Hippity-Hop, etc. Heck, let's put the low level cut-off at 50 Hz.
6.283*50*300 = 94.25 kOhm
So this looks like it will work. But what it mean? It means that if you like to hear any bass, you need a whopping huge plate choke that doesn't exist.
Or you can choose a different valve/tube than 6SF5 (probably with a much lower mu). Or maybe just stick to the 100 kOhm resistor and save yourself the hassle...
If you do want to try plate choke, then I would look at checking out the Russian 6e5p (triode strapped). Yes the mu is only around 30, but it has a really small 1 kOhm plate resistance and is nice and linear. It's still *affordable* too. You could use an Electra Print AC8037 with it its only 70Hy so you might consider AC8034 which is 100Hy (note: I never tried them). They cost $50 and $70 respectively, and you would of course need a pair. Obviously the cheapest thing is the valve/tube in this arrangement... Make sure you bias it to run it at 20mA or so at least. Your power supply will need to handle that extra current. It's just an idea.
🙂
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The 417A (or 5842) is indeed a better choice. It has a mu around 43-44 and a plate resistance of around 1.7-1.85 kOhms.
Of course, direct coupling can be tricky if you want to use a plate choke.
This is one way do it though:
My New 45 SET (choke-loaded 6SN7, direct-coupled)
🙂
A Lundahl LL1667/5mA is small, light and 800H. It has a Z of 100Kohms at 20 Hz. Probably enough for a 6SF5.
Well, at 10Hz you have roughly a load of 50 kOhm. Not spectacular really, and it doesn't solve the problem of the high output impedance you will get with 6SF5 or 12ax7, etc.
Still really not ideal to drive 2a3 grid... Of course that is only my opinion.
That’s precisely why the hybrid mu follower is a good option for high ra and low current triodes
If choke/IT is the preferred way (and if DHT is allowed as driver stage as well) then I’d vote for 4P1L and 2P29L with filament bias. The latter is easier to implement and isn’t microphonic at all. Both with a 1:4 SUT will give you the best performance at reasonable gain, 150-200vpp and sound marvellous.
Only caveat is that the source has to be able to drive the input capacitance of the driver
Ale
I realise that the 6SF5 has a very high plate resistance and may not be the ideal tube as a driver for the 2a3. But some people like it and if you are set on using this tube with a plate choke I think the Lundahl LL1667/5mA is a viable option. This is however not something I would do myself. I am in the DHT into DHT camp. My upcoming build (for which I have collected all the parts) will be a 1:4 Lundahl input transformer into a 4P1L or a 2P29L (haven't decided yet) into a Monolith IT-01 Summit into a P15/250 into a Monolith S-9 Summit. Both stages in filament bias.
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