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:
The OPT has a couple of feedback windings to provide 10% CFB. I set the valve rather hot, I know. Probably a tad too much. Should be 35W maximum. You can do GU-50 here and get more power, cheap and easy.
The driver is a gem. The EF184 is a superb pentode when triode-strapped. Can do very well on the same levels of a D3a and is (yet) somehow still cheaper. Cathode degeneration plays well and easy here in the hybrid mu-follower.
Some people would say: “pentode and cathode feedback, arghh!!”. Trust me, good path down this way when feedback is local of course.
Although the stage can do 8W and THD will jump up to 2.9% is nice to see the triode-like harmonic distribution. Here is the view at 1W:
If you push the output stage to its limits, you will get this:
Bandwidth is over 65kHz. A simple and nice amplifier. With different arrangements on feedback like plate to grid with cathode driven via a PMOS and increasing the FB ratio to 20%, you can get less than 1% at 7W output. But that’s a rather more complex amplifier. This is simpler.
Hi Ale,
What is the difference between cathode feedback and global negative feedback?
Bruno Putzey says, amongst other things “Nested feedback is functionally equivalent to global feedback.”
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
stay safe
tim
Hi Tim
Thanks for sharing this, haven’t read the article and will add it to the list. Nice bedtime reading though!
I’m following my ears here. I found very little gNFB amplifiers which I liked truly their sound. I take phase lagging on cascading stages and imperfections of the OPTs may be the reason, I don’t know.
Best arrangement for me is “plate to grid” or aka parallel-parallel feedback. That doesn’t take the OPT with it and is a simple and yet effective local loop.
CFB uses the transformer, but still is local. I liked its sounds every time I heard one amp using it.
Others with more experience may chip in here..
I’m not saying you have to like the gNFB sound; as long you have experimented and listened before dismissing it. Most people just repeat what they have heard–NFB (not just global) is bad–which is patently untrue–it is different (lower distortion, wider bandwidth etc) but it isn’t bad.If you are interested in local feedback then consider current feedback as used by FrankB (he uses 100% current feedback) and by JC. OPTs usually do limit the amount of gNFB that can be used but don’t prevent its use. Considering FB’s designs; how much gNFB can a 470K feedback resistor give?
If you have time to waste here is a DIYAudio forum discussion on Bruno’s paper. It goes in the usual circles though does help to clarify some oints.
https://www.diyaudio.com/forums/the-lounge/306022-bruno-putzeys-paper-negative-feedback.html
Hello, where to get OTP? Thanks.
Hi. Googled the transformer TTG-41433 SE Cathode FeedBack, with no result, but: This has to be a PP output transformer, due to the CFB winding. As the PP has no air gap to deal with the magnetic field (no canceling out by the 2 pair of winding with DC!) you will need a resistor, similar-sized on the PP unused winding to make it suitable.
It’s an SE OPT custom made. That’s why you can’t find it. See attached picture.
And yes, G3 is connected to cathode, however I’d supply 12-15V instead to square the curves at lower voltages and reduce distortion at high output levels.
Forgive me, I am not native speaker: What means squaring, why do you think this is good? Could you tell mire about G3? Some ETF ppt says pentodes sounds better connected as triodes. So G3 to negative, G2 to anide via some resistor.
Your custom made transformer has 2×5% CFB, right?
Have a read through this
https://www.diyaudio.com/forums/tubes-valves/357559-corona-ultra-low-distortion-a2-dht-se-amp-prototype-19.html
Just saying, that understood point of squaring… The Anode may 400V, but when it swings that much, Anode from tube perspective goes down, so important that area… I understand this now. In the meanwhile I think my RL12P35 will be driven by a C3G, normal resistor load, cathide bias + a plate to driver plate direct NFB and an 7kOhm like output (the Ra 27Kohm is suggesting me that I should go up. DC workinpoint Voltages the same you suggest. My measurements also not showing much better…
Why 5kOhm? Why not 3.5kOhm? (Sorry for the questions. Learning through these)
G3 is at cathode I guess?
-Made measurements with my Roetest, see that the bias is right ~82mA (-28Vg1, 240Vg2, Ua does not limit)
Also the calculated THD by Roetest is optimal between -25 and -30V, so -28V seems to be right
Read the proposed forum about Ug3, but do not get how this is impacting flatness (with your words squaring)
I see the squaring, but this is at low Anode voltage. Checking with various G3 (0,12,25) I see no flattening like change in the curves at the working conditions
Or I missinterpret your graph and the horizontal is not the anode voltage?
-For Vg1 why not simply using a cathode bias with ~100uf below the CFB?
R=U/I=28.1V/0.085A= 330Ohm for the cathode. (Unsure on the DC drop by the CFB, but guess okay to have it’s adder to the -28.1V)
Or why not do a DC couping, so remove C2, but with proper supplies put everything lower in the first stage?
-Why is EF184 G3 connected to anode? Why not cathode?
Found this article interesting: http://www.audiodesignguide.com/New2A3/ETF06TS.pdf
-Ferrite beads: What type? Really on all pins? Both tubes? (except heater)
Did you saw high freq ringing without them?
-For Ig2, do yout think a simple resistor split + capacitance is fine? Or maybe have on the transformer that voltage available is practical?
-For heater I plan AC with simple soft start of the transformer via a resistor. What is your experience?
(I am somehow a believer better sound of AC heated, but have plans to build an demo unit to proove my theory.)
I hope there will be part 2, actively thinking to build one amp with the best available sound
Were you wondering to have a plate to driver plate feedback via a resistor? I mean, the 1% THD at 1W is not good enough.
What is the benefit of your Gyrator compared to a simple resistor on the plate?
Hello Ali, Have you actually built or do you plan to build this amp, or are the opts just lying magnificently in a drawer? This design looks interesting to me and I’ld love to ‘hear’ more about it. Fully aware that none of us could realize all of our projects in just one lifetime…
All the best, Simon
Hi Simon, not yet in full. I have some EL152 sockets made by Jakeband which should be with me shortly and likely build a swappable version between the RL12P35 and the EL152. Have to add a positive supply for the G3 to lift it 12V to get better response at high power levels.
Cheers
Ale