CCS Loaded Parafeed Output - Design Considerations?

[L0rdGwyn]

I see thanks, I assume some feedback was used?  I’m afraid I may have to live with the HF rolloff in my design, perhaps I should have looked into using feedback myself, but I suppose -3dB at 16.5kHz isn’t horrible.

[Paul Birkeland]

Yes, I used feedback between the plate of the 801 and the plate of the first stage driver tube. 

[L0rdGwyn]

Got it, thanks PB.  My OPT can also be wired for 6.5K:8ohm rather than 11K:8ohm which I'm sure will help my HF rolloff.  I bypassed my 1K grid stoppers along the signal path with a 100ohm resistor and took FR measurements, no change, so I think it is safe to say it is due to leakage inductance / stray capacitance of the OPT.  I think I am going to look into using a 6.5K impedance, altering my bias point, and adding NFB.  Simply changing the primary impedance to 6.5K and my output impedance will be far too high, damping factor less than 2 with 8ohm speakers.  NFB is new to me, we'll have to see if it will fit in with the rest of my design.

[L0rdGwyn]

Going off of what I said above, I think I will give this bias point a try, 350V 50mA -9Vg.  Below is a load line with 6.5K primary, this is at 87.5% of max plate dissipation.

I guess it is time to learn about NFB, been putting it off, hope it will gel here.

[Paul Birkeland]

This thread has an approach that is far less intimidating than others:
https://www.diyaudio.com/forums/tubes-valves/356952-output-tube-plate-driver-cathode-feedback.html

Remember that you need lots and lots of gain so that when you add the feedback network you end up with enough leftover.

[L0rdGwyn]

Awesome, thanks for the link, I'll do some homework.  The measured mu of my CCS loaded 841 gain stage is 29, so it can swing a hefty amount of voltage into the grid of the 801A.  With the new bias point, will need about 100Vpp to drive to full output, so I should have an extra 50Vpp of headroom with a standard DAC input of 5.6Vpp, so hopefully that will be enough.

[Paul Birkeland]

29 ain't gonna do it! You'll want to move to a pentode up font to give you a lot more wiggle room. 

The first schematic in the link I sent you shows a pentode driver that will have an amplification factor over 1000x.  That sounds like a lot, but 1000x is only 60dB

[L0rdGwyn]

Nothing is ever easy, is it?  Okay well no point in speculating on what I can do without the knowledge of how/why to do it, let me do some reading and come up with a plan bummed I wouldn't be able to use my 841 driver, guess I am going back to the drawing board.

[L0rdGwyn]

So rather than going straight to a total input stage overall PB, I decided to try something else first, NFB from the secondary of the OPT to the cathode of the driver stage, the 841.  Picture I found attached.

I used 500ohm on the 841 cathode bypassed with 10uF film cap + 50ohms to ground  (R2 in the diagram) .  This got me my 500Va / 9mA / -5Vg bias point and LF extension down to 20Hz.  I tried bypassing with an electrolytic first and it sounded horrible, the film cap sounds great.  I then altered the value of the feedback resistor (R1) from the OPT secondary.  I tried 2K, 1K, and 500ohms.  Obviously I am sacrificing gain from the driver stage for other benefits.  Here are the results:

No NFB
Output impedance: 3.95ohm
HF -3dB: 16.5kHz
THD at 1W: 0.78%
Power output: 5.85W

R1 = 2K
Output impedance: 3.47ohm
HF -3dB 18kHz
THD at 1W: 0.71%
Power output: 5.85W (still enough gain to drive the 801A to clipping)

R1 = 1K
Output impedance: 2.50ohm
HF -3dB 20kHz
THD at 1W: 0.67%
Power output: 5.3W

R1 = 500ohm
Output impedance: 1.02ohm
20kHz at -2.3dB
THD at 1W: 0.61%
Power output: 4.3W

What do you think?  Other than the decreased gain, not sure if there are other issues caused by using feedback resistors as low as 1K or 500ohms, the 1K results seem to be a nice compromise to me and gets me a 1:3 damping ratio with 8ohm speakers.

[Paul Birkeland]

Putting feedback around the output transformer will make the amplifier at best conditionally stable, and at worst it could oscillate.  I just had a home brew amp someone brought in my workshop the other day that oscillated at 150kHz just sitting with a speaker hooked up.  This is not a desirable outcome! 

We would tend to recommend the shortest loop you can possibly use to get the job done.  Cathode feedback is a super short loop, but that requires a very special output transformer and doesn't work so well with parallel feed.  Output tube plate to driver plate is a very short loop, and it's what we used on the Seductor (and what I have used on several personal projects).  Output tube plate to driver cathode is a bit longer of a loop but far easier to implement, and that's where I would recommend starting.  Output transformer secondary to first stage cathode is the longest and most problematic loop.

Still, your experiments are a decent demonstration of why the 841 has to go in favor of a pentode.  There are directly heated pentodes...

[L0rdGwyn]

Point taken, PB, I have not ruled out the pentode!  Just wanted to see what could be done here.  The amp appears to be stable, at least as it sits here in my garage, no oscillations on my scope and it sounds quite good.  But I will look into the pentode further with plate to cathode feedback.  I appreciate your $0.02.

[L0rdGwyn]

I am going to mock something up with the EF37A in LTSpice, seems a popular choice as a pentode input with NFB.

[L0rdGwyn]

I took your pentode suggestion seriously PB, I have been doing lots of reading and I am working it up now.

I was looking over some of the work Gary Pimm did on CCS loaded pentode stages.  Looks like he came up with two different ways, the first with a high value resistor in parallel with the CCS, the other with the resistance to ground (picture attached).  I am looking at using the EF37A or 6J7G on the input, I mocked up a model using plate to cathode feedback with Pimm's "resistance to ground" method.  I need to take a closer look at the "paralell resistor" method, I am doing something wrong in LTSpice, since there is very little current across the resistor, there is no Vdrop and I am getting full B+ on the tube plate.  I am sure the answer is obvious but my brain is getting tired.

In my sim with 250K to ground connected to the 6J7G plate, the open loop gain of the amp is roughly 59dB and the closed loop gain is roughly 44dB, so about 15dB of NFB.  This leaves me with a mu of about 30 from the driver stage (open loop mu ~165), plenty of swing to drive the 801A to full output.  With this setup, I would rewire my OPT to 6.5K:8ohm and a 370V / 50mA / -11 Vg bias point on the 801A.

I penciled in 50pF on the phase compensation cap, but will need to dial in the value.  Overall I think this looks pretty good!  I know it isn't accurate, but in LTSpice I am getting 0.25% THD at 1W, 0.56% THD at 3W.  If that is anywhere close to real life results, that is an incredible improvement.

Will check my work a few more times and dial in part values, have some tubes on the way to give this a try.

[Paul Birkeland]

I would suggest bypassing R6 and rerunning the sim.  Try 1000uF.

As far as plate voltage goes, you need to set the CCS to deliver adequate current for the plate load resistor plus the plate current of the tube itself, so that can be a little tricky. 

[L0rdGwyn]

I'll give the bypass cap a shot, PB.  Yes, setting the CCS has been a bit challenging!  A little bit of guess and check to get the correct plate current.  I'll report back when I have some real world measurements.