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active loading with parallel feed?

74.220.74.160

I've been looking over Gary Pimm's article on active loads for tubes, and saw his example of CCS loading on an output stage (see link). He uses a CCS above the plate on the output tube to handle the DC current, and a parallel feed output transformer, with a DC blocking cap, parallel to the tube. The triode handles the AC component by working into the load presented by the primary of the output transformer. Now, in the standard parallel feed output transformer configuration, the total inductance is the parallel sum of the output transformer and the plate choke. When you replace the plate choke with a CCS, as Gary shows, there is no parallel inductance component. So the total inductance will be much higher (set by only the output transformer). In my case, a full stack Ni TFA-2004 output transformer will have quite a lot of inductance. Is there any problem with high freq response when operating this way? It looks like a nice idea to me, but I haven't seen may people using this approach. Maybe it just doesn't sound as good? Any thoughts?
Thanks,
Dan
Gary Pimm, active loads (Open in New Window)

Follow Ups:

how about a gyrator as an active load? - dhsettim 21:17:52 02/14/11 (2)
Y'all,
how about a gyrator as an active load?
http://www.diyaudio.com/forums/tubes-valves/143687-gyrator-question-9.html#post1863890

tim
- RE: how about a gyrator as an active load? - DanK tubes 22:38:46 02/14/11 (1)
  Hi Tim,
  Interesting idea, but I'm still trying to wrap my head around it. That's quite a long thread over on DIY Audio. In Gianluca's example, a 45 parafeed SET with a CCS active load would require a B+ of more than 700V and the CCS would need to dissipate 17W. Not so practical a solution. In short, what's the benefit of a gyrator over a CCS? Lower B+ requirement, less power dissipation..?
  Thanks,
  Dan
  - RE: how about a gyrator as an active load? - dhsettim 21:14:12 02/15/11 (0)
    I dunno Dan, my guess would be that similar voltages and power would be required. I think a gyrator and ccs just sound different rather than one better than t'other. But I have no experienceof gyrators. Michael Koster has written extensively in avariety of posts over at DIYAudio so maybe send him an email. Seems to a clever dude. tim
    
    Edits: 02/15/11
Thanks everyone! - DanK tubes 14:37:32 02/13/11 (0)
Great comments. Sounds like it's worth experimenting with, if I can get my B+ up high enough.
Dan
RE: active loading with parallel feed? - Paul Joppa 23:33:43 02/12/11 (0)
It sounds great this way, no question. Finding the right parafeed cap is still a difficult problem though - the OPT inductance is all over the map as a function of frequency and voltage. The solution as far as I can see lies in the losses - eddy current and hysteresis in the transformer core. These are never quoted as part of the standard transformer specs, and are not exactly easy to measure, or to interpret.

There is no high frequency problem, believe me!

The idea certainly goes back at least to Doc B's "Soul Sister" preamp (1998). Tucker did a 45 amp at the Vancouver VSAC this way, which is still the best 45 I've heard - yes, I believe it was a TFA-2004Ni output. I'm working on a few variants currently, and the Bottlehead Repro tape preamp uses this approach with an Altec 15095 output transformer.
RE: active loading with parallel feed? - gluca 22:31:43 02/12/11 (7)
Dan

you want to use the parafeed calculator (mine or the more complete by voltsecond) to have an idea of what is going on with a CCS. Links to the spreadsheet have been posted here a number of times.

With CCS the limiting factor at low freqs is indeed the primary inductance of the output transformer. Keep in mind that the CCS needs to drop the whole voltage swing of the output tube that is your B+ must be 100s volts higher than the plate voltage and this requires a big heatsink. I'll make an example. Suppose your B+ is usually +300V and suppose you are aiming at 3W into a 5k load, that is 122VRms or 770Vpp, that means your PSU must be 300V+770/2=685V and the CCS must dissipate 17W if you bias the tube at 40mA. Numbers explain that the whole idea is not very practical.

All that said, I have used parafeed OPT and CCS in all my preamps and that is something I strongly suggest every time.

Gianluca
- Gianluca. Could you calculate a rough estimate of the B+ - Bas Horneman 08:16:04 02/13/11 (6)
  I'll need for my SE 6c33c? I am going to use a CCS. Output transformer will be the parafeed MQ EXO-800. Which has a primary of 800R. A airgapped design would have around 270VDC and 250mA of current for the final stage.
  
  I don't understand how you go from the load and power to rms and then to Vpp.
  
  Thank in advance.
  
  To infinity and beyond!!!
  - RE: Gianluca. Could you calculate a rough estimate of the B+ - mqracing 09:36:50 02/13/11 (4)
    Hi Bas:
    
    you were asking;
    
    "I don't understand how you go from the load and power to rms and then to Vpp."
    
    let's say you have a 2 watt amp (45 tube for example) working into a 5000 ohm load (primary impedance).
    
    first multiply watts time impedance
    
    in this case 2 times 5000 equals 10,000
    
    then to get the volts rms take the square root of 10,000
    
    in this case the square root of 10,000 is 100
    
    so your signal volts (at full power) will be 100 vrms.
    
    peak volts is volts rms times 1.41
    
    peak to peak volts is volts rms time 2.82
    
    if you want to know the signal current
    
    take the volts rms and divide by the primary impedance
    
    in the illustrated case it would be 100 divided by 5000
    which equals 20 ma rms.
    
    MSL
    
    Builder of MagneQuest & Peerless transformers since 1989
    - Thanks Mike! nt - Bas Horneman 15:29:48 02/13/11 (3)
      .
      To infinity and beyond!!!
      - Using the calculations I arrive at a B+ of around 444V - Bas Horneman 12:09:44 02/14/11 (2)
        
        In Reply to: RE: Thanks Mike! nt posted by Bas Horneman on February 13, 2011 at 15:29:48
        
        that means that the B+ for Mike's 6C33C is probably on the low side with the b+ of 300V.
        To infinity and beyond!!!
        
        RE: Using the calculations I arrive at a B+ of around 444V - mqracing 06:07:11 02/15/11 (0)
        
        In Reply to: RE: Using the calculations I arrive at a B+ of around 444V posted by Bas Horneman on February 14, 2011 at 12:09:44
        
        Hi Bas:
        I should have taken more time in my initial post to state more clearly that this design (at this stage of development) was presented as a "topological" schematic. By that I mean--- what I wanted to do was to show a "black box" topography of what the circuit might look like and then present it to my customer in China for his consideration.
        
        So in no way is it a complete design--- notice that there is not a plate voltage noted for the anode of the 6C33C and the actual value which we had in mind is, in fact, substantially less than 300VDC. So a lot of detailed values have been left out (and in some cases not yet fully calculated) since this was intended to be a study document.
        Which,thanks guys, I asked for and received some comments and suggestions. All of which are appreciated.
        
        Builder of MagneQuest & Peerless transformers since 1989
        
        Edits: 02/15/11 02/15/11
        
        RE: Using the calculations I arrive at a B+ of around 444V - Bas Horneman 12:15:55 02/14/11 (0)
        
        In Reply to: RE: Using the calculations I arrive at a B+ of around 444V posted by Bas Horneman on February 14, 2011 at 12:09:44
        
        Assuming a 12Watt amplifier.
        To infinity and beyond!!!
  - The planned heatsink - Bas Horneman 08:38:01 02/13/11 (0)
    To infinity and beyond!!!
    
    Edits: 02/13/11

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