Parallel feed (choke load) in output stage

[ReinoutdV]

Good morning again,

yes i know.....i did already posted pictures of this amp earlier. But that was on a 833-tube thread. And this is about ultra parafeed. And this amp uses a 833-tube in an ultra parafeed schematic.

Again: please use a high quality cap. Value: araound 5 uF gave me good results.
Formula wise:

C = 1 / (6,28 x I x F)
C = capacitor in F
I = ompendance tube in Ohms
F = desired "fall off frequency" (sorry....i do not know the correct phrase)

For the 833:
C = 1/ (6,28 x 5000 ohms x 10 Hz) = 0,000003185 F = 3,2 uF

Much less will impair bass-quality.
Much more is not bringing stability.....on the contrary

Hope this helps ?

Reinout

[audiousername]

Originally posted by ReinoutdV
The proposed schematic is a nice combination of ultrapath (Jack Elliano, Electraprint, etc) and parallel feed. This combination is sometimes referred to as "ultra parafeed". It is certainly not new, Western Electric used it already in 1938 in their 92B-amplifier.

Please help me understand this.... does the "Ultra-Parafeed" nomenclature refer to the (conventional) capacitor in series with the OPT primary or that the primary is returned to the cathode of the output valve instead of to ground.

As an aside, I believe but the use of what is now knon as "Ultrapath" was mentioned by John Broskie in the TubeCad Journal in an article something like "lowering the SE amp's output noise", prior to the Jack Elliano article.

Originally posted by ReinoutdV
Ultra parafeed does have some nice features:
- the coil will take care of DC-component. So you can optimize the OPT with more exotic cores. Or use a smaller winding to decrease the resistance. Either way......the OPT can be better (and there is no air gap as well).
- the extra cap does seem to smooth out a lot of power supply irregularities.

Of course there are trade-offs no surprise:
- the coil is not a standard choke !
- there will be an extra cap between the OPT and the cathode. This must be a quality item ! I do favour a nice oil cap here......but that is a matter of taste.

This is true... Also of note, I believe that the reasoning of returning the OPT primary to the cathode instead of ground is to stop the PS from being in series with the signal (not sure on this though)

Originally posted by ReinoutdV
Lynn Olson wrote in VTV (issue 16) a very informative article about these topologies under the name "Ultrapath, parallel feed and Western Electric". This very article made me base my 833 amplifier on the proposed topology "ultra parafeed". And it does work superbly !

I am not familiar with this article, however if it means that you are using the cathode-follower output stage (as in the schematic proposed by Yagoolar) that must be one very special drive stage!

By the way, splendid looking amp! Only wish I had the patience to make something that looked that good...

[yagoolar]

Originally posted by rcavictim


Where do you guys find this stuff?

Here you are

Pride and Prejudice

I also recommend the article: "Mathematical Derivation of Parafeed Output Stage" from this page.

[rcavictim]

audiousername,

The ultrapath slogan does indeed refer to returning the AC signal to the cathode of the output valve. The parafeed design places the output valve in the mode of operating as an active shunt to the power supply applied to the output xfmer. Like a shunt regulator stage if you will on a DC power supply. The power supply is not in series wit the output stage in this configuration and is in fact isolated from the output stage by the plate choke which gives this circuit greater PS noise immunity as part of it's charm.

The regular output stage configuration uses the output valve as a series pass element to change the power applied to the output xfmer. The PS is in series with the output stage and any PSU noise/ripple is impressed upon the output signal.

In the ultrapath config the fact that the cathode resistor is not there to do anything good for the audio component, it is only there to provide operating bias for the output valve, is recongnized for what it is and eliminated from the audio circuit. If you wire the return of output xfmer to ground, including the resistance of the cathode R you effectively place an extra R in series with the output xfmer primary that only reduces the % of available audio power that can be taken out of the amplifier. You also introduce a small amount of negative feedback in the output signal which is the changing voltage across Rk.

As stated the two concepts of Ultrapath and Parafeed are nothing new, just perhaps the slogans used to describe them. I employ both these techniques in a parallelled SET amp I designed which uses eight triodes (four 6080's) in parallel in the output stage. BTW, I use 4 uf off the cathode of each individual triode stage for the output coupling caps all going to the bottom of the output transformer primary. High quality metal can, polypropelene and oil motor run caps have been purchased but not yet tried to replace the 3.5 uF mylar dry caps employed in the prototype breadboard which sounds marvelous!

[pedroskova]

rcavictim,

Would love to see a schematic of your amp. I've recently switched my line stage to this configuration, using a 12B4A into a toroid opt(ala Manfred Huber). I'm using a cheap ccs instead of the plate choke.

I've been leaning toward a ppp parafeed 2A3(pppp2A3?) but am open to other topologies.

[audiousername]

Originally posted by rcavictim
The parafeed design places the output valve in the mode of operating as an active shunt to the power supply applied to the output xfmer. Like a shunt regulator stage if you will on a DC power supply. The power supply is not in series wit the output stage in this configuration and is in fact isolated from the output stage by the plate choke which gives this circuit greater PS noise immunity as part of it's charm.

The regular output stage configuration uses the output valve as a series pass element to change the power applied to the output xfmer. The PS is in series with the output stage and any PSU noise/ripple is impressed upon the output signal.

Thanks for that. I'd never thought about it that way..

[ReinoutdV]

Hi Audiousername,

i wished i could explain the ultra-parafeed like Rcavictim....

Just to hint at the capacitor:
- in a ultrapath-topology the cap-value will be in the 10+ region. It's often used in preamps and don't be surprised to see values between 10~100 uF.
- in the parallel feed and ultra-parafeed you'll see values around the 5 uF. The cap has a somewhat different function here in combination with the coil (as Rcavictim pointed out).

You asked about the driver. You're right about the special quality. But i'm not using a cathode-follower.
The 833 has a huge grid and really likes to see "power" in order to function. So i choose a 300B (can sound really nice) with a stepdown interstage. That also gives me the wanted low impendance....power !

Added is a simple drawing of the shematic:
- XLR in;
- input transformer;
- 6SN7 push-pull;
- interstage;
- 300B "classical SE";
- interstage stepdown;
- 833;
- opt.

Besides nice sounding this amp will also provide the heating during the cold winter days......

Reinout

[j45yip]

Hi all,

Should I load the secondary winding of my existing output transformer of my 45 SE amp with a dummy 8 ohms resistor if i were to wire up another parafeed optx in the amp? Is it harmful to just leave the secondary open?

Thanks in advance for any sound advice.

[Shoog]

"Should I load the secondary winding of my existing output transformer of my 45 SE amp with a dummy 8 ohms resistor if i were to wire up another parafeed optx in the amp? Is it harmful to just leave the secondary open? "

Yes - voltages in the secondary of an unloaded transformer can spike very high - infact to damaging levels, which could destroy your transformers. However loading your transformer with a 8R resistor will halve your availble power output. Think about a 22R load.

Shoog

[j45yip]

hi shoog,

thanks for your reply. but why the value of 22R? how about the rating?