How to figure output Z of SRPP?

[smbrown]

Can someone give me the process for computing the output impedance of a SRPP preamp both when the output is taken from the cathode of the upper tube, and when the output is taken from the plate of the lower tube? Thanks!!

 

[first]

I want to know too

 

[tubetvr]

Given that both tubes are identical the Zout at the cathode of the upper tube is:

rp * (rp + R)/ (2*rp+(u+1)*R) where rp is internal anode resistance and R is the cathode resistor value. For a 12AX7 with 1.2k cathode resistor this gives ~16k

The output impedance at the lower tube anode is higher and will be (rp + (u+1) * R) / 2 if the lower tube cathode is not decoupled and will be:
((rp+(u+1)*R)*Rp)/(2rp+(u+1)*R) if the lower athode resistor is decoupled. Typical values for a 12A7 will be ~92k with no deoupling and 47k with decoupled cathode resistor

Regards Hans


PS, this is described in chapter 11 of "Vacuum tube amplifiers " by Valley and Wallman where the SRPP is first described and analysed.

 

[smbrown]

Thanks! In fact I'm using a 6072 and had an opportunity to measure it at lunch. The bottom is not bypassed. The output at the bottom of the upper tube cathode is about 15k, which seems to work out okay. Kind of a higher output Z than I'd like, but sure sounds nice as a preamp.

 

[tubetvr]

Yes, 15k with a 6072 sounds about right. A mu follower will give lover output impedance and higher gain than a SRPP but it doesn't have the same output current capability and the distortion rises very quickly when a mu follower is loaded too hard, (e.g. when driving an output stage which clips and start to draw grid current) the SRPP is much better when loaded hard.

Regards Hans

 

[Merlinb]

Here's a tutorial on the SRPP:
http://www.valvewizard.co.uk and go to SRPP

 

[Geek]

I have this sitting on my HD for a reference. Can't remember where I got it from.

 

[tubetvr]

Here's a tutorial on the SRPP:http://www.valvewizard.co.uk and go to SRPP

Interesting site with a lot of useful information but unfortunately it contain a common error when describing the output impedance of the cathodyne phase splitter, it gives output impedance values that are different for cathode and anode when in reality these are equal when both outputs are loaded equally.

Regards Hans

 

[smbrown]

Thanks for all the great feedback, very appreciated. My little project was to just try a 6072 in a SRPP as a preamp. I used 2k R's and this resulted in about 1.2ma of current, and 15k Zout. In listening last night a friend and I thought it sounded very nice until pushed a bit harder, then it started to get that over driven sound to it. I'm thinking that the fault is likely a combination of the lower current and highish Zout. The input to the power amp is 100k, but the capacitance of the cables and the input tube on the power amp comes in at about 280pf, which I think is challenging the slew capability of the preamp SRPP stage. I really like the sound of the 6072, but not very much grunt there. Maybe switching to 1k and around 2ma would do it. I'll play some more.

 

[plovati]

Interesting site with a lot of useful information but unfortunately it contain a common error when describing the output impedance of the cathodyne phase splitter, it gives output impedance values that are different for cathode and anode when in reality these are equal when both outputs are loaded equally.

Regards Hans

Please could You explain a little bit?
Thank You

 

[tubetvr]

This has been discussed several times before but I think it is important to get rid of misunderstandings and I think that the cathodyne has gotten a bad reputation because of this. Actually the cathodyne is one of the very best phase splitters that exist, it has excellent balance, low and equal output impdeances and when compared to any other phase splitter implemented by using a dual triode it has highest gain.

When analysing the cathodyne it is a common mistake to see it as either a cathode follower or as an ordinary common cathode amplifier with cathode feedback. If you calculate the output impedance separately in this way you will find that indeed the output impedance at the anode is much higher than at the cathode.

What is wrong with this analysis? The answer is that you need to think about how a phase splitter is used, in the analysis described above we analysed cathode and anode outputs separately without considering that in reality both outputs are loaded.

We come up with 2 distinct cases, one is when both outputs are eqyually loaded as is the normal case when the phase splitter is loaded by a separate drivers stage ala Williamson or by a output stage that runs without grid current. The second case is where we have unequal load e.g when an output stage start to draw grid current. The only case that is not valid or realistic is the first desribed one where the outputs are analysed independately of each other.

The result of case A with equal loads is that we find that both output impedances are equal and lower even then they would have been from the cathode at its own. The earliest text that describe a calculation for this is by Preisman http://www.aikenamps.com/cathodyne.pdf

but it is also described in later editions of Morgan Jones book

The output impedance for equal loads is (rp*Zk)/(Zk*(u+2)+rp) where rp is internal anode resistance and Zk is cathode resistor

If you should doubt what Preisman or Morgan Jones has written you can do what I did, I measured a real phase splitter and came up with values very close to these calculated ones.

Regards Hans

 

[plovati]

Thanks

Great clarification..
Many Thanks.

 

[smbrown]

Tubtvr, great post, thanks. So if I wanted to make a lowish Z out, balanced preamp, would the cathodyne phase splitter be a good answer? Could I use it SE or Balanced w/o consequences?

 

[tubetvr]

So if I wanted to make a lowish Z out, balanced preamp, would the cathodyne phase splitter be a good answer?

Maybe if you are sure that both loads are always more or less equal, a better idea I think is to use a cathodyne coupled to cathode followers in order to get some load isolation for this kind of application.

Could I use it SE or Balanced w/o consequences?

No you can't as the ouput impedance is dependant on if the outputs are eqyually loaded or not, if you use only one output e.g the cathode the output impedance is higher than it is when both outputs are equally loaded.

Regards Hans