Murdock

Lorenz V41 output transformer
« on: September 04, 2019, 05:34:18 PM »
Hey,

I'm looking at the schematic of the Lorenz V41 preamp.
And the output transformer got me wondering.
It states 17H and 110 Ohm for the primary.
And the anode choke has 100H...
Can somebody please explain to me, why the output transformer inductance can be so low? It has an reactance of only 4k3 Ohms at 40Hz. The plate impedance of the EF12 isn't that low, isn't it?
Is the choke somehow taking over the "work"?  ::)
Other output transformers with a similiar topology have way more inductance (V76, V241,...)

And no, the 17H is not a typo or anything. I have an original here and measured it...


PRR

Re: Lorenz V41 output transformer
« Reply #1 on: September 04, 2019, 10:37:15 PM »
.

Re: Lorenz V41 output transformer
« Reply #2 on: September 05, 2019, 12:12:46 AM »
The actual output impedance of that triode-strapped ef12 will be quite a bit lower than you think as it's configured with a gain of about -1.    Note the two 300K resistors. 
To calculate the actual impedance you'd need to know Ra and open loop A which require a bit more than my thumbs and tired brain to work out due to there being a partially bypassed cathode and other bits involved.
Best guess: 1K - 2K

Edit:. If I had a unit in hand as you do, I probably wouldn't even bother doing the maths.  Just find it by putting a mid band signal through the amp, shunt the output after the blocking cap but before the transformer with a pot and twiddle it until the output falls by 6dB.    Voila! 
« Last Edit: September 05, 2019, 12:37:21 AM by Winston O'Boogie »
D. J. H.

PRR

Re: Lorenz V41 output transformer
« Reply #3 on: September 05, 2019, 12:59:13 AM »
The LARGE-signal impedance is still 8k or so.

Driving 4k load is inefficient and nonlinear.

Putting heavy NFB around it makes the nonlinearity much less, and bass response much better, but does not fix the inefficiency.

I would guess this was not a big problem for the application.

It roughs-out to as much as 0.4W out for 1.6 Watts of plate heat. 0.4W is +26dBm, which is really very high for a single small triode. It is probably >5% THD at this point, and near 1% under NFB. So I bet it is rated more like +20dBm, 0.1W, at some more-pleasing distortion. And another 6dB unexpected peak isn't clipped, just blunted.

PRR

Re: Lorenz V41 output transformer
« Reply #4 on: September 05, 2019, 01:08:58 AM »
> shunt the output ....a pot and twiddle it until the output falls by 6dB.

This (-6dB) rarely works well with heavy NFB. At minimum you must watch for clipping.

If an amp is known stable no-load, then comparing voltage no-load to nominal-load and doing math gives an answer without upsetting the amp much. Or if nominal 600r, compare 700r and 500r, do math. (US line-amps would certainly swallow 450r-900r without minding a bit.)

Perhaps of more interest is the bass extension with the low-H transformer. 17H and 4k imply 40Hz. Tube Mu is like 24. Badly loaded like this, gain may be 8. So the inverting NFB leaves 4:1 of error reduction, indeed coming near 1k-2k. Now the bass extension is to 20hz or less. Radio gear rarely aimed for subsonics. (It can blow-up AM modulators.)

Why this design is different from other high-H designs? Who knows? New designer. Old designer with a new idea. An over-stock of transformers that had to find homes.

Re: Lorenz V41 output transformer
« Reply #5 on: September 05, 2019, 01:39:51 AM »
Cheers PRR 😊
I'm not actually sure what nominal load on the secondary would have been on these amps.  Possibly 400 ohms as I've seen reference to that with later German broadcast gear, but it may have been intended to be bridged by circa 2K as was also common on European desks at the time.
Max "ausgangspegal" is +18dB being 6.2V so we're not dealing with a different reference such as was employed at the likes of EMI and Decca.

It's a bit of a curious one to me why they went with this output arrangement since usually you see nothing but exemplary rationale behind these German designs.  Seems to me that a tube, a choke, and some passives could have been spared and the output could have been taken from the prior tube's choke loaded plate into a suitably wound transformer.
Edit: current having been upped to a nominal 7mA in the proceeding stage first of course.





« Last Edit: September 05, 2019, 08:29:08 AM by Winston O'Boogie »
D. J. H.

Re: Lorenz V41 output transformer
« Reply #6 on: September 05, 2019, 08:17:03 AM »
I missed that this was a Lorenz V41, built in Leipzig  which was, at the time of course, in East Germany.
It seems to be a bit different from the West German V41.
The Maihak V41 schematic I looked at shows 2 identical output stages strapped onto the proceeding valve for feeding 2 isolated lines.  That would explain why the output wasn't just taken from the proceeding valve.  It looks like Leipzig just didn't implement this 2nd parallel output.
Maihak schematic is here:   http://audio.kubarth.com/rundfunk/index.cgi

And there's an earlier schematic of the ELA V41 that has only one output stage like your Lorenz but utilized a higher ratio output transformer.  That schem is attached. 

So, the plot thickens!  😉
« Last Edit: September 05, 2019, 08:20:30 AM by Winston O'Boogie »
D. J. H.

CJ

Re: Lorenz V41 output transformer
« Reply #7 on: September 07, 2019, 03:50:05 AM »
what frequency is tied to the 17 Henry figure?   sometimes the standard freq for inductance is 500 or 1000 Hz, in that case, the low freq inductance would be a ton higher since this is a non gapped core (cap blocks DC so no need for a gap)

what kind of meter did you use to check H?

try running 40 hz thru the primary with a series ammeter,  write down voltage and current, do Ohm's law for XL,  divide by 6.28 and then divide by frequency to get H.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Lorenz V41 output transformer
« Reply #8 on: September 07, 2019, 04:30:14 AM »
what frequency is tied to the 17 Henry figure?   sometimes the standard freq for inductance is 500 or 1000 Hz, in that case, the low freq inductance would be a ton higher since this is a non gapped core (cap blocks DC so no need for a gap)

what kind of meter did you use to check H?

try running 40 hz thru the primary with a series ammeter,  write down voltage and current, do Ohm's law for XL,  divide by 6.28 and then divide by frequency to get H.

Good call C.J.  Maybe Murdock can measure his output again.   
D. J. H.

Murdock

Re: Lorenz V41 output transformer
« Reply #9 on: September 07, 2019, 10:41:51 AM »
Thanks for all the replies and information!
Sorry for the misunderstanding. But I have only an original output transformer, not a whole V41 unit...

The inductance measurements are measured with 40Hz. The input transformer primary is stated as >2.8H which equals the stated input impedance of >700 ohms...

And I measured 14H with 100Hz.

Distortion at 1000Hz and 55dB amplification is <0,4% at +12dB (3,1V) and <1% at 18dB (6,2V).
Output impedance is <60 Ohms.
Frrequency response 40 - 15000 Hz +-1dB

Does the anode choke somehow reduce the tubes output impedance?
« Last Edit: September 07, 2019, 10:58:37 AM by Murdock »


Re: Lorenz V41 output transformer
« Reply #10 on: September 07, 2019, 11:47:49 AM »
Thanks for all the replies and information!
Sorry for the misunderstanding. But I have only an original output transformer, not a whole V41 unit...

The inductance measurements are measured with 40Hz. The input transformer primary is stated as >2.8H which equals the stated input impedance of >700 ohms...

And I measured 14H with 100Hz.

Distortion at 1000Hz and 55dB amplification is <0,4% at +12dB (3,1V) and <1% at 18dB (6,2V).
Output impedance is <60 Ohms.
Frrequency response 40 - 15000 Hz +-1dB

Does the anode choke somehow reduce the tubes output impedance?

Thanks for the clarification 😊
The results you give certainly tally with the schematic's stated maximum output level of +18dB.
The plate to grid negative feedback will be lowering the impedance as relates to small signal frequency response.  I gave a quick guess at it being in the 1K - 2k area and PRR's more thorough understanding says that's about right.  But as he also stated, for large signal purposes, it's not a very efficient scheme.   
Can't remember which particular later V series amplifier it was but, maybe using your transformer with signal taken from a choke loaded cathode might be one other way to utilize it in something you build.  It's probably a nice piece of iron if used within its limitations 👍
D. J. H.

Murdock

Re: Lorenz V41 output transformer
« Reply #11 on: September 07, 2019, 12:03:51 PM »
Thanks for the info!
I thought about using it in the successor amp Lorenz V241 which also has a plate choke. It uses an ECC85 which I think has an even lower output impedance. But on the schematic the output transformer has an Inductance of 150H... 
Do you think the V41 output transformer would work in that application?




Re: Lorenz V41 output transformer
« Reply #12 on: September 07, 2019, 12:31:35 PM »
Thanks for the info!
I thought about using it in the successor amp Lorenz V241 which also has a plate choke. It uses an ECC85 which I think has an even lower output impedance. But on the schematic the output transformer has an Inductance of 150H... 
Do you think the V41 output transformer would work in that application?



It took me a while to find where the negative feedback was leaving from and heading to on that schematic,  I went looking down near the bottom 10M resistors etc and was completely baffled for a minute 😄
Once I'd stopped chasing my tail,  I looked at the rest at saw that it looks remarkably like the REDD47 scheme!

I would hazard that, given that the output valve is probably around the same plate impedance as the triode strapped ef12 on the V41,  but that there are 2 in parallel so we're looking at half the value, it would be a little better.  Although I don't see that the shunt feedback is near the same amount on this so, six of one, half a dozen of the other?

I'd hate to say go for it and have you commit money and time and then it not be up to scratch.   Do you have any way of lashing things up and trying the output stage driving that transformer without a big commitment?

Edit: just did a fingers and thumbs calculation based on the info on the schem and it looks like impedance of the output anodes is around 8K on that amplifier so...   Hmm?
« Last Edit: September 07, 2019, 12:49:19 PM by Winston O'Boogie »
D. J. H.

PRR

Re: Lorenz V41 output transformer
« Reply #13 on: September 07, 2019, 01:43:54 PM »
Does the anode choke somehow reduce the tubes output impedance?

No. (It is 47K in parallel with much lower impedances; insignificant.)

Re: Lorenz V41 output transformer
« Reply #14 on: September 07, 2019, 10:33:46 PM »
Heya Murdock,
took another look at the V241 schematic and think you might be better just grafting the output stage from the Lorenz V41 onto the end of it.  We know the transformer works when fed like that, albeit the tube isn't working efficiently.  You could substitute another EF86 for the EF12 without too much bother. 
The V241 looks like a nice circuit

To clarify:  I "think", you keep both the EF86 and ECC85 from the V241, and add another EF86 working like the V41 end stage, it'll be fine. 
« Last Edit: September 07, 2019, 10:42:34 PM by Winston O'Boogie »
D. J. H.

Re: Lorenz V41 output transformer
« Reply #15 on: September 08, 2019, 10:02:30 AM »
There is a series resonance of C20 and primary inductance. fr is 27 Hz.

Re: Lorenz V41 output transformer
« Reply #16 on: September 08, 2019, 10:03:18 AM »
diagram

Re: Lorenz V41 output transformer
« Reply #17 on: September 08, 2019, 03:09:12 PM »
There is a series resonance of C20 and primary inductance. fr is 27 Hz.

Ha! Never thought to look at that, good catch Andreas.  I haven't seen that used on anything other than old tube radiogram tbh. 
C.J. is our resident transformer guru but I would think it's quite a narrow Q given the inductance value and, without damping, some possibly quite big peaking to occur there yes?  And hence also saturation.
Wonder how much of it was left to load impedance to keep under control somewhat...

P.S.  can't see your 2nd attachment
D. J. H.

Re: Lorenz V41 output transformer
« Reply #18 on: September 09, 2019, 02:03:26 AM »
sorry

PRR

Re: Lorenz V41 output transformer
« Reply #19 on: September 09, 2019, 02:40:58 PM »
> without damping, some possibly quite big peaking to occur there yes?

If we are still looking at V241..... The ECC85/12AT7 is triode and lowish rp. Also the DC choke shows 70V drop from B+ so has like 10k dead resistance inside. Assuming the OT is similar, we get -1dB @ 50Hz and -3dB @ 26Hz, with 6dB/oct slope at first then steepening below 3Hz. All hardly affected by load/no-load. This neglects the NFB around the ECC85, so it may be flatter at 50Hz and more-bump at 3Hz, but can't be much NFB so won't be a big bump. Seems good to me.


 

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