V-241 from scratch

I have been wanting to make a good vintage mic pre for some time and the discovery of the Lorenz V-241 schematic from 1958 has now made this a possibility.

The interesting part about this design, is the use of a 150 Henry choke for the output stage.  The V series mic pre's need special chokes made to order as they are 400H and over.  You can buy a Hammond 150H choke for around $17 plus shipping.  The reason why you can use this lower level of inductance is because the twinned ECC85 has an rp of around 9k,  instead of the very high rp of a pentode.  I have tested it and it's flat down to 10Hz.

Another factor that has made this a possibility is an affordable source of MuMetal foil from South Korea.
This arrived today, beautifully packed on plywood.


I have put one of the new LED lightbulbs next to it for scale.  It cost $60 including shipping.
The V-241 has all its transformers and chokes in MuMetal boxes because it is crammed into a small rectangular box.


I shall be making mine in a 2u rack mount case, but I will still have to pay great care to layout and screening to achieve the lowest noise levels.

DaveP

Very interesting! I'll definately follow this thread.
Which transformers will you use, if I may ask?

*subscribed* 

Which transformers will you use, if I may ask

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I have a Jenson  JT-115K-E60 on route as we speak, this is the double MuMetal case version which should reduce noise at the front end to a minimum.

I have not made up my mind about the OPT yet, it will be 15k: 600, but I've not decided on manufacturer yet.  I'm open to suggestions.

Thanks for your support

DaveP

DaveP said:

I have a Jenson  JT-115K-E60 on route as we speak, this is the double MuMetal case version which should reduce noise at the front end to a minimum.

I have not made up my mind about the OPT yet, it will be 15k: 600, but I've not decided on manufacturer yet.  I'm open to suggestions.

Thanks for your support

DaveP

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I‘ve used this output transformer in my v241 rebuild. It works fine for me.
http://diy-tubes.com/index.php?route=product/product&path=25_72&product_id=201

Yes that would probably do the job, but I really want an OPT in a MuMetal case, a Sowter 1290 maybe my best option.

DaveP

Sounds good! The original V241 is for me one of the best preamps i‘ve heard in my life.
I‘ve heard many preamps.

I'm very pleased to hear that Eric ;D
It will be a lot of work just for it to sound lousy!
Thanks

DaveP

Ericbazaar said:

I‘ve used this output transformer in my v241 rebuild. It works fine for me.
http://diy-tubes.com/index.php?route=product/product&path=25_72&product_id=201

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I just saw in the schematic that the original used 1:18 input and 17:1 output transformers...
Did you change anything in the circuit or did the lower ratio transformer worked right out the box?
Or is there another schematic?

Murdock said:

I just saw in the schematic that the original used 1:18 input and 17:1 output transformers...
Did you change anything in the circuit or did the lower ratio transformer worked right out the box?
Or is there another schematic?

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I‘ve measured booth transformers of the original one. Input was 1:20, Output was 18:1. In my Build i use 1:10 Lundahl at the Input and these Russian Transformer (Datasheet 7:1+1 but i measured 8,2:1+1) at the Output. Only Change i‘ve made is to drive the second Stage ECC85 with lower Plate Voltage and higher Current 200V Plate @ 11mA (Grid Voltage -3,5V) instead of 250V @ 6mA (Grid Voltage -4V) from the original Schematic.
Frequency Response is flat from 15hz - 20khz. The rebuild sounds close to the Original. No Hum at all. I‘ve build with EZ80 rectifier Tube and AC Heater like the Original Schematic.

The ratios don't matter much.  They were built for 200 ohm operation, but nowadays we might choose 600 ohms so the ratios change to suit.

DaveP

Subscribed.

I have only seen 1 of your builds Dave, but I am highly anticipating this one after seeing the first (Its the Comp that EMRR is selling on your behalf. Crazy metalwork!)  ;D

DaveP said:

The ratios don't matter much.  They were built for 200 ohm operation, but nowadays we might choose 600 ohms so the ratios change to suit.

DaveP

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sorry, I'm not experienced in electronics. But you say ratio doesn't matter Much. Why is that? what advantages or disadvantages does a higher ratio transformer have? lower noise? higher gain? why did they use such high ratio?

what advantages or disadvantages does a higher ratio transformer have? lower noise? higher gain? why did they use such high ratio?

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If you look at the ratios on the schematic...........


You will see that the turns ratio of the IPT is 1:18, this means that 10mV from the mike becomes ~180mV on the grid. this is usually noise-free amplification which is very welcome.  However, 1:18 is also  18 squared for the impedance so its 1:324, this means that 200 ohms becomes  64.8K which is ok for a tube amp input.

At the output things are reversed as the ratio is 17:1 so the impedance ratio will be  289: 1.  If the load is 200 ohms, then the amp  will be working into 289x200 = 57.8K  The maximum output voltage is stated at 3.1V, so at the anode/plate it will be 17 x 3.1 = 52.7V

Now suppose we want the amp to be working to 600 ohms rather than 200.  A common OPT type is 15k:600 which is a ratio of 15000/600=25, so the amp will be working into 15k rather than 57.8k which is probably fine.  25 impedance ratio is 5:1 voltage ratio so the 52.7V on the tube will now be 52.7/5=10.5V at the output.

The amp itself will always be amplifying between 10-316 times but the transformers on either end can modify that as I've shown

It is much simpler than it looks, best thing is to copy it out yourself on paper

DaveP

I sent an email to Cinemag with a request for details as they weren't on their website....................no reply.

When I was at work, all emails had to be answered in less than 24 hours.......my conclusion, Cinemag don't need my business.

So I ordered the Jenson OPT in the end, it is the only one they make and is very highly regarded,  Total cost for IPT and OPT $292

That is serious iron!

DaveP

The V-241 is an interesting circuit. It seems to have elements of the V72,  V76 and REDD37 and as far as I can see it has included the best bits of them all.

It has the second stage plate inductor of the V72 et al. This allows you to use a separate output transformer and alter its turns ratio for different applications without altering any other parts of the circuit.

It has the variable NFB circuit of the V76 which allows the gain to be varied over a wide range without compromising stability.

It has the paralleled triode output stage of the REDD47 with local NFB. This both helps minimise distortion and allows a lower value plate inductor to be used.

No wonder it sounds good.

The only odd thing is the use of the ECC85 tube. This was one of the first tubes designed specifically for FM radio reception; one triode for the VHF oscillator and the other for the mixer and was introduced in 1954. I guess they would have used the ECC88 if it had been available but it was only introduced in 1958 so perhaps too late to be included in the design. I wonder if they ever did a later version with the ECC88?

Cheers

Ian

The only odd thing is the use of the ECC85 tube. This was one of the first tubes designed specifically for FM radio reception; one triode for the VHF oscillator and the other for the mixer and was introduced in 1954. I guess they would have used the ECC88 if it had been available but it was only introduced in 1958 so perhaps too late to be included in the design. I wonder if they ever did a later version with the ECC88?

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I wondered about that too, but the mu of the ECC85 is  55 as against 33 for the  ECC88.

While you are online, I was planning to do without the input cap and grid resistor after the IPT, or at least change it from 2M to 1M as my IPT is only 1:10.  That would give me an industry standard 10k input Z.  Any thoughts?

DaveP

DaveP said:

I wondered about that too, but the mu of the ECC85 is  55 as against 33 for the  ECC88.

While you are online, I was planning to do without the input cap and grid resistor after the IPT, or at least change it from 2M to 1M as my IPT is only 1:10.  That would give me an industry standard 10k input Z.  Any thoughts?

DaveP

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I have a feeling you cannot do this. You would have to connect the transformer across exactly where the 2M resistor was - but this point will vary with the NFB at the lower gain settings which means it really needs to be referenced to 0V.

Cheers

Ian

Yes I see that now, incidentally, I have discovered why the schematic is incorrectly marked as 90V on both the Va and Vg2.

Checking the original schematic of 1958 (pre preferred values version) I noticed that Ra was originally 300k, not 390k.  I have found this kind of thing before after a revision.  They change the component values, but forget to change the voltages.

I checked the circuit on the breadboard and it is indeed 90V with 300k, but with 390k it's 63V.  I don't know why they changed it so much, 330k would have been OK.

DaveP

I have made a few decisions about some modifications, following Ian's comments.

I have always liked the V76 front end, it's a very elegant way of combining feedback with gain reduction.  I tried this on the breadboard and it works perfectly.  This change allows me to lose the input cap and 2M grid resistor as well.

I do not like on principle switching DC in an amp, so that was a no-no for me with the V-241 in its original format for the 20 and 25dB gain reduction..  There also needs to be a 160V differential between the plate of V2 and the plate of V1, to maintain the original level of DC feedback, the 360k Ra makes this possible.

The Hammond 150H choke does not have the DCR of the original, so the voltage drop is less, in effect this makes both tubes run from 270V which is not good practice.  I got around this by making V1 plate resistor 360K instead of 300k or 390k, this puts the plate voltage at 106V with the V76 cathode arrangement.  In order to keep the 160V DC differential the V2 plate needs to be 266V and its B+ 289V.  this sorts out the HT supply problem at the same time.

This is my draft schematic, not yet finished, but shows the details I'm talking about.



It also gives a gain range of 20-55dB over the 8 switch positions.  The feedback string can be neatly mounted between the switch positions as is good practice.  The top cut caps of the original are no longer needed (-3dB ~24kHz)  The lower value feedback resistors are not finalised yet, but get things in the right ballpark.

I will calculate the ripple on the power supply in my next post.  It uses 420V 105C  12,000 hour Rubycon caps which are excellent value for money at around £2.60 each.

DaveP