V-241 from scratch

[DaveP]

Morgan Jones formula for calculating ripple on the HT line:-

0.01 seconds x current in A/ Capacitance in Farads =Vripple pk-pk.

My power supply for the V-241 just uses overkill because the caps are cheaper for 5.

Current draw is 8.148mA

So we have 0.01 x 0.008148/ 0.00018 x 2.828  =  0.16Vrms

Each dropper resistor is 2.7k and the reactance of 180uF is 17.7 ohms so each stage is a voltage divider.........

0.16 x 17.7/2717.7 = 0.00104V

Next stage goes down to 6.8uV and final 2.7k gets down to 44nV.

For the supply to V1 we have a 33k dropper and 180uF which gets ripple down to 23pV, way less than the noise of the plate resistor.

This is important because the PSRR of a pentode is really poor,  but in this case there is capacitance to spare.

DaveP

 

[Murdock]

No news  really want to see your progress! 
Also I just noticed something on the schematic. At the choke you can see "150H" and "10k".  Thought that would probably be inductance and resistance. But if you look at the output transformer there is also a "150H" at the primary. Is that the inductance of the trafo? If yes, why are there no markings on the other transformers?
Also, could you tell us, where you get you mu-metal sheets?

 

[DaveP]

Sorry to disappoint, but I'm still waiting for the parts to arrive.

Local bureau de poste lost my Jensen IPT for a day, then I had to pay 37 euros import duty!  I collected it today however.

I said that the schematic I posted was not finished, I only posted it to illustrate the changes I made.  The finished schematic will be posted soon.

This is where I got the MuMetal sheet:-
https://www.ebay.com/itm/171173026902

It was very well packed on a plywood sheet, like a work of of art!

The choke I'm using is 150H @8mA +/- 15%, I am using it at around 7mA so it should be at least 150H.

https://www.tubesandmore.com/products/filter-choke-hammond-open-bracket-150-h-8-ma

It will take me some time to design the chassis and the layout, but I will get there in time.

DaveP

 

 

[Murdock]

Hey, thanks alot for the link! Pretty thin. Will you use several layers or combine it with steel shields?
Also, I meant the original schematic.

 

[DaveP]

I will use it in combination with other layers (I also have some copper foil) because even though the layers are thin, the magnetic flux is weakened by having to pass through several dissimilar metals.  I discovered this during tests before I made the U73b
https://groupdiy.com/index.php?topic=59242.0

DaveP

 

[Murdock]

I will use it in combination with other layers (I also have some copper foil) because even though the layers are thin, the magnetic flux is weakened by having to pass through several dissimilar metals.  I discovered this during tests before I made the U73b
https://groupdiy.com/index.php?topic=59242.0

DaveP

thanks a lot, really nice thread and very useful information!
can't wait for your progression!

 

[DaveP]

The Jensen OPT arrived yesterday.

This is supposed to be one of the best OPT's in the world.  So if this mic pre is lousy, it won't be the fault of the transformers.

I can now start on the chassis design.

DaveP

 

[DaveP]

I started on the Faceplate today, it will be 2u and I'll try to build the rest of the chassis around it.

I covered the sheet of 3mm Ally with masking tape and measured off 3.5 inches for 2u.  The tape is useful for marking and protecting the surface from scratching, getting it off the reel in one piece is an art in itself.

Carefully cutting about a mm away from the line.

Then half an hour of filing down to the line!  The ruler tells you where you need to file most.

When it's perfectly flat, hold some emery paper round a file and smooth the edge, carefully take off all the sharp edges to avoid slicing your finger later.

Marking out all the drilling positions, use a square for this to do both sides at once.

The hole cutter makes very neat 12mm holes  for the on/off switch.  The dimensions for the rack mount slots can be found here.
https://en.wikipedia.org/wiki/Rack_unit.  I don't think this has been written by anyone who has ever cut the slots, other wise they would have included how long they should be,  I mark the quarter inch hole centres 0.5 inches from the edge, then draw tangential lines to the edge.  The centres are 1.75 inches apart. from the centre of the plate.  Filing them neatly is the hardest part of the whole task. :-\

This is the  faceplate from the rear almost finished.  Still to do: countersinking 18 fixing holes, hole for the LED (when it arrives) and the hole for the gain switch,  I don't know the exact position yet.

All for now

DaveP

 

[DaveP]

A lot of work has been done, but not much to show for it! :-[

But here, at least you can see the size of the chassis as the build progresses.

This will be the first chassis I make entirely from scratch, all the others have been modifications of commercial boxes. that source has now dried up unfortunately.

There will be several sub-divisions inside this box for the power supply and capacitors.

Onwards and upwards

DaveP

 

[DaveP]

I made the rear panel today and started cutting out the holes, I'm still to decide on the exact position of the output socket.

The first of several internal divisions is in place.

I now have all the components so I can plan the detail of the layout.

I have used some of the best capacitors available for the audio signal and the longest life electrolytics for a long trouble free life.

Some of you may not know that the lifetime of an electrolytic doubles for every 10C below its rating.  So 2000hrs at 85C becomes 32,000hrs at 45C. 

I have used Vishay's 118 AHT Series which are 4000hrs at 125C, so they will probably outlive me

I will post the complete parts list and schematic after its finished.

DaveP

 

[Ericbazaar]

Hello Dave,

very good work, but why no Case like the original one? This looks so much better then these standard 19" Cases :'( 

cheers
Andreas

 

[DaveP]

why no Case like the original one? This looks so much better then these standard 19" Cases

Thanks Eric,
I did consider this, but a potential buyer preferred a rack mounted version.  19" also gives more distance to keep sensitive areas away from power supplies.  The original V-241 only had a S:N ratio of around -69dB, which is not really good enough nowadays.
This is often because it's hard to keep hum out of the audio chain in such cramped conditions.

Best
DaveP

 

[Ericbazaar]

Thanks Eric,
I did consider this, but a potential buyer preferred a rack mounted version.  19" also gives more distance to keep sensitive areas away from power supplies.  The original V-241 only had a S:N ratio of around -69dB S-N, which is not really good enough nowadays.
This is often because it's hard to keep hum out of the audio chain in such cramped conditions.

Best
DaveP

Interesting, have you measured this on a original one? -69dB is only a little bit better than a good working M15a Tape Maschine. I have booth here, two V241 and one M15a.  Picture up is a rebuild. Booth preamps are complete recaped, i changed importand old resistors and ceramic caps and cleaned the Poti.
All Tubes are new.

The preamps have no noise or hum at all, same noise Level than a Neumann V476b or RFT MV810/2 at the same Levels. These are very good noise free solid state Amps. Of course recaped and cleaned.
I have no expensive Measuring Equipment, but the V241's are the quitest Tube Preamps i've ever heard.

P.s. My rebuild is a little bit quiter than the original one, i installed no shielding, only change i made is the toroid is build outside, backside of the Case.

 

[DaveP]

Interesting, have you measured this on a original one? -69dB is only a little bit better than a good working M15a Tape Maschin

No I have not measured one, I'm  going on the report in the V-241 thread. (attached)

Best
DaveP

 

[Ericbazaar]

No I have not measured one, I'm am going on the report in the V-241 thread. (attached)

Best
DaveP

Okay thank you, this is a Test from a german Magazin called „Studio Magazin“ translated to english. The tested V241 has a Hum Problem, these Preamps are build for 220VAC and must be new callibrated with the installed „dehumer“ (two 100ohm potis) you can bring the Hum down to a minimum. Better use a variable Transformator, i didn’t know the word in english : (in german „Stelltrafo“)Hope they did that before the test

 

[DaveP]

This is my draft schematic.

I've called it a 241-76 because it has a V76 front end.

I may have to adjust some components during testing, but in a rough breadboard circuit this works OK.

The metalwork on the case is almost complete but it takes quite a while to imagine where all the components will go,

You only get one shot at a prototype like this so you have to be very sure where you drill holes or it will end up a mess.

DaveP

 

[DaveP]

The vertical panels are now designed.

Panel 1

This carries the heater TX and both of the rectifiers.

Panel 2

This is a horizontal panel for the big heater capacitors.

Panel 3

This holds the heater dropper resistor and some of the  HT caps, the rest fit on the reverse of panel of panel 1.

These panels are ready to wire, but I still have the main panel to fit and design for the tubes and the rest of the components.

I'm not sure how much time I'll have have over Christmas due to family commitments, but steady progress will be made.

DaveP

 

[letterbeacon]

DaveP threads are always good value! Nothing to add here apart from this looks like another amazing project and I'm looking forward to seeing the results!

 

[DaveP]

Letterbeacon, :-[

The layout was sorted out over the long Christmas break, here are the supports for the chassis plate.

Here's the chassis plate in position.

These are the main components in place.

Now I know where to put the output socket.

Notice the massive polypropylene 10uF 630V cap!  I must be turning into an Audiophool 



These are the rough positions of the screens

The screen around the choke must be cut to size and it will then have a MuMetal sheet lining to stop the radiation of the output signal back to the input section.

The tuner box will likewise be cut to size to screen the gain switch resistors.  I have found from experience that these can be a source of noise and hum pick-up.

Now that these components have their positions fixed, I can drill the front panel for the switch and then prepare for painting and lettering.

Best wishes for the New Year to all groupdiyers

DaveP

 

[DaveP]

The chassis design and build is finally finished.

In the end it was easier to make a box to shield the feedback gain switch.

This is with everything in place.

There is also now a shield around the choke to stop it radiating back to the input and causing oscillation.
Wires on the panels have been pre-connected ready for final assembly.

Design from scratch takes a lot of time and thought, wiring in all the components is the easy part, which is what I'm looking forward to next.

I will be trying out some of the MuMetal and copper foil too

DaveP