V-241 from scratch

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I received the mic pre and have been recording with the 241-76 mic pre now here for about a week on as many sessions as i can.
And i can say with certainty that its  very classy sounding mic pre indeed !

It has its own character but at the same time sounds to my ears quite flat.

Its been up against some pretty big names, as i have here 6 other mic pre's here that i use regularly.
My favourite for vocals over the last few years has been either of my API's (the 512 or Capi 312) both of which are great. And the V241-76 sounds very bit as good as those and has its own character - which is in short - big and round!

I am especially pleased with how it work well with my vintage C12.

Thank Dave for your great builds, i use them all regularly on my productions here each week .

Ian

 

[Alm6mountain]

Great to hear! I am currently in the process of building one. Waiting on the cinemag transformers to be built, so it won't be done for a couple more weeks.

Right off the bat I realized that my power transformer didn't have a center tap for the 6.3vac filament. Do any of you guys know if you can use an artificial center tap (2 200r resistors from each side of the transformer to ground) in this situation? I have done some research and it seems like if I only have one ground reference I should be good, but I want to make sure before shorting something out. Thanks!

 

[DaveP]

I have not tried that on a high current circuit, but I guess they would have to be rated for the return current if you are making a DC filament supply.  An ac filament supply will not give you such a low noise spec.

Good luck

DaveP

 

[My3gger]

Using LDO regulator and schottky diodes can get you close to 6,3V on heaters. LDO drop doesn't take much power from PT.
I found EH makes new ECC85/6AQ8. Looks well built like some more expensive EHX, so it is good news for those who didn't know it.

 

[Alm6mountain]

Thanks for the help! Very excited to have just finished building this amazing preamp! I tested it with my Apex 460 modded to a C12 and was blown away by the huge low end and clarity on the vocals and acoustic I recorded! I added a switchable Cinemag DBX DI transformer and had a blast playing my  50's Silvertone bass through it. It blows the Altec 1566a I built out of the water.

I am looking forward to running some tests on this version to see how it compares with the others built. I am expecting a higher noise floor/AC hum because i have not shielded the power transformer or cables, but the Cinemag CM-75101A input transformer with a mu-metal casing seems to be doing a great job!

I am planning on making a HPAD output switch so I can get some crunch  while running a good level to my interface.
I was thinking of a 1K impedance on both sides of the pad to bridge the transformer output (output load is around 80 ohms?) But I would love to hear if anyone has other values I should try.

Dave- thanks again for sharing this incredible pre with us! I hope to share the results of my tests this weekend along with some sound clips!

 

[Murdock]

Looks and sounds nice!
Did you build it according to the original schematic or Dave's modded one?

 

[DaveP]

@Alm6mountain,

It's great that you have made your own V241 and that this project has been helpful.

Another guy's post about the V241 inspired me, and so it goes on, this is why this forum is so good.

Best
DaveP

 

[Alm6mountain]

Looks and sounds nice!
Did you build it according to the original schematic or Dave's modded one?

This is almost identical to Dave's modified 241-76 schematic found on the previous page. The only changes were to 1 resistor value on the high voltage and 1 resistor on the filament because I am running a a Hammond 115V power transformer at 120V. However, there is no shielded wires at this point.

 

[Alm6mountain]

Attached is my result from the frequency response test.

I am not sure if my way of measuring is the best that I could have done.  I loaded the output of my interface with a 1.5K pad that fed the preamp. (I added a crude diagram of the signal path attached as well) I used the RightMark Audio Analyzer software with a sine sweep. I have used this a method as a simple way to test builds, and it seems to give me the results I would expect, but I am sure there has to be a better way. I don't currently have a signal generator other than my computer and my oscilloscope is from the stone ages. How do you guys get accurate tests?

 

[DaveP]

A stone age scope is fine!

You just need a basic sig gen with a constant output level.

Take readings at 10, 20, 50, 100, 200, 500, 1k, 2k 5k, 10k,20k, 50k. off the scope.

Convert your readings to 20LOG( *) in Excel then relate them to the reading at 1k

Put them in a table in Excel and choose smooth line graph option, then format the frequency to log scale.

Most amps are totally flat in the mid ranges, but curl up or down at either ends of the spectrum.

DaveP

 

[Pief]

@Davep

Nice work. I was reading up on your project because i.m building a preamp with an Ef86 at the first stage aswell.

I started a thread about this. The Olmsted brdcstr thread.
Wondering if you could shed some light on what to do.

I was wondering. I got my project free of hum. But not free of hiss.

The ef86 is pushing a signal that is really hot and hissy.
Did you experience the same problem?

I have a gain pot between ef86 and second stage.
Thinking about repositioning it before the first stage to control it a bit better. Don.t wanna run a pad just to keep it under control.

Any thoughts?

 

[DaveP]

@Pief

I'll reply on your thread

DaveP

 

[Murdock]

@DaveP

I would like to build a stereo pair of this preamp and thought about building one power supply for both units.
Would that be advisable? Is it even be possible with the anode choke?
Also, if I would use the original psu schematic, would I need four heater windings  and four "hum eliminator" potis? It probably isn't really advisable to use one poti for two tubes, right?

 

[guavatone]

@DaveP

I would like to build a stereo pair of this preamp and thought about building one power supply for both units.
Would that be advisable? Is it even be possible with the anode choke?
Also, if I would use the original psu schematic, would I need four heater windings  and four "hum eliminator" potis? It probably isn't really advisable to use one poti for two tubes, right?

I would split the HT with 2 1n4007s after the second RC stage on Dave’s PS.  You will have to decrease the dropping resistors on the first 2 RC stages. 4 heater windings is overkill and probably doesn’t exist.  1 humdinger worked for V series amps, so you should be fine. I’m not sure why they did dual heater supplies with such low current needed. 

I get 1.4A for Filament Supply.

 

[Murdock]

I would split the HT with 2 1n4007s after the second RC stage on Dave’s PS.  You will have to decrease the dropping resistors on the first 2 RC stages. 4 heater windings is overkill and probably doesn’t exist.  1 humdinger worked for V series amps, so you should be fine. I’m not sure why they did dual heater supplies with such low current needed. 

I get 1.4A for Filament Supply.

Hey guavatone,  thanks for your reply.
But I don't quite understand the "split the HT with 2 1n4007s after the second RC stage" part. 
Could you please elaborate on that?

 

[guavatone]

Simply put a diode on each channel’s B+ branch in series. It’s supposed to minimize crosstalk.

 

[Alm6mountain]

Hey everyone. I finally had some time to test some pres of mine over this holiday and I found something a little strange with the EF86 current draw in my build which is exactly (i hope)  following the final schem. It appears the plate on mine is pulling about .4mA instead of .7mA. I tested this and found similar results with 2 different EF86's. Is this acceptable and within the range of proper function? Or is something amiss? BTW, it sounds great the way it is running...

 

[guavatone]

I wouldn’t worry about it if it sounds good.  BTW where did this .4 mA spec come from?

 

[Alm6mountain]

I wouldn’t worry about it if it sounds good.  BTW where did this .4 mA spec come from?

It came from reading voltages and using ohms law to determine current from drop of voltage over a resistor. I also checked my work using a tube bias calculator. the result was the same.

 

[DaveP]

0.4 mA is just about optimal for an EF86 and many other recent pentodes. 
For best noise, the G2 current should be about  20% of that.  The early pentodes had closer G2 filaments and could only manage 33% or 25% and noise from the partition current was not so good.  EF806 is particularly good in this respect.

DaveP