V-241 from scratch

[djn111]

Hi,

Very nice build en inspiration

But if you ever build a other one, I will give you a nice Elma switch for the gain switch ;-)

DJN

 

[DaveP]

Too late, the second one is almost finished!

Thanks anyway

DaveP

 

[Murdock]

Hey Dave,

have you looked at the Lorenz V41 preamp?
It is the predecessor to the V241 and V72 preamps.
As I was lucky enough to get two original V41 output transformers for 50€  : I would like to build a blend of the V41 and V241.
I thought about using the same tubes as the V241 but the gainswitching of the V41.
How would you implement the gainswitch in to the V241 schematic?
Would it be as easy as just to put it after the output cap (C9 in the V241 schematic) or before as you did it? 

 

[DaveP]

I thought about using the same tubes as the V241 but the gainswitching of the V41.
How would you implement the gainswitch in to the V241 schematic?
Would it be as easy as just to put it after the output cap (C9 in the V241 schematic) or before as you did it? 

Yes, I know about the V41, the gain switching of the V76 was developed from it and is almost the same.  It is an elegant combination of feedback and cathode bypass gain changes.

Wise to use a later pentode type as the EF12 type has a much higher g2 current (partition noise), check the data sheets.

Breadboard the circuit, then try connecting before and after the cap, the results may well be different.  I think I found some instability coonecting after the C9 cap.

Note that the feedback string is in an earthed metal box and that C17 has been put in for a reason maybe to tailor the response or cure a stability issue or both.

Find a triode that has the same spec as the E12 triode wired. An EC90 is close, but there are many other possibles.

Good luck

DaveP

 

[Murdock]

Yes, I know about the V41, the gain switching of the V76 was developed from it and is almost the same.  It is an elegant combination of feedback and cathode bypass gain changes.

Wise to use a later pentode type as the EF12 type has a much higher g2 current (partition noise), check the data sheets.

Breadboard the circuit, then try connecting before and after the cap, the results may well be different.  I think I found some instability coonecting after the C9 cap.

Note that the feedback string is in an earthed metal box and that C17 has been put in for a reason maybe to tailor the response or cure a stability issue or both.

Find a triode that has the same spec as the E12 triode wired. An EC90 is close, but there are many other possibles.

Good luck

DaveP

Hey Dave, thanks alot for your answer!
I wanted to use EF86/E80F and ECC85 like the V241. Already got a suitable choke and don't want to buy two more.
I saw that your (V76) gainswitching was similiar but that also adds two more electrolytics... Can you maybe explain why?
In the orignal there is only one electrolytic in the whole circuit and I wanted  to try to keep it that way, maybe even use a wet tantalum for that spot.

Also in one Maihak V41 they tied the input transformer secondary directly to grid without caps. Can you maybe explain the difference between the Lorenz and the Maihak input topolgy?

 

[DaveP]

I saw that your (V76) gainswitching was similiar but that also adds two more electrolytics... Can you maybe explain why?
In the orignal there is only one electrolytic in the whole circuit and I wanted  to try to keep it that way, maybe even use a wet tantalum for that spot.

I copied the V76 circuit as far as possible, I figured if its good enough for them, its OK for me and not directly in the audio path, for that I used poly prop caps throughout.  Tantalum has not got a good reliability record.

Also in one Maihak V41 they tied the input transformer secondary directly to grid without caps. Can you maybe explain the difference between the Lorenz and the Maihak input topolgy?

I can only guess really, I can think of two reasons. 1: they used a different input transformer that had some secondary effects that needed correction (after the war everything was in short supply so you just used what you had). 2: It was developed for a studio environment that had a lot of stray fields like tape machine bias or camera scanning so it needed to be filtered out.

On that subject: notice the chokes are all in boxes (mine too) as if they radiate back to the input circuit you get an unwanted feedback path and possible oscillation.

The V76 was connected directly so I would copy that version, resistor thermal noise on a grid is bad news, 10M is to be avoided!  It is not easy making ratios like 1:18, so they have tried to correct the response I guess.

DaveP

 

[AndreasSchwarz]

Also in one Maihak V41 they tied the input transformer secondary directly to grid without caps. Can you maybe explain the difference between the Lorenz and the Maihak input topolgy?

Hello,

the first tube in the ÜV268 works with grid leak bias (W1 10 MΩ).

Andreas

 

[smilan]

The earth/ground at the IEC socket goes direct to chassis as is the regulation I believe. The amp earth bussbar is only connected to chassis at the input socket (Merlin's design). The amp earth does not touch the chassis anywhere else.

I had to increase the output cap from 8uF to 10 because I couldn't source the lower value in that quality.

All the other caps are as the schematic except there is no cap on the input grid as I am using a V76 front end.

I will post the final schematic later as I may need to do some tweaking after testing.

DaveP

Hi Dave, what type of caps did you used for C3 and C10? The original ones are large oil caps.

 

[DaveP]

I can't answer you unless you specify which schematic you are referring to

Best
Dave P

 

[opacheco]

I can't answer you unless you specify which schematic you are referring to

Best
Dave P

DaveP

How many schematic do exit for in total?

Thanks
opacheco

 

[DaveP]

Hi Opacheco,

There is the original from 1958, then the Lorenz from 1960.
My version uses a V76 front end with a V241 output stage, that's why it's called a V241-76.
I made this change because the original switches DC through the input tube which is a recipe for noise.
The output stage is very good and possible with modern available parts.

best
DaveP

 

[opacheco]

Hi Opacheco,

There is the original from 1958, then the Lorenz from 1960.
My version uses a V76 front end with a V241 output stage, that's why it's called a V241-76.
I made this change because the original switches DC through the input tube which is a recipe for noise.
The output stage is very good and possible with modern available parts.

best
DaveP

Dear DaveP Thanks for your response! but do you have the schematics of these versions?

Thanks again
opacheco

 

[DaveP]

The original 1958 is on page 4, my version is on page 5 and the 1960 is on page 8.

best
DaveP

 

[opacheco]

The original 1958 is on page 4, my version is on page 5 and the 1960 is on page 8.

best
DaveP

DaveP thanks for the response and help!!…I forgot I had read all this post in the past

opacheco

 

[opacheco]

Dear DaveP, what schematic are you talking about here??

It still isn't completely clear to me, for example i wonder what are R1,2,3 and C1 on schematic on p.2. R2 and R3 look like biasing resistor and voltage divider for R1. I'm probably wrong on this assumption and don't know function of C1.
V72s also has two cathode resistors, although that is so just to set the gain. C13/50uF on V76 is more clear, it should change open loop gain for NFB resistors setting gain.

 

[My3gger]

This is probably revision of V241-76 as he called it. Schematic is similar to V241, i'm sure Dave will add it and reply when he finds time.

 

[opacheco]

This is probably revision of V241-76 as he called it. Schematic is similar to V241, i'm sure Dave will add it and reply when he finds time.

Ok My3gger, thanks for your comments!!

opacheco

 

[My3gger]

Ok My3gger, thanks for your comments!!

opacheco

You are welcome, my schematic would be wrong if you didn't ask... I rather wait for designers to post their own schematics, going through all pages i've seen Dave already posted final one. My question was about schematic from p.2 (i might be wrong about this one), note it is NOT FINAL one as posted on p.5 with some differences to the output stage.
Did you build one from final schematic? I'm very interesting in knowing how it sounds compared to original which is great, we felt more highs would make it a lot more useful.

 

[opacheco]

You are welcome, my schematic would be wrong if you didn't ask... I rather wait for designers to post their own schematics, going through all pages i've seen Dave already posted final one. My question was about schematic from p.2 (i might be wrong about this one), note it is NOT FINAL one as posted on p.5 with some differences to the output stage.
Did you build one from final schematic? I'm very interesting in knowing how it sounds compared to original which is great, we felt more highs would make it a lot more useful.

I am planning to built one but in the future, no now!, I have many projects in another’s themes for now!

Thanks
opacheco

 

[paulcheeba]

Does anyone have the schematic for the first version of the Maihak V41? i.e. Not A or B as a post fix.