Apex 460 mod with some Telefunken ELA M 251 influences

[grantlack]

I was in the same boat not so long ago. Still am in many respects  ;D.
When in doubt, google and datasheets are good companions. The assumption that something isn't a direct sub is actually a good one, but thorough checking and anecdotal knowledge from  'prior art' can get you where you want to be a little quicker and a little cheaper.

 

[saxmonster]

Are there any cleaner higher dpi, schematics versions of 251.  I cant read some of the numbers because of the scanned pictures, writing is so small and distorts when you enlarge.

-Scott

 

[e.oelberg]

I'm a real amateur and don't know much about electronics. Anyay I built 2 Elam 250 and I'd like to change a few things.
I could design a small PCB for the relay switches. Is anybody interested ? If so what is the inner diameter of you mic body ?
nicholas

 

[saxmonster]

My GE 5 Star 6072 arrived today, man Brent is quick.  I ordered it on friday, wow.  Now to get the transformer and caps. I order my c12 a week ago or so.  That will take the longest.


-Scott

 

[saxmonster]

Nicholas,
My power supply has a switch on it for the different polar patterns so I will not need it but I will take a measurement for you of the inside diameter of the tube cover.

-Scott

 

[wave]

My GE 5 Star 6072 arrived today, man Brent is quick.  I ordered it on friday, wow.  Now to get the transformer and caps. I order my c12 a week ago or so.  That will take the longest.


-Scott

Scott,
Congrats! Brent is a good guy. Did you talk to him about the ECC081s?

Dave

 

[saxmonster]

Dave,
I didn't talk to him about the Ecc's  I can't really swing that price right now so when I get some cash flow up maybe in a year or so I can replace the Ge with one of those Ecc's.  I can then sell the GE on ebay or maybe brent would want it?

Thanks
-Scott

 

[saxmonster]

Is there a place somewhere here I check in the meta but no luck.

I need to know capacitor conversions.

Such as uf or ufd converted to Pf.  Do i just add 6 zeros?

does Pf = P  and uf = ufd?

Also what does 1 mike equal in a clarity cap brand?

Also as far as the 6072 tube does pin one correlate to plate one or is pin 6 plate one?  Just wanted to double check.

 

[saxmonster]

Dave Thomas sent me this email and I will be following this to the T.


Hi Scott, forget the 251 schematic!!!!!!  The existing 460 schematic is far superior with just a few simple changes.

There are a lot of myths and misconceptions on Gear Slutz and other Forums.

The plate only circuit is inferior to the 460 circuit when optimized correctly.

The front end (head amp) is identical to the 251 except the cathode bypass capacitors are not required in the 460 circuit.

The ELA M251 circuit has very little if any effect over the sound signature except the very low end a very high end.  The sound signature is created by the capsule.

There are a couple of flaws in the original 460 circuit.

1) C9 & C10 should be removed as they cause the distortion to increase at 11khz by 4 times compared to the distortion at 1khz.

2) C6 & C7 can also be removed

3) The tube should be a 60762a

4) The transformer should be a BV11 (5:1) ratio and not the 12:1 T14.

The components in the 460 are much better than components used back in the day.  Today we have much quieter and more accurate resistors and capacitors.

I first starting servicing audio tube gear in the 60's and I was always changing bad capacitors.  Today we have better power supply regulation and components.


The 460 has a head amp with the same values as 251 but with the more elegant C12  9-pattern  selection switch.

The 460 uses the second half of the 6072a as a CF output stage configured as a CCDA (constant current draw amplifier).

I have heard so called Vintage microphone GURU's and Gear Slutz Elitist's tell folks that they have never heard a Cathode Follower circuit they liked.

However, Bill Putnam who designed the LA2 tube compressor drives the output transformer from the cathode.  Interestingly, the CCDA circuit in the 460 comes from the output stage of a 1961 State of the Art McIntosh Tube HiFi preamp.

Why does the 2-stage 460 circuit work better than the original circuit?

1) The output of the plate in the first stage is coupled directly into the grid of the second stage and the load on the plate is reduce over 20 times. 
Therefore, cathode bypass capacitors C6 and C7 are not required and the gain only drops 4db.

2) The two halves of the 6072 are opposite in current draw so the current being drawn from the power supply remains constant.  In the single stage circuit as the current draw changes the voltage on the plate will also change slightly and the gain will vary accordingly.  The optimized 460 circuit has much more "punch" and transparency.

3) The output impedance of the cathode is 20 times lower than the plate and a 5:1 transformer can be used instead of the 12:1.  There is 8db less loss in the 5:1 than the 12:1.
The result is that decreasing the gain by 4db and decreasing the loss in the transformer increases the headroom and signal to noise.

4) The CF output stage swings the entire B+ plate supply and is not limited by a plate resistor.


The other important thing is to reduce the power supply B+ voltage of the 460 power supply.  Change Z5 to a 61v Zener from a 72 volt zener.

This will give you the proper polarization voltage on the CT12.

In the ELA M251 C4 is too small at 20 mfd and should be at least 200ufd to produce a flat response down to 20hz.
However, the German Broadcasting Industry wanted the low end response limited for Radio Broadcasts.

The ELA M251 also used a bypass capacitor (C2) between the plate and ground and this rolls out the very high end 15khz about 3db.

However, in the ELA M251 circuit the amount of roll-off will vary depending on different microphone preamp loads.  Today we have much higher impedance preamps.

To duplicate this roll-off with the CT12/460 circuit a 1000pf can be connected from the first plate to ground to give the same curve because the first plate is buffered from the input to the microphone preamp by the CF output stage the HF roll out remains consistent no matter what kind of preamp is used.


However, from a producers point of view I would rather have the HF rise at 14khz and EQ it myself if required.


One of my favourite microphones the Soundeluxe U99 also used this 2-stage CF output circuit.

Cheers, Dave

Also included this,

Hi, here is the circuit for the 460.  The transformer is in the bell housing and you must remove 4 screws to pull it and the 7 pin connector away.

The green wire from the transformer goes to pin 6 and the black wire goes to pin 5 of the 7 pin connector.  Just hold the new transformer in with some hot glue.

Remove C8 which is a 1ufd/450 volt.  Hot glue the 2.2 ufd/600v to the circuit board.
One side connects to pin 8 of the tube or where R8 connects to the circuit board and goes to pin 8 of the tube.  R8 is a Red Violet Orange resistor.  The red wire from the transformer goes to the other side of the 2.2ufd capacitor.  The white wire from the transformer goes to ground or pin 4 or 7 of the 7 pin connector.

C9 and C10 are not required with the new transformer an should be removed.

Remove C6 and C7 which are also not required with the 6072 tube and the new transformer.

 

[saxmonster]

Ok so I started looking for parts to swap in this mic.

For C4 should I stick to Polystyrene?  Dave says to use a 200ufd but doesn't say the voltage so I am thinking around 250vdc?  The apex has a 1000p/630vdc but I think the volts do not need to be that high.

I found this for C4 will it work?

http://www.mouser.com/Search/ProductDetail.aspx?R=TVA1720virtualkey61320000virtualkey75-TVA1720

For C8 Should I also stick to Polystyrene? 

Will this work

http://www.mouser.com/ProductDetail/EPCOS/B32774D4226K/?qs=UAZYTDUJKnRuXG13Ked%252bbDRINO9lCJ8kL%252brfXydeuvs%3d


For the first plate to Gnd roll off cap will this 1000pf cap work? He doesn't say what the volts are again.

http://www.mouser.com/Search/ProductDetail.aspx?R=CM06FD102FO3virtualkey59850000virtualkey598-CM06FD102FO3

 

[wave]

The choice you made for C4 is Electrolytic. If you want a styrene cap for C4 dont' use that Mouser part.
The choice you made for C8 is polypropylene. Polyproylene is ok but once again not styrene. also the size of that polypropylene is approx
1.23" long and 1.25" high. That's kinda big to go inside a mic body.

Are you sure he didn't mean 200pF for C4?

Dave

 

[saxmonster]

Dave,
Yea its pretty hard to find polystyrene at these values.  The email I got from him did say 200ufd for C4.  But no voltages or what it should be made out of.  I do know that electrolytic is bad.  Sorry i didn't notice that.  Polystyrene is the best correct?

I checked all the normal places but where is a good place to get polystyrene?


-Scott

 

[e.oelberg]

still there are some fun things with the elam. The seperated backplate has advantages in snr. The cardoid is 3 db quiter than
in a remote switch (chinese, brauner or similar curcuit) with figure of 8 the same. with 500m from grid to ground you'll have
masses of bass.

 

[wave]

You should email him back and ask him what voltage and type it should be.

This is where I got my styrenes for my U87 build

http://www.justradios.com/orderform.html

They have a $15 minimum order but there's a good selection of caps and some resistors too. The styrenes are 630V but they are small and will fit in a mic circuit with ease.

Dave

 

[pasarski]

The email I got from him did say 200ufd for C4.

C4 of the Apex/ELA M or what schemo? He says 20mfd (20 microfarads). That's the cathode bypass cap in ELA M (haven't seen a scheme where it's C4). He says you don't need that cap if you keep the cathode follower. These are usually electrolytes I suggest to re-read his mail couple of times and make sure you don't go and change wrong parts.

 

[saxmonster]

pasrski,

I have read his email alot do you think where he says in blue is a misprint, it should say 200mfd instead of 200ufd?


"In the ELA M251 C4 is too small at 20 mfd and should be at least 200ufd to produce a flat response down to 20hz.
However, the German Broadcasting Industry wanted the low end response limited for Radio Broadcasts.

The ELA M251 also used a bypass capacitor (C2) between the plate and ground and this rolls out the very high end 15khz about 3db.

However, in the ELA M251 circuit the amount of roll-off will vary depending on different microphone preamp loads.  Today we have much higher impedance preamps.

To duplicate this roll-off with the CT12/460 circuit a 1000pf can be connected from the first plate to ground to give the same curve because the first plate is buffered from the input to the microphone preamp by the CF output stage the HF roll out remains consistent no matter what kind of preamp is used."


Is he talking about the ela m251 circuit here and that I should not change the c4 or add the 1000pf between the plate and gnd in the apex circuit.  Its seem that I should add the 1000pf between the plate and gnd because I want the roll-off and also change the C4 to 200ufd or is it 200mfd?

Sorry if I don't get what he is saying I tried emailing him about this stuff when I first got the email but never got a response back.  It seems that he just copied and pasted this email to me, which i can see if a lot of people are asking him about it.

 

[pasarski]

pasrski,

I have read his email alot do you think where he says in blue is a misprint, it should say 200mfd instead of 200ufd?


"In the ELA M251 C4 is too small at 20 mfd and should be at least 200ufd to produce a flat response down to 20hz.
However, the German Broadcasting Industry wanted the low end response limited for Radio Broadcasts.

The ELA M251 also used a bypass capacitor (C2) between the plate and ground and this rolls out the very high end 15khz about 3db.

However, in the ELA M251 circuit the amount of roll-off will vary depending on different microphone preamp loads.  Today we have much higher impedance preamps.

To duplicate this roll-off with the CT12/460 circuit a 1000pf can be connected from the first plate to ground to give the same curve because the first plate is buffered from the input to the microphone preamp by the CF output stage the HF roll out remains consistent no matter what kind of preamp is used."


Is he talking about the ela m251 circuit here and that I should not change the c4 or add the 1000pf between the plate and gnd in the apex circuit.  Its seem that I should add the 1000pf between the plate and gnd and also change the C4 to 200ufd or is it 200mfd?

Sorry if I don't get what he is saying I tried emailing him about this stuff when I first got the email but never got a response back.  It seems that he just copied and pasted this email to me, which i can see if a lot of people are asking him about it.

mfd=mF=ufd=uF (though ufd and uF are "wrong"). Don't know why he mixes them in one sentence. I noticed that he's referring to the coutant org schemo when talking about ELA M circuit. By C4 he means the cap that is C6 and C7 in Apex schemo and C6 in the hand drawn.

In Apex schematic C4 is the capsule to grid coupling cap which at 1000pF is fine. Polystyrene for "best" sonics. The cap from plate to ground is blurry in the coutant scheme and C11 (100pF) in the hand drawn. That's the one he says could be implemented in the Apex circuit with 1000pF value. I'd leave it out or try different values between 100~1000pF.

 

[wave]

That being said you can get all those values in styrene from justradios

Dave

 

[pasarski]

20uF cathode bypass cap in ELA M makes a nice anti-proximity effect low shelf filter BTW (for those who fear the equalizer at least). And some people seem to prefer the ELA M and other classic plate fed circuits over "punchier" and more transparent Apex circuit. I don't have an opinion because I have never tested the Apex circuit (stock or corrected) with a proper capsule. But I'm not a fan of these step by step mods, It's more fun to test different things, mix different topologies and maybe learn something on the way.

 

[MagnetoSound]

One thing is for sure - an 251 clone is one thing, and just changing out a few component values (or types) in an Apex 460 circuit is something else.

I'm not advocating one over the other, just pointing out - with your thread title in mind - that a tweaked 460 does not an ELAM make. And there are a lot of 460 mod threads already.