EP-PRE revisited

Good Guest · Post by **Good Guest** » Sun Oct 16, 2011 5:57 pm

The 24 volt power supply is key also...not 9 or 18 but 24 especially with all the attenuatin going on with the series of resistive networks going on after the fet...some say they are acting as phase shift networks...but they are just simple attenuation networks. The output is 180 degrees out of phase to the input of the fet anyways ...which is a natural operation of the fet in common source configuration like in the ep3.

Xplorer · Post by **Xplorer** » Sun Oct 16, 2011 6:04 pm

my echoplex has 22 volts, not 24, coming into R4. after R4, it's 13 volts. So .. am i right if i say that it's just a matter of adjusting R4 value to have the right voltage after R4, coming into the fet ? no need for 24 volts or 15 volts or 18 volts ... if only 13 volts are required, is that right or am i missing something ?
oh ... perhaps minus 22 or 24 volts are required on the ground ... mh ...

Good Guest · Post by **Good Guest** » Sun Oct 16, 2011 7:11 pm

Yeah that's right 22vdc....the voltage drop to 13 0r 14 is from the fet and R4....just dumping any fet in there and placing a resistor and adjusting it too 13 is not right ..you need a scope..or you can use your ear...or you'll end up clipping a top or bottom of your signal instead of enjoying the headroom...and if you were to send a hot signal to the fet you would want it to clip symetrically , or asymetrically? ..so just adgusting to 13 volts won't work ..you need the exact fet and exact 22k and exact 22vdvc to do it that way. Now if waveforms for the ep3 were available showing the clipping characteristics , that would be differant and fet circuits could be designed to match at least those characteristics.

The same goes if you used any old fet and placed a 22k there ..depending on the fet you may not even get a signal. Once you change that 22k you also change the impedance and how it reacts to certain frequencies also... now you know why people don't bother ..they just get the darn thing to pass a signal and call it a day and sell it... they don't even know what circuit Eddie used.

Buy the way it clips assymetrically when overdriven ..just like a smooth fuzz unit.

Xplorer · Post by **Xplorer** » Sun Oct 16, 2011 7:39 pm

it starts to be tough science for me !
it's a new tis58 in there ... 13 volts isn't right ? i didn't hear my echoplex clipping, i don't understand. i know it has some background noise on the echo at higher level but for the rest it sounds ok so .. ( ? )
what voltage should that be in there ? mmh , lopoking at an old paper from mike battle, it says D : 8,2 volts , S : 1,58 volts and G : -0,03 volts ( "varies" )
symetric and asymetric, clipping, i'm not educated enough to understand this, i'm sorry.

that's also why i want to build a separate preamp, 100 % correct, so i'll compare it with my echoplex without touching it yet. cause if i want to improve something and iit goes less good, i won't be very satisfied. so i'm going there little by little, first : the preamp.

leadguy · Post by **leadguy** » Sun Oct 16, 2011 10:15 pm

Well, all transistors and tubes have to be DC biased to allow AC signal swing operation to function properly.

The AC signal swings around the DC bias and if the DC bias is too high or too low for a particular tube or transistor or if the AC signal is too large and swings too much around the DC bias then the tube or transistor clips the AC signal producing a square wave like output and that is distortion.

Biasing a particular Jfet can be done by using it's spec sheet.

The Echoplex Jfet is biased for clean operation but can be overloaded into clipping by a large AC signal input just like anything really.

For how to do the output, look at how the output is for this jFet and there is the answer http://www.till.com/articles/GuitarPreamp/" onclick="window.open(this.href);return false;

Ed's EP-3's are around the 255xx serial number range which is also the EP-3 with a compressor board range.
How do I know this?
By Ed's EP-3 logos and also by knowing that he had these EP-3 logos in May 1977 and knowing the serial numbers of the wider EP-3 logos which Ed had.

Good Guest · Post by **Good Guest** » Sun Oct 16, 2011 10:41 pm

Well said Leadguy

---------------------------------------------

From the Tillman write up
"I should point out that FETs in general suffer from a serious lack of manufacturing consistancy. The FET VGS and IDSS, the parameters that determine the bias point, can be anywhere over a 5-to-1 range and still be within spec. That's pretty awful, but such is life. It's an engineering accomplishment to design a circuit that can function exactly the same over a wide variation of component parameters. But I can't guarantee that in this situation; there's not enough supply voltage headroom. "

----------------------------------------------------------------------------------------------

The fet in the ep3 suffers from the same problems except , because of it's high operating voltage suppy has an advantage over the tillman, when it comes to headroom...So if you hear of a manufacturer " handpicking fets" at least you might be getting some value. Hand picking is expensive tho when it comes to rare and valuable fets ....

At least with the tillman you got lots of info and tech support of the simple circuit...I'd switch the 2.2 k to 3.3k and adgust the drain resistor by ear till I found a fet that works as close to the 22k value as that determines the output impedance , actually anything in between 10k and 22k would be OK and call it a day ..and you end up with a great 9 volt boost with a EVH flavor. Once those parameters are met then you can change the input to copy the ep3's and or the output cap..

Xplorer · Post by **Xplorer** » Sun Oct 16, 2011 10:58 pm

interesting ! do you know how to measure a fet ? i recently learned how to guess wich pin is G with a multimeter, but i really don't know how to choose a fet, as it's so different from one to another one. and that's maybe why my ep3 has this higher voltage on D.
i feel frustrated not to understand how to use a scope, and knowing what to do then. i don't have any anyway. that could be useful to make my ep3 even better, dead silent.

Good Guest · Post by **Good Guest** » Sun Oct 16, 2011 11:36 pm

Xplorer wrote:interesting ! do you know how to measure a fet ? i recently learned how to guess wich pin is G with a multimeter, but i really don't know how to choose a fet, as it's so different from one to another one. and that's maybe why my ep3 has this higher voltage on D.
i feel frustrated not to understand how to use a scope, and knowing what to do then. i don't have any anyway. that could be useful to make my ep3 even better, dead silent.

You would probably benifet from building the tillman...obtaining some j201's that are readily available and pinouts etc...and hooking it all up like an ep3 use a 9 volt battery supply.....THEN hook up a drain resistor like a 100k pot and adjust till you hear signal and play around with that...record the operating resistance ...if it isn't beween 10k and 22k insert another j201...till you find one that works at those drain resistance levels...you'll get a good handle from that...your ears will tell you lots ..you don't really need a scope unless your going to be a bigtime marketer of ep3 copy pedals with standards that should be met.

You don't really need a a scope to measure a fet ...rg keen used to have a nice rig setup using a multimeter and battery that would measure some parameters ...do an internet search for it..jeez after doing all this you'll be all primed for building yer own phase 90 with 4 matched fets..

Xplorer · Post by **Xplorer** » Mon Oct 17, 2011 12:08 am

ha ha !
yeah, a pot for the drain resistor sounds nice. but i'll have several 2n5457, good for the ep3, and i have a few tis58 left.
yes, i saw something from rg keen but it was to measure some germanium transistors. but i'll take a look again, he might say something about the fets.

to my ears my echoplex sounds perfect, i don't want to change it, even if it's out of specs if i understand.
I just want to make an exact ep3 preamp clone, not a tillman ( for now ), and compare it. and i just want to improve it by removing some noise :

i think that i'm gonna disconnect the head board, and disconnect the motor circuit, to see if it does anything good ( but being naïve and not well educated, i hope that it's not gonna change all the voltages on the rest ! ) and i'll take a look at this Dc filtering board, mine seems to be a variant, perhaps improved i don't know. i should try it with stock specs. what do you think ?

all i want is to be able to increase the volume pot at max without hearing an undesired background noise like a vacuum, o i'll know the at lower levels, the echo will have a much better quality.
the t70 nortronics board .... well i don't know what it does, and if it can bring noise.
the transformer : someone seem to have screwed something into it a bit, so i should perhaps replace it.

But sooner, i'll have a look at how my two echoplex will sound in chain, when i get a chance to find another 230v - 120v converter. and by the way i should perhaps simply have a new transformer that accepts directly 230 volts.. and have a voltage regulator or something ...

or i should experiment all this on the second echoplex without touching this one yet. but does anybody has some spare echoplex pieces ? i'd need a bridge and a sliding chassis for the write/erase head combo, cause this one came with 6 heads in line, fixed.

in the end i think that i'll can compare the preamp clone, and both echoplex and i'll tell you, if i can do it one day ...
and i'll mod my ep3 to have both preamp versions, with some switchs ...
i already have a cool feature i invented by accidnt, that allows three positions : signal and echo - SOS - and echo + sos playing the signal and what was "echoed".

a speed variation for the motor could be cool ... very long echoes ... special effects ... tape flanging with the other ep3 etc ...

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 10:43 am

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 12:34 pm

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 10:13 pm

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Metropoulos Forum

EP-PRE revisited

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