Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Effects => Topic started by: goldstache on March 20, 2017, 04:31:08 pm

Title: My EP-2 Project
Post by: goldstache on March 20, 2017, 04:31:08 pm

I love the dry preamp on my old EP-2 Echoplex.  I like it so much I decided to make it as a stand alone preamp. 

I started by just straight cloning the preamp portion of the echoplex.
It sounds great. I use a 6eu7 and ran the preamp off a SMPS to get my B+. 

I have some questions...........

The schematic attached is for the beast I have.  Though, in my actual EP-2, many of the parts are different from the schematic.  I tried both the schematic and the actual units topology. 

There are a few errors in the original schematic.  R16 is a 220K  and R17 is a 4.7K  I'm ass-uming anyway, as those values make more sense.

Here are the value differences from unit (lifted and measured) and the schematic:
Unit:
C8- .02uf
R17- 5.6k
R16- 220K
Schematic:
C8- .22uf
R17- 470k (i'm thinking 4.7k is what they meant)
R16- 220k

After breadboarding them out I finally got a taste of the output of the stage without the loading of other associated parts that are part of the echoplex's delay circuit.  I figured this issue would arise. 

I ended up tweaking the snot out of what's there until I liked it. 

KEEP IN MIND THAT I'M ONLY USING THE PREAMP STAGE CAP COUPLED TO SOME DIVIDERS TO THE OUTPUT JACK, NONE OF THE OTHER CIRCUITRY IN THE ECHOPLEX.

On the inputs of the 6EU7 tube there is a 100k//100k voltage divider/grid leak configuration.  If I want to put a volume control here would I just put a 1M pot like a volume control and get rid of the 100k's?  I know this will effect a current relationship, or at least I think it will. 

I can't seem to wrap my head around the right way to go about adding a volume control here.  Ideally I would like the full input gain as if a 1M grid leak was there to ground and be able to get the standard 100K/100K Vdivider sound as a base (zero setting) on my volume pot and extended gain as I cranked it up. 

Anybody want to take me through some options.  I know once I have it working I have some miller stuff to sort out too, but I just want to get my head wrapped around how to do it. 

I also made a PT2399 delay circuit to use in conjunction but that's down the road as I just want to get the preamp working. 

Thanks

Title: Re: My EP-2 Project
Post by: HotBluePlates on March 20, 2017, 11:48:26 pm

See your other thread (http://el34world.com/Forum/index.php?topic=21721.0).