Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on March 20, 2017, 04:59:28 pm
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I posted this in the Effects section of the forum, but since it is tube, I thought I'd try here as well.
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
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Please show us a schematic rather than make us switch back and forth between this message and a stock schematic.
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... 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. ...
There were lots of different versions of at least the EP-2 and EP-3 (and possibly the EP-1 as well). See if the attached schematic matches your EP-2 (and check the serial # of your EP-2, if the sticker is still present).
I took the schematic from the Echoplex Owner/Service Manual (http://www.synfo.nl/servicemanuals/Maestro/ECHOPLEX_OWNER-SERVICE_MANUAL.pdf) but most are already in the Echoplex file (http://el34world.com/charts/Schematics/files/effects/MaestroEchoPlex.pdf) in the Schematics linked below.
Except C8 is different; could be a holdover from the EP-1 circuit (attached).
How is the Volume implemented in your EP-2? These are different between the EP-1 & EP-2 schematics. If you really love the sound of your EP-2, I'd copy how it implements the Volume (which might be "Echo Balance" in your EP-2). Tell us what comes between C8 & the Output Cord.
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I had the same thought once regarding my EP3. I think someone even built/builds a pedal that is just an EP3 dry channel. Since I prefer to tinker with tubes so I did what you are talking about, basing it off the EP2.
In the schematic you posted, note that R14 is 470K. I'm not sure if that is correct, maybe it was 470? (makes more sense). On your later example, R14 is 5.6K.
Either way, based on your numbers, you might start here:
(http://i.imgur.com/IMNyY4K.jpg)
There isn't really a reason not to make R15 500K-1M. I don't think anything to the left of the grid input resulted in any Mike Battle magic. I'd make R13 22K. I drew R15 500K in there to take the place of the record level pot, but it might be superfluous.
Your volume control is the 500K pot (adding the two 220K mix resistors together from the final EP2 output section). You might find you have too much gain and decide to replace the pot with a resistor on top of a pot.., like a 220K resistor + a 250K pot, or a 470K resistor and a 50K pot. It really depends how much gain you end up with at the output (and how much gain you want to cap it off at).
if you want we can talk about the power supply as well.
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Attached is close to what I have on the board. TerminalGS, thanks for the sketch up. I think the front end plays a part as it acts as a voltage divider. Signal>resistor>grid and resistor to ground. On my EP-2 it's a 100k/100K setup for a division of half. I like that setting and sound, but want to be able to increase the gain by toying with that setup. I'm thinking I might go for a B250K pot wired like a volume control. But ideally I would like the stock input sound and then from there be able to turn it up to no attenuation. I just can't seem to figure out how to do it best.
The tube is close to the jack so I don't think RF will get me too bad.
As I mentioned I plan on implementing other circuitry in the build (delay, reverb, or something) But for now I'm just tackling the dry sound.
All the passive series resistors from plate to outjack are 220K and I'm using the echo balance topology where ground is on the wiper of the pot. Thanks.
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(http://i.imgur.com/QPjZpAJ.jpg)
May be I'm wrong but this seems the preamp
About the 470K cathode resistor, looking to the other schematic seems the right value is 4.7K
This is the other preamp
(http://i.imgur.com/4DtfLKY.jpg)
Franco
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I'd do it exactly as the stock preamp does, T-attenuator & all. See attached.
... On my EP-2 it's a 100k/100K setup for a division of half. I ... want to be able to increase the gain by toying with that setup. ... I would like the stock input sound and then from there be able to turn it up to no attenuation.
... I'm using the echo balance topology where ground is on the wiper of the pot.
I wouldn't tinker with the input resistors; nor would I try to get "extra gain" at least until after the original circuit is built & auditioned.
The "T-attenuator" I referenced is the same as the "grounded wiper" set you noted. I've retained that in the lash-up schematic.
I can't repeat enough that you don't want "close to what I have" you likely want exactly what's in your EP-2's preamp. The EP Boost (which copies the EP-3 preamp in a pedal) doesn't copy just any old EP-3 circuit, it copies the very earliest of them that isn't even in the Service Manual.
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Attached is close to what I have on the board.
In that schematic, I don't think you want a .0068 cap followed by a 18K resistor. That isn't going to get you to unity, at least not evenly across all frequencies. The low frequency roll off is about 1.3KHz. I'd stick to the original coupling cap followed by a 500K resistor, then to the 100nf||220K. and on to your output.
Also, your dry to the delay box: I think you need to omit the .022 cathode by-pass cap, otherwsie your delay box will only get partial frequency input (only low freq.). With your ".01 (start here then tweak)", depending on your input Z of the delay box, which might be very low, you might need to go way up on the coupling cap.. like 1uf or 10uf +/-. If your delay can take direct signal from the guitar patch cord, you might do that out front of the 100K/100K divider.
HPB has better advice than my previous schematic, start exact with Battle's design, then tweak.
I'd put any volume attenuators well after all the EP2 components with a pot.
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Good stuff. I will try some of these snippets and update!
Thanks everyone
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Any update?
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It deviated from the snippet of the actual EP-2 dry preamp. But it sounds great and I have it built up as just a stand alone effects type loop. I'll scan it off when I get some time.