Building a Preamp

[Liquids]

I've got a tube-preamp prototyping chassis (it's legitimately cake pan) running in the last week or so.

I love my amps (Ampeg V4) and I run any amps I play clean - very clean - and loud as possible in the setting, using drive from pedals I've crafted myself into a rack reverb/delay for space and depth...it's the happiest with my tone I've ever been, and it's been my arrangement for quite a while now.

Still, I thought it would be fun to try out some actual preamp circuits now that I am comfortable with tubes and HV...

The idea of the preamp/chassis is to eliminate the 'redundant EQ' most tube pedals and rack preamps I've used suffer from when they have a typical passive EQ control themselves, but are also run into the front of an amp with an additional EQ, which can make the preamp's EQ a liability.

Sticking to the core circuits, followed by a way to shave high frequencies and/or independantly control low frequencies in an attempt to match my clean tone without affecting the overdriven tones mids, which my amp takes care of nicely on it's own.

First preamp circuit I wanted to try was the Dumble/D'Lite...many of my favorite plays use Dumbles and Dumble-type amps, and get tones I like to hear.

I'd wired up a Dumble circuit before in a bandmaster head and didn't dig it. But I later learned that didn't at all know how to set the OD trim at the time (now I see here that 25-33k to ground is the key) and it was 'dry' since I didn't mess with effects loops.

For the gain levels I was going for, I found it very mushy/fuzzy in the low end kind of like Sonny Landreth - who's tone I like 'when he is wearing it,' but wouldn't want to imitate that aspect of it. Likewise, despite the snubber caps, the high end was also pretty gross to my ears when satisfying amount of sustain was reached.

I felt the sound at the more moderate gain levels (That cliche RF and LC zone) 'works' to my ears tonally and in my hands, but was still a bit mushy and I wanted more usable sustain than where that left me, sadly. Tweaking between 33k and 22k to ground on the OD trim helped refine the tone range, but it wasn't much of a solution for what I wanted.

So next I have turned to auditioning with circuits that are more engineered to 'high gain lead' (Soldano, Carvin Legacy). This was more curiosity than anything though the legacy has sounded good and toneful to me in clips of people playing one not trying to sound like Steve Vai.

I've found these circuits a bit more pleasing, tight, from the first note...and especially after my own tweaks. Surprising to me, because I am really a blues, borderline blues-rock player, by no means a high gain EVH/Vai/Satriani/or shred player. Way more gain that I need is on tap - and it can get nasty in a completely different way to my ears - but the high gain sound is smoother than the dumble offered. Especially in the legacy, moderate to large value snubber caps on the plates and such are all over the place, as opposed to the 'be careful' kind of feeling about using too much capacitance in a dumble circuit. I've removed most of the caps even as I found the circuit unnecesarily DULL (!) in the highs where I tend to set it, but even with most of them removed, I still find it sweeter and warmer than the Dumble circuit was at corresponding medium-high gain levels...

The best find so far is that I found that turning some set grid resistance into a pot between the third and forth gain stages - where the Dumble OD pot is - allows me to dial back the gain to the more moderate gain levels where the Dumble circuit 'worked best to my ears. However,I could get the tone sweeter and tighter to my ears, especially with some passive baxandall bass boost after the fourth stage (which the Legacy actually has available as well), to tune it from it's stock 'tight' sound with no EQ, to fat.

In the end (though I'm still auditioning, tweaking and prototyping), I was surprised that - at least for me - while the Dumble has a lot of potential gain, but it doesn't seem to do that well with it...it needs to be tamed to work for my kind of tones.

That said, I do like the Dumble sound on record and in theory, and I'm finding that with some intelligent tweaking that may break from the standard circuit, re-thinking the design with the intended goal in mind - smooth but clear tone at various gain levels - may yield the same if not preferable results.

Anybody else here go 'back to the drawing board' with their Dumble amps and stray significantly from the core design for similar sounds with good results?

[Luthierwnc]

I'm a big fan of V Ampegs (and Geminis too) and have rebuilt a couple of them. It is pretty easy to rewire the extension amp outs in back to the Dumble-style Send/Return. IIRC, you don't need to mess with the traces at all but I can't remember if one of the extension jacks was a shorting jack -- so you might be out the expense of a switchcraft.

The reason I mention it is I'm working on a preamp that has a switchable input to either the preamp or to the main amp input. The signal exit goes to the another side of the same switch. These amps are usually built to have an external active loop so the loop-out jack would go to the return jack on the Ampeg if you don't make a Dumbleator to go with the system.

A V4 is probably a great power amp to drive this preamp. Big, clean and very efficient. Tear your face off too. You would lose the reverb, though. sh

[Structo]

Welcome to the forum Liquids.
As you have discovered there are many ways to arrive at a solution for amp tones.

I think that part of the Dumble charm is a bit of looseness of the bottom end.
You start tightening that up and pretty soon you are off the reservation.

The ODS type amps are not for everyone and I think that is where a lot of the love/ hate type debates originate.

The Dumbles seem to have a lot of mystique or folklore around them.
Partly due to the reclusive builder who likes to remain the man behind the curtain.

My own experience and experiments have led me in circles sometimes.
The ODS amps take a bit of knob twirling to get them to sound right for player.
It's not one of those amps where you set everything to 6 and you instantly become Robben Ford or Carlton.

The power amps are kept clean for the most part and the magic (or lack of) happens in the preamp mostly.

The power supply is not real formidable and when I made attempts at stiffening the filtering it changed the overall character of the amp, for the worse.

So I guess the first thing a person should do is outline what he wants tonally from an amp.
Will this amp be for studio work only or for live gigs as well, and so on.

I think you are on the right track, make various preamps and then patch them to a clean power amp.
That would give you the ability of having a Dumble or Soldano or Fender or any other amp you may want.
That would be a great studio tool.

[Liquids]

Luthierwnc - my V4 is actually a V4-b, so no 'verb I took what I could get with limited cash, an amazing price, and the motivation to fix her up...and it's by far my favorite amp. I have two now, although the recent one is in pieces and awaits some TLC to get her running again. But I love these amps - I sold my '68 Fender Super Reverb after I fixed the V4-b up; with NO regrets; it's that good. I've got a reverberocket 2 recently and on and off the bench as time permits - another little-appreciated but great warm, clean amp there.

Anyhow, so as I said I use delay/reverb at the rack unit (TC g-major) which is glorious and flexible, good for guitar though probably prefers a buffered signal like most rack effects. I *always* send it a buffered signal (another story) but I say that for those who have had bad experiences with 'tone sucking' rack effects...it's simply a matter of impedance/buffers.

Given that, the preamp is just another piece in my chain....I could do the same kind of switching thing you mention and have thought of it. You're right, the V4 is an amazing power amp in and of itself when run via the dual send/return jacks, so running a full fledged preamp & EQ in is a wonderful arrangement. I would, however, need to build a box that would simultaneously switch the preamp in/out of my chain, AND route the signal from the end of my chain (the rack) between the amp input and the amps 'return' if I wanted to go between going into the regular input and running the preamp into the V4's power amp section.

Then, depending on desire volume level, It could be problematic running the preamp's signal to match without causing them to clip...so I'd probably be best running the preamp out at low levels not unlike a pedal, and arrange a make-up gain stage before going into the 'power amp' like a loop...but at that point I'm mostly back to using it more or less like an in-line pedal. I've thought this through many times!

My current plan and configuration is the most hassle free I have come up with, though there is no one right way to do it. Everything may be a compromise...at the same time, I'm happy with what I'm getting from my pedals and from this stripped down preamp simply run into the amp with some basic filtering to keep it flexible.

Structo - I was a bit hesitant to post my personal findings here since they aren't a 'glowing' review of Dumble preamp but your response and open mindedness put me at ease.

As far as studio tools - that is part of what inspired this project; in addition to always wanting to hear these 'classic' preamps in a poor-mans way, and reduce the variables (power amp, room, output transformer, effects loops, etc), I started playing with a band that works on originals for fun but rehearses and rolls tape every week in the songwriters dedicated recording studio. H

Having amps at one's disposal is an amazing asset in that regard (and it does have a few) but having a few more tools for diversity of classic guitar tones to put to tape is equally inspiring for building and recording.

[Luthierwnc]

The schematic I posted shows a 4PDT switch controlling the both ends of both preamps. Two sections control whether the instrument signal goes to the preamp or the main amp input and grounds the other side. A third section puts the amp send on one throw, the preamp output on the other and the pole goes to the amp return or out to the loop -- so basically a bypass in the normally closed position. Those V Ampegs have half of a 12DW7 as a recovery stage so it would hit the inverter harder than a regular Dumble clone.

That said, one of the beautiful things about D-amps is the active loop. You can replicate some of the OD with the right pedals but nothing I've heard adds the fullness of the post-master loop. That would also be the new home for your G-Major. A Dumbleator is also a much easier and better-documented project than a standalone preamp. I'll bet your amp would kill it.

As long as you have the Ampeg open, consider replacing the tone stack module (the one that looks like a biscuit). Ampeg used really cheap parts on those and it is pretty simple to slap in individual components. Here is link on that and lots of other useful V4 mods:

http://www.ampegv4.com/forum/viewtopic.php?id=301

I hadn't thought about it before this, and maybe some of our theorists could chime in, but it looks like you could put a switchable bypass resistor to ground at the bottom of that James/Baxandall stack as a poor-man's PAB. Put it at the junction of the resistor and cap, put a 220k resistor to ground and bypass it to ground in the normally closed position on the ultra-low switch. Food for thought,

Good luck, Skip

[Structo]

meh, don't worry about it.
Teh Dumbellz aren't for everyone.

Look in the technical section for more diversity.

[ToneKarma]

Sorry to tune in from aside (and first post too), but I am hatching the plan to build myself a preamp (only) as well ...

I am currently wondering if it would be necessary to us a cathode follower after OD2/V2 or if utilizing the "FX Send" of the original layout would be sufficient enough to incorporate this output signal into a rack system.

In regards of a possible cathode follower I thought about perhaps using the "Send"-half of the Dumbleator circuit.

Any thoughts will be much appreciated ... thanks !

[LeftyStrat]

i think you definitely want a CF in there, and the first half of the Dumbleator would certainly do the trick as well as add some flexibility.

In fact, since you would have a leftover triode, you might consider using it to provide two independent Dumbleator sends. This would allow you to send one to any parallel/time-based fx and another for straight to the power amp, with independent control of level and brightness.

[markusw]

You might also want to consider to use a high voltage MOSFET as a "cathode follower" instead of using another tube, e.g. a TO-92 LND150 as used in the Metro amp zero loss fx loop.

http://www.geofex.com/Article_Folders/m ... tfolly.htm
http://www.hawestv.com/amp_projects/amp ... _200v1.htm
Also something about LND150:
http://music-electronics-forum.com/t3493/

In fact in the zero loss loop it's not a "cathode follower" (or actually source follower) but a "triode stage" with a strong feedback from plate (drain) to grid (gate) that reduces the gain of the stage to much smaller than 1. IIRC it uses a 2200k grid/gate resistor and a trim pot set to about 26k as a feedback resistor. Therefore it doesn't have a send pot.
Anyway, you could just use a LND150 configured as a cathode/source follower as on R.G. Keen's site or use a "triode stage" as in the metro amp loop with just as much feedback to reduce the gain to 1 and add a send pot.

Peace,

Markus

[ToneKarma]

i think you definitely want a CF in there, and the first half of the Dumbleator would certainly do the trick as well as add some flexibility.

In fact, since you would have a leftover triode, you might consider using it to provide two independent Dumbleator sends. This would allow you to send one to any parallel/time-based fx and another for straight to the power amp, with independent control of level and brightness.

Thanks LS!
I have thought about utilizing the other half of the tube for something "useful" and a 2nd "send" would be kind of handy. I definitely do not need a recovery stage.

However, the Dumbleator II shows that the two triodes of the "send"-tube were simply used as one. See picture below from another topic.

[img656]http://i205.photobucket.com/albums/bb17 ... Update.jpg[/img]

[ToneKarma]

You might also want to consider to use a high voltage MOSFET as a "cathode follower" instead of using another tube, e.g. a TO-92 LND150 as used in the Metro amp zero loss fx loop.

In fact in the zero loss loop it's not a "cathode follower" (or actually source follower) but a "triode stage" with a strong feedback from plate (drain) to grid (gate) that reduces the gain of the stage to much smaller than 1.

Peace,

Markus

Danke Markus ,

this is a very interesting subject that I will dive into!
Still, my gut feeling wants to stick to a tube approach for a CF.

However, mentioning transistor based ideas: for my preamp build I am actually planning to use the Jack Orman booster as a replacement for the original FET-preamp.
I will also give the "infamous" (though IC-based: TL072 & 4558D) Korg SDD-3000 preamp a try and hear how it blends into an ODS preamp circuit. Could be a positive surprise.

Cheers,
TK