Designing an amp - similar to Lex. Sig. 284 - need help

[mad hatter]

Hi

New to this forum. I'm looking for some help finding someone who would be willing to help in my development of a small watt class a tube amp. I've looked around a bunch and have not found what I want. I'm after an amp design with the following:

1-5 watts, vol, gain, bass, mid, treb, presence, reverb.

The closest thing I've found is a schematic for a studio amp called a Lexicon Signature 284 but it's a stereo design. I'm only interested in mono.

Further, I think I'd rather have a fender style tone stack. I want to build this thing but I'm not a EE - just a lowly ME. I'm good for the building part but the circuit design changes are over my head. Willing to learn and have many questions.

Any one out there willing to do some circuit surgery with me?

Here's the schematic

Lexicon Signature 284 Schematic.pdf

I'm not sure what I'm asking for is out of line or not. Is it a few minutes work or something a person should get paid for? If so I apologize. This is a very low budget project.

Thanks in advance,

Mad

[d95err]

Take the preamp from the princeton reverb and put a Vibro Champ poweramp on it. Just skip the phase splitter stage of the princeton and connect the third gain stage directly to the 6V6. Just change the 6k8 resistor in the tonestack to a 10k pot and you've got a mid control.

I would skip the presence control. If you want to be able to make it brighter, you could add a switch for a bright-capcacitor over the volume control, or a switch to select smaller values on the cathode bypass caps. Increasing the 10p cap that bypasses the 3M3 resistor would also add brightness.

If you really want a conventional presence control, you could experiment to find a cap/pot value to bypass the 47 ohm resistor, or change the configuration of the last gain stage to that of a Tweed Champ and use more conventional presence cap/pot values.

[GibsonLover]

I have a Signature 284 and I've borrowed a few circuits from it. Yes, it is stereo, but just build half of it. It is an incredibly versitile amp and well suited for recording for that reason. Notice that they cascade 4 gain stages - that's why the EQ is at the end of the chain. If it were at the beginning, all of the distortion (when dialed up that way) will mask the effect of the tone controls. I don't know if you are firm on a Fender tone stack, but this one - especially the presence control - works very well. I noticed it will nail just about any sound well, except for the clean Fender sound, probably due to all of the cascaded gain stages (although it is also quiet as a mouse). It has an FX loop so that you are not married to one particular reverb. Obviously, you can plug anything in there...
The Princeton reverb route suggested is good too, just by changing the output to an SE stage like the champ. It all depends on what type of music you want to play and what you'll use the amp for. The Princeton gives you reverb with one knob - is that good for you, or would you like to use your Lexicon processor? :-)
I would just as soon use an amp for its gain, sans reverb, and plug FX in a loop or use before the amp. The stereo version is cool because one channel can be you 'pure' guitar signal, while the other you can dial in a little delay or reverb.

[mad hatter]

Thanks for the replies.

Maybe I should rethink mono and just go stereo and simply build a 284 replica. It's a bit more than I wanted to do but what the heck. I'm not looking to record. I just like playing at home but I want a good tone. I wonder if I could simply switch in/out a bypass of the gain stages and add a different tone stack if I want clean fender. More knobs but I could care less. What do you think?

It looks like most of the schematic is complete except for specifics of the transformers. Where can I find info on them? I don't need the cute little magic eye thing and I don't think I need record outs so I don't need all of those secondary winding in the PT.

Another question is the EL84. How different of sound is it from a 6V6. Are they interchangable? I haven't stumbled across a tube to wattage or sound comparison.

One more, this thing uses a bridge rectifier. Is there a tonal difference to that of a tube?

Thanks,

Mad

[GibsonLover]

I have two stereo amps, the 284 and one dual SE 6L6 that I designed with a Gibson preamp. I use those two much more often than my other 15 amps (combos and heads) that I also own, so I'd say go for it. Frankly, to do all that work, stereo is one more tube and one more OT, so why not?
The 284 looks like a busy schematic, but you already realize that you can use the circuits you want. The actual chassis is pretty compact and well thought out in a 2U rack. If you are going to pursue a PCB layout you could have the same results.
There are plenty of knobs already, but if you were to do some switching out of gain stages, they could be added to the back. Personally, the Clean Fender comment I made is a pretty minor point, and your effort to change it may not be too rewarding.
If you find another schematic with the same tube compliment, you could copy that PT. The 284 is pretty special with its secondary winding options. The same OT's are commonly used for 6V6 and EL84's. Tube selection is pretty subjective, but they will take different sockets. Pick your poison.
The rectifier i wouldn't worry about either, if you suspect you may be missing some of the sag response of a tube rectifier. That comment comes from my love of the Presence control. While not a sag control, I'm thoroughly impressed with the effect it has on the attack of your notes. With a tube rectifier, just like built in reverb, you get what you get. This circuit gives you a lot of variation inbetween without insisting on all-tube, keeping the design and layout simple.
Good Luck!

[mad hatter]

Thanks again GibsonLover.

Do you have any photos or diagrams of the layout of the 284 you could share? I have images of the front and back but that's it.

There seems to be a third rectifier design (probably many more than that really) that I have seen and I wonder what the tonal difference is. See attached Fender AB763. It's a set of 3 diodes in series on each leg of the PT. For all of me I can just go ahead with the bridge but I wonder about this one as the AB763 is a classic design. What's shown is a bandmaster but this is found on other fender amp like a bassman too.

bandmaster_ab763_schem.pdf

Mad

[GibsonLover]

The three diodes are a cheap way to use diodes that don't quite have the necessary voltage rating individually. You can consider each set of three diodes as one if the single diode has a high enough rating by itself. It's a safety feature. The configuration shown is still fullwave. The Bridge configuration is fullwave also, but is used where your PT does not have a center tap on the B+ winding. If you have a B+ center tap, use what you have shown.
I have my 284 mounted in the top position of my recording rack, so I can easily pop the cover and take some photos if you give me a couple of days to get at it. I also have a cool circuit mod that I've been dying to try, but have not gotten to it yet. I was going to use it on a new build that would be a copy of the 284 preamp, but if you want to give it a go, please feel free. Basically, I wanted the amp to be a little more performance friendly by replacing the Boost switch (part of the Volume knob) with a Drive knob. As-is, there is a drastic change in volume between max vol/no boost compared to switching the Boost circuit in. There is lots of gain there when Boost is activated, but then you have to dial the Volume down again. The mod I have will remove the switch and put a dual gang Drive pot in. Set Drive at minimum and it acts as if the Boost is off, advance the Drive pot to dial as much saturation as you want.
I'll attach a schematic and few photos sometime in the middle of the week.

[GibsonLover]

Lexicon 284 chassis shot and schematic mod

Here's a gut shot of the Lexicon 284. I'm not much of a photographer, but it's all there. The back PC boards hold the input and output jacks, dummy loads, and speaker sim circuits, two for stereo. The PC board next to the PT is the +/-12V regulator circuit for the recording outputs. The long board in front is the preamp with B+ filtering on the right end. The tubes are mounted on a bracket with ceramic sockets. If you were interested in just the preamp and dual outputs, the layout gets pretty simple, with plenty of room to wire the preamp using a turret board. The top and bottom covers are solid, and is probably the only reason it includes a fan. Cut some vent holes above and below the tubes and a fan should not be necessary.
My schematic shows the changes around the first tube with a pretty simple re-wire and minor resistor changes. There are Before and After circuits so you can see what changed. The Boost switch is replaced with a dual gang Drive pot. Minimum Drive = Boost off. Advancing the Drive control is essentially a variable Boost instead of Boost ON / Boost OFF.

[mad hatter]

Thanks much for the pic and pdf. I've been out so I appologize for not responding earlier. I appreciate your help.

Wish me luck.

[DR_CHOPZ]

Great info Thanks,
Was searching for Lexicon schematic and located this thread.
I picked one up few months ago and finally got around to putting it through its paces (Just ordered new tubes). Anyway, I discovered that when the Left Out is at max, it starts to smoke & cuts out! Mid-way is ok and the right ch. is okay. It smokes near right side front. Circuit board is clean underneath. Manual says not to connect speaker outs unless connedted to cab. (I'm going into console, but previous owner may have?). Lastly, any info on bias point? Any help appreciated. Thanks

[blewgrass]

lexicon sig 284

I couldn't help you fix yours, but to the OP - I own this amp too. It is one of the best sounding tube amps I've played with. I'm only a luke warm fan of the EL84 sound, but this amp may do it the most justice. I think this would be a killer build! It would be a neat 2 x 10 combo in a pine cabinet with a lexicon effects processor built in the combo. This amp was originally designed to "show off" what the lexicon effects boxes could do. Although many wound up with this amp which retailed for $1200 or so, It never caught on commercially. I scored mine used and only wish there was a way to mount this for gigging purposes to a lightweight combo. I built a pine 2 x 10 cabinet for this thing and it's super cool to play with all the effects.

Enjoy the project and let us know how it works.

[GibsonLover]

Lexicon Signature 284 voltages

I was asked for the tube voltages. These were taken left to right from the front, as if you were reading the tube chart in the chassis:
PIN V1 V2 V3 V4 V5
1 166 149 181 284 284
2 - - - - -
3 .817 1.02 1.33 7.89 7.86
4 - - - - -
5 - - - - -
6 162 184 181 - -
7 - - - 287 287
8 .84 1.96 1.34 - -
9 - - - 285 285

I can't figure out the formatting, but there are 5 tubes and
5 entries per line

[ILMUNG]

Hello GibsonLover ,
I am from Begium and the reason why I contact you is because I just got a lexicon 284 + a McTyre Bluesmaker .
I know only a few things about amps but I want to experiment with the tubes .
So my question is : do I need a biasing for the 12ax7 and the el34 by changing the tubes ?
Is there a schematic available where voltages and current is noted ?
What is your experience with tubes for the lexicon 284 ?
Many thanks for your answer .
My e-mail is gvanassche@abriso.com
Geert