Wreck based circuit... question

[riscado]

If I change the rectification on an express from a diode bridge to a gz34, and remove the negative feedback, what would you guys advise me to do first in order to keep the amp under control...

Changing the rectification will lower B+ a bit, but removing the negative feedback in an express will most certainly cause instability. So let's say I really want the negative feedback off, what should I aim for firstly in order to control the amp? Maybe a small voltage divider in the preamp?

thanks

[UR12]

If the amp is stable now, you may find that removing the negative feedback will have less of an effect than you think. Of course your presence control will quit working but I doubt that you will see a hugh difference in gain.

[riscado]

Hi Dana, thanks for replying...

I'm not really trying to get more gain (even though I realize that will be an inherent result of removing the neg. feedback). I seem to realize that most amps I have played that lack the neg. feedback seem to have a response that I enjoy more, they feel looser and seem to swirl and feedback even at clean settings...

I wanted to try and enhance these characteristics in the express, by removing the neg. feedback (like an experiment, just to see what happens) but I would like the amp not to go wild on me, since there is a lot of gain on tap from that preamp design.

thanks

[paulruby]

Changing recto will lower B+ by 20 to 30V. This will take off some edgeyness in the top end and make the bass a bit more mushy. But, it will be a subtle difference, not make or break.

Removing NFB will have bigger impact. It will primarily loosen up the bass and it may get farty. I would suggest you experiment with the amount of NFB via the feedback resistor instead of simply removing it.

[riscado]

thanks I'll try that...

[ampdoc1]

On the first Express clone I made, I put in a three way switch to vary the negative feedback. Mostly, though, I used it with 0 feedback. Not farty at all, just a little looser.

ampdoc

[riscado]

I might have done this question already, but have any of you ever added an extra gain stage to the wreck circuit?

So it would have first gain stage-tonestack-gain recovery stage and then two more gain stages? The original design has more gain than I could ever use, so I'm asking this just for kicks not that I really intend to try it...

thanks

[ampdoc1]

Not trying to hog this thread, but the first one I built I added a gain stage before the PI. It also included some switching options on the power, a power "scaling" type control on the output tube heaters, EL, and changes in the preamp tubes to reduce noise. A great sounding amp in the end, different,... but still had all the touch sensitivity and playability that it started with.

I've attached a schematic.

A built another variation a couple of years ago that used the Express preamp feeding a cathode follower, then added a Dumble-type OD stage, and switching. It was not as sensitive as the Wreck, but had a little more of the Dumble sustain thing.

ampdoc

[riscado]

nah don't worry you're not hoging, that's pretty much what I was asking about, cool to here the description... I'm thinking about building a dumble ods too ,just to be able to check the differences between these two very sought after amps, even though I do realize they're very different.

[dartanion]

You can try the other OT taps. 16 ohm tap will give you the highest NFB voltage and most attenuation whereas the 4 ohm will be the lowest NFB voltage at half that of the 16 ohm tap.

[paulruby]

Not trying to hog this thread, but the first one I built I added a gain stage before the PI. It also included some switching options on the power, a power "scaling" type control on the output tube heaters, EL, and changes in the preamp tubes to reduce noise. A great sounding amp in the end, different,... but still had all the touch sensitivity and playability that it started with.

I've attached a schematic.

A built another variation a couple of years ago that used the Express preamp feeding a cathode follower, then added a Dumble-type OD stage, and switching. It was not as sensitive as the Wreck, but had a little more of the Dumble sustain thing.

ampdoc

Without looking all that closely, the shared 10K cathode resistor on two cascaded stages is a very strange setup. that creates a very odd feed back (or feed forward depending on the setting of the 250k volume pot) between the stages. Those are certainly not independent gain stages bt are very interactive like that.

[ampdoc1]

No argument Paul. But, I didn't use the second volume as a front panel control, but an internal adjustment to regain some drive on the end of the line. It seemed to give back what was lost using the lower gain tubes, without the hiss of the original circuitry.

ampdoc

[Fischerman]

ampdoc,
Is that MV drawn correctly? It seems like you'd put it on the other side of the PI input cap. The way it's drawn...it seems like it would affect the PI balance as you adjusted it (and maybe even the bias of that triode?)...but if you put it before the PI input cap it wouldn't...it would just attenuate the signal going into the PI.

riscado,
I've added a cathode follower at the end of the Express pre...however my Express pre had already been changed to a Komet pre. It's essentially a Concorde preamp...or at least, what I've deduced to be the Concorde preamp...I've never seen a Concorde schematic (or a Concorde amp for that matter). Of the 3 (Express, Komet, Concorde) the order in which I preferred them was: Concorde>Komet>Express. They all have similarities...also, I learned a thing or two along the way and was able to incorporate what I learned into the Concorde but not the Express. Maybe I need to just start over.

[ampdoc1]

Fisch,

I'm pretty sure it's drawn as installed. That's a circuit I devised myself, and it works as well as any pre-PI MV I've heard,...no problems with upsetting the bias or balance of the PI. I posted this a couple of years ago on AMZ and had several responses that this circuitry had been in one of the TUT books earlier. I don't have any of those, but it might be interesting to see how he set it up.

ampdoc

[paulruby]

That MV is perfectly fine. The bottom of the the MV pot is the PI bias ref voltage to avoid the need for another coupling cap from MV to PI.