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What’s up with this poor forum?!
I just ordered one of these, I think the design is highly unique and intriguing.
I don’t have the amp or schem yet, so I’m wondering specifically how the negative feedback is configured. To the uninitiated it would look as if it’s simply a wire from the output straight to a pin. Holy crap, could that really be it!? I’m assuming there’s a pretty high resistance somewhere in that loop.
I was also thinking that could be a great setup for a presence and/or resonance control.
What do you think? Anyone out there?!Charles,
Thanks for your interest in the Percolator. I hope you enjoy building it.
The NFB loop is connected from the output to the cathode of a previous gain stage via a resistor (you’ll see the schematic when the kit arrives). You are correct, a good way to mod the circuit is to manipulate the NFB loop with either a feedback level control, some tone shaping (via an RC network), or a switch to disconnect it for extra gain in the midrange.
I’ve done this sort of thing with several different amps in the past, but it’s been so many years now since I designed the Percolator that I can’t remember the specifics of my experiments with it’s NFBL. Feel free to experiment with it yourself!
Have fun and take care.
-BrachWell, I finished the amp, and then ruined it. Man, it was nice and loud! although it had a muffled tone. Soooo, I went through 3000 changes to get some brightness. (I’m a Vox guy)
Somehow I got carried away fudging with the circuit and ended up with a near 100% clean amp until the volume was turned up to around 9 out of 10.(And still a bit muffled)
I decided I was an idiot and I replaced all the changes with all the original spec components.
Well, now I can crank it to 10 and its max is about at 60Db.
All test point voltages check right on except at R8, which should read close to 70v and I’m at about 90v. All resistors measure expected values within 1% ish.
I tried 3 different tubes, speakers, input equipment, and even upgraded the output transformer, no changes.I’ll tell you what though, I’ll keep at it, this is a really cool amp design!
Quick update, I’m getting closer, It’s somewhere around R10, which makes sense, considering the preamps are fine, it’s in the power amp section, my readings for the power amp resistances to ground are wacky.
I gotta tell you though, this design is about bullet proof!Oh Geezus, What a dumb mistake, I wasn’t using my auto-range meter. I put a 220 ohm resistor in the 220K ohm position! Too much ground on pin11! That being said, it still played well with no damage to any circuitry.
As a side note, I tried a 100K resistor in that position and I get a bit earlier break up and it’s still louder than I need.I shoulda just kept my mouth shut. This is a GREAT little amp.
Good! I’m glad you figured the problem out.
It’s good to hear you are enjoying it! Hopefully building it and tweaking it was a lot of fun in itself.
Take care.
-Brach
All done and enjoying it immensely! I used some neg feedback for an always on Prescence and a 3 way “power switch”. up position is full power with asymmetrical Germanium diode clipping, middle is full power and down parallels resistance to 66k on R10 for a medium output cut, this allows me to push the preamp section a little harder.
I installed a Kustom Defender 5 output transformer for 16,8 and 4 ohm on an output switch. I used the 4 ohm tap for the presence circuit.
I also lowered the value of c7 to 300pF, still cuts ultra high noise, but it also seems to brighten up the amp a bit. I raised the value of c1 to .068 uF to let a tad more bass into the front.
As you’ve stated, it’s hard to describe the tone of this design but I’d say it’s a mix of an early 5f1 or GA5 and a Vox Night train.
Thanks again, I’m now playing this more than my AC30!I’m so glad you are enjoying your Percolator!
Those mods sound nice. But I hope you ran a load line on the circuit to see if your transformer is within range to work for it. I don’t know anything about the Defender 5, but I hope that mod isn’t damaging your tube. Another customer tried to change the output transformer on this amp and ended up killing the tube and a resistor or 2. So I’m nervous about customers modding the OPT now, but you sound like you know what you are doing.
I made the frequency response of this amp a little wider than most amps, to make it sound “fatter” and “bigger” than it really is. After all these years, I can’t remember what frequency cut-offs I used. Most amps I design, I limit the bandwidth to allow it to fit in the mix within the context of a band, so it will cut through the mix better and be heard easier…more mid-range. I didn’t figure the Percolator would be played in any kind of ensemble, but rather in someone’s bedroom by themself where it’s nice to have a wide bandwidth, or in the studio where you can EQ it to fit into your recording.
-BrachI can definitely tell the tone of the percolator was very specific. I decided to expand on that just a tiny bit to closer match my particular tone tastes. As far as letting more bass in it’s just a bit extra to fatten up my single coils.
You got me there, I didn’t load test the OT. I went with basic knowledge and some educated guess work. My guess was that the compactron needed somewhere around a 6k load and I think this transformer is pretty close to that. (I have a whole collection of power and OT transformers)
I have about 3 hours on the new OT with no issues yet. If I did make a mistake, It would just be another project, I have your OT set aside in case.
I’ve been recommending this amp to my buddies, they usually look to me for gear advice. I have a pro player coming soon to really put it through its paces. When I get some more gear cash, I think I’m going to be another one of these, hopefully you’ll still have one!
When my buddy comes over, I intend to record him playing it and put it up on Youtube. (although I only have one crap video of my dad playing and zero subscribers!)
*EDIT* I’m charles99 on Instagram.
