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SEplex style build questions (fizz, fuzz, fart?)
SEplex style build questions (fizz, fuzz, fart?)
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Old 17th December 2017, 09:24 PM   #1
jarrodthebobo is offline jarrodthebobo
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Default SEplex style build questions (fizz, fuzz, fart?)

Hey guys, I'm pretty new around here! I actually joined up here because I was having trouble with this DIY amp of mine and was having a couple of odd issues with it.

The amp in question started its life as a bit of a project one of my buds made up for me; it was an Epiphone Valve special that he gutted (aside from the power and output trannies) and built a brand new all tube amp circuit inside. It was great for cleans, but not much else honestly. It also used 3 12ax7s, 12 utilized for one clean channel, and 1 for a different, separate clean channel.

I have since modified the 2 12ax7 clean channel to the topology that the SEplex has, and although it sounds pretty great, there is some odd sort of fuzzy/hissy/farty quality to the distorted tones. Clean and crunch sound very good, but the overdriven tone sounds like it has some sort of 'swirl' to the sound, and its more harsh on the lower strings.

At full gain, chords seem to 'compress' in volume. I've upped some of the grid stopping resistors in the preamp, and the grid stopper for the el84 already seems very high (100k resistor in series with a 2k resistor?? Not sure if that is a mistake or not but there DEFIANTLY is no lack of high end in this amp) so I'm not certain its lack of blocking resistance. Also, all coupling caps are .02, just like in the SEplex so I'm not certain its that either.

Could it be that the output transformer just can't handle the amp as it is? It doesn't seem to be dependent on volume; just the pre-gain.

Thanks for any help you can provide me with!

ON A SIDE NOTE:

If I wanted to say, convert the power section to a dual el34 design (I have a 100w PP power tranny and output tranny hanging around from a busted amp), would I have to make huge changes to the power supply as is, or should that be able to remain unchanged? I'm not very well educated with power amps and how transformers, both output and power are rated and utilized. I do have a relative understand on pre-amp circuitry however and realize I'd likely have to convert the clean channel 12ax7 into a phase inverter if I made this push pull; but that's not such a big deal to me.

Thanks!

-Jarrod
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Old 18th December 2017, 02:00 AM   #2
audiowize is offline audiowize  United States
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Can you post a schematic of what you have now?

A 100K grid stopper isn't acceptable for an EL84. A 100K grid leak resistor on an EL84 is OK. My guess is that you have a 100K grid leak and a 2K grid stopper.
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Old 18th December 2017, 02:14 AM   #3
PRR is offline PRR  United States
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Quote:
Originally Posted by jarrodthebobo View Post
...If I wanted to say, convert the power section to a dual el34 design (I have a 100w PP power tranny and output tranny hanging around from a busted amp), would I have to make huge changes to the power supply....
I have a 9HP lawn tractor. Can I put a 300 horsepower Mustang engine in it?

Actually, the guy up the street "did". There is a lawn-tractor race club around here. Gol-danged V-8 on top of the teeny front tires. But LOTS of mods to the driveline. And I suspect he can't really get all 250HP through the small rear tires to the ground; it may be more smoke and noise than actual GO.

No, the original power is basically all you will get until you replace Major Parts.

With OT and PT, you are halfway there. Will it fit? Can you build the more complex design? (I would suggest you explore the present amp for practice before going big.)

As for your actual question: you should give a link to this "SEplex", because not everybody has every plan on the Web memorized yet; you should detail what you think you really built (often, after a long thread, it turns out there WERE significant changes), with pictures to show what was really built.

Last edited by PRR; 18th December 2017 at 02:16 AM.
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Old 18th December 2017, 05:43 AM   #4
jarrodthebobo is offline jarrodthebobo
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Click the image to open in full size.

Whoops, sorry about that! I thought I had hyper linked "Seplex" up there but I guess it didn't! Helps to have the circuit, haha.

Since posting this I've played around with a lot in the circuit and have fixed it a bit; mainly by following the schematic a bit closer at certain points, and removing others.

For example; On the last stage (the 'fake' pi v2b), I had a switch which allowed you to select a bypass capacitor for the cathode which allowed for 20uf, .68uf, and finally nothing. Remove this little circuit help immensely with the mush-y issue I was having. There is no cathode bypass cap here now, and I feel like a cathode bypass cap here probably isn't a good idea gain staging wise as it seems to drop too much signal onto the output tube, but who knows; maybe I just fixed something else while I was in there??? I don't feel like putting a cap back in there because it sounds pretty good at the moment, haha.

Another thing I had done was remove that 100k resistor going to the grid of the el84 (it wasn't a grid leak, it was in series with the master volume). I bumped the grid stopper resistor up to about 5k. Not really sure what this really did as I oddly didn't really notice a change in high end response (which I thought this would alter dramatically). That weird fizzy high end is still here though; but a bit less buzzy. I also removed the 470k resistor on the master volume lugs, as well as the 500pf cap as I didn't feel they were necessary.

After the volume control (v1a) I added a 470k grid stopper with a 470pf cap bypassing it just for experimental purposes; again I didn't feel like too much high end was cut with this mod, which is a bit odd considering 470k is pretty significant for a grid stopper. A bit less 'mush' with this mod too.

V1a's cathode bypass cap is on a switch so you can choose between 20uf, .68uf, or no bypass cap. I really like this little addition here, and found it didn't really add any noticeable noise when used.

The 'boost' circuit after the tone circuit is a bit different for me; since I ran out of 3 pole switches, I used an on-off-on dpdt switch with the middle lugs wired to ground in series with a 220k resistor, and the other to the exit point of the tone circuit. One side of the switch is bridged to just allow the 220k to operate per normal. The other side of the switch has another 220k resistor to make a sum of 440k for a little extra gain boost. This little circuit works, but it's kinda unnecessary. The gain added isn't really enough to warrant use.

Finally, for a bit of fun, I have a 25k pot on a switch wired in parallel with the tone slope resistor so I can play around with the mid point shaping. Again, not super usable, but interesting to mess around with. I want to try converting this to series so I can increase the slope resistor rather than drop it.

There is an artificial center tap that I put on; 90ohms for each resistor on the heaters of the 6v6 socket, going to the cathode. Defiantly helped quiet this amp quite a lot.

Other than that the circuit is the same as the SEplex pre-amp wise. I havn't touched the power amp circuitry disregarding the grid stopper. I don't exactly remember the resistor values on the cathode of the 6v6 and the el84, but I DO remember that they matched the 'recommended" values for these tubes (I think 6v6 was about half the resistance of the el84; but I'm not sure).

I'll take some pictures in the morning of my messy rats nest here. I'm surprised the amp is so quiet; because I don't don't have neat wiring here! The only time you really here audible hum is when the master is cranked or the pre-gain is cranked; at mid levels this dude is dead quiet. I thought SE amps were supposedly super noisy just based on principal!
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Old 18th December 2017, 08:03 AM   #5
GeorgK is offline GeorgK  Austria
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Have you also tried different tubes?
I have found that the common chinese 12AX7 by Shuguang can sound really nasty in some amps featuring that DC-coupled tone stack driver stage (V1b+V2a). Not sure why this is the case and some amps react more to this than others.
Just an idea of course...
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Old 18th December 2017, 09:46 AM   #6
jarrodthebobo is offline jarrodthebobo
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Yeah ive played around with a small handull of 12ax7s, 6v6s, and el84s and havnt really noticed a reduction in fizz. I should post a clip when i get the chance
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Old 18th December 2017, 12:07 PM   #7
GeorgK is offline GeorgK  Austria
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What looks a little odd to me is that the master volume pot is included in the NFB loop. Behavior of the power amp will therefore change with its position. Have you tried omitting NFB to see if something changes?
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Old 18th December 2017, 03:36 PM   #8
Printer2 is offline Printer2  Canada
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Conjunctive, zobel filter on the EL84.

5.6k-10k, with 0.01-0.022uF in series across the output transformer primary.
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Old 19th December 2017, 01:06 AM   #9
PRR is offline PRR  United States
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Quote:
Originally Posted by jarrodthebobo View Post
....On the last stage (the 'fake' pi v2b), I had a switch which allowed you to select a bypass capacitor for the cathode...........
Not a "PI"; the driver. We usually dedicate a gain stage after all controls and before the power bottle.

There's NFB at that cathode. Putting a cap there screws it up.

Agree that having a "master" inside the loop makes it a very different amp up or down. People do that. Hey, if we wanted predictable response we could use chips and transistors.
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Old 19th December 2017, 03:15 AM   #10
jarrodthebobo is offline jarrodthebobo
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I'm a bit confused; how exactly is he master 'inside' the NFB loop? There isn't really anywhere else the master can be put in this circuit and remain effective, is there?

Thanks!
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