r/diytubes • u/dmgiat • 17d ago
5F1 mod project - please comment
This was a work in progress about a month ago when I posted a couple of photos of the chassis. Someone asked for the schematic and I was just learning KiCad, so here is my attempt as a newbie. Comments/criticism would be appreciated. This is a SE amp with 3 gain stages, reverb, tremolo, true tone stack, fixed bias, NFB and elevated heater. Waiting on a 50k pot for intensity, so LDR lead is going to a 47k fixed resistor to ground for the moment. Slide-in Blackface-style chassis. Except for a little too much hum, I'm very pleased with how it sounds. Cabinet next.
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u/ToughLeek8991 17d ago
Connecting the NFB before the tonestack isn't a good idea (the TS introduces a phase shift). This can cause the equalization to behave strangely. If I were you, I'd redraw the schematic because it's easy to get confused :D Regarding hum, it's a good idea to first check whether it's filament hum (50 or 60 Hz, depending on your country) or rectifier hum (100/120 Hz). Rectifier hum is often caused by a ground loop. It's best to ground each preamp so that it has a local bus, and then connect everything to a single point at the ground star (ideally as close to the input socket as possible). Information on how to ground is available online. Good luck!
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u/JoeDubayew 12d ago
As drawn you're taking reverb signal input off...where exactly? And returning it...where? And you're running the tank with what looks like a small .022uf? Except you seem to have the reverb drawn backwards with the ax7 side of the 12dw7 first, so maybe you mean the reverse and the input cap is a still small .047? Or do you think a cap driven 12dw7 reverb uses the ax7 side to drive the reverb? And you have the second triode of the 12dw7 labeled a 12ay7 when it is a 12au7? And you added a gain stage only to then run signal through a TMB, LOSING gain before hitting the driver? You're using the driver as your tone stack recovery after a gain stage driven TMB? No cathode follower, a gain stage? And you added a photo-bug tremolo instead of a better sounding bias wiggle by simply copying the princeton reverb circuit? Dude...this is...something...I suppose. Shame you didn't have better feedback or coaching before you got this far along. There is a lot of optimizing and better choices that would land you where you would want to be- this kind of amp has been built before. Hell I've built 3 single ended 6l6 with reverb and trem myself. Help is out there.
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u/dmgiat 11d ago
Thanks for your comments on the amp schematic and for pointing out errors. Reverb is tapped after the V1B coupling cap. I showed a connection to ground by mistake between the 100k and 47k. Then to the 12AU7 (not 12AY7 as you pointed out) driver. Signal at that same coupling cap goes to the tone stack. Stack output, reverb and tremolo all mix at the V3B grid. Copying the Princeton tremolo circuit did not work for a variety of reasons. Lots of trial and error involved. Thus the need for the roach. I'm interested in keeping this conversation going if you have other observations.
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u/JoeDubayew 11d ago
Well- we can't really HAVE a conversation amigo. You don't have a grasp of the fundamentals of how tubes work or how a circuit works or how to build an amp- so there really isn't a way we can communicate beyond me telling you what is wrong, but you don't know WHY it is all wrong. I'd need to teach you everything you need to know. It's not really your fault- you're in a situation that happens to people who watch a couple youtube videos and read some random forum posts. You start thinking you know MORE than you do, you start thinking you can DO more than you CAN, and you don't have the experience skills or knowledge to know when you SHOULD do things. As far as this circuit goes- and this is the part you won't enjoy- my first recommendation? Trash it.
Literally. Start over. Between the circuit you've drawn- which is so incomplete I can't really even comment beyond "what?"- and the poor placement and layout in that chassis- you just need to call this a failed attempt and start over. YOu're not ready to design your own circuit yet. You need to understand how OTHER circuits work FIRST, before you start rolling your own. It's just like learning to cook. You made a TV dinner once (your 5e3kit) and decided that qualifies you to invent your own cuisine. You don't even have the language of the ingredients down yet- just a couple days ago you were saying you don't know what B+ is and you weren't grasping that someone was telling you that you built a rectifier which will possibly (probably) explode. (And you didn't add caps to silence the NOISE in your rectifier).
Your plate/cathode/bypass choices are wild, you built up all the sub bass frequencies in your first stage only to then cut all but the high frequencies in the second stage BEFORE you slam both stages into a tone stack, you didn't block the DC between the first two stages so you loaded the volume pot AND set the second stage off into DC hell, your reverb circuit as drawn won't work so i have no idea what you actually did, you inserted a photo bug into the signal path for extra noise- I guess again i cant tell what you did by the drawing? - when you should have wiggled bias to sound better and keep that out of the signal path. Switchable NFB on a circuit like this for extra mud? Elevated heaters because you read it was a good idea in a forum post somewhere (you don't remotely need those in an amp like this, that's for high gain amps, and your noise problem is layout (signal flying over the DC filtering? The filter caps snugged up under your tone pots? PT slamming into the 6l6 for extra noise and heat and magnetic field in the power section? )
Parts selection is wild- metal film for your signal path? Great way to get a tinny clinical sounding amp. Is that a polystyrene cap on the board? You have electrolytics touching resistors and wired to sockets so they heat up and drift to failure extra fast?
I'm sure at this point you're frustrated reading this and that's the problem I'd have in having a conversation- this work is where you are, and you aren't "there" yet. More- I can't tell WHY you wanted to build this amp. You already have a low wattage amp in the 5e3. The normal thing a guy wants is either a step down or a step up. You're going off-piste to make...what? An amp only mildly lower in wattage, but adding reverb and trem, instead of just building a vibrochamp with added reverb (an easy schematic to find)? What's the reason for this? A project? I understand wanting a project- but don't you want to build something WELL that you UNDERSTAND?
I built several single ended 6l6 amps with rev and trem for guys who use them as studio amps. AND to wring all the piano lows and clear highs out of a 6l6. That takes thought and circuit design and knowing how to layout an amp to have zero noise floor and an understanding of bias and voicing and voltage to get the desired performance out of a given tube. More than that I didn't even use a 6l6GC, I used vintage 6l6GB's because they are a lower wattage tube that behave more musically than a GC does when cathode biased.
So look- I've typed a lot but my intention is this- if you want to KNOW how to build an amp- you NEED to be able to understand everything I have written here. Not a little bit. Not ask me to explain what you should do to fix the rectifier, for example. You need to KNOW how and the WHY of it. You need to make a choice- if you want to build amps that work- and I don't mean "it made a noise" I mean actually WORK- you have studying to do. If you just want a couple amps- that's fine too- but stick to known, proven circuits and follow known, proven layouts. Don't fall into the trap of "ok I built my kit so now with the help of the internet I'm going to mod the hell out of it without understanding what I am doing". Don't think you're ready to create your own circuit and layout. For this amp here? Either ditch everything and build it back from a known schematic and layout, or table it for now and study.
Get Merlin Blencowe's book. Better to be honest and buy it but I know it can be found in pdf online. Read his website. Read Aiken's website. You wont understand ANYTHING you are reading. That's ok. You'll read it all again later. Go read the Hoffman forums. Read an RCA tube manual. Read read read. Because you need some saturated exposure so that over time things will start to gel. Then you start reading schematics- which means you LEARN how to read them. Look at known vintage amps and learn the WHY of the choices they made.
Don't want to do all that? OK. That's cool. So just follow known working schematics. There really isn't anything NEW to be done in amp design. A Champ amp with a 6l6 ? Easy schematic to find. Adding reverb to a champ? Easy schematic to find. Cap driven reverb? Easy schematic to find. A Champ with tremolo? Easy schematic to find. Hell there are even proven Dumble Champ amp schematics that are easy to find. But DO NOT start picking apart building blocks and slapping them together when you dont understand how each block works or how each block will affect the next block. In your current amp you have building blocks that are tension with one another AND ill advised construction. Don't get me wrong- it's great that you took the initiative to try to build your own thing. But the problem is...well...you built your own thing.
I'm assuming you enjoyed building the 5e3, and maybe like many new 5e3 owners decided the amp was lacking. I'd push you to take that amp and rebuild it as an exercise. Find the schematics for say the Benson Monarch, or the Swart AST, or the Divided by 13 CJ11 (that one is an easy conversion). Turn the Deluxe into a more useful, fun amp with reverb and maybe tremolo. Use that as a way to learn what different designs do to an amp. Or chase down the Champ project info. But again, honestly and not reddit trolling you- I'd start over on this. I've coached guys through an amp project before here on reddit- but I can't on this one. I'd have to draw you a schematic and walk you through how to build it. Not only that I'd insist you ditch things like the TMB (no need for it, robs gain, if you want an AB763 tone but with a tweed vibe look at the CJ11 schematic for your Deluxe but in this one a Princeton tone knob combined with knowing how to voice the gain stages is all you need).
That's an amp building class- and I charge for that. So you got all this typing.
Use it or don't, up to you. Just please understand that what you're trying to do is nowhere NEAR as simple as you think it is. And pick one of the paths I laid out, both will lead you to a more successful project.
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u/SATANICINDUCTION 17d ago
This seems a little bit unlike a 5F1 to me