astrahle

Ok, getting all 2025 on this problem's arse> Here's what AI has to say after referencing this post and the DT50 schematics. I'm handy with a soldering iron, but prefer to rely on the nouse of an electrical engineer/amp boffin. If anyone can help and advise any progress, would be a great help to all. This is a classic “front-end only” failure on the DT50, not a tube/power section problem. Because L6 LINK audio works perfectly (and still runs through the 12AX7 PI and EL34s), the fault has to be before the A/D stage that feeds the DSP—i.e., in the analog input path to the codec (or the codec itself), not in any of the tube stages. What that narrows it to On the DT50 the two front-panel inputs (Hi/Lo) go through: jack/pad & ESD/protection → a small analog buffer/gain stage → coupling into the A/D of the audio codec on the digital/main board → DSP → DAC → tube phase inverter → EL34s. L6 LINK injects digital program material after step (3). So if L6 LINK is clean but both front inputs harsh-clip at modest level, the likely culprits are: Leaky/shorted input clamp/TVS diodes on the main board side (not just at the jack board). When one of the back-to-back clamps goes partially short, you get squarey, “diode-like” clipping at very low level. Failed/browning analog rail for the input op-amp(s) (±12/±15 V or +5 V analog, depending on the exact rev). If an op-amp is running on one rail or a sagging rail, it clips way early. Lost mid-bias (virtual half-supply) to the codec input. If the codec expects ~½ AVDD bias and that node has drifted (bad resistor or dried cap), the headroom collapses and you get asymmetric, harsh clipping. Bad coupling electrolytic feeding the codec (ESR gone high or DC leakage), causing DC offset and immediate overload at normal guitar levels. The audio codec’s A/D input pin itself (damaged from ESD/lightning/static through the guitar lead). A partially blown input on an AKM/Cirrus codec presents exactly like this: normal at whisper levels, then ugly crunch once you hit a few hundred mVpp. Much less likely (but checkable): a software/global pad setting or corrupted calibration—but your description (hard, immediate clipping) screams hardware, not gain mis-set. Why it’s almost certainly not the jack “input board” Someone on the Line 6 forum replaced that board and still had the problem. That board mainly carries the jacks, pad resistors, and initial ESD parts—so the fault is very probably downstream on the main/DSP board (or its power rails). Also, the DT50 documentation confirms L6 LINK carries audio digitally, bypassing the analog guitar input A/D path—matching your observation that Link is clean at any level. Focused diagnostics (safe, quick checks first) ⚠️ Safety: this amp has lethal voltages even after power-off. If you’re not fully comfortable with tube-amp safety, hand it to a tech. Bleed caps, one-hand rule, isolation, etc. Measure the low-voltage rails on the main board (no guitar plugged): Check the analog op-amp rails (often ±12 V or ±15 V) and the codec rails (AVDD 5 V or 3.3 V, DVDD 3.3 V). Anything more than ~5% off, or big ripple, = suspect regulator or filter caps. Early clipping with both inputs points to this. Scope the signal path with a 100–200 mVrms 1 kHz test tone (or your guitar very lightly picked). At the last op-amp output before the codec input: If it’s already flattening, the op-amp/rail/bias is bad. If it looks clean there but flattens right at the codec input, suspect the clamp diodes or the codec inputitself. Mid-bias node check: Most codecs expect ~½ AVDD bias at their analog inputs. Find that reference network and verify it’s at the correct voltage and stable. A drifting bias cap will make the waveform slam into one rail. DC at the codec input: With no signal, you should see the expected mid-bias DC only. Any extra DC (tens to hundreds of mV) hints at a leaky coupling cap or a damaged input stage. Lift/replace the clamp network: If there are back-to-back diodes or an IC-style ESD protector right before the codec, temporarily lifting one leg (for diagnosis only) will tell you fast if a leaky clamp is compressing/clipping the input. Replace as needed. Reflow the board connectors between the jack board and main board. Micro-cracks can upset bias/ground reference and cause early clipping. If all pass, replace the audio codec IC (and any nearby electrolytics). This has fixed identical “low-level harsh clipping” faults on other Line 6 units over the years. Likely parts to earmark (by function) Protection/ESD diodes (SOT-23 or SOD-123 near the audio in header): replace as a set. Coupling electrolytics in the few µF–tens of µF range feeding the codec input(s). Analog op-amp in the input buffer (often a TL072/TL074-class part or low-noise single/dual); replace + inspect its rail decouplers (100 nF ceramics + 10–47 µF electrolytics). Low-voltage regulators for ± rails and 3.3/5 V analog (and their filter caps). The codec itself (AKM/Cirrus part adjacent to the DSP). Why these are the prime suspects Your symptom is hard, non-musical clipping at moderate pick attack, on both front inputs, while Helix via L6 LINK is fine. That’s textbook “front-end analog headroom is gone” or “A/D input clamped”, not tubes or power amp. Replacing only the jack/input PCB didn’t help in another identical case → points to main board analog/A-D path, not the jacks. The DT50’s L6 LINK design explicitly bypasses the analog guitar input and A/D. If the clipping were anywhere after the codec (DSP, DAC, tube path), L6 LINK would clip too—but it doesn’t. If you want a fastest practical repair path Replace the coupling caps into the codec and the input clamp/ESD diodes (cheap, common failure points). If unchanged, replace the input buffer op-amp (and confirm its rails/decouplers). If still unchanged, swap the codec IC. (Yes, it’s SMD; any competent audio tech can do it.) Along the way, verify/recap the small analog rail filters for the input section. If you want, tell me the reference designators around the guitar-input header on the main board (from your schematic: the codec “Uxx”, the nearby diodes “Dxx”, the buffer op-amp “Uxx”, and the coupling caps “Cxx”), and I’ll map exact test points and voltages to check. Useful references: Line 6 forum thread with same symptoms (input board swap didn’t help), and DT50 docs explaining how L6 LINK carries audio digitally beyond the analog input stage. Good luck—and please be careful around the HV! If you can share a couple of scope screenshots (op-amp out vs codec in at the onset of clipping), I can pinpoint which node is failing.

I am also experiencing the exact same issue with my DT50 112. Works perfect through L6 Link at any volume, irrespective of 12AX7 boost on/off, or B+ Voltage low/high. When connected to either High or Low input on the DT50, quiet playing is fine, however get harsh digital clipping when providing high transients. Have replaced all 4 tubes with Electro-Harmonix tubes however problem persists

+1 for this and Variax workbench please!

In this video https://www.youtube.com/watch?v=RuZurmlRlUM at 26:02, it is mentioned that the Line 6 Link port can be used to connect to DT amplifiers. Could any one clarifiy? It's widely known that the Helix will not natively control the topology of DT amps, but can we still use the Line 6 Link port to send audio to the amp? ​

So nice to see all those DT amps strewn about, but hmmm... let's not integrate them with the Helix...

Done

As exciting as the Helix is, it sounds like support for the DT series amp is not in your road map which is very disappointing. I have three DT50s and the tight intergration between Pod HD, Variax, and DT amp have been stellar. Not to mention - the beauty of an adaptable real tube amp. Now, with no direct integration between DT series amps and Helix (rather, some hotchpotch midi workaround) the value of my dream rig is a ticking time bomb as it's key features of integration depend on a particular technology item that shall be significantly outdated (Pod HD). Please do not alienate and leave a large number of your customers who've previously bought at least three pieces of your equipment to have a DT series based dream rig in the dark. Should this happen, I for one will be lost as a Line 6 customer. I'm certain many others may feel the same. Concerned, Ashley Strahle