Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cboysen on November 03, 2021, 06:36:26 am
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Hi, this is a list of some lo-fi tips and personal opinions, that could have helped me to a quicker and better solution when designing, building, modding or fixing amps in the past. Hopefully this will come in handy to some. Feel free to ask questions or add your own tips in the comments. Needful warning: These tips assume a basic level of safety. Sticking your hands inside an amp that's turned on can be lethal. Don't tinker unless you know what you're doing
Nice to Have tools
- Attenuator. I highly recommend building a small reactive attenuator, like the one from M3 by JohnH at the marshall forum. Having the ability to listen to the amp at any volume setting, may reveal hidden problems and design flaws. It also serves in great conjuction with a listening device, when you don't want to listen to the speaker signal.
- Listening device. A small device capable of extracting the AC signal at any point in the amp. This makes it far easier to figure out where unwanted noise may originate from, and also reveal when, where and how the amp is breaking up.
- Looper pedal. A signal generator for your listening device, and a valuable tool for finding loose connections when poking inside your amp while it's on.
- Poking stick. Always have one hand in your pocket. Use a wooden sushi stick or a pencil to tap components and move wires. Also, a pencil can be slightly conductive at the tip, which may come in handy when looking for parasitic noise.
- Current Limiting Lamp. When starting up any amp for the first time, always use a current limiter. This has saved me more than one transformer in the past.
Trouble Shooting Tips
- Isolate the problem. Imo, step one of any issue, is to find the approximate area of fault. Depending on the problem, it's often useful to pull all the tubes or otherwise cut off sections of an amp. Then insert one section at a time, from output to input. If the problem appears when a section is inserted, that section or something around it, is most likely a part of the problem.
- Always assume it's a tube problem first. Did you buy NOS tubes for your newly build amp? You're likely in for a ride of fun trouble
- Take a step back, and think. My wife always say "why don't you leave it for a while?". I hate to admit it, but most of my problems have been fixed with a fresh pair of eyes (and ears).
- Don't play the hail-mary guessing game. Like before, be smart about your decisions. Use your knowledge about tube circuits and basic electrical theory to make qualified decisions about mods or repairs. Go on forums, read a few good tube books (Merlin Blencowe i.e) or scour the internet for a solution, before picking up the soldering iron. Otherwise, you may end up spoiling your fine newly build amp for no good reason, and down the line create more problems by all the prodding and reheating of components.
- Tap away. When all that is said, use that poking stick, mentioned earlier. Microphonics, cold solders, shorts, arcing, parasitics, you name it. You'd be amazed how much you can figure out, before taking out the big guns (read: oscilloscope).
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Design and Modding
- Prepare and review. This may be obvious, but can't be overstated. When designing an amp, make sure everything fits before buying and assembling. Make sure your layout and diagram is without obvious errors. Use Visio and/or expressSCH to plan ahead. Building the amp is the fun, but fixing a bad sounding or faulty expensive new build is the worst.
- Steal or get help. This may sound quite condescending, but unless you've build and modded many amps, in which case this list isn't for you, don't think that you can somehow magically come up with a schematic and layout that sounds better than what's already out there. Leo Fender and others spent years building and modding their amps, with countless of iterations, where only some made the market. Expect to do the same, if you want to make your own design. A boring but safer approach to a great sounding amp, is to start off with a known layout and diagram, and mod it afterwards to your liking. If you do plan to radically change the amp, consult the internet. It will save you countless hours of headache.
- Less is more. This may only be true for clean amps, but it's my opinion that any time you add something, you lose a bit of 'clearness' to the tone. So you want reverb in your amp? Sure, prepare to use a big value resistor that drops a lot of dry signal and high end, to make the reverb audible when summed again. So you want 4 inputs for that cool jumper-channel sound? A single triode without the summing resistors, may produce a clearer and stronger signal with more high end content. So you want to use that second triode for some parallel input magic? Unless you truely make it parallel, tying grids, plates and cathode, you will most likely end up with two triodes fighting each other, and a considerable volume and tone loss, especially if you plan to only use one grid at a time. Consider using only one or two triodes before the phase inverter, or prepare to attenuate the circuit a lot to avoid an overdriven amp at low volume settings. If you want overdriven tones with a 5-6 gain stages, I highy recommend using a relay-type switching circuit. And even then, the mere fact that tubes are inserted, will affect the power supply, and hence the tone of the amp, for better or worse.
- Attenuate that signal! Contrary to what is stated earlier, a great sounding amp may have multiple stages of attenuation through out the circuit. Not only will this decrease the noise floor, but make you able to turn up the volume knob, getting those highs back lost to the lo-pass filter effect of a low-set volume knob (I don't recommend the use of bright caps). Highs at the beginning of the amp, will counter highs lost due to attenuation later. This will often give a better feel, and may smoothen the overdriven tubes late in the amp. For example, the ab763, is massively attenuated with a tone stack after the first triode, a reverb circuit after the second triode and a tremolo circuit after the third triode. Still a great sounding amp to most people.
- Attenuate that signal again! As a side note, I'm the firm believer of attenuation in amps, but consider attenuation before the coupling cap. As seen with the input triodes of the lo-power tweed twin, or the 6g3 brownface. Using lower gain tubes, split plate loads, or mixing resistors before the coupling cap, may reduce the loss of highs from the RC filter effect. If you do need to sum a signal, maybe use a second triode, and sum the signal by tying the plates? Not necessarily a better sound, but a different one non the less.
- It's all about that RC filter. Selecting a smaller coupling cap or reducing the size of the volume pot/grid leak resistor, either way you shave off some bass. Reducing the ripple voltage in the power supply by increasing the capacitor or making the dropping resistor bigger - Either way, you lower that cut-off point. Make sure it's lower than your RC cut off at the AC side though (plate and cathode).
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- Don't overdo the filtering. I strongly believe that a great sounding amp, is obtained by using the right amount of filtering. Early tweed amps and Vox amps used small filter caps and large dropping resistors due to financial reasons. They sounded great though. Increasing the filter cap, will often result in a more modern and stiffer feel, but can quickly make things lifeless and sterile. The same goes for that choke.
- Beware of small filter caps. On the other hand, choosing small filter caps, will lead to a greater ripple voltage in the power supply. If designing single ended amps, this will cause 120/100hz hum at the speaker, which is easily solved by an extra RC filter before the output transformer B+. If you consider using a standard negative feedback from the speaker out in your amp, prepare to increase those filter caps. Any ripple voltage will reflect to the secondary side of the output transformer, and with enough NFB, that will send 120hz straight in to phase inverter, and possibly upstream to earlier triodes, and send your whole amp into motorboating and other fun activities. So next time you want to give your cool tweed deluxe build some NFB, you may want to reconsider.
- Watch out for those parasitics. So you went and made your own design anyway. Before you go buy for a 1000€ worth of materials, make sure you don't have high impedance lying next to low impedance circuits, and that Lenz law don't send magnetic fluctuations into nearby components. For example, the output coupling cap before the mater volume, lying next to the coupling cap of the phase inverter, or long runs of grid wire picking up noise a long the way. Consider placing V1 close to the input jacks, and if you dare, don't place everything on a nice turret board, with a lot of parallel components ready to couple. All of this takes a lot of time and planning... maybe that Hoffman layout isn't so bad after all?
- Watch out for ear-fatique. After spending hours replacing that coupling cap, tonestack cap or cathode bypass cap for the millionth time, to find the right amount of bass and feel in your amp, you may end up fooling yourself. Chances are, you ears are getting tired and tomorrow the amp will sound vastly different. Personally I don't spend more than 45min to an hour when tinkering for better sound. After that I can't trust what I'm hearing.
Kind regards
Christian