• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Finally built a prototype

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I'll start off by saying I wrote a long post, and unfortunately took too much time. The site made me log in again and discarded what I wrote, maybe it was for the better, but sorry if this one sounds a little more sterile and I am no longer describing possibly important details. I'm writing this out in notepad before trying to post again.

First.. Thanks to everyone who helped me with design and unfounded fears around certain details. I decided that I can only model so much and just have to breadboard the damn thing. I only did a one-channel prototype, as I built it on an acrylic sheet with standoffs and only had so much space. The full design so far is using an 12AX7 and two KT88s, with an unloaded B+ of 530V.

I powered it up and used an old iPhone(burner phone, just in case) to give a signal, using a crappy Best Buy brand speaker(it's all I have right now and honestly wouldn't mind blowing if something went wrong). It sounded "fine" right off the bat. No discernable distortion, very fair frequency response, decent volume - all of this subjective. I have to say I did well up a bit when I first heard it make a sound. Honestly like the first time I heard my son cry when he was born. I guess I was just surprised it worked the first time and didn't catch on fire! I was able to read about 100dB out of the speaker when I turned the iPhone up to max. I didn't put a tapered pot on the input, nor measure the voltage output of the iPhone, and actually assumed I'd need to put another stage in. I was just surprised.

I'm going for hi-fi and wondering where to go next with the limited equipment I have. I have a couple of multimeters, an old Tektronix 300MHz analog oscilloscope, and a frequency generator.

The first thing I want to make sure is that I don't have any mega-frequency oscillations. I can figure that out. I do have some line hum. I'm using some breakout boards for the tubes that I screwed into the acrylic. Between that and the lazy wiring, it's physically impossible to dress the wiring in the way I would like to. There may be some other adjustments I can make, but I can address that when the time comes.

I verified the biases. They were a couple dozen volts off from what I had calculated, but from what I've read, that's to be expected, the tolerances with these things are awful.

So I guess my questions are trying to find out some objective truths about what I have.

Heat - Things started to smell hot. Now, I've never owned a tube amp, let alone designed/built one. I expect heat and have read some cases where someone grabbed a tube too soon and was burned pretty bad on their fingers. I'm not concerned about the temps so much, but it might be worth asking. I've been using my IR thermometer to measure temps. From least to most: the power transformer core raised up only 8ºF from ambient temp(80ºF at the time), surely fine. Other coils showed no appreciable temp increase. The 12AX7 went up to around 120ºF. The KT88 went up to around 180ºF if measured from the top, but in the "guts"(measured basically in the middle of the body of the tube) it measured nearly 380ºF. I'm not sure if this was glass surface temperature or radiative temp from inside. I turned off the lights and all I could see is the coil, no hot plating, as far as I know in my naive state.

Power consumption - As we know, this is complicated with the variety of frequencies and dynamics involved in audio. I'm not trying to come up with a single number like audio manufacturers do for bragging rights, but I would like to know how to measure how much power I am dissipating in the KT88 and output transformer to make sure that I'm not slowly killing them. I have looked around online about how to do this, but most of the resources are the same BS metrics that some manufacturers pull to get a max wattage, and I haven't found any that would help someone like me determine if I should "tone it down a bit" to protect the equipment, and also make sure the quality is decent.

Frequency response - I know this is a little more complicated, especially when hooked up to an actual speaker. I have a 100W non-inductive 8ohm resistor that I could hook this up to. This might remove some of the dynamics we'd normally deal with, but I think there is something lost in there as well. I also don't have a frequency analyzer, as mentioned, just an oscilloscope, some multimeters, and a signal generator. Can I approximate frequency response using this at all, even manually? Even with a non-inductive load, I fear that objective measurements on that will shoot me in the foot once put into an actual system.

Distortion - other than visually comparing input and output on the oscilloscope, or even if that is an okay way to do it, any tips in this arena?

KT88 - triode or ultralinear for HiFi? I have a switch that puts the plate between 100ohms to B+ and 1.3k to the 40% CT on the OT. I have to find a better way, other than ears, to discover the difference. Maybe one of the upper questions can answer that. I did a quick(20 second gap after discharging/re-heating) test between the two, and that may have been enough time gap to not notice a subjective difference.

Anyway, I appreciate any advice/tips I could get.

Thank you all again. I'm really excited to be on this adventure.
 
I will eventually upload some photos of my plexiglass hack job, and asking for schematics implies that I've built it directly from my original plan - I haven't. I'm still tweaking some things on the fly. I'll share what I ended up with. I was just asking for some basic overall tips from what I've observed so far.
 
The smell of burning is usually a good indicator of a fault, usaully a short circuit like a backwards electrolytic but valve amps do get hot, old valves that have been in storage often smell on first power up. So don't think there's anything worry about here but still worth keeping the old nostrils free of encumbrances.

Power consumption/overworked transformers. Transformers that are struggling usually are noisy, and get hot. To work out power used in a OP stage you have to do a few measurements and a few sums, see Uncle Doug on Youtube, he's done several video's on this and other subjects. Though guitar amp centric his knowledge still applies.

Frequency measurement, easy. Signal generator into the amp, a small 8 or 10r 1w resistor on the OP in a cup of water. Only do this with AC tests. Sig gens often have range switches, EG x 10, x 100, x 1000 etc. Stick it on 1khz (or 400hz with speaker tests) adjust display on scope till it fits all the screen, flick the range sw, if the displayed trace halves, you've found the 3dB points at which the amp rolls off. You can turn the frequency up/down to get a better idea, but toggling between the ranges gives you a quick idea.

There's a lot more more to properly testing for frequency response, especially in relation to stability, it can take hours of work, but your ears and a quick test as above will give you a rough idea. See - amplifier-frequency-tests for more info, there's also the Tektronix Cookbook available as a free PDF and a book of audio testing by Metzler, both can be found as free PDF's online.

Distortion, see the books above or download Soundcard Scope which is a free scope for PC, with you can test FFT and distortion which is as accurate as a HP3903B audio analyser.

Lastly, it's often impossible to find differences between different circuits or triode, pentode, UL unless you do this sort of thing all the time or are a mixing tech, sound engineer. For home use, use your ears, if the result doesn't make you grin like an idiot and start dancing about like a Grandad then it's not worth bothering about.

Hope that helps, Andy.
 
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The smell of burning is usually a good indicator of a fault, usaully a short circuit like a backwards electrolytic but valve amps do get hot, old valves that have been in storage often smell on first power up. So don't think there's anything worry about here but still worth keeping the old nostrils free of encumbrances.

I have been running it for a few hours by this time. Seems the smell may have died off and I haven't suffered any noticeable degradation. I'm thinking me handling the tubes and possibly the cathode resistor on the KT88 probably deposited something on them that needed to work out. I'd like to clarify that by "hot", I was referring to that subtle indescribable secondary smell of something getting heated, but not to the point where I'd call it "burning". I think I may be good here!

Power consumption/overworked transformers. Transformers that are struggling usually are noisy, and get hot. To work out power used in a OP stage you have to do a few measurements and a few sums, see Uncle Doug on Youtube, he's done several video's on this and other subjects. Though guitar amp centric his knowledge still applies.

Thank you. I found one of his videos on doing the measurement, and I did a facepalm after watching it. I was so hung up on the phase shifting between volts and amps, and meanwhile I had literally the same exact non-inductive 8ohm 100W resistor on my bench, as he used in his video.

One thing I would like to mention, however, is that he mentioned multiple times to rely on the multimeter measurement over the oscilloscope. A lot of common multimeters won't handle AC measurements well on frequencies over a few hundred Hz. Indeed, mine said I was dropping a bit over 22V@1kHz, which would imply I was outputting around 60W out of a SE KT88 setup on a 30W Hammond OT. Don't think so. Using the scope and doing the RMS conversion yielded an actual value of around a few volts dropped.

Frequency measurement, easy. Signal generator into the amp, a small 8 or 10r 1w resistor on the OP in a cup of water. Only do this with AC tests. Sig gens often have range switches, EG x 10, x 100, x 1000 etc. Stick it on 1khz (or 400hz with speaker tests) adjust display on scope till it fits all the screen, flick the range sw, if the displayed trace halves, you've found the 3dB points at which the amp rolls off. You can turn the frequency up/down to get a better idea, but toggling between the ranges gives you a quick idea.

There's a lot more more to properly testing for frequency response, especially in relation to stability, it can take hours of work, but your ears and a quick test as above will give you a rough idea. See - amplifier-frequency-tests for more info, there's also the Tektronix Cookbook available as a free PDF and a book of audio testing by Metzler, both can be found as free PDF's online.

Awesome. I did basically this, doing an "audit" across many frequency points as I had the scope hooked up to a dummy load. Obviously this isn't sufficient to pick up on harmonics and other artifacts, as I've read about in the books, nor an accurate representation of a speaker, but at least I was able to eliminate any obvious WTFs.

Distortion, see the books above or download Soundcard Scope which is a free scope for PC, with you can test FFT and distortion which is as accurate as a HP3903B audio analyser.

Great info, I will look into this once I get all of the other things out of the way.

Lastly, it's often impossible to find differences between different circuits or triode, pentode, UL unless you do this sort of thing all the time or are a mixing tech, sound engineer. For home use, use your ears, if the result doesn't make you grin like an idiot and start dancing about like a Grandad then it's not worth bothering about.

Sound advice. I figured out a process to do a faster A/B switch to make it more obvious. UL gave me a decent low and high boost that was good for things like R&B and other dance-type of music, whereas triode mode had the frequency response that gave the warmth needed for classical music. So I need to put a switch on this bad boy, because it really comes down to what I am listening to.

I was already grinning like an idiot and dancing around like a Grandad when I turned the thing on for the first time, it didn't blow up, catch fire, or sound like dookie! :D
 
A couple dozen volts off? So 24 volts off? I find it hard to believe that this is (just) a matter of tolerances. Can you show us your calculation?

After further observation I found that the biggest problem was B+ voltage under load and without. Back when I first posted here, I tried to model the power supply as well as the amplifier itself in LTspice, to which every one said "why bother?" and I agreed, so my model had B+ around 530V, which is what my real world circuit was at unloaded.

Under load, it dropped down to around 468Vdc. Since LTspice treats a voltage source as perfect and can never be bogged down(especially since the power source itself isn't modeled), it was still up at 530V. By adjusting the voltage source to match the real world, all the other DC voltages came into alignment.

Now whether a 62V drop from 530V between loaded and unloaded is normal, is another question. :D
 
Another newb question about when I was doing my dummy load test. Again this is my first experience with tube amps in general, let alone building one. I could lightly hear the signal coming from somewhere, kind of like when you put your ear next to a phonograph without speakers attached?

I'm sure it's an obvious answer to you folks. Locality-wise, this was coming from the KT88, OT or the dummy load. Since I'm still getting comfortable with the high voltages, I didn't dare get close enough to try to figure out what it was. Is this normal? And where is it coming from? I'm suspecting the OT as the KT88 is in a vacuum and the dummy load was only at about a tenth of its capacity.
 
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