18W TRex build - dark sounding

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Re: 18W TRex build - dark sounding

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 05/06/21 12:38 pm
Josh, you have no idea how many engineers I've worked with (or after they left) who got this wrong. Sometimes their calculations went the other way, and all the bias resistors on dozens of ICs burned up after the product was in the field six months or so.

Just to throw out one more thing (I know that's what you were hoping for), using a resistor that's too large can have consequences. Sometimes it's a good thing to have the resistor burn and open when the component it feeds fails and shorts. Otherwise, other components in series with the resistor, such as other dropping resistors, power supply diodes, etc.) will also fail. That's how solid state works - one transistor shorts, and a whole raft of them send up smoke signals. :lol:

Jack
Thanks. Yes, I'm aware of the second part, but in guitar amps it's almost always better to be overrated than underrated. Though I think I saw you or somebody else discuss this in one of our screen resistor conversations? Maybe an underrated screen resistor blew, and saved something as a result. I forget.

Thanks,
Josh
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SiRaff
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Re: 18W TRex build - dark sounding

Post by SiRaff »

I've made a few tweaks based on advice on this thread and wanted to circle back with some findings. I have now finished work on the circuit (and don't want to revisit it unless something goes wrong!) and all that remains is to order a faceplate and make the whole thing look the part. It's currently just a blank chassis with lots of gummy glue residue on the front (yum), which is a hangover from its days as a humble MG50DFX. Overall I'm really delighted with the amp and it has some great sounds. Funnily enough, my favourite sound is a Strat neck pickup through the Wreck channel at around 4-5 on the Volume (gritty, but not distortion territory). It's lovely and articulate, with a beautiful compression when you dig in. I'll post sound clips soon.

The original issue was a dark sounding Normal channel with not all that much gain. This has improved with a couple of tweaks, although to my ears its still reasonably dark and I would always run the Tone control at full. The tweaks I've ended up making are:
- Swapped the 10n in the Tone circuit for a 22n
- Swapped the 22n coupling cap on the Normal channel for a 10n to let less bass through
- Unbalanced the PI by replacing the 82k with a 100k plate resistor. It does sound better this way, and the removal of NFB is indeed more noticeable.
- Turned the amp up! I've been testing the amp with an attenuator down to bedroom levels. The attenuator is a simple homemade affair, it does have a treble peaking cap but even so...the amp sounds brighter at higher volumes. I'm sure those Fletcher-Munson curves have some role to play in this.

I've made a small number of other tweaks based on feedback/advice from this thread, and general tinkering.
- Increased the cathode bypass cap in the PA to 2200uf/35V. This did tighten up the bass a lot, you can really hear it on palm muted notes.
- Added another sag resistor in parallel to drop it down to ~110 ohms (I was too lazy to order one).
- Added Merlin's tube-buffered FX loop. I know a loop in an 18W is pointless, with the OD being generated in the PA. But I had a spare tube socket, I was curious, and the loop is extremely transparent. It doesn't affect the tone or feel of the amp at all. One benefit for me was that I could add a cheap and nasty footswitchable gain boost, because the loop includes an extra gain stage after the Return jack. I simply added a voltage divider after the cathode follower (before the Send), and then wired the footswitch so that it parallels a resistor into the lower half of the divider (thus reducing gain when engaged). Voila, a safe lead boost with minimal effort. It is a little subtle (I chose values to give ~30% boost), and there's no LED to say when it's on. But it's there and it adds variety.
- Sorted out some of those bad solder joints. I got a new 65W iron. My soldering technique is still horrible, but it's the best I can do for now. It works now and if it breaks later, I'll fix it up.

I can't think of any other tweaks I've done. I'll edit this if anything comes to mind. If anyone is curious about the TRex (I guess there haven't been that many builds), happy to give more details!
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SiRaff
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Re: 18W TRex build - dark sounding

Post by SiRaff »

I forgot to mention I’ve also added VVR (I used the AmpMaker kit). It was a bit of an experiment, overall I like it. I scaled the whole amp - it was too much faff to add a separate power chain for the preamp, and scaling only PI & PA seems to come with its own issues from what I’ve heard (eg the need for a PPIMV).

At lower settings (loud TV kind of levels), it’s very usable. The overall gain definitely reduces compared to running at full B+, but as long as the VVR isn’t set too low, the amp is still responsive and the tone is good. It seems that the way to see VVR is that you’re not getting exactly the same amp at lower volume (the way our ears work, that’s impossible anyway). You’re getting a close cousin, which hopefully will behave in a similar way and have usable tones of its own.

At the very lowest settings, VVRing the whole amp seems to be disappointing. The amp becomes dull and not very responsive. But we’re talking conversation levels here. A better solution might be medium VVR setting plus a little attenuation. If I had the appetite, I’d go back in and install a half power switch (or maybe 1/10th power? Which would halve the volume?). Anyway, I’d be interested to hear how these observations compare to others who have tried this!
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SiRaff
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Re: 18W TRex build - dark sounding

Post by SiRaff »

I've discovered that I have the "buzz" problem that Paul Ruby discovered (the one described in this thread: viewtopic.php?t=18869).

His suggested fix with zener diodes seems to have cured it completely, so I'm posting this in case it helps others who might have the same issue.

When I was investigating, I struggled to find sound clips of the infamous buzz sound so it was hard to work out if this was really the problem. It's only by trying the fix (admittedly very cheap and easy to do) that I confirmed it. So, I've recorded a sound sample with (first half) and without (second half) the fix.

It's a little hard to hear the difference on the clip, probably because it was recorded on a phone. You can hear the buzz sound most clearly when I hit the individual low G notes in the second half of the clip (the sequence was: open G chord, G power chords, individual G notes, D power chords, high G note). It sounds kinda like a phaser, a high pitched "peeeesssshhh" sound that gradually decreases in pitch as the notes decay.
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TriodeLuvr
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Re: 18W TRex build - dark sounding

Post by TriodeLuvr »

SiRaff wrote:
Wed 06/09/21 3:18 pm
I've discovered that I have the "buzz" problem that Paul Ruby discovered (the one described in this thread: viewtopic.php?t=18869).

His suggested fix with zener diodes seems to have cured it completely, so I'm posting this in case it helps others who might have the same issue.

When I was investigating, I struggled to find sound clips of the infamous buzz sound so it was hard to work out if this was really the problem. It's only by trying the fix (admittedly very cheap and easy to do) that I confirmed it. So, I've recorded a sound sample with (first half) and without (second half) the fix.

It's a little hard to hear the difference on the clip, probably because it was recorded on a phone. You can hear the buzz sound most clearly when I hit the individual low G notes in the second half of the clip (the sequence was: open G chord, G power chords, individual G notes, D power chords, high G note). It sounds kinda like a phaser, a high pitched "peeeesssshhh" sound that gradually decreases in pitch as the notes decay.
One of the scope displays in Paul Ruby's PDF shows this.

http://www.18watt.com/storage/18-watter ... fo_311.pdf

Scroll down to "Speaker Load Comparison," and the lower trace tells the tale. Both the positive- and negative-going halves of the waveform are clipped, and the clipped areas clearly contain another (higher) frequency. It appears to be roughly twice the original, which indicates rectification is occurring. This secondary frequency will mix back with the fundamental to create a slew of higher order harmonics. The harder the amplifier is pushed, the greater the amplitude of these products, and the more audible the higher frequencies become. This is almost certainly the reason for the apparent pitch decrease as the notes decay.

I haven't had time to study Ruby's mod in depth. My only question, really, is how it counteracts the increase in bias that occurs simply due to increased current through the cathode bypass network. That in itself is sufficient to create crossover distortion in AB1 amplifiers, and it won't be affected by his modification. A little time with the simulator will probably reveal what's happening.

Anyway, congrats on your work and the success of all the mods!

Jack
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ZeroZeroZero
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Re: 18W TRex build - dark sounding

Post by ZeroZeroZero »

Would you mind sending me pics of your merlin tube fx loop layout? Im not a stud at figuring out how to lay one of these out in a schematic. And i assume that an unmodded 1974x circuit will have the same routing as yours? Thanks for any help!
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