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PostPosted: Fri Dec 04, 2020 1:07 pm 
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I've been through a couple missteps with my Triwatt build:
Then, I perceived the amp had too much high-end, or maybe not enough low-end. Based on this thread, and this post specifically, I changed the 1nF cap in the tone stack (C6) to a 47nF. I thought that had too much bass, so I tried a 22nF. And then I got a little neurotic, and decided I hadn't spent enough time with the amp with the corrected operating parameters, so I put C6 back to its stock value (1nF).

Somewhere along the way, I thought the amp was sounding a little dull, i.e., losing high-end. I checked the power tube bias, and it had drifted quite a bit. I don't recall the values, this was several months ago. I went ahead and replaced the power tubes, thinking they may be marginal given the abuse they've seen (see above, i.e. having red-plated several times during initial build (albeit briefly), and then run at the higher KT66 voltage during rehearsals). Since then, I've had JJ 6V6S power tubes in there, biased at -20 mA and -19 mA. The bias remains stable.

That was a long-winded preamble to say where I'm at now: I feel like the amp is a little light on the high end (i.e. maybe a bit lacking in the higher frequency part of the tonal spectrum). This could be a perception/expectation issue. I don't have any frame of reference, except Internet hearsay. But, in most of what I've read on the "Hiwatt sound", it's considered a "bright" amp. If I read about settings people are using to get various tones, the treble is typically dialed back (below noon), the bass dialed up, and if the linked inputs are used, the bright input is also dialed back (relative to the normal input). For me to get what I feel is a "balanced" tone, I have the treble up around 3 o'clock, and the bass dialed below noon. Likewise, I have the bright input dialed up well above the normal input.

Note this is all at "bedroom volumes" - the master volume is down around 8 o'clock, maybe the highs come out with higher volume? Also note, I'm not using the overdrive at all.

I'm hoping to have some time this weekend to take it out of the cabinet. (I used electrical tape in a few places in the build, I want to replace that with some self amalgamating tape.) At a minimum, I can do another sanity check of my work against the layout diagram, and check voltages. But I thought maybe my vague and utterly non-specific rambling might trigger some thoughts of additional checks I can do while it's on the bench. ;)

Thanks for any thoughts or feedback!


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PostPosted: Sat Dec 05, 2020 5:59 pm 
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:oops: Well, it appears I left C6 as 10 nF, rather than the default of 1 nF! Now I can't remember if that was deliberate, or if I made a mistake (i.e. meant to do 1 but accidentally did 10). Either way, it's definitely down to 1nF now.

I also noticed that one of the power transformer leads wasn't soldered! I don't know how I missed that! It was secured mechanically fairly well (stripped wire wrapped tightly around the turret). It may have worked fine for quite a while, though, knowing my luck, it would fall off right in the middle of a gig.

I also removed all the electrical tape I originally used, and replaced it with self-amalgamating tape. (I just learned about this stuff, it's awesome.)

Anyway, the lesson here is, don't go messing with modifications until you're 100% sure everything is correctly set up in the default state, and that you've spent a good amount of time feeling out the default state. Of course that's a lesson I already knew, but am apparently destined to relearn it time and time again. ;)

I only spent a little time playing on it after make the changes above, but I can definitely hear a difference. I feel the tone of the 10nF is well imprinted in my memory, since I've been playing it this way for quite a while. What I perceive is that before correcting C6, it wasn't so much a lack of high end, but too much low/mid frequencies coming through (at least that's the effect of the filter difference of 1nF vs 10nF). Of course tonal preferences are very subjective, but I feel that when C6 is increased, it removes some of the immediacy and "punch" that Hiwatts are famous for; it also seems to take away some of the note definition "sparkle". I need to spend more time with it in the corrected state to say for sure.


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