johnnyreece wrote: ↑Mon May 06, 2024 1:36 pm
-My tremolo is satisfactory, however, there's not a huge difference between the speeds. I put my frequency counter on it, and on slow my range is 3.1-6.21Hz. Fast is 3.96-9.26Hz. Would it be worth it to increase R25 to slow down the slow range a bit more? Practically, how low can this frequency really go?
Hmm -- I've neither the frequency nor depth of experience sluckey brings. However, it seems reasonable to up that resistor to 680k or even 750k so the sum of the pot plus resistor is closer to the original. You'll lose some range into higher frequencies, but get closer to the lowest available. Subbing that 6SL7 has changed overall oscillation dynamics, which confounds perfect comparison.
didit wrote: ↑Mon May 06, 2024 3:39 pm
However, it seems reasonable to up that resistor to 680k or even 750k so the sum of the pot plus resistor is closer to the original.
Good guess! I swapped the 220k with a 680k (which actually measures over 700k), and that was good. My range is now (from memory) 2.6-4 Hz on slow, 3.9-9.6 Hz on fast. I think I'm about to call it done. Need to get that 680k soldered in (I was just clipping in different values temporarily). Then I'll take some pics. Hopefully going to get some faceplates made soon. If there's any interest, I can take some more measurements with other power tubes and post them, as well as some preamp voltages.
johnnyreece wrote: ↑Tue May 07, 2024 1:47 pm
My range is now (from memory) 2.6-4 Hz on slow, 3.9-9.6 Hz on fast.
That's a very useful frequency range.
Thanks! Yeah, I was playing over the weekend, and I don't see a need for any faster speed. I don't know how much lower it could practically go, but it's fine enough for me as-is. Glad I kept tinkering with it to get it dialed in.
Speaking of which, I finally ran some 6V6 and KT66 tubes this weekend. It forced me to modify the bias resistors (again). I temporarily clipped in a parallel 1k resistor to the one that's currently in there. I now have the option for 176 ohms (EL34, 6L6), 600 ohms (6K6), and 375 ohms (6V6, KT66). I found my favorite combo over the weekend: KT66 and GZ34. I couldn't find a setting I didn't like with those installed. A side bonus of the GZ34 is the warmup time. Unlike the 5U4, it doesn't slam my power section on startup. That's something I wanted to ask: My electrolytics are rated for 450V, but with a 5U4, my B+ nears 500V while warming up. Is this okay, or should I get higher rated caps?
johnnyreece wrote: ↑Mon May 13, 2024 12:14 pm
... My electrolytics are rated for 450V, but with a 5U4, my B+ nears 500V while warming up. Is this okay, or should I get higher rated caps?
Their short term surge voltage rating might be up to 20% above their max working voltage rating.
So as the near 500V will only last for 20 seconds or so, a time or 2 per day (?), I think most ecaps should be able to accommodate it.
johnnyreece wrote: ↑Mon May 13, 2024 12:14 pm
My electrolytics are rated for 450V, but with a 5U4, my B+ nears 500V while warming up. Is this okay, or should I get higher rated caps?
I would be nervous. I like to measure the AC voltage on pin 4 or 6 of the rectifier tube, multiply that number by 1.414, and chose filter caps with a voltage rating that exceeds that number.
Thank you both for your replies. Since it was one in favor of swapping it out and one in favor of leaving it as-is, I decided to look up the data sheet (which, I admit, I should have done in the first place). According to the manufacturer, these are 450VDC working; 500VDC surge. Steve - your suggestion would put me around 510 in the high power option. Knowing the surge rating, does this seem unsafe? They're BMI caps, if that means anything to anyone.
Well...as I try to figure out whether to submit another parts order or not, here are some pictures of it as it sits. Please note, I accidentally bent the LED leads to where they're almost touching, but I have bent them back apart. Some of the pics will reflect that. Anyone have a better option for how to connect those?
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So, I've learned a few things about making faceplates on this build. First - I don't like the larger font. Easier to read? Yes. I think it looks too cartoonish, or something, though. Other thing? Make sure you know your cabinet's dimensions. A couple of things are cut off or pert-near cut off because of how it fits in the cab. Just because it fits on the plate doesn't mean it won't be obscured by something! Oh, and in a similar vein, if you want to use a chicken-head knob, make sure your label doesn't get covered by the knob. Plus, an oldie (but goodie): Measure twice; cut once. You can see where I boogered up and have a secondary "Off" label showing through about an inch to the left of the actual "Off" switch. Anyway, here's a pic. I used an old head cab I built years ago. Pretty sure it was my first one.
IMG_6875.jpg
As far as the 6K6GTs go? I ran it pretty hard for about half an hour. They didn't blow up. I suppose I could do more of a stressful test, but that's for another time.
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Final update on this: I downloaded a free decibel meter app on my phone. Had the amp in cascade mode, with gain halfway up and master at full tilt. Running through 4x12 cab.
First run, 5Y3 rectifier, 6K6GT power tubes, and Low B+ setting yielded 110 dB. Second run, GZ34 rectifier, 6L6GC power tubes, High B+ setting...a whopping 3dB higher at 113. Color me surprised. I wasn't expecting a massive difference, but I *did* expect it to be more than 3dB.
Helmholtz wrote: ↑Thu Aug 01, 2024 2:31 pm
Well, 3dB more means doubled power.
Yes, I had just thought that having much lower B+ (using lower secondary taps along with a 5Y3, which drops even more voltage) coupled with a power tube that has a much lower dissipation max (8.5w vs. the 6L6GC 30w) would result in a bigger difference than 1/2 power. I've not actually measured the output, but I'd have thought I was closer to 1/3 or 1/4 power. Oh well. Still a fun experiment!
