Post by thetragichero on May 20, 2020 21:17:44 GMT -5
this is not going to be my next build but my next-next build. just planning it out and looking for thoughts
what is definite: single-ended 6l6gb (output primary 5k secondary 8k) class a cathode bias power transformer (133-0-133) i pulled from some old test equipment... would give 376v unloaded b+ after bridge rectifier clc power supply filtering before the plate node (4h 90ma choke) 350v plate 250v screens for ~10.8w 15% thd (straight from data sheet... 5k is close enough to the 4k2 for me)... or see below lol combo cabinet made out of pine (besides the plywood baffle) for vintage vibe transformerless reverb (I've done it before with a 12dw7 and higher impedance tank. I'm up to my eyeballs in tanks)
what i am mulling over: making it really old school and using some pentodes for preamp tubes (I've got 6sq7, 6sj7, 6ag7 and a folder full of schematics for old Gibson amps, etc)
speaker(s) - i wish i could have a secondary of 4 or 16 ohms as i have some nice pairs of 8" and 10" speakers but they're all 8 ohm. I've got a 10" speaker that i like or two pairs of 8" speakers that in series/parallel would be 8 ohms. i could add another power tube in parallel to require half the primary load but i definitely wouldn't have enough b+ current without jumping up to a larger choke (online power supply calculator puts me at 70-80ma with 1 6l6gb and 3 preamp tubes) there IS a 250v plate/250v screen/2k5 primary listing in the data sheet for 6.5w 10%thd. this would allow either a pair of 8" (which I'm sure i won't be able to sell since 8" speakers get a bad rap) or a pair of 10"... but would have to either dump b+ or use some of the other pt taps (20v and 30v) to bump up b+ while using full wave rectification
so far that's the reason for this thread: i kinda like the idea of a 4x8 combo. did some googling and found a couple 4x8 combos. teisco and the like. on the other hand 2x10 has been used a lot more for a reason so I'm open to suggestions/thoughts
i kinda like the idea of a 4x8 combo. did some googling and found a couple 4x8 combos. teisco and the like. on the other hand 2x10 has been used a lot more for a reason
I guess it comes down to who are you trying to satisfy, yourself, or a potential customer. If you're going to sell it, you're right, 2X10 is probably more attractive to a buyer. As to the bad rap 8" speakers get, I think it's the breakup when overdriven- 8" ers can sound brittle when overdriven. OTOH, I also wonder how much of that bad rap is that players heard crappy 8" speakers driven by crappy amps- you know, it was the first amp one had, and since you didn't sound like Jimi or Eric or whoever your hero was, you blamed the rig . . .
So, if it's for yourself, get 4 decent 8" speakers and give it a try.
I also wonder how much of this is comes down to pure ol' macho-man b.s., the whole "my cabs bigger than your cab" mindset. After all, how many roadies have had to wrestle a stack of 4 X 12 cabs onstage, while the guitarist is actually playing a Twin reverb miked into the FOH?
You do realize, don't you, that distortion of any kind, including harmonic, is totally dependent on the bias voltage? You can supply a Gawd-awful amount of plate voltage, or starve it half to death, and still get whatever distortion figures you wish, thanks to how the grid operates to influence the flow of electrons between the cathode and anode.
While a good spectrum analyzer is the best method for looking at harmonic structures, an o'scope can do in a pinch. Check out the following link for some YT vids:
Most of the ones I (quickly) reviewed were at least useful, if not downright spot-on. (I admit, I'm spoiled by having access to MilSpec gear for a number of years - that was the bee's knees in getting accurate figures.)
Post by thetragichero on Aug 28, 2020 2:16:46 GMT -5
hadn't forgotten about this had the power transformer fall into my lap from a Thomas organ. WAY overspec but that's perfect. entire b+ filtered through a pi filter (CLC went with 22uf-whatever the 90ma choke from the big boy fenders-80uf) so STOUT. unloaded b+ of 416v should be fine. looking for around 350v on the plates. 5k ot primary. close enough to the 6l6g datasheet numbers. will probably need to adjust the power tube cathode resistor because it's the stock 470r from a vibro champ (this is basically a vibro champ with a James tone stack) my first attempt at making my own eyelet board seems to have worked okay
Post by thetragichero on Sept 1, 2020 2:32:03 GMT -5
after having to swap the output transformer leads (due to positive feedback).... it works!
like every James tone stack, treble pot is wired backwards but that's only two leads I'll switch later tremolo works but without the footswitch plugged in i get some oscillation so I'm thinking maybe a leaky cap (used a bunch of scavenged parts so I'm okay with that. will just keep the footswitch plugged in) only limited testing but gets decently loud but clean unless you dig in. little range on the bass control but the extremes are both nice settings so I'm cool with it. neither too thin nor too boomy out of the 2x15 I've got a few potential 10s that I'll use in the combo, which is my next step. think i have just enough 3/4" ply left for it. the baffle will be angled slightly up to keep from firing at me ankles
Post by thetragichero on Sept 1, 2020 17:04:07 GMT -5
combo cabinet made from all scrap. I've gotten a lot of mileage out of that 3/4" sandeply (two heads, a 1x15 cabinet, and now this). top is pine (which i forgot i can't get to move once doweled so had to fix with glue). front slats are cypress (baffle is going to screw into them from the inside) wood putty to fill some voids/knots
Post by thetragichero on Sept 1, 2020 21:27:59 GMT -5
HAD SOME SPARKING could not figure what it was, no fuses blew, nothing indicating a short on lightbulb limiter. only happened while playing through a speaker (but not a dummy load) finally was able to replicate it with the chassis guts-side-up had to extend one of the ot leads to reach the tube socket and it's right at the heatshrink joint time to do extra extra extra heatshrink (on a new joint)
had to extend one of the ot leads to reach the tube socket and it's right at the heatshrink joint
Heatshrink tubing, particularly the type with a rough texture, tends to fail at lower voltages than normal plastic insulation of a similar thickness. Compounding the problem is the fact that the plate connection can swing far above the power supply voltage when the tube moves toward cutoff under high signal conditions.
Also, you can reduce the potential for punching through if you make your splice such that there aren't any pointy ends of the wires at the surface. 'Globbing-on' solder isn't elegant but can help.
Post by thetragichero on Sept 2, 2020 0:46:53 GMT -5
i also had used solid core wire (18awg) which i think exacerbated the issue. made sure there were no pointy ends (think you nailed it on the head) and then one later of standard black heatshrink followed by three of the clear stuff (one and two sizes up from the black) that wire ran right next to the screen supply wire that goes to the vibrato tube plates. both were replaced (as that had some charring on the insulation) and i also moved them
used my handy dandy ehx switchblade to hear between the vibro champ and this build (albeit through a closed back 1x15) and I'm happy. cleaner louder and doesn't fart out the tremolo is different but still quite nice
using so many scraps and odds and ends means I'll probably have about fifty bucks into it. much more comfortable leaving this at church vs the vibro champ (although i can see myself borrowing this for recording from time to time)
Post by thetragichero on Sept 4, 2020 23:34:58 GMT -5
okay so it's done, save for maybe some little tweaks. had some high frequency oscillation problems and it was because i had a small capacitor from the power tube grid to cathode (local negative feedback) that i copied from the vibro champ schematic (wasn't on the schematic i had drawn). removed that and lost the dog whistle but then was getting blocking distortion, so i installed a 1k5 grid stopper. also right now it is biased at like 130% plate dissipation with a 250 ohm cathode resistor. might go in search of a 330 ohm or so resistor if i burn through the three 6l6g i have (470 ohm was like 85/90%), although i imagine a 6l6gc would be on the cold side. I'll leave it for now ended up choosing a 1980 organ pull rola usa speaker as it sounded best vs fisher alnico speaker and pyle driver 10" (probably my favorite speaker)