'lisan al-gaib' - pentode preamp vox, american-style

[thetragichero]

had been watching a toroidal power transformer on ebay for awhile and decided to pull the trigger (less than 32 bucks shipped)
has 216vac 120ma and 12vac 800ma secondaries so i decided on 12v heater tubes to keep it simple (I've run 6v heater power tubes with their heaters in series before on a 12vac heater tap but wanted to keep this as simple as possible)
so my tube lineup is 12au6 preamp tube, 12ax7 long tailed pair phase inverter, and two 12v6gt (6v6gt with 12v heaters, snagged a pair for like twelve bucks on ebay... cheap!) power amp tubes in push/pull. 8k:8 ohm output transformer from a hammond ao-39 organ. decided on a pretty hefty 9h choke (12au6 plates can pull 10ma! will feed the preamp after the screens and lower dropping resistors in the power supply mean less voltage drop)
controls will be simple: volume control, no standard tone control but a vox-style cut control after the phase inverter, and a loose/tight toggle switch that makes global nfb switchable (820 ohm/47 ohm nfb lifted from a fender deluxe reverb). so you can get either vox sounds with no nfb or more classic fender sounds with nfb
i figure the guitar's tone control can be useful for mitigating brightness on the preamp while the 'cut' control can keep harsh harmonics in check. with no bass control i was mindful of the cathode bypass cap on the preamp, settling on 2.2uf instead of a more standard 22uf value. there's a 10k resistor before the volume control as sort of a placeholder (my schematic lists it as ?k) as this style preamp is capable of a TON of gain and this'll help dial it down if needed



taken last night after finishing the board. I've since drilled the chassis for the transformers and iec socket along with painting the top of the ot metallic blue. choke and 7pin socket for the 12au6 should be here tomorrow (today), once i figure placement I'll fashion a plate to cover the square cutouts (power transistor heatsinks were there originally) and get this guy painted and labeled!

[thetragichero]

all wired up. hooked up to scope and i get about 17w at full bore. currently biased at 79% of max plate dissipation. I'll do tone test tomorrow and pickup some different value cement resistors at the electronics surplus store on Tuesday when they're open to see if cooling it down had a beneficial effect (used an online calculator to get 230 ohms as the ideal bias resistor value, have 180 ohms in there right now)
loaded plate voltage of 276vdc. heck might even investigate whether i can bias class a



unsure whether this will get a 10" or 12" speaker. that's all part of the tone testing phase

[thetragichero]

first tone tests very promising. a little bright at lower volume settings, and the negative feedback switch doesn't have as drastic an effect as I'd like. so i am planning to reduce the bright cap on the volume control from 470pf to something lower along with add a bypass capacitor on the negative feedback resistor to reduce the treble in that mode
didn't notice any redplating so for now I'll keep the bias as is

[thetragichero]

final fighting with tone shaping. already purchased everything besides the transformers to build a head version with a blank aluminum head chassis. inside the amp placement will be a lot more ideal when i don't have to fit in an already swiss cheesed chassis. tan tolex 😎

[thetragichero]

a ha! why was my negative feedback not working? ac shunt cap on phase inverter went to ground instead of the junction of tail and nfb resistors!
still tweaking but huge mystery solved

[thetragichero]

haven't done a lot of work this week as i played both a youth and young adults service at my old church, but parts for the head version are trickling in (all the iron got here today, chassis is expected tomorrow, tan tolex probably Monday). with no nfb there is plenty of bass (although a little floppy because.... no nfb) but with it switched on there's not a lot of thump. okay for most guitarists not trying for a metal sound but on the head I'm going to decrease the first stage cathode cap while having a second, larger one in parallel with a 100k pot between it and ground for a quick and dirty "bass" control. I'm also considering stopping by the local trophy shop to see if they'll make faceplates for it. figure if it's doable and not too expensive it'll look sharper than waterslide decals while also supporting a local business (I'd imagine they've gotta be hurting right now). this might force me to finally learn inkspace which is needed but frankly i loathe time in front of the pc

[thetragichero]

chassis (and remaining caps i needed) came in today so the boards for the head version are done. test fit for placement so i can drill mounting holes



gonna have to wait a bit to make a combo cabinet and cabinet for the head since I've gotta make some headway on the house and tend to be a bit of a madman building amps

[thetragichero]

while still prioritizing work on the house, I'm looking to get the combo ready for church next Sunday. 3/4" sandeply for the outsides. using 1/2" sandply for the baffle to hopefully coax a little more low end out of it



also stopped by the local trophy shop and looks like it'll cost less than twenty bucks for both front and back plates for the head. offcuts from the combo cab are just about the perfect size for top and bottom of the head shell. i love maximizing $50 pieces of wood (so far this sheet of 3/4" has been used for two combos, another head that's on the backburner, and the head version of this amp with some to spare)

[thetragichero]

was not very productive today. spent some time installing ubuntu on an ancient lenovo ideapad (given for free) that is just too bogged down with xp. I've got a cheap ssd coming for it and will likely run ubuntu without a dual boot of windows since I've gotten duncan's psu designer and tone stack calculator installed via wine. not expecting this to be a multimedia machine but for those, a few web-based calculators, and hopefully diy layout creator will be much easier than using the phone

anyway, i got the speaker baffle cut and predrilled for mounting holes along with adding these strips of 3/4" pine doweled and glued to the sides (mainly to hide the chassis mounting lip). tomorrow I'll rout the edges and cut the rear baffles so that i can tolex later in the week

[thetragichero]

tolex on, just gotta hit the hardware store up for various fasteners

[thetragichero]

just gotta make up a speaker cable for it!

[thetragichero]

made a cable for the speaker, plugged it in, and got sound! will put it through paces tomorrow going over for songs for church but so far so good!

need to get the faceplate layout for the head version in a web-friendly format (don't think i could post .svg) and get over to the trophy shop to get em made. that's what's holding up the head build, because there's no sense bolting anything down when I'm waiting to drill holes through the faceplates (and will spray a little adhesive between the faceplate and chassis)

[thetragichero]

sounds great! loud enough and i like that the loose/smooth switch has an audible effect... nothing i hate more than wasting a switch on something i can't hear the difference

hopefully the site's upload thing resizes my images because they're 2"x13" 600dpi

[thetragichero]

picked these up today!



waiting for my metal nibbler tool to come in to cut out the iec socket (that's the big white box) and drill the holes. two name badges are for the head and cab

[thetragichero]

used my new cheap metal nibbler to make the hole for the iec socket. transformers and tube sockets in, next is install the boards and hook it all together

[b4nj0]

Coming together well and a nice looker Tradg. You are doing some great stuff with all these basket case bottle banks.

I know just what you mean about wasted switch locations, I have a sweet little Tech21 Trademark10 amplifier and one of the three cabinet/loudspeaker emulation switches does nothing that I can hear. I queried Tech21 and they replied to the effect that "it's subtle". Ha!

e&oe ...

[thetragichero]

haven't forgotten about this, just been working on other stuff
mains and heaters wired, was going to install the main board when i discovered a little problem... can you spot it?



find it yet?

okay, I'll tell ya

board was made for input and controls for a combo, where the chassis would be upside-down and the input would be on the other side of the chassis, so it's basically a mirror image. guess I'll have to make another combo and a board for this build oriented the correct way. any other option is not gonna cut it for me, wires crossing where they shouldn't be. happens i guess ;-/

[thetragichero]

okay new board has components tack soldered into place, got inputs wired up along with the power tube screens, we're moving now

[thetragichero]

back on this after a long absence working on other stuff



all power supply nodes accounted for. leads for the controls in, just have to wire up volume and treble cut

[thetragichero]

everything wired up besides output transformer and negative feedback

[unreg]

I’d surely like to hear you play through your lisan al-gaib when it’s finally done, thetragichero. You are so diligent, smart, and talented; it’s incredible trying to comprehend/read all of your lingo/descriptions-above-my-head used. You were so funny 😂 when you asked me to guess what was wrong, and then provided the highly technical answer that was amazing. 😁

Actually, it was amazing bc I was looking for your highly technical answer, but it turned out to be an interesting mirror image problem.

[thetragichero]

think I'm better building 'em than playing 'em, but that's never stopped me before

output jacks wired, primaries tack soldered to power tube plates (will shorten and solder permanently after i do some testing to confirm that my negative feedback switch doesn't turn on positive feedback and cause oscillation)

[unreg]

Apr 1, 2021 2:14:57 GMT -5 thetragichero said:

doesn't turn on positive feedback

feedback can be positively charged? Otherwise, feedback has always been negative for me. A big goal of running a soundboard during a play is to use turn its trim and equalizer knobs/move the slider carefully to AVOID feedback. Bc no one wants to hear feedback during a play.

Obviously, you must be talking about a different type of feedback.

[newey]

Apr 5, 2021 14:42:53 GMT -5 unreg said:

Obviously, you must be talking about a different type of feedback.

Not really, he is just using the term in a broader sense than you are. You are talking about audio feedback, which is one type of positive feedback. It is positive because it is in phase with the output signal, and is additive- the gain with the feedback is greater than the gain without it. Negative feedback is out of phase with the output and reduces the output.

But in either case, there is a "feedback loop"- an output signal is being "fed back" into the input, whether acoustically (mic or guitar signal feeding back through amp speaker, for example) or electrically (which is what tragichero is talking about).

Bc no one wants to hear feedback during a play.

Really? "Hair", "Tommy", "We Will Rock You", "School of Rock", all those big Broadway productions- I haven't seen them all but I imagine there was feedback in some of them!

[unreg]

Apr 5, 2021 15:21:21 GMT -5 newey said:

Apr 5, 2021 14:42:53 GMT -5 unreg said:

Obviously, you must be talking about a different type of feedback.

Not really, he is just using the term in a broader sense than you are. You are talking about audio feedback, which is one type of positive feedback. It is positive because it is in phase with the output signal, and is additive- the gain with the feedback is greater than the gain without it. Negative feedback is out of phase with the output and reduces the output.

But in either case, there is a "feedback loop"- an output signal is being "fed back" into the input, whether acoustically (mic or guitar signal feeding back through amp speaker, for example) or electrically (which is what tragichero is talking about).

Aahhh, makes so much more sense after reading your extremely helpful words newey!

Apr 5, 2021 15:21:21 GMT -5 newey said:

Bc no one wants to hear feedback during a play.

Really? "Hair", "Tommy", "We Will Rock You", "School of Rock", all those big Broadway productions- I haven't seen them all but I imagine there was feedback in some of them!

😀 Good point. My perspective came from running soundboard for many church Christmas plays... there, feedback is problematic... hurts the soundboard operator, the crowd’s ears, the “actors/actresses”’ concentration (they aren’t really actors and actresses).

Broadway is an entirely different ballgame.

[newey]

Point is, feedback isn't inherently bad, it can be used creatively, too. Ask ashcatlt . . .

[ashcatlt]

With a lot of the stuff I end up mixing, it can be tough to tell if the feedback is my fault and something I should fix or their deliberate choice and something I should embrace. Or sometimes it’s my fault, but it helps.

[unreg]

Thank you newey ashcatlt; you have opened up my mind; feedback isn’t inherently bad. 😊

[newey]

But we've derailed Tragic's wonderful amp building thread. The pertinent question for our thetragichero, however, is what does Frank Herbert have to do with guitar amplifiers?

[thetragichero]

environmental impact of tube production notwithstanding, he'd be okay with them as they are not made to imitate the mind of a human
have a pushbutton switch to replace the bass pot as it was basically an on/off switch regardless of taper/resistance value, and also picked up a 130r 10w resistor to replace the 200r cathode resistor with (120-125r was what i was looking for but can't be too picky at the electronic surplus store)