20ish watt tube power amp build

[thetragichero]

had an aggravating few days diagnosing and then replacing the alternator in my fiance's car. of course the least convenient bolt to get to was the most difficult so i had to wait until this morning to get a bolt extractor kit to remove the jacked up bolt. success, it works. but man did that zap my time and energy. so not much amp work



got the IEC socket, fuse, power and standby switches wired up. waiting on the power supply choke coming tomorrow so i can figure out where the rest of the power supply nodes will go

[thetragichero]

earlier today cut the main piece of tolex for the 1x15 cab. think I've finally learned my lesson to never order anything less than 3 yards at a time so that i can cover things with one piece



package from ce distribution FINALLY came and with it the power supply choke i needed for this 2x6V6 power amp build. installed that along with the bias pot and the majority of the power tube wiring (save for the heaters). decided against the dual gang type master volume and went with the trainwreck type 3 ppimv (1MA pot in between the inverted and noninverted outputs of the pi} much simpler

[thetragichero]

have to tap out for the night because i ran out of solder. phaser inverter about half wired up, including the presence circuit

[thetragichero]

ups finally came with my solder so i was able to do more than just remove components from salvaged pcbs (I've got a TON of low gain germanium transistors now just waiting to become Darlington/sziklai pairs for fuzz usage) and breadboard dirt boxes



back to 2x 6v6 power amp. i think all that is left is to finish wiring the heaters and fire 'er up. the output transformer secondary leads are quickly tack soldered in the event that they're wired backwards and the negative feedback loop is really a positive feedback loop. I'm excited to try running a fuzz box directly into it

[reTrEaD]

Feb 25, 2020 3:07:18 GMT -5 thetragichero said:

the output transformer secondary leads are quickly tack soldered in the event that they're wired backwards and the negative feedback loop is really a positive feedback loop.

Good call.

Always think ah
e
a
d
.

[sumgai]

Feb 25, 2020 3:07:18 GMT -5 thetragichero said:

the output transformer secondary leads are quickly tack soldered in the event that they're wired backwards and the negative feedback loop is really a positive feedback loop Gawd-awful loud oscillator.

There, fixed that for ya.   (Which is why reTrEaD's praise is valid, for this very reason.)



sumgai


* For those not in the know, the simple definition of an oscillator is an amplifier that has positive feedback.  Circuits that are intended to be oscillators have controls within them to vary the frequency and loudness.  Circuits designed for amplification don't have those controls, thus they can get waaaay out of hand, very quickly.

[thetragichero]

I've pretty much figured out the Hammond organ output transformers so those I've had luck just wiring up but this one i had to swap the secondary leads. issue with the bias circuit that i haven't had time to troubleshoot but that's why the cheap Chinese 6v6 tubes went in there.. red plating not nearly as heartbreaking as if i used my "good" tubes
i think i measured 17 transistors (about half were pretty much all leakage and the other half hover around 60 hfe with decently low leakage) until i got aggravated and stopped lol

[thetragichero]

let's see if i can move everything over here

edit: done. all posts moved from coffee shop thread relating to this build. pain in the rear on a phone

[thetragichero]

okay so the reason my bias circuit isn't working: this is a non-center tapped secondary with a bridge rectifier, so the negative rectified voltage is being sent to ground instead of through my half wave rectifier
found instructions for a capacitor-coupled bias supply which should get me what i need

[thetragichero]

tried it with 6v6, 6l6g, and 6l6gc using my rat clone as a preamp
it works, the bias is set way cold (smallest negative voltage i can get is -50v), so i will reduce the capacitor connected to the transformer secondary from 100nf to 68nf (initially had 100nf and 47nf in parallel that wouldn't adjust beyond -90v)
the volume and presence controls are like on/off switches so that needs some figurin
but all in all it makes sound! pretty good sound!
when i don't have a sleeping spouse down the hall i will test with actual preamp out signals

[thetragichero]

biased to ~60% of the max plate dissipation for 6v6 it sounds great being fed directly by my rat clone. loud enough to distort my phone's mic. sounds good with 6l6g and 6l6gc as well. still have some tweaks and to test with other preamps (not loud at all connected to my acoustic 140's booster out, but I'm not sure what sort of output that provides)
since it is daytime and i can make noise i am working on the design for a headshell for it that will fit the 51" long piece of emerald green tolex i have. will hopefully use some of the 3-sided corners i mistakenly ordered a ton of. picked up a cabinet pull from home depot to use as a handle

[thetragichero]

box together, once dry i can introduce to the router table and figure out exactly what i need to do to secure the chassis to the enclosure

[thetragichero]

recorded a demo (in the hallway because it was so durn loud!) using the preamp out of my peavey classic vtx combo (traded a fuzz pedal i made for it... mr big time over here!). volume control was connected to the wrong area of the circuit so there's a little more travel than there was before. scrapped the previous presence control and used the later version of the fender bassman presence pot (25kB with 4k7 from lug 3 to ground, 100nf capacitor between ground and lugs 1 and 2). much happier
i've gotten it to work with my rat clone dimed into the input as well
have to work on the enclosure. right now i have a box with the edges routed. have to figure out the rest of it

[thetragichero]

was able to use a bunch of scraps for this. that big space at the top of the front will be a single piece of ply
emerald green tolex. should the piece of ply be dyed green instead of covered in tolex?
probably won't get any further until Monday: recording a worship service at one church tomorrow, rehearsing at another after that to record an Easter worship service early Sunday

this'll be a relatively heavy amp but I'm okay with that. small price to pay for rock solid construction

[thetragichero]

enclosure is done, just gotta install the chassis. should've come up with a name and metal badge for the middle of the front but i don't feel like wasting a whole sheet of waterslide for one build. whatever meng





(while that piping line sure looks nice it's more for function rather than form: i used leftover tolex and would not have a piece the proper dimensions to not have a seam on that piece. so now the seam is decorative)

[thetragichero]

aaaaaaand done

[JohnH]

Just wanna say, love this! Nice work indeed

what do you have in mind for its future?

J

[thetragichero]

i have been using it to record, my rat clone straight into the input, but i built it with the intentions of selling it. got who originally asked about it is no longer interested but I'm satisfied with what it turned into considering everything but the choke was scrap or stuff on hand (down to the wood and tolex). those undersized transformers from the fender excelsior were perfect for this

[frets]

Trag,
You’re work is superlative.