6L6GC Guitar Amp DIY

[CJ]

we are going to try a 6SJ7 glass pentode into a 6SL7 phase splitter into a pair of 6L6GC power tubes backed up by a ultra linear output transformer,

this would be a Vox type front end with a Fender type output stage,

done a lot of marshall, fender and vox, now time for combo-nations.

we want a good amp for rhythm guitar, not a def tones crunch-meister.

this means plenty of natural compression with cathode biased pwr tubes and the magic vox ef86 pentode funk, are you with me so far?

ef86 family means "handle with care", amplification can come at a price, ie: noise.

so we simply drop the screen voltage and increase plate resistor to 330 K to drop B+ and noise while retaining most of the amp factor.

OPT is good from 5 to 80K, bottom end should be good,  ;D

we will lose amp factor by using 6L6 instead of EL84, so we must now simply run the amp wide open, oh darn! 

 

[CJ]

just one amp stage and straight into the inverter,

use 2 colors of wire to keep heater wires phased,

use all green and you won't know fanny anns after about  3 twists,

cathode resistor JB welded to chassis for heat sink, low voltage so no insl needed,

6 amp bridge so we need no surge resistor with 100 amp surge rating,

choke and then a pi filter, ultar-linear saves screen resistor and cap,

turrets on the sockets on this model,

volume, tone, and hi-cut, no master,

 

[CJ]

$4.79 for some 3/4 angle, DIY core brackets, no end bells means less hi end loss,

bolts are regular steel, no need for brass or stainless unless you want to be guided by the gilded lilly, jus some tape around the bolts is fine,

commercial xfmr will look like this but have 1/8 inch drive pins installed thru the brackets and into the core about 3/8 inch to bond the bracket to the transformer because the core is cut from a strip of steel and built into a 3 phase or single phase assy with no holes in the steel, 1/8 inch Alum tie plates are clamped between the brackets which usually bow in the middle since the bolts ride outside the core,
but stamped lams have holes for the bolts so drive pins not needed,

 

[analag]

5AR4/ 1N4007 bridge rectifier for slow start and more of that compression you talkin bout dude. Also eliminates the need for silly caps, I mean snubbers around the sandstate.

 

[CJ]

that looks good!

1) protects rectifier from surge
2) protects first lilter cap from over voltage
3) provides a little sag to the pwr supply,

5y3 might even take the load,

 

[analag]

Yep I played with it some years ago. First cap was a Solen 8uF IIRC, worked great.

 

[man-bot]

Another cool amp - very interesting!  I'm just finishing up a 5B6 build as my first foray into octal preamp tubes.  Its requiring some tweaking to get the amp there tonally.  I used a 5U4G rectifier (as is standard on the 5B6) and the sucker will sag when cranked (not like a 5Y3) but defo gets into some Neil-ish territory.

I'd be interested to hear how your high gain octal pre-amp section sounds. The 6SC7 pre was pretty dirty at the stock plate voltages, cranking them to the 240v area got it more useable for what I like (and a more useable volume control)

Once again - very cool!  Looking forward to see how this turns out.

Mike

 

[CJ]

ok this amp still has the vox sound from the first stage, with less dirt from the pwr tubes,

this is not an amp you stick a box in front of,

it has the tone all by itself, no matter what guitar you plug in, or which pickup you use, it will always bring out the best qualities of that instrument.

no tone control because you don't need one. just a bright switch,

VOX is the only amp i know of that takes the pwr tube B+ after the filter choke,

i have a deluxe reverb choke in there and it seems to handle the 140 ma plate current alright,

DCR is about 110 ohms so we generate about 2 watts of heat, should be ok,

found a mod that will rally jack up gain:

put a cap across the 47K to ground resistor that is found on the 6SN& inverter.

there is no neg.  feedback in this amp, except that non bypassed 47 k.

i also am using 1200 pf instead of 250 pf for the treble switch, too thin with the 250 pf,

here is the schemo i am using, plate voltage is 480  on the pwr tubes,

inverter and preamp tube voltage is about 380, using a 330 K plate resistor on 6SJ7,

this amp wires up in about 3 days since there is no reverb or vibrato and 2 less tube stages compared to a Fender.

 

[iomegaman]

I am watching this with vested interest, it sounds exactly like the amp I want, having just finished Guavatones Orange 86 I am really enjoying the tube/transformer side of this art...this might be my next project since I am kinda tired of building mic pre's...

 

[ed rees]

Badass. I have all the tubes and iron kicking around, so sign me up! ;D

 

[CJ]

this amp sounds real cool with two speakers, one bass type and one celestion mid type,

a bypass cap on the 270 ohm cathode resistor for the pwr tubes does not change the sound so you save yourself a 25/50,

OT: have you seen Telstar, the Joe Meek story, what a killer movie!

ritchie blackmore in court, mitch mitchell has a shotgun held at his head, clem canettii, a drummer i have never heard about, drummer who almost joined led zep and plate on the kinks you really got me, Heintz, Fury, the tornadoes and outlaws,
a reverb pan made from a space heater, too much fun check it out!

 

[iomegaman]

I have an old Meissner "recorder-phono" unit that I think I will strip the chassis and case and put this amp in...sorta vintage with plenty of room for speakers as well...the tranx might even power this, need to check it out since it has 3 6sj7's in it...

 

[CJ]

you could use 6V6 to reduce heater current,

or use the VOX AC-4 amp which has just one power tube,

most of the VOX sound is in the EF86. Bulldog spk and EL84 add to the mojo,

there might even be a EL84 in that record player for the pwr amp,

Webcor made a portable record player with Phillips EL84, yum yum! 

 

[iomegaman]

yeah this thing has 7 tubes, 2-6SJ7, 2-6SH7, 1-6SF7, 1-6SC7, and 1-5Y3GT, the cool thing is the chasis is pretty easy to rape into whatever I want and it frops right back into the case which has room for a speaker or two...vintage Frankenstein...

 

[CJ]

portable suitcase amp, i could see Leadbelly setting one up on Beale St, 

wow, Fender has done the Ultra Linear thing already,

people either love it or hate it, too clean they say,

6SJ7 warms this version up, along with the cathode bias.

tubes are running hot, -38 volts bias, 480 on the plates and no screen resistors,

135 watts from the UL Fender? 

here is a pic of the Fender UL output section>

 

[CJ]

this amp has been tweaked and about to be bolted up,

using a metal 6SJ7 after the glass version proved too microphonic,

installed a 0.22 musicap across the 47k to ground resistor on the phase inverter,

just that small of cap really boost the volume,

since i am using 6L6gc instead of EL84 like vox, i need more voltage to drive the tubes,

so the cap really helps out, do not have to build another stage into this thing,

sounds great, crank it up to 10 and it sounds like you are using a stomp box, only without all the garbage on the high end,

re-biased the pwr tubes by adding more cathode resistors, bias is now 41 volts,

not much different than the 38volts i had before, but since the cathode resistance is now 360 ohms, i dropped my plate current down to 110 ma instead of 142 ma with the 270 ohm resistor, this is an old Bugera chassis that somebody did not like, so i think the pwr tubes are re-labled JJ's, they put out a lot more current than a pair of RCA black plates, the tubes were handling the 142ma, but they smelled hot, now it is running a bit cooler,

using a 1uf/630 volt non polarized cap for the forth and final filter which feeds the first tube, it is that big gold thing on the top rt, you do not need a 20uf cap on the first stage, Fender just uses that value as it is the last section of a 20-20-20uf multican, no hum issues with the 1 uf cap,

we have a silver mica color dot cap and a ceramic NPO across the tone switch for the best of both worlds, mica can be harsh, ceramic can lack detail, so we mixed them together,

 

[CJ]

that huge ultralinear xfmr that i built was kind of a waste on this amp, so i built another OPT,

this one is done on a 1 x 2.4  stack of 625 EI, way smaller than a typical 6L6 OPT, but if you design for 100 or 200 hz instead of 20 or 40, the core gets small fast,

i was hoping for some xfmr saturation and i think i got it when this thing is cranked,

just a pri-sec-pri, so the top end rolls off at about 10k which limits the guitar tone to a more listenable spectrum, so both low and hi end are more in the guitar range,

we have 1600 turns pri and 89 bi fi wound sec, 17 k gauss at 100 hz, so we are hitting the core pretty good,

since we ditched the ultralinear xfmr, the pwr tubes are wired like a standard Fender, B+ off the first filter cap, screen volts taken after the choke, then 2 more filter sections, 1 for the inverter and 1 for the 6SJ7 preamp,  just 1 volume knob, that hi cut tone control on the standard Vox circuit is a waste as it is always on 10 so i left it out, plug and play, no controls to mess with,

 

[CJ]

ok the 6SJ7 just was not workin, if you sit the amp on top of the speaker cabinet like everybody does, then the vibration means that the tubes have to be solid, the 6SJ7 could not handle being on top of the cab, too microphonic, so we yanked the first tube socket apart and put in a 6SC7 like they use in the tweed deluxe, and the results were pretty good,

this thing sounds like a Marshall with the vol knob on 4,  ;D so we are very happy as that was what we were workin for,

OPT is small so we are getting crunch from the phase inverter, power tubes, OPT and speakers, which means it will put away any master volume circuit out there,

really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong,  ;D  Pops had his own fuzz box permanently  installed behind his larynx,

no cathode bypass cap on the pwr tubes for more mojo,
6SC7 has a common cathode, so we used a big bypass there as if not, you get pos feedback = oscillator,

never seen a circuit with the 6SC7 used for stage 1 and 2, usually it is used for ch1 and ch 2 inputs which means there will not be any pos feedback, we might be getting some residual pos feedback by doing this as the cap can not get rid of everything,

not a big fan of the Blackface tone stack, this circuit works nicely and handles the Les Paul/Fender transition better, used a 100K tone pot instead of 1 meg, was trying to lose a bit of gain, don't know if that worked, but it did not affect operation of the tone control.

Hoviland Musicap and Mallory and ERO caps for coupling, transformer specs are included,

phase inverter stolen from the VOX AC 15, 6SN7 is a great tube for inverter circuits, ran out of 1 Meg so using 820K for one of the grid resistors, no biggy,

running the first cap, 47/500 at 496 volts, as this seems to enhance the sound a bit,

1 uf/600 non polarized seems to be enuff for the first stage,

we knocked the B+ way down with big resistors for the preamp stages and inverter, can not stand the Mesa Boogie, one of the reasons it sounds so harsh is the 400 volts they run on the first preamp tube, there might be some sag built in with the large filter resistors and the small filter cap feeding the first tube.

choke is for Fender Deluxe Reverb,

darn left out the cap value on the pwr xfmr pri, it is a .0068 @600, gets rid of line garbage without shocking anybody,
schematic>

 

[CJ]

new pic of the new circuit,

 

[CJ]

this amp used to be a Bugera, they make a real nice cabinet and a great metal chassis but unfortunately everything else inside leaves much to be desired, although the 6L6 tubes sound pretty good, i do not know who makes them.

sounds good with the Tele and the SG,

now all we need is some cosmetics and maybe a place to play that will not attract the PoPo,