Thinking of building Gibson GA77

[Willabe]

Here's the new drawing with TS switch that DL posted. If any one sees any mistakes please let me know.

PSU coming next.


             Brad      

[Willabe]

Ok, here's the PSU drawing.

The Tx set is on the way, should be here Tues. or Wed. I need them 1'st to do my layout drawing to scale.


          Brad    

[Willabe]

This same circuit will be present on every VOX amp that contains a Top Boost section.
If you're curious to hear how the tone controls were meant to respond it's not a difficult thing to do. Just unsolder the ground connection from the Bass pot. You can leave it floating or join this terminal to the wiper to make it a variable resistor.

I drew it out for comparision with the tweed style TS wiring.

Do you think they will function the same? If so then I would only need a SPST switch instead of a 3P2T switch to lift the ground connection to switch between the 2 TS's.


             Brad      

[Willabe]

Called to see where my transformers where and they didn't have the PT in stock and had to make it. So they shipped the set on the 29'th.         

Should have them by Fri. or Sat.


              Brad     

[sluckey]

Do you think they will function the same?

Of course. The circuits are identical! Well, except you're using a log pot for bass so the taper will be opposite.

You have the bass pot wired as a variable resistor and the two ends connect to the same point in each circuit.

[Willabe]

I thought so, but the wiper and "x"/"0" end of the pot are swapped for what they feed.

Forgot about the log taper, guess I could go with a linear taper pot for the fix.


  Thanks,   Brad   

[DummyLoad]

brad, did you buy the merc. mag stuff to have it cloned or build with?

question: would you be so kind as to post the both transformer dimensions, the end bell to end bell width isn't as important as the lamination stack size; height, width & depth. also, please measure the DCR of all the windings and the unloaded voltages of all the secondaries with 120VRMS measured applied to the primary.
would you also consider measuring the OT as well, run it up on a variac until you get 1 VRMS on each of the secondaries record the primary VRMS for each secondary @ 1 VRMS.

my thoughts are that we may be able to source some iron that's current production and close enough to not care ( i think i have the PT close on the breadboard), however, the last real variable is the OT.

pics next to a ruler help too.  :-)

either way, much thanks in advance.

--pete

[Willabe]

I bought it to build with.

And sure I'll post the measurements you want when I get them. Maybe today.

Did you ever get that 2M pot?


             Brad     

[DummyLoad]

I bought it to build with.

And sure I'll post the measurements you want when I get them. Maybe today.

Did you ever get that 2M pot?


             Brad     

yes, they were delivered wednesday - been playing with blackfart head up until last night, so much soldering and un-soldering, i ruined the PCB and scrapped it in frustration.

i'll get the tone network installed on the breadboard amp and report back tonight or tomorrow morning. 

--pete

[Willabe]

i'll get the tone network installed on the breadboard amp and report back tonight or tomorrow morning.

Cool.


           Brad     

[Willabe]

The Tx/iron set just got here, 3:28 PM CST. They where sitting ~30 miles from me at the UPS place since 1:28 AM Fri. night/Sat. morning.    

Their from Mercury Magnetics tone clone line, here's some measurements.

Laminations;

GA90-P;
PT = 3 3/4" height, 3 1/8" width, 1 9/16" depth. End bells, 2 3/4" height, 2 1/16" width, 3 1/2" depth. Has shield end bells.

GA70-0;
OT = 2 1/2" height, 3 1/32" width, 1" depth. Bobbin, 1 1/2" height, 2" width, 2 1/8" depth.

GIB-C-5H;
Choke = 1 5/8" height, 1 7/8" width, 3/4" depth. Bobbin, 15/16" height, 1 1/4" width, 1 3/8" depth.

DCR;

PT; HT, 370-0-370, Rd./Rd. = 155.4 ohm, Rd./RY = 74.4 ohm, Rd./Rd.Ylw. = 81 ohm, 6.3-0-6.3 @ 5A = all readings 0.0 ohm on lowest setting, 5.0v @ 3A = all readings 0.0 ohm on lowest setting. Primary = Blk./Blk. = 3.2 ohm. Has internal shield brought out with white lead.

OT; Primary = Brn./Rd. = 158.0 ohm, Blu./Rd. = 170.7 ohm, Brn./Blu. = 329.0 ohm.
Secondary = 8 ohm. Blk./Grn.  = 0.1 ohm.

Choke; Blk./Blk. = 173.2 ohm.

I'll take ACv readings next and post #'s.


               Brad        

[DummyLoad]

GA90-P;
PT = 3 3/4" height, 3 1/8" width, 1 9/16" depth. End bells, 2 3/4" height, 2 1/16" width, 3 1/2" depth. Has shield end bells.

on the breadboard:
PT=
290CX - Doug's NSC041316  3 3/4" W 3 1/8" H 2" D (laminations) 650V CT @ 180mA - seems beefier by dimensions alone...

OT=
Hammond 1750J - 3 1/16" W 2 9/16" H 1" D (laminations) seems comparable to the GA-70-O - slightly larger lamination area and roughly same size bobbin area. this hammond piece is rated @ 35W
alternate:
Doug's NSC018343 - 50W  3 3/4" W 3 1/8" H 1 1/2" D (laminations) much larger than the GA-70-O and probably why it sounds so good clean.

CH =
Hammond 193H  2 1/2" W 3 1/16" H 1" D (Laminations) 5H@200mA DCR 65R - again, larger than the gibson part. closest hammond part i could match up to GIB-C-5H is the hammond 156L - 5H@75mA DCR 135R 2" W 1 11/16 H 3/4" D

i'll stick with the iron used on the breadboard with the "upgraded OT", thank_you_very_much! i think we nailed it with the substituted iron.

--pete

[Willabe]

Ok, I hooked up a small 6.3 Tx and ran it up to 1.004vac feeding the OT secondary. Used 2 meters to watch the 1.004vac and the OT primary readings. 1.004vac stayed very steady. Only moved from 1.003vac to 1.005vac.

Wasn't sure how to measure so I measured 2 ways;

OT primary, Brn., Rd. (CT), Blu. Secondary, only 8 ohm tap, Bl., Gn. common.

Brn. to Rd. = 14.18vac

Blu to Rd. = 14.19vac

Brn. to power cord/wall safety ground = 7.11

Blu. to power cord/wall safety ground = 4.35 ( Tx/meter did not like this keep moving up and down.)

Rd. to power cord/wall safety ground = 3.487


            Brad    

[DummyLoad]

ok - so blu <> red 14.8V & brn <> red 14.9V both w/ 1V applied to 8R secodary.

looking for Ra-a; IOW, plate to plate Z, so blu to brn = you have 14.2V start to CT + 14.2V end to CT = ~28.4V; (28.4/1)sq * 8 = 806 x 8 = 6542 or roughly 6.6Kohm a-a to 8 ohm. so ya, it's a clean 25W amp - it likely hits 35W when it's distorting.

--pete

[Willabe]

ok - so blu <> red 14.8V & brn <> red 14.9V both w/ 1V applied to 8R secodary.

Yup.

But where did you get 14.2V?


            Brad    

[DummyLoad]

ok - so blu <> red 14.8V & brn <> red 14.9V both w/ 1V applied to 8R secodary.

Yup.

But where did you get 14.2V?


            Brad    

i'm a dumbass. i missed transposing the numbers. 29.8sq * 8

[Willabe]

Close enough, probable put's it closer to 6.6K?

Just thought I missed something with the math.


               Brad     

[Willabe]

Hey Pete, did you get a chance to put in the 2M treble cut ciucuit?

I know you have a few projects going on right now.


            Brad     

[DummyLoad]

Hey Pete, did you get a chance to put in the 2M treble cut ciucuit?

I know you have a few projects going on right now.


            Brad     

not yet. i'll get it done tonight, sorry, but haven't had much desire to play on the breadboard the last few days.

--pete

[Willabe]

No hurry Pete, when you get to you get to it.

I've been working on Sluckeys Warbler for the past 3 days. Gonna post my layout drawing with a few questions right after this.

GA-77 is next so I'm just gathering information on it.


              Brad     

[DummyLoad]

ok, i installed the 3 components for the primary tone stack to make my proto amp a GA-77... IMO, it is redundant. you may like it since it can make the amp really bright; the downside is it does shave some gain.

i went back to the GA-70 configuration. if you install this in your amp may i suggest you add a disable switch to cut the wiper and 500pF cap out of ckt. (DPST)

this GA-77 arrangement is reminiscent of a 5E3, only without the mixing resistors. another couple of suggestions if may; add a disconnect switch to Ck of V2a; and try a 2uF there instead of a 20uF - with the bypass disconnected you get a greater range of control with the guitars volume; hence more perceived headroom. i like it with dougs NSC 50W iron and the 6L6GCs. really very nice clean tones and howls like a banshee when you open it up. sorry, but i don't own any pedals so i'm not much help there.

it is coming down the end of posting i'll make with this amp on the breadboard, i'm cleaning it off for the dual SE, if you have any last test requests, please let me know by saturday evening as that's when i'll likely dismantle the ckt.

--pete

[Willabe]

ok, i installed the 3 components for the primary tone stack to make my proto amp a GA-77... IMO, it is redundant. you may like it since it can make the amp really bright; the downside is it does shave some gain.

Then it's like a Fender bright switch across the volume pot but they made it variable.

this GA-77 arrangement is reminiscent of a 5E3, only without the mixing resistors.

Looks like a 5F6A Bassman pre with a 5E3 TS to me.

When you turn it down, away from the 500pF cap does it cut the treble to get a dark Jazz tone? Maybe they thought that the TS didn't have enough swing to it so they added the 2nd treble bleed?

Or maybe since back then if they were running a guitar and a mic for a singer at the same gig they wanted to give a little more flexibility to the 2nd channels without adding a 2nd whole TS?

Or maybe it just ended up being deemed a failed experiment?

One last question. The 2 volume controls should be interactive just like on a tweed bassman with a mid scoop. Have you played around with them?


            Brad          

[Willabe]

really very nice clean tones and howls like a banshee when you open it up.

As good as a BF SR AB763 and Tweed bassman 5F6A or maybe even a little better?

it is coming down the end of posting i'll make with this amp on the breadboard, i'm cleaning it off for the dual SE, if you have any last test requests, please let me know by saturday evening as that's when i'll likely dismantle the ckt.

The only questions I have left at this point is about the TS switching for going from Gibsons version to the Tweed style, with the mid boost pot and 5E3 treble bleed. And how they work in combanation with the interactive volume controls.

These 2 TS's by switch with the 5E3 treble bleed or variable bright cap together with the mid boost pot and the interactive controls mid cut/boost should give a lot of range clean or distorted.

It may be that the TS switch can be done with a SPST instead of a 3P1T? But with the SPST the bass pot may have to be a linear instead of an audio as Sluckey pointed out. Unless we use a 1MRA. Drawings are posted below.

It would be great to sort out these last few questions and get your thoughts on how the above sounds to your ear before you take it out of your prototype rig. I don't know at this point which TS you have in there, if you have the mid boost pot included with the TS while testing the treble bleed TS?

i went back to the GA-70 configuration. if you install this in your amp may i suggest you add a disable switch to cut the wiper and 500pF cap out of ckt. (DPST)

another couple of suggestions if may; add a disconnect switch to Ck of V2a; and try a 2uF there instead of a 20uF - with the bypass disconnected you get a greater range of control with the guitars volume; hence more perceived headroom.

Your suggestions would increase this range of flexibility.

i like it with doug's NSC 50W iron and the 6L6GCs.

I will definitely try a SR OT. I bought the Mer. Mag iron set in hopes to get a base line as close to an original to start with. I also wanted to get the right voltages using a 5V4, little more sag/sustain? OTOH a GZ34 is a great dual rectifier tube and is still in production.



             Brad      

[DummyLoad]

attached schema is as built - the mid control is all i did test, it's much like a marshall T&B - try .1 and .047 instead of .022u. a 25KL pot sounded best to me; i tried a 50KL as well - wired mid as a VR under the 10K.
wiring in a switch to the breadboard with the current layout would require near complete rip-up and re-route of existing layout. not gonna happen; sorry.  

ok, so we know that tone stacks are subjective for the most part. start where i did and experiment to your liking. i've carried the torch this far, time to turn it over to you and your ears.

...i sure hope we have similar taste...

 

regards,

--pete

[Willabe]

wiring in a switch to the breadboard with the current layout would require near complete rip-up and re-route of existing layout. not gonna happen; sorry.  

start where i did and experiment to your liking. i've carried the torch this far, time to turn it over to you and your ears.

Pete, Thank You for all you've done in prototyping this build. And for all the info you posted on it. I gotta tell you that I was thrilled when you took an interest in this amp.

I'm not trying to butter your bread on both sides, but I know you have built, repaired, rebuilt, tweaked and prototyped many different amps and designs through the years. So I believe you have developed a very good set of ears at this point and have a very broad understanding of how an amp should respond when playing through it. Your experience is rare and valuable IMO. You've traveled down the road far enough not to be swayed by the run of the mill.

No a near complete rip-up and re-route is not necessary. I had no idea it would involve that much.  

Yes, I will carry the torch forward          and report what I encounter.        

Seriously, you have given me a HUGE head start.

I have the iron set and a few different sized blank chassis to pick from so I'll start measuring this week so I can start my layout drawing.


         Thank You,     Brad    

[DummyLoad]

You've traveled down the road far enough not to be swayed by the run of the mill.

i'm extremely hard headed. 

thank you. i was/am happy to help - when i saw the circuit of this amp it piqued my interest + we had a voltage chart; i was fairly certain we could nail the tone with off_the_shelf_parts. from what i have on my breadboard, it's likely fairly close to an original.
it's still on the breadboard; i don't have the drive right now to tear it down and i'll be on the road for 2-3 days, so until then, should you have anymore tricks you want to try that aren't too invasive, then post up.

hint - i wonder how this thing would sound with a pair of 6550/KT88 and the appropriate iron? point it at the drummer... <evilgrin>

Yes, I will carry the torch forward

and well lit it will be. post up pics in this thread as you progress - sort of like an chronology from the lab to finished product.
be careful, smoke some rosin, and have fun.

respectfully,

--pete


 

[DummyLoad]

bump!

any progress brad?

--pete

[Willabe]

Yes.    

Layout drawing is almost finished. I need to put in the output PA stage, bias, NFB loop/pres. control, speaker jacks and power switch/fuse.

I need the layout to scale for the face/back plates. I tape them to the chassis and drill both at the same time. That way they line up perfectly. I did this on my 5G9 build and it came out great.

I have a friend at church who is a graphic artist and knows the vector program inside out. He's also a very good jazz guitarist and is going to help me. (He played through my 5G9 with a Tone Tubby 12" hemp cone and my Fender stand alone verb. and he loved it.) I think I'll go see him Sat. and he'll knock it out in a half hour or so with me looking over his shoulder. The engraver needs the layout in a vector form. Inkscape is a bit much for me. I did figure it out enough to get the plates made for my 5G9 but I went in to Inkscape a few days ago and I've forgotten how I did it.  

I'll probable learn more from my friend and way faster then me poking around for hours to learn how it works.

I also got side tracked with making the layout/schemo drawings for Sluckeys Warbler and a clone of the Gibby/Vox vibrato/tremolo as a stand alone unit. Warbles finished, Gibbys 3/4's finished.

I'd like to have the plates made for all 3 at the same time.

I'm going to build the GA-77 stock and see how I like it. I then will try it with a PT/OT from a BF SR or a tweed Bassman.

I can also see that either one of the vibratos will fit in the builds chassis, so I'll gator clip it in and see how it sounds.

I also would like to try dropping the normal channel and putting in a 5879 pentode.

Anyway I'll keep posting any progress.


                Thanks,    Brad          

 

[Willabe]

I'm going to talk with mojo about modifying their British front mount 18w 1 x 12" for my build.

I'll get a price on them changing the tolex, grill cloth, piping, handle, speaker baffle cut out to a 15" and chassis cut out needs to be lengthened. Should be under $500.

Maybe a 2 tone tolex in gator or ostrich for the top with a piping bead seperating the bottom in a cream or brown, cane grill, with their heavy duty brown leather handle?


                  Brad           
                     

[DummyLoad]

I'm going to talk with mojo about modifying their British front mount 18w 1 x 12" for my build.

I'll get a price on them changing the tolex, grill cloth, piping, handle, speaker baffle cut out to a 15" and chassis cut out needs to be lengthened. Should be under $500.

Maybe a 2 tone tolex in gator or ostrich for the top with a piping bead seperating the bottom in a cream or brown, cane grill, with their heavy duty brown leather handle?


                  Brad           
                     

that would be a nice cabinet for this amp. if you would, please let us know what kind $$ they'd want for a duplicate.

keep smokin' that rosin...

--pete

[Willabe]

that would be a nice cabinet for this amp. if you would, please let us know what kind $$ they'd want for a duplicate.

Yeah, if I pick the right tolex, grill cloth, piping and handle. Along with the chicken head knob colors and face plate colors. They have a very good selection to choose from. I really want to dress this build up right.

I'm going to ask them for a 1 x 12" and maybe even 3 x 10" baffle option price. 3 x 10" might sound real good.

I will let you guys know what they quote me on price.

The base price on that cab is $317.15 and the "everything up charge" is $25.00, which is tolex, grill cloth, piping, handel and feet. (I like their big rubber feet.) The baffle change is $45.00 and the chassis cut out is $25.00. Cab is 20"H x 24"W x 9"D, my chassis is 20" x 8" x 2.5. So the cabs going to have to be deeper.


Looks like I'm up to $412.15 and that's without deepening the cab. They might have another option since so much would be changed? It might be cheaper to start from scratch and get a custom made cab?

Going with a tweed cab is an option but then I'd have to change my chassis and a bit of the layout. I do like that it's made out of pine as apposed to baltic birch and the tweed cabs floating speaker baffle. But I prefer the British cabs front access to the controlls.      


               Brad      

[DummyLoad]

Looks like I'm up to $412.15 and that's without deepening the cab. They might have another option since so much would be changed? It might be cheaper to start from scratch and get a custom made cab?

412USD seems to be a fair price. total custom may not be as low?

do you really need deeper cabinet, or is deeper cabinet required by the larger 1x15" speaker?

--pete

[Willabe]

do you really need deeper cabinet, or is deeper cabinet required by the larger 1x15" speaker?

No, not for the speaker. But.... maybe I can re-think this?

My chassis is 8" deep + the 2 chassis trim/face/back plates at 1/8", so ~8 1/4" and their cab that I'm looking at is 9" deep. Their front panel is 1/2" + ~1/32" for the tolex wrapped around the back side of it and the cab has the tolex wrapped around it too. (Some tolex is thicker than others?) Then there's the back panel, I think it's 3/8" + 1/32".

So, 8"+ 1/8" + 1/8" + 1/2" + 3/8" + 3 @ 3/32" (tolex) + 1/32" (grace/round up) =  8" + 3/4" + (3/8" + 1/8") 1/2" = 9 1/4".

Now I could leave the face/back plates out in front of the cab and not tucked/sandwiched  behind the cab like a Fender brown/black face. Which would give me an extra 1/4". I like to have what I was taught as a carpenter as "grace" where you hide things by over lapping them when you have a very small tolerance/measurement.

But.....   if I have them drill the cab for the chassis mounting screws, I bet they could nail it on the money.

I was planing on sending them the chassis with the face/back plates attached so they could make it fit along with drilling the chassis mounting holes.


                Brad      

[DummyLoad]

So, 8"+ 1/8" + 1/8" + 1/2" + 3/8" + 3 @ 3/32" (tolex) + 1/32" (grace/round up) =  8" + 3/4" + (3/8" + 1/8") 1/2" = 9 1/4".

gotcha! make it an even 1 foot deep? that way a 15" speaker with the deepest basket known to man will fit too...   

but then again... 1 x 12" + 2 x 10" might be nice... or, 3 x 10".... or...      

--pete

[Willabe]

but then again... 1 x 12" + 2 x 10" might be nice... or, 3 x 10".... or...    

Will you stop already?  I'm confused enough as it is!!!!!   


              Brad      

[Ed_Chambley]

Gettin it down to brass tacks I see.  I hope this amp just sings as you have really worked to get the info together.

Since you are confused already, what about a piggyback style head and multiple cabinets to match how that old refretted Strat feels from day to day.  You would have your forward controls and any cabinet your pocketbook can afford.

[Willabe]

Yeah Ed, that's an option too. Plus then I would have the tubes/chassis right side up.


                Brad      

[Willabe]

All right, I finished my layout drawing. I drew this out, some what, in 3 different sized chassis. Trying to get the iron and tubes to have some space to breath/cool between them. I think this chassis size will work and I can get it in a stock mojo combo cab. They will change the face/back pannel openings for a small fee. I'm going to go with a 12" speaker as that's stock and changing to a 15" (or any different baffle cut out) is $45.

The small bias PT is because the clone PT MM makes does'nt have a bias tap. I might just go with pulling the bias from the PT secondary like on a Marshall.

B+ droping R's will have to be played with.

Has a KOC wired galactic ground, at least as far as I understand how to wire it.

The far right end of the drawing is cut off, as it's a little too big to crop. If I resize under 50 x 50 it starts to leave out some of the lines in the drawing.    

Second drawing shows the iron set placment.

If anybody sees any mistakes please let me know.


              Brad        

[Willabe]

I was looking around on the web for different tolex than what's the standard colors and I found this gator pattern. I don't know if it can be glued to wood. Their sending me free samples in 5 different colors, should have them be the end of the week.

I called them and they can't tell me any more than what's on the web site.        All it says is it's 100% polyurethane. They show a couch covered with it.

I really like this pattern and color.


             Brad            

[Willabe]

Here's the mojo cab I'm going with. I'll probable have in done in a 2 tone with a brown/gator on top and 5", 6" down the sides with a lighter colored ~ tan for the rest. Nice dark brown leather handle.


                  Brad      

[DummyLoad]

reads like a nice finished product. please keep us posted.

--pete

[Willabe]

Update;

Well I just got back from my friends house.            

He did the art work in a vector program for the face/back plates for both the Warbler and GA-77.

Man is he good at what he does! It was a real pleasure to see someone with his experience and talent work in the vector program on a computer.

Next, call the engraver and e-mail him the files.

I also got some more color samples today in the mail of piping and bead for the cabinet. I have more coming next week.


                Brad      


Edit; They look bad because I cropped them in Windows 7 paint.

[Willabe]

Well since I'm doing 2 builds at once, this 1 and a Warbler, I'm ending up posting the same update at this point in both threads.

So here's the link to the update on my face/back plates.

http://www.el34world.com/Forum/index.php?topic=10681.msg148641#msg148641


               Brad     

[Willabe]

Face/back plate update;

http://www.el34world.com/Forum/index.php?topic=10681.msg148666#msg148666


            Brad     

[Fictitiousfreedom]

I know its been about 15 years since anyone posted here but i happen to have a Gibson Ga-77RV Vanguard and if anyone in the future is interested in a build of these i have some info that could help.

A friend of mines engineer grandpa found it in the dump 30 years ago. It is missing the combo cab, speaker, and reverb tank. It took me awhile to get it working, i replaced the filter caps first which had already been replaced, then the heater wiring and coax, then i yanked the switches for on-standby-Off and the ground switch and replaced with a single dpdt. After months of nothing i found one of my brand new from mouser nichicon filter caps was DOA and all i needed to do was replace it. It is still a little unstable and noisy, especially the Pots. I'm making a layout and want it to be as original as possible before i replace anything else. Its a great clean amp, probably the best I've played. Ive only played a few AC30s and i actually think it sounds better than the few I’ve heard.

I plan to build a nice head cab or combo for it but need to figure out the reverb tank first. If anyone knows a reverb tank that will work with this amp id love to know, all the ones i have are very far from ideal.

Glass
5u4GT, 2-6L6GCs, 1-12au7, and 3-6EU6

Iron
Power - GA-90-P1 166103
Output - GA-70-02
Choke- E-3021S 116114
Reverb - E6400 166103

Also the bass pot is grounded as in the schematic and it looks to be that the way from the factory, its pretty obvious what had been modified so im pretty confident about that.

Anyway, hopefully this info can help someone. If anyone has questions let me know. My layout and other info will be uploaded to my website when complete. Www.Fictitiousfuzz.com/tubeamps

Thanks, hopefully its ok i post in this ancient thread, i found it while looking for this amp so i figure it could help.

[sluckey]

I know its been about 15 years since anyone posted here but i happen to have a Gibson Ga-77RV Vanguard and if anyone in the future

I suggest starting a new thread.

If anyone knows a reverb tank that will work with this amp id love to know

Measure the DCR of the reverb transformer. If lower than 10Ω then the common Fender 4AB3C1B should work just fine.

[Willabe]

I know its been about 15 years since anyone posted here but i happen to have a Gibson Ga-77RV Vanguard and if anyone in the future is interested in a build of these i have some info that could help.

That looks like it's a nice amp from the schematic. But it's different than GA-77 amp I built.