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Hoffman Amps Forum image Author Topic: Thinking of building Gibson GA77  (Read 46961 times)

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Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #50 on: December 29, 2012, 12:08:52 pm »
ok so plan on 2 wiring errors. hey, sue me, ok. ;) i had V3 running off the screen power node. i suspected there was a wiring error when comparing voltages to the original. still haven't bumped up the plate idle dissipation of the output stage. V3a still looks suspicious - the last gain stage. original plan says 80V at the plate and 3.4V at the cathode - i'm reading 46V at the plate and 4.1 at the cathode.

this thing is LOUD and CLEAN. like C&W clean tones. takes a lot of crank on the channel volumes to start braking up well and it has a cracking/sparkling overtones when over-driven hard but does growl at the point of breakup. not my cup of tea for a hard rock amp. with the treble control full up and on the bridge p/u it's makes your ears bleed it's so bright. overall, for a blues & country player, a very nice amp; to get anything close to rock tones you have to push it hard. i have a cross-line MV on this build not indicated in the schematic and it's likely why my ears aren't bleeding now.

attached final build schematic with updated voltage chart. i fiddle with the gain some, i know i'd like more gain and that's just my cup of tea though, so to speak...

 :icon_biggrin:
  
--DL
« Last Edit: December 29, 2012, 06:05:36 pm by DummyLoad »

Offline tubenit

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Re: Thinking of building Gibson GA77
« Reply #51 on: December 29, 2012, 01:25:27 pm »
I would think using the 12AU7 for the 3rd gain stage and the phase invertor would keep it pretty clean sounding?

Have you tried a 5751 or 12AY7 in that position?  And if so, how did that sound to you?

With respect, Tubenit

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #52 on: December 29, 2012, 01:40:42 pm »
Have you tried a 5751 or 12AY7 in that position?

actually was going to try 12AX7/5751 in V1, maybe even a 12BZ7 for V2 and a definitely a 12DW7 for V3 with the medium mu section for the concertina. that ought to "wake it up".

btw, as is, this is a decent sounding jazz amp.

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #53 on: December 29, 2012, 03:13:27 pm »
this thing sounds really nice - i'm impressed, but i couldn't take it all the way up

this thing is LOUD and CLEAN. like C&W clean tones. takes a lot of crank on the channel volumes to start braking up well and it has a cracking/sparkling overtones when over-driven hard but does growl at the point of breakup.

overall, for a blues & country player, a very nice amp; to get anything close to rock tones you have to push it hard.

btw, as is, this is a decent sounding jazz amp.

Well alright then!      :blob8:  

The way your describing it DL is just what I was hoping for after re-reading TQR's review of the amp.

Be a while before I can get to it but I'm convinced enough at this point to build it. (I'm working on my shop right now and I might have my bench put back together today? Moving everything is still in boxes.   :BangHead:    :laugh:    )

You wouldn't have a pair of NOS 5881/6L6GB's to pop in it for comparison?   :undecided:

Might sound a little different than the black plate RCA 6L6's? Little less headroom/earlier break up, less output?

I think your both right about trying different 12 _ _ 7's/5771 in different positions. Makes the amp more versatile for different sounds if/when needed.

DL I see the way you wired up the bass pot in new updated schemo. I think that's the way Gibson wired it but I'm not positive. Could they have wired it as in the drawing below? With the "o" end of the bass pot standing on the 10K midrange R?      :dontknow:


                  Brad      :icon_biggrin:

Edit; The wiring of the bass pot in this drawing is wrong. Do not use it. See DL's reply #54
« Last Edit: January 24, 2013, 11:18:20 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #54 on: December 29, 2012, 06:14:32 pm »
tone stack won't work as we've drawn it - it leaks DC into grid of last AF amp.

i detached all the schematics in all previous posts as they are WRONG and will potentially damage the 12AU7 if followed.  :BangHead: my apologies for any confusion i may have spawned.

attached are the corrected versions. this is hot off the breadboard - the treble AND bass controls now function as they should. nice control with this TS btw...  :icon_biggrin:

i included the OT wire colors for proper phasing, assuming of course, that you build it with doug's iron.

--DL


Offline HotBluePlates

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Re: Thinking of building Gibson GA77
« Reply #55 on: December 29, 2012, 07:40:23 pm »
You wouldn't have a pair of NOS 5881/6L6GB's to pop in it for comparison?   :undecided:

Might sound a little different than the black plate RCA 6L6's? Little less headroom/earlier break up, less output?

I've got a fair number of 50's-60's Tung Sol 5881's. I've never really noticed an obvious difference in output power when using them in an amp vs 6L6GC's. There is some tone change, maybe some distortion-quality change.

I'd chalk up the lack of power difference to the fact that 6L6GC's are still 6L6's; if you want to get more power output, you'll probably need to up the supply voltage (and maybe alter the loading) and take advantage of the plate dissipation rating difference.

Some day I'll do a breadboard system like DL and Richard and verify the notion I'm getting at: Most would install 6L6GC's and idle hotter to make up the difference in plate dissipation, but that lowers the amount of grid bias and the size of input signal before distortion. Since there's no change of mu between 5881 and 6L6GC, less input signal means less output plate voltage swing. So it seems idling hotter might get you a bigger current swing, but smaller voltage swing, possibly net about the same actual output power.

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #56 on: December 30, 2012, 05:59:51 pm »
tone stack won't work as we've drawn it - it leaks DC into grid of last AF amp.

i detached all the schematics in all previous posts as they are WRONG and will potentially damage the 12AU7 if followed.  :BangHead: my apologies for any confusion i may have spawned.

I had a hand in that too, sorry guys. I knew you'ld be able to figure out how it was supposed to be wired up. It was not clear in the TQR review, only that in the schemo it was wrong.

the treble AND bass controls now function as they should. nice control with this TS

Great!   :blob8:  

Is the crackling gone now that you mentioned earlier? But with the top end chime still there and in balance now with the bass end so as not to be so hard on your ears?


                   Brad     :icon_biggrin:

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #57 on: December 30, 2012, 06:17:51 pm »
I've got a fair number of 50's-60's Tung Sol 5881's. I've never really noticed an obvious difference in output power when using them in an amp vs 6L6GC's. There is some tone change, maybe some distortion-quality change.

When I still had my BF SR years ago I tried a set of sovtek 5881's, NOS Philips 6L6GB's and NOS GE 6L6GC's in it with a friend of mine. Plate voltage ~470dcv all 3 sets biased to 35mA. The sovteks sounded flat to us, he loved the GE's and I loved the Philips GB's the best. GE's were too clean to me comparied to the 6L6GB's and not as warm. The 6L6GB's sounded fuller/warmer? More hamonic content? It was very noticable to me at the time. Which is also why my friend didn't like them as well as the GE's.     :dontknow:

verify the notion I'm getting at: Most would install 6L6GC's and idle hotter to make up the difference in plate dissipation, but that lowers the amount of grid bias and the size of input signal before distortion. Since there's no change of mu between 5881 and 6L6GC, less input signal means less output plate voltage swing. So it seems idling hotter might get you a bigger current swing, but smaller voltage swing, possibly net about the same actual output power.

That's interesting.    


                    Brad       :think1:
« Last Edit: December 30, 2012, 08:29:32 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #58 on: December 30, 2012, 06:29:23 pm »
is the crackling gone now

somewhat, but still sparkles when driven hard on note fade..i'm ok with that kind of distortion.  V3A readings are now closer to what's on the original documentation.

i fried a 7247 fiddling with tone control, wasn't paying attention and leaked ~200VDC to grid of V3a; it flashed orange flame in death! - first tube i've fried on the breadboard in LONG time. a moment of silence please for the 7247 that has given it's life in the search for tone nirvana. 7247 #1 has joined the thermionic gods... too bad too, it was mullard.  :worthy1:

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #59 on: December 30, 2012, 06:46:13 pm »
somewhat, but still sparkles when driven hard on note fade..i'm ok with that kind of distortion.  

OK, sounds good. Gota have a little distortion.

is the crackling gone now

V3A readings are now closer to what's on the original documentation.

That's good too.

is the crackling gone now

i fried a 7247 fiddling with tone control, wasn't paying attention and leaked ~200VDC to grid of V3a; it flashed orange flame in death! - first tube i've fried on the breadboard in LONG time. a moment of silence please for the 7247 that has given it's life in the search for tone nirvana. 7247 #1 has joined the thermionic gods... too bad too, it was mullard.  :worthy1:


Ouch!  :m15

Thank you DL for taking the time to breadboard this amp and giving us your thoughts on it. All in all it sounds like a winner to me, gotta build it.


                     Brad       :icon_biggrin:

                 

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #60 on: January 01, 2013, 09:10:55 pm »
Would this be how to wire a mid boost to this amps tone stack, so when the mid pot is on 0 the mids are stock?

I don't think it's right because now the bass pot is standing on 35K instead of 10K.


                Brad      :icon_biggrin:

Edit; Do not use this circuit. It will not work. See response #62 from Sluckey and from DL #63.
« Last Edit: January 24, 2013, 11:22:31 pm by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #61 on: January 01, 2013, 09:27:49 pm »
Or maybe this one?

               Brad      :think1:

Edit; Do not use this circuit. It will not work. See response #62 from Sluckey and from DL #63.
« Last Edit: January 24, 2013, 11:21:36 pm by Willabe »

Offline sluckey

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Re: Thinking of building Gibson GA77
« Reply #62 on: January 01, 2013, 09:37:44 pm »
Both of those drawings put dc on all the pots and into the grid of V3A.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #63 on: January 01, 2013, 09:43:29 pm »
Would this be how to wire a mid boost to this amps tone stack, so when the mid pot is on 0 the mids are stock?

I don't think it's right because now the bass pot is standing on 35K instead of 10K.


                Brad      :icon_biggrin:

brad, no, i don't think so. that circuit leaks DC into last AF amp. uncouple the pot from the 100K resistor.

if you want a mid control - make the 10K  a 10K or 25K or even 50K pot. with a 50K the mid dip get up to around -8dB.  please see attached schema and f sweep trace.

edit: sweep made with 25k control stacked on 10K - you can trim 10k as well, should you so desire.

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #64 on: January 01, 2013, 09:44:47 pm »
Ok thanks Sluckey.

I think I figured it out. How about this one?


                   Brad      :think1:

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #65 on: January 01, 2013, 09:52:27 pm »
Thanks DL, I was writing as you posted.

I'd like the mids/tone stack to be stock with the mid pot off/on 0.


                  Brad      :icon_biggrin:

Offline HotBluePlates

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Re: Thinking of building Gibson GA77
« Reply #66 on: January 01, 2013, 09:54:35 pm »
Would this be how to wire a mid boost to this amps tone stack ...

How much do you like that mis-wiring of the Bass control in the Gibson (and Vox' copy of it)?

If you just love it, then you can't do it, because the Bass pot doesn't stand on the Mid resistor. Of course, if DL's present drawing is correct to his breadboard, he didn't use the original funky Gibson circuit either.

If you want midrange control, and are willing to ditch Gibson's error, then wire it as a Fender tone circuit, and do a Mid control in the standard way.

Or, you bypass the Gibson ton circuit completely, along with its midrange dip. By comparison, you have a big midrange boost, and the tweed-Fender-like tone control in the one channel shapes the overall sound to trebly or bassy but retains the mids, just as it does in tweed Fender amps.

If this was a different Gibson model using a T-filter tone network (which creates a mid-dip), then I could tell you for certain how to add a Mid control to that (cause I've already done it in my Standel build).

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #67 on: January 01, 2013, 10:06:58 pm »
How much do you like that mis-wiring of the Bass control in the Gibson (and Vox' copy of it)?

If you just love it, then you can't do it, because the Bass pot doesn't stand on the Mid resistor. Of course, if DL's present drawing is correct to his breadboard, he didn't use the original funky Gibson circuit either.

What I've read is that the draftsman made a mistake in the drawing but Gibson never wired them that way at the factory. But Vox did. I've never heard/played through either amp.    :dontknow:

I'm gonna put my money on DL's version at this point, I bet he's got it right.

I think my 3'rd drawing will work.


                      Brad       :icon_biggrin:

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #68 on: January 01, 2013, 10:14:28 pm »
If you want midrange control, and are willing to ditch Gibson's error

was gibson's error limited to the schematic? was it ever implemented?

schema and f plot with error. notice curves with the wiper of control at 0.

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #69 on: January 01, 2013, 10:22:51 pm »
was gibson's error limited to the schematic?

Yes.

was it ever implemented?

Not by Gibson but Vox didn't know the drawing was wrong and lifted the TS for their top boost circuit, so the story goes.

http://www.el34world.com/charts/Schematics/files/Vox/topboost_preamp.pdf

schema and f plot with error. notice curves with the wiper of control at 0.

That looks like a big dip in midrange.


                Brad     :think1:
« Last Edit: January 01, 2013, 10:28:06 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #70 on: January 01, 2013, 10:38:38 pm »
Not by Gibson but Vox didn't know the drawing...

then it seems with the error implemented, the bass control would basically behave like a switch on the VOX?

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #71 on: January 01, 2013, 10:44:11 pm »
then it seems with the error implemented, the bass control would basically behave like a switch on the VOX?

 :dontknow:

Never played through one. Guess it's part of the Vox ac30 top boost mojo?


               Brad     :icon_biggrin:
« Last Edit: January 01, 2013, 10:49:54 pm by Willabe »

Offline HotBluePlates

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Re: Thinking of building Gibson GA77
« Reply #72 on: January 02, 2013, 11:43:14 am »
was gibson's error limited to the schematic?

Yes.

was it ever implemented?

Not by Gibson but Vox didn't know the drawing was wrong and lifted the TS for their top boost circuit, so the story goes.

I take Brad's word on it. Never saw a GA-77 or GA-70 in person, so I dunno. I did have an AC-30 at one time. The tone circuit seemed kind of fiddly, and with blue celestions it tended to be very dark when clean. You'd need to crank it up and/or roll off the bass to get the sound everyone thinks AC-30's make.

schema and f plot with error. notice curves with the wiper of control at 0.

That looks like a big dip in midrange.

Don't hear with your eyes. Or, look at a Fender tonestack in Duncan's Tone Stack Calculator.

The sound you're used to hearing in blackface amps is that big mid-dip. It shifts and dips a little more/less depending on how the treble and bass controls are set.

By comparison, the "midrange-heavy" sound of small tweed Fenders is much closer to actually being "flat" with no mid-dip.

The Gibson amps with a T-filter tone network have a mid-dip created by that, then additional Treble/Bass controls. I hadn't looked lately to see if the type of tone stack used in those amps is like the Fender circuit, which would make an even bigger resulting mid-dip, or if it's more like a James/Baxandall.

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #73 on: January 05, 2013, 07:23:11 pm »
ok, update. got a bit of the bug over the last 4 days so haven't had much desire to fiddle with anything. revisiting this amp on the breadboard just a short while ago i noticed that i had left it powered up for the last 3 days at least - more likely 4; around the time of my last post on this thread, 01/01/13.

so here goes; i found another pair of used 7247s - one was a GE and dead out of box, so it promptly hit the circular file, the other is another eico/mullard - damn this is a sweet sounding but loud amp. brad, give the 7247 a spin around the block - i think your gonna love it even more.

i'm going to fiddle with the input stage, and then the OT iron. i have a hammond super reverb replacement and doug's SR multi-tap OT - shooting for less core saturation and just to hear difference in voicing of this circuit with different OT. both are 4K so, i doubt there'll be much difference, other than in the low octave with the larger OT.

--pete

 

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #74 on: January 05, 2013, 08:38:38 pm »
Glad to hear your feeling better DL.

I'll keep the 7247 in mind and give it a try when I build the amp.

It will be good to see what you think of the SR OT in this amp.


              Brad      :icon_biggrin:

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #75 on: January 15, 2013, 03:22:24 pm »
i'm going to fiddle with the input stage, and then the OT iron. i have a hammond super reverb replacement and doug's SR multi-tap OT - shooting for less core saturation and just to hear difference in voicing of this circuit with different OT. both are 4K so, i doubt there'll be much difference, other than in the low octave with the larger OT.

Did you ever get a chance to try the SR OT?


            Brad      :icon_biggrin:

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #76 on: January 15, 2013, 09:53:21 pm »
i'm going to fiddle with the input stage, and then the OT iron. i have a hammond super reverb replacement and doug's SR multi-tap OT - shooting for less core saturation and just to hear difference in voicing of this circuit with different OT. both are 4K so, i doubt there'll be much difference, other than in the low octave with the larger OT.

Did you ever get a chance to try the SR OT?


            Brad      :icon_biggrin:

not yet. i'll try it out tomorrow. 

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #77 on: January 16, 2013, 05:19:02 pm »
I think I found a 4'th schematic error that I didn't notice before?

Switched speaker jack is wrong. The way it's drawn when you plug in a jack you disconnect the + OT secondary from going to the speaker? Not good.


         Brad       :think1:

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #78 on: January 16, 2013, 05:25:50 pm »
I've been trying to figure out why Gibson added a 2'nd treble control to the second channel on the GA-77.

I think it was meant as a type of variable treble boost instead of a treble boost switch with a set bypass cap value? It is tied across the volume pot.


                 Brad     :think1:
« Last Edit: January 16, 2013, 08:10:21 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #79 on: January 16, 2013, 10:01:24 pm »
i'm going to fiddle with the input stage, and then the OT iron. i have a hammond super reverb replacement and doug's SR multi-tap OT - shooting for less core saturation and just to hear difference in voicing of this circuit with different OT. both are 4K so, i doubt there'll be much difference, other than in the low octave with the larger OT.

Did you ever get a chance to try the SR OT?


            Brad      :icon_biggrin:

not yet. i'll try it out tomorrow. 

--DL

if i were to build this amp the hoffman SR OT is the one i'd use - the multi-tap one. it is a very neutral sounding clean with really good bass response. this circuit i think is better than the AB763 for clean playing. yes, i'd go that far, and i love the AB763 pattern - i've owned two AB763 amps. it can be a VERY bright amp, you'll need to back off the treble considerably for blues or jazz playing. the only thing missing is reverb, i don't think we could graft in reverb without upsetting the tone too much, just front end it with the delay pedal or a revibe. my guitar is a 99 american standard strat w/ seymour duncan pickups - gets that country twang on very well. i also played with a godin LG - the plain jane 2 humbucker model - sounds fabulous when cording jazz.

ok, so then when you're playing "stand by your man" with the band, this amp is the one thing that won't embarrass you...  :icon_biggrin:

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #80 on: January 17, 2013, 12:10:59 am »
hoffman SR OT is the one i'd use - the multi-tap one. it is a very neutral sounding clean with really good bass response. this circuit i think is better than the AB763 for clean playing. yes, i'd go that far, and i love the AB763 pattern - i've owned two AB763 amps. it can be a VERY bright amp, you'll need to back off the treble considerably for blues or jazz playing.  sounds fabulous when cording jazz.

Wow!

What speaker are you using?

It came with a single Jensen 15". They put in an Eminence Legend 15" after they blew up the Jensen and said they loved it.

ok, so then when you're playing "stand by your man" with the band, this amp is the one thing that won't embarrass you...  :icon_biggrin:

Ok got it.    :laugh:  


                  Brad      :icon_biggrin:

« Last Edit: January 17, 2013, 12:25:39 am by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #81 on: January 17, 2013, 02:55:28 am »
it can be a VERY bright amp, you'll need to back off the treble considerably for blues or jazz playing. - gets that country twang on very well.

Was this with the stock tube line up and with the presence control all the way off?

And just thought of something. The GA-70 Country and Western didn't have the 2'nd treble control.

The GA-77 Vanguard did. It was aimed at the jazz players. They needed a treble cut where the country guys didn't?

Did you ever get the 2M pot to try the 2'nd tone control?


                  Brad      :think1:
« Last Edit: January 17, 2013, 03:09:27 am by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #82 on: January 17, 2013, 05:00:38 am »
I found this on the schemo drawing TS wiring error;

The VOX type Top Boost circuit response is used in the TC-15. This is well-known for it’s interactive nature.
When VOX re-designed their AC30 and removed the EF86, they added a Top Boost circuit to regain some of the high end sparkle the amp had lost. What was used was the high gain circuit from the 1954 Gibson GA-77 "Vanguard" - An Amplifier known for it's 'high gain' channel. The interesting point is that the original Gibson diagrams happen to have an error on the Bass pot, grounding one end that should be left floating.
Glen Lambert - 2002. For more information see:
http://www.geocities.com/vintage325/topboost.html
This error was carried across to the VOX JMI 'Brilliance' unit. This affects the way the tone controls interact and the effect of the Bass control on mid band. Normally it would be desirable for the Bass control to effect only the Bass content, but in this case the Bass control will effect the midrange and to some extent the Treble when it is at it's extreme travel. Even in correct form the circuit is not ideal. It's a compromise to keep the component count low, but the error does make it slightly harder to control Treble and Bass separately.
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.



              Brad      :icon_biggrin:
« Last Edit: January 17, 2013, 05:38:41 am by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #83 on: January 17, 2013, 05:23:35 am »
But here's a monkey wrench for the wiring of the TS gears;

p.390- After Tom Jennings sent Derek Underdown on the quest for more top end, Dick came across the tone control circuit that Gibson used in their GA70 and GA77 amplifiers, and, for all practical purposes, nicked it.

p.391- There has been, over the years, some speculation that the Gibson schematic contains an error that JMI copied. This is only half true. Yes, JMI copied the schematic exactly, but there was no error- the GA70’s and GA77’s circuits match the schematic. It was a circuit variation that is was not what people expected to see, so it was assumed to be a mistake. Glen and I have discussed this, and he is now aware that there was no mistake.


Here's the link where it came from;

http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0


                Brad      :dontknow:
« Last Edit: January 17, 2013, 05:38:19 am by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #84 on: January 17, 2013, 07:21:08 am »
it can be a VERY bright amp, you'll need to back off the treble considerably for blues or jazz playing. - gets that country twang on very well.

Was this with the stock tube line up and with the presence control all the way off?

presence full down. (darkest setting) MV full up. bass about 12:00 treble about 11:00.

And just thought of something. The GA-70 Country and Western didn't have the 2'nd treble control.

The GA-77 Vanguard did. It was aimed at the jazz players. They needed a treble cut where the country guys didn't?

and why you need to back off the treble considerably to make it work for jazz.

Did you ever get the 2M pot to try the 2'nd tone control?


no, i didn't, sorry!  :sad2:  complete brain-fart on my last CE-dist order. i'll have to get one from AES.

i'm running the GA-70 on the protoboard. - two channel level controls, bass, treble, presence, and and cross-line MV.

Wow!

What speaker are you using?


JBL K120

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #85 on: January 17, 2013, 09:52:18 am »
A JBL K120, aren't those a very bright sounding speaker compared to an old Jensen?


                  Brad     :think1:

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #86 on: January 17, 2013, 11:32:01 am »
A JBL K120, aren't those a very bright sounding speaker compared to an old Jensen?


                  Brad     :think1:

bright is relative - i use the JBL K120 as a test speaker - with this amp it's very bright. you can tailor the roll-off points to match the the speaker of choice. or, just do like i do, roll off on the treble knob. 

jensens are too dark for my taste - don't care for most - the P12Q is OK.. i like some celestions (vint. 30), fane, EV (evm12), JBL D,K and E 120, 130 series, and Altec 418,421.  i have a eminence red coat that i use as a first fire-up test speaker that sounds OK and and en eminence texas heat (purportedly has celestion vint. 30 tone) that i have not evaluated yet. i also test higher wattage amps with a banjo center 4x10 el-cheap-o with chinese 10"s that can only be described tone-wise is bad in every aspect; it's a sacrificial and loaner cabinet. saving my pennies for a vintage hi-watt 4x12 with fane speakers.

--DL

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #87 on: January 17, 2013, 12:36:08 pm »
bright is relative - i use the JBL K120 as a test speaker - with this amp it's very bright.

Ok. I want to get that Eminence Legend 15" for this build. I've wanted to try a 15" speaker for a long time now. And I have a Tubby tone hemp cone 12" that I bet will sound great with it too.

saving my pennies for a vintage hi-watt 4x12 with fane speakers.

Those are supposed to sound great and are loud!


               Brad     :icon_biggrin:   

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #88 on: January 20, 2013, 02:28:10 pm »
bump!

any progress?   

 :icon_biggrin:

--DL

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Re: Thinking of building Gibson GA77
« Reply #89 on: January 20, 2013, 06:16:33 pm »
DL, I sent you a PM.

I talked with David at Tone Quest Report to see if he or his tech had looked at how the TS was wired. Said he didn't but didn't remember if his tech Jeff Bakos did or not. I'll try and send him an e-mail.


            Brad     :icon_biggrin:
« Last Edit: January 20, 2013, 10:59:37 pm by Willabe »

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Re: Thinking of building Gibson GA77
« Reply #90 on: January 21, 2013, 05:25:05 pm »
Well I talked with Jeff, shot in the dark, said he didn't remember about the TS wiring.  :BangHead:        Said he has only seen 2 in his life as a tech.  

He was a nice guy and was generous with his time even though I was not going to be a customer.

I have also e-mailed 1 of the 2 sellers on fleabay that have a GA-77 up for auction. Maybe he'll take a look inside and let us know how the TS is wired?

I'm trying to get my hands on an old one, but my funds are tight. I'll see what happens, might get luckey.

If I don't get it I'll use that $ to buy a Tx set for the build.


              Brad      :icon_biggrin:  
« Last Edit: January 21, 2013, 05:32:41 pm by Willabe »

Offline 6bq5

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Re: Thinking of building Gibson GA77
« Reply #91 on: January 21, 2013, 07:13:24 pm »
i've repaired/refurbished one here locally for a friend a few yrs back and the tone circuit is indeed wired like the "error" in the schematic.    rh

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Re: Thinking of building Gibson GA77
« Reply #92 on: January 21, 2013, 11:09:27 pm »
i've repaired/refurbished one here locally for a friend a few yrs back and the tone circuit is indeed wired like the "error" in the schematic.    rh

Woaw, back up the bus.     :laugh:    

From the "Gibson Amplifiers 1933-2008" book, they say they where made from 1954-1961 GA-77 (1220) and GA-70 1955- 1958 (ver. 1, 160, ver. 2, 172). In total 1552 out the door, if the records are true.

Do you remember any more?????? Did it sound good????  What speaker???? How did the tone circuit work, good, touchy????    

Does your friend still have it???? Can you get a hold of it again, maybe get the OEM transformer numbers?????  Pics of the TS wiring to put the myth to death?????

I talked with Patrick a few times at Merc. Mag. and finally figured out they do have the full iron set cloned. But he told me that the PT they got in to rebuild last Sept. (which completed the set) had no bias tap. I said it's in the schemo, he said Gibson was notorious (in there experience) about changing the amps circuit from it's scheme and not updating the changes.

With that there could be other changes that Gibson made that might not be known because they didn't update the schemo.

Now we know that DL has protoed this amps schemo with Doug's iron with more than great results, including 2 different OT's from Doug.    :blob8:

Now I do think that anything Gibson (or any other amp company) changed through the years has been more than experimented with and documented here (and other fourms) through the years.

Still it would really be great to add any versions of the original Gibson's true amps wiring circuit to our forum.
            
        Brad     :icon_biggrin:  

  

« Last Edit: January 22, 2013, 08:10:49 am by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #93 on: January 23, 2013, 08:08:10 am »
Well there was a GA-77 in bad shape up on fleabay that I bid on and lost. It ended up going for $710. +$100 shipping.   :w2:   They can have it at that price and I wish them well with it.

Said it light's up but no sound, speaker was changed to a Marsland 15", tolex was gone/replaced with some kinda black vinyl? But to me most importanly I had e-mail the seller and asked him for the OEM part #'s on the iron. He wrote back said he could only find #'s on the choke and I belive the choke was correct. But who knows if the PT and OT were correct?

I was willing to go to $400 and did. Oh well, guess I'll wait for the next buss to come along.


            Brad      :laugh:  
« Last Edit: January 23, 2013, 10:03:33 am by Willabe »

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #94 on: January 24, 2013, 01:02:08 am »
Ok, moving on. Gonna buy the iron set tomorrow, should have it next week.

So I started doing the drawings. I will need the iron first to do the layout with the chassis.

TS is per Gibsons schemo to start with. I added a 25KL mid pot.

Here's part 1;


               Brad       :icon_biggrin:  
« Last Edit: January 24, 2013, 12:25:16 pm by Willabe »

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Re: Thinking of building Gibson GA77
« Reply #95 on: January 24, 2013, 01:28:34 pm »
I drew up 3 versions of the TS.

1. From Gibsons schematic, with possible wiring error that Vox did use in their top boost TS.

2. Same as above but with added 25K mid boost control.

3. Version DL came up with. I'm gonna call it tweed style. But with added 25K mid boost control.

After seeing the TS sims that DL posted and with what he said about how the TS controlls worked after he re-did them tweed style I think it would be good to have the option to use both.

How can I add a switch to go from version 2 to version 3?


                Brad      :think1:
« Last Edit: January 24, 2013, 01:42:56 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #96 on: January 24, 2013, 02:27:51 pm »
i think this will work - you'll need a 3PDT switch.

--DL


Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #97 on: January 24, 2013, 02:43:40 pm »
Wow, thanks DL.  :icon_biggrin:    Man you drew that out fast. Looks correct to me.

I knew 3 wires had to be moved, so I should have realized that means 3 poles. And each wire had to be moved from point A to point B, so double throw.


               Brad     :icon_biggrin:   

Offline Willabe

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Re: Thinking of building Gibson GA77
« Reply #98 on: January 24, 2013, 08:34:35 pm »
I re-did the way I drew the TS. After DL drew out a version to fix the possible Giby wiring error I was able to see it's like a Fender TS. I couldn't see that with the way Giby has it drawn.


           Brad     :icon_biggrin:
« Last Edit: January 24, 2013, 08:41:48 pm by Willabe »

Offline DummyLoad

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Re: Thinking of building Gibson GA77
« Reply #99 on: January 24, 2013, 09:14:30 pm »
Wow, thanks DL.  :icon_biggrin:    Man you drew that out fast. Looks correct to me.

I knew 3 wires had to be moved, so I should have realized that means 3 poles. And each wire had to be moved from point A to point B, so double throw.


               Brad     :icon_biggrin:   

YW! slow day @ work...  :icon_biggrin:

--DL

 


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