Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on December 22, 2012, 03:16:58 pm
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I was going through my old Tone Quest Report mags. and they had a review (Dec. 2007) of a Gibson GA77 (GA70) and GA40.
I already have the iron for the GA40 laying on a shelf but I'm very interested in the GA77 from what they said about it;
"The instrument channel begins to gradually swell up at about 4 on the volume control with the type of clean distortion that allows notes and chords to bloom without being dominated by buzzy pre-amp distortion. Increasing the volume coaxes the GA77 to open up and breath with a guttural roar that is thick and saturated, yet still remarkably clear and defined. Unlike many smaller Gibson amps, the GA77 remains bright and trebly instead of turning dark and sodden with dominant midrange frequencies as the top end disappears."
"The most striking features of the GA77 are it's naturally bright, musical character, full, rich clean tones, and an exceptionally smooth, busted up, overdriven voice that is simply the stuff for hard rockin' rhythm, chunky blues and stinging solos."
"Oh, and the GA77 is a fine, fine tool for polka kings." :laugh:
This is the amp that Dick Denny of Vox copied the CF tone stack for the Vox top boost including the wiring error in Gibsons schematic. They grounded 1 side of the bass pot that should have had no connection. (I also think the on/off/standby switch is drawn wrong. Top part of switch, 1 wire from 1 of the 220K grid return R's, top + push side, should be moved to the center position for the stand by to work?)
It's rated at 25w, 5V4 rect., C/L/C pi filter feeding the OT CT, PP 5881's in grid bias, NFB loop "frequency" knob "that acts as a subtle presence control", 12AY7 2 channel pre (with standard Fender values), 2'nd channel has a second tone control, 12AX7 CF tone stack, bass/treble, 12AU7 split load PI (with standard Fender values) Jensen 15" speaker (they put a Eminence Legend 15 after the original 54/55 blew, they say it's the best 15" made today for guitar ITO.)
They said it will take a GZ34 rect. and all 12AX7's just fine if wanted.
Low power 5881/6L6GB with a single 15". Sounds like a nice little amp.
What do you guys think?
This is the "Vanguard".
http://www.el34world.com/charts/Schematics/files/gibson/GA-77.pdf (http://www.el34world.com/charts/Schematics/files/gibson/GA-77.pdf)
This is the same amp "Country and Western" no 2'nd tone control on pre.
http://www.el34world.com/charts/Schematics/files/gibson/GA-70.pdf (http://www.el34world.com/charts/Schematics/files/gibson/GA-70.pdf)
Brad :icon_biggrin:
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what, you're iron's not smoking yet? from V3a on it looks like a twin 5E8 circuit - including the NFB and i love the NFB scheme. should be much fun.
--DL
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It looks like there's an error on the GA-77 schematic at the Power/Standby switch.
Specifically, there's no difference between Standby and On; the clever standby which shorts the push and pull output tube grids together is only activated in the Off position. Seems like that should be jumpered to the Standby position as well.
Also interesting that one channel gets the "Top Boost" tone circuit, but also has a simple small-tweed type tone circuit as well. I keep looking for how both wouldn't be active but it seems to me both would be active on the one channel.
Note your 3H choke is gonna need a pretty big current rating since it handles output tube plate current, along with the rest of the amp.
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It looks like there's an error on the GA-77 schematic at the Power/Standby switch.
and the input jack on the lower channel (3rd down) is missing a wire between the 68K GL and the jack tip.
i'm thinking that replacement iron for a pro-reverb/tremolux/vibrolux would produce favorable results. for the inductor, i'd try the hammond 193L or 193N, leaning more to the 193N. lower cost alternative might be the 159T.
kick out a bit more gain with 12AX7/5751 or 12AY7 in V3 instead of the 12AU7 - other than the usual places to hack, looks like a good place to dial in some more overdrive.
--DL
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kick out a bit more gain with 12AX7/5751 or 12AY7 in V3 instead of the 12AU7 - other than the usual places to hack, looks like a good place to dial in some more overdrive.
Yeah, but that may be part of the magic TQR was trying to describe with "a type of clean distortion".
I'll tell you the Standel I built is so clean due to the 12AX7->12AT7->12AU7 (and probably the JBL) that I now hear how fuzzy Fender clean really is.
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what, you're iron's not smoking yet?
:laugh: In the middle of moving, unpacking now. I got too much stuff! Maybe I'll be able to find what I'm looking for now.
from V3a on it looks like a twin 5E8 circuit - including the NFB and i love the NFB scheme. should be much fun.
Dog gone if you ain't right.
NFB values are a little different, the Gibson has more NFB?
http://www.el34world.com/charts/Schematics/files/fender/TWIN_5E8.pdf (http://www.el34world.com/charts/Schematics/files/fender/TWIN_5E8.pdf)
It looks like there's an error on the GA-77 schematic at the Power/Standby switch.
Specifically, there's no difference between Standby and On; the clever standby which shorts the push and pull output tube grids together is only activated in the Off position. Seems like that should be jumpered to the Standby position as well.
(I also think the on/off/standby switch is drawn wrong. Top part of switch, 1 wire from 1 of the 220K grid return R's, top + push side, should be moved to the center position for the stand by to work?)
That's what I thought, thanks.
Also interesting that one channel gets the "Top Boost" tone circuit, but also has a simple small-tweed type tone circuit as well.
The GA77 has it but the GA70 does not. :dontknow:
I keep looking for how both wouldn't be active but it seems to me both would be active on the one channel.
The review said both work on the second channel. I wonder why they felt they needed a 2'nd (overall treb) tone controll on channel 2?
Note your 3H choke is gonna need a pretty big current rating since it handles output tube plate current, along with the rest of the amp.
Yes, I'll bet Merc. Mags. makes the 1 for the 5E8, that would work. I'll look at Hammonds list 1'st.
Thanks, Brad :icon_biggrin:
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kick out a bit more gain with 12AX7/5751 or 12AY7 in V3 instead of the 12AU7 - other than the usual places to hack, looks like a good place to dial in some more overdrive.
Yeah, but that may be part of the magic TQR was trying to describe with "a type of clean distortion".
I'll tell you the Standel I built is so clean due to the 12AX7->12AT7->12AU7 (and probably the JBL) that I now hear how fuzzy Fender clean really is.
agreed. JBLs are also one of my favorite line of speakers. i was just stating that would be a place to start tweaking, if he wanted to tweak.
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NFB values are a little different, the Gibson has more NFB?
i believe less.
Yes, I'll bet Merc. Mags. makes the 1 for the 5E8, that would work. I'll look at Hammonds list 1'st.
doug sells tremolux/pro/bandmaster iron - have some and used it and it sounds wonderful.
OT= 022848
PT= 022798
now all you need is a sufficiently rated inductor. happy rosin smokin' :icon_biggrin:
--DL
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It looks like there's an error on the GA-77 schematic at the Power/Standby switch.
and the input jack on the lower channel (3rd down) is missing a wire between the 68K GL and the jack tip.
There's an error in the tone stack too. Vox copied it for their top boost.
This is the amp that Dick Denny of Vox copied the CF tone stack for the Vox top boost including the wiring error in Gibsons schematic. They grounded 1 side of the bass pot that should have had no connection.
So that's 3 mistakes in the schemo drawing, do I hear 4? :laugh:
i'm thinking that replacement iron for a pro-reverb/tremolux/vibrolux would produce favorable results. for the inductor, i'd try the hammond 193L or 193N, leaning more to the 193N. lower cost alternative might be the 159T.
Ok, or maybe 1 of the tweed 5881/6L6GB OT's? Merc. Mag. has a tone clone copy of the Gibby, but big $$.
But do you guys think it's really only a 25w amp? PP 5881/6L6GB with grid bias, 417dcv on the plates/screen. Looks like plenty of small bottle signal to put out more wattage? Schemo says -45dcv for bias, on the cold side?
Do you think they limited the output with the OT's primary Z? And/or with a lot of NFB? I can't see a reason for this amp to only be 25w from the schemo?
kick out a bit more gain with 12AX7/5751 or 12AY7 in V3 instead of the 12AU7 - other than the usual places to hack, looks like a good place to dial in some more overdrive.
Yeah, but that may be part of the magic TQR was trying to describe with "a type of clean distortion".
That's a big part of why I think I will build 1. The older I get (53) the more I lean towards the cleaner side. (Main guitar is a rosewood fret board Strat and I live on the neck PUP.) I gig with a BF SR for years mostly playing blues/blues rock but it was pretty loud and there's something to be said for the warmth of a tweed amp. Still it's nice that with this amp you can change around different 12 _ _ 7's and the rect. tube for versatility.
I'll tell you the Standel I built is so clean due to the 12AX7->12AT7->12AU7 (and probably the JBL) that I now hear how fuzzy Fender clean really is.
Now that's funny. New zero line. (One mans ceiling is another mans floor.)
Thanks DL and HBP! Brad :icon_biggrin:
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Ok, or maybe 1 of the tweed 5881/6L6GB OT's? Merc. Mag. has a tone clone copy of the Gibby, but big $$.
they claim 430V B+ and at -45V on 6L6 it won't make much over 35 watts. the pro-reverb iron is rated @ 35W. if you want even less OT saturation, use the SR OT it's rated at 50W with the same primary Z.
i may breadboard one this weekend - i have all the aforementioned iron and a suitable inductor + we're off for a 1-1/2 weeks. :smiley:
--DL
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they claim 430V B+ and at -45V on 6L6 it won't make much over 35 watts. the pro-reverb iron is rated @ 35W. if you want even less OT saturation, use the SR OT it's rated at 50W with the same primary Z.
That's what I'm wondering about. Gibson only raited it at 25w? (Altho TQR said it "sounds' louder. But he's played through a LOT of amps and his ear IMO is very good at this point.)
Question is and to me this is a big question;
What did Gibson use for the OT's primary?
Did they use something different than what Fender and others used at the time for 5881/6L6Gb's to keep it cleaner sounding? Why did they rait it at only 25w when at the time every one was being quite loose with output specs?
i may breadboard one this weekend - i have all the aforementioned iron and a suitable inductor + we're off for a 1-1/2 weeks. :smiley:
That would be great DL!
You've got a great amount of breadboarding under your belt and ears to match.
I find this amp very interesting it's a Gibson but it has a lot of Fender in it at the same time. I don't know which company put out which circuit first?
Brad :icon_biggrin:
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NFB values are a little different, the Gibson has more NFB?
You can't just look at resistor values, you have to look at the speaker load.
As DL said, less NFB might be what you'd think, cause the series feedback resistor is bigger, resulting in less feedback voltage with the same signal at the speaker.
But the Gibby circuit is using an 8Ω load and the Fender has 4Ω. Assuming same output power, there's less voltage across the Fender speaker load. 35w output across 8Ω implies 16.7vRMS, and 11.8vRMS across 4Ω.
Gibson used double the series resistance, and you might think that's because the speaker impedance is doubled. But I just showed the resulting voltage at the speaker isn't double (it's x 1.414), so the feedback divider in the Gibson will result in less NFB than Fender's circuit.
PP 5881/6L6GB with grid bias, 417dcv on the plates/screen. Looks like plenty of small bottle signal to put out more wattage? Schemo says -45dcv for bias, on the cold side?
It sounds cold compared to the 5E8 Twin with 395v plate, 385v screen and -32v bias. It doesn't sound cold compared to the 5F8A Twin with 395v plate and screen and -41v bias.
I'll tell you the Standel I built is so clean due to the 12AX7->12AT7->12AU7 (and probably the JBL) that I now hear how fuzzy Fender clean really is.
Now that's funny. New zero line. (One mans ceiling is another mans floor.)
If Dave Funk was more willing to schmooze guitar mag reviewers who made technically-incorrect statements in their reviews, Thunderfunk amps might be a widely-available and sought-after boutique brand today. He did after all build some of the Trainwreck amps to Fischer's specs when Ken was too ill to build them himself (and they're still legit Trainwreck amps).
I had Dave work on one of my amps when I and he lived in Nashville, and when I was first trying to learn anything about tube amps. Thunderfunk amps were used by some of the heavyweight session and touring guys there. Mid-late 90's country had plenty of places with ultra-clean guitar parts, and the amps were prized for maintaining clarity and cut even at high volumes (though they could get distortion tones too).
But when Dave's amp was reviewed in one of the mags, the writer seemed to want what other boutique makers were doing then, which was cloning tweed Fender amps (blackface amps weren't ridiculously expensive like they are now). The writer said, "the clean sounds are Fendery-enough" but pretty much only had something positive to say about the reverb sound (the "12 Spring" reverb) which was acknowledged as vastly superior to other amps.
Well, Dave's articles in Vintage Guitar at the time often ripped on the inaccuracies of their other columnist, Gerald Weber, as he also did in his book (although in a veiled way). Dave wrote in a letter the following issue that said the statement about the cleans being 'Fendery enough' "... proves you're not qualified to do listening tests."
Anyway, his amps could do clear and clean even with complex chords, in the same way some pedal steel sounds are incredibly clean. It seems informative to point out the Standel 25L15 was probably first intended as a steel amp (that's who Chet bought his Standel from, a steel player).
Anyway, it seems like Dave just wasn't into selling people on an idea or hype. He was more into the technical side and removing mystery, and wasn't afraid to call people on their B.S. It's unfortunate, but it seems his distaste for marketing prevented technically superior amps from becoming sought-after.
That said, "fuzzy clean" is a sound I fell in love with in my first tube amp (a '67 Princeton Reverb), and set it apart from a good solid state amp.
Sorry for the thread hijack.
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The 1956 Gibson catalog (http://www.preservationsound.com/wp-content/uploads/2012/02/Gibson_Elec_1956_cat.pdf) claims the GA-70 Country and Western has an "output of 25 watts with reserve up to 35 watts to cover peaks."
Anyway, you build the GA-70 and you'll be all set to play at Bob's Country Bunker.
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[Sorry for the thread hijack.
:hijack1:
Are you kidding me?
Sing your song brother, tell your story brother!
Anyway, you build the GA-70 and you'll be all set to play at Bob's Country Bunker.
"Where gonna do a favorite of the band right now, Stand by your Man".
Brad :l2:
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You can't just look at resistor values, you have to look at the speaker load.
Yes but, the 5E8 has a 5K (much more bleed?) presence pot to ground and the GA77 has a 50K (much less bleed?) pot to ground?
Brad :think1:
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[Sorry for the thread hijack.
:hijack1:
Are you kidding me?
Sing your song brother, tell your story brother!
Anyway, you build the GA-70 and you'll be all set to play at Bob's Country Bunker.
"Where gonna do a favorite of the band right now, Stand by your Man".
Brad :l2:
Stand By Your Man -Blues Brothers (http://www.youtube.com/watch?v=Psm96Dn9KII#) this version?
:icon_biggrin:
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Yes!
Brad :laugh:
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drafted an editable schematic.
--DL
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The 1956 Gibson catalog (http://www.preservationsound.com/wp-content/uploads/2012/02/Gibson_Elec_1956_cat.pdf) claims the GA-70 Country and Western has an "output of 25 watts with reserve up to 35 watts to cover peaks."
Now that seems more like it. 35w amp.
Brad :icon_biggrin:
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My D'Mars is cathode biased using 5881's and is around 23 watts with 337v on plates. It can get LOUD & benefits from the PPIMV and
D'Lator.
with respect, Tubenit
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DL,
Can you please post the editable SCH version of the GA-77?
Thanks, Tubenit
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My D'Mars is cathode biased using 5881's and is around 23 watts with 337v on plates. It can get LOUD & benefits from the PPIMV and D'Lator.
Thanks tubenit, I seemed to remember you and Geezer having a 23/25w build or 2 with 5881/6L6GB, lower plate dcv and cathode bias.
One of the main reasons I was TQR said it was only 25w with 5881/6L6GB. I already have 2 amps with 6V6. I thought maybe Gibson used an OT with an odd primary Z and that's how they droped the output wattage. Oh well, I'm still gonna build it.
I might put the PPIMV in because 35w is too much for me these days at home. My 5E3 and 5G9 are too loud at 15/17w.
Brad :laugh:
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Man tubenit you and DL are fast at making a drawing! :laugh:
I don't know about the layout but there's a couple of parts values that need to be changed from DL's drawing.
1. Cap to ground on channel 2's treble cut tone control is .005.
2. Cap to ground on presence pot is .1.
Brad :icon_biggrin:
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Fixed.
Brad :icon_biggrin:
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Brad,
THANKS for reviewing and the edit! I attached the revised SCH to your post.
Now .................. if DL will post his SCH version ..................... :dontknow: :icon_biggrin:
With respect, Tubenit
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THANKS for reviewing and the edit! I attached the revised SCH to your post.
No thank you! :laugh:
I'll look at the layout latter this morning (?) when I get some time.
Now .................. if DL will post his SCH version ..................... :dontknow: :icon_biggrin:
He will, DL's got the next week and a half off. He said he was going to breadboard it. It will be great to hear how he thinks the amp sounds.
Thanks Brad :icon_biggrin:
Edit; Spell check changed DL to Dj, fixed it.
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brad & jeff, here are the edits - had a couple of part values wrong and node connection dots were missing.
--DL
THANKS DL! Appreciate your help and contribution.
with respect, Tubenit
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OK, IF .............. I am understanding DL's schematic ........... it appears that the 5881's are hardwired in triode mode?
I would install a pentode/triode switch if that were the case.
I also would add more filtering and PPIMV.
Original or with the mods .......... either way, I think this could be a really super nice amp!
With respect, Tubenit
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OK, IF .............. I am understanding DL's schematic ........... it appears that the 5881's are hardwired in triode mode?
No their pentode, just Gibson didn't use screen grid R's and fed the screens from the same B+ node as the plates. Just like a Champ did, but I think the GA-77 should have at least seperate screen R's. (I was planning on a seperate B+ node with a screen R of at least 470 up to 1K for each screen.
But a triode/pentode swich might be nice for some of the guys if they like/need it? Very nice Tubnit.
DL, Gibson schemo lists 5V4 for rect. tube, you have 5U4.
Brad :icon_biggrin:
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fed the screens from the same B+ node as the plates
Unless I am missing something (& maybe I am?) ........................ based on the GA-77 (Gibson schematic) & DL's schematic ........
I think "triode" in a pentode/triode switch has the screens fed by the same node as the plate?
Perhaps the OT between the node A and the screens connected to the plate wires make a difference? Maybe Sluckey or someone who would know can weigh in on that?
With respect, Tubenit
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Perhaps the OT between the node A and the screens connected to the plate wires make a difference?
That's the way I understand it and the screen should still have a screen grid R in triode.
Brad :icon_biggrin:
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DL's drawing simply shows the screen supply and plate supply are the same B+ node. A lot of cheap amps used this design. But that does not mean the tubes are hard wired for triode mode. Triode mode means the screen is connected to the plate, either directly or sometimes thru a resistor. There is a difference.
And you don't have to use the same B+ node for screens and plates when using a triode/pentode switch either. Your last drawing does not specify whether or not the same B+ node is used. Here's a drawing that uses separate nodes.
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Sluckey beat me to it.
Brad :icon_biggrin:
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fed the screens from the same B+ node as the plates
I think "triode" in a pentode/triode switch has the screens fed by the same node as the plate?
Where do the screens connect to the OT primary?
Think of it this way:
When in triode mode, the screens must connect to the plate, so they are connected to the same point on the OT as the plates (in push-pull, that's either end of the primary).
In pentode mode, the screens are not directly connected to the plate, so they might be at a B+ node. Or, if the plates and screens share a B+ node, the plates connect to the ends of the primary while the screens connect to the CT. That puts them at a different point on the OT primary.
Further, since ultralinear is a mode in-between pentode and triode, the screens are conencted to some mid-point between the OT primary CT and the ends of the winding. Different tapping points (20%, 40%, etc) might yield slightly different results.
EDIT: Yet another way (maybe the most important) to think about pentode/triode mode:
In a pentode, the screen is held at some constant voltage. The plate may bounce up and down during a signal, but the screen should be nearly-constant.
That's because the screen voltage impacts the plate current; it's the steadiness of the screen voltage (and the resulting plate current) that gives pentode plate curves their shape (as different from triode curves).
If screen voltage varies while the plate voltage (or current) is varying, then it will impact the plate current. Connect the screen to the plate (through a resistor or not), and as the plate voltage drops with increased plate current, the screen voltage will drop and reduce the plate current. This might sound like a paradox, but really what you wind up with is triode operation (in triodes, plate current also drops when plate voltage drops).
So ask yourself if the screen voltage will change at all (connected to a B+ node, probably not). If yes (say, in UL or with a sizeable screen resistor), ask how much? If not too much (compared with direct connection to the plate), then it is probably not triode mode, but possibly an in-between state like UL. Note, as screen resistor while the tube is in triode mode still only sees screen current, and has a voltage drop to match.
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switch to triode mode through 100ohm resistors?
brad & jeff, my thoughts are that replication of the tone of this amp would closest to that of the original if we were to replicate the circuit as close the the original as possible. thoughts?
:icon_biggrin:
--pete
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my thoughts are that replication of the tone of this amp would closest to that of the original if we were to replicate the circuit as close the the original as possible.
Yes I agree. I'd like to see what it sounds like as it was.
But, with that being said Tubenit as always is looking to help make any design more accessible to more builders? And that's a good thing.
We can add different ideas latter?
Brad :icon_biggrin:
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Unless I am missing something
Well, I was waaayyy off on that one and stand corrected. Thanks for the clarity and explanations.
And building it original first makes sense to me with the exception that I would add the "D" node which functionally should not sound any different except possible less hum. I like really quiet amps.
I will look forward to seeing the build pics.
With respect, Tubenit
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brad & TN, do you guys like the input jack in the left (fender) or right side (marshall) i have a preliminary chassis layout using fender tremolux/pro-reverb iron. nice fitment on a 17.5" x 8" x 2.5" unnamed-tube-store blank .09" thick aluminum chassis. it may fit into 5E8 combo style chassis by weber. it would be nice to have the chassis without panel engraving.
have the PS built and filaments strapped on BB. should be lighting it up on friday at the latest.
--DL
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I'm a Fender guy, like the jacks on the left. I can't read right to left nor can I drive on the left side of the road. :icon_biggrin:
I have 5 nice chassis here of a few different sizes in aluminum. I'm not crazy about tweed chassis anymore myself.
have the PS built and filaments strapped on BB. should be lighting it up on friday at the latest.
Very cool DL can't wait to hear what you think about how the amp sounds/handles.
Remember the bass pot in Gibsons drawing is wrong. It was not wired like that on the amp. I think the bass pot is supposed to stand on top of the 10K R, like a normal Fender TS.
Any body know where we can find a 2ML pot for channle 2's treble cut controll?
And building it original first makes sense to me with the exception that I would add the "D" node which functionally should not sound any different except possible less hum. I like really quiet amps.
I agree with tubenit, that amp was under filtered to me. I think it's a good idea to add another B+ node or 2. I was planning on it.
Brad
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http://www.tubesandmore.com/products/R-V38-2ML (http://www.tubesandmore.com/products/R-V38-2ML)
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Thanks Sluckey! That'll work and the price in nice too.
Brad :icon_biggrin:
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brad & TN, do you guys like the input jack in the left (fender) or right side (marshall) i have a preliminary chassis layout using fender tremolux/pro-reverb iron. nice fitment on a 17.5" x 8" x 2.5" unnamed-tube-store blank .09" thick aluminum chassis. it may fit into 5E8 combo style chassis by weber. it would be nice to have the chassis without panel engraving.
have the PS built and filaments strapped on BB. should be lighting it up on friday at the latest.
--DL
Nice drawing, what program did you use to create it, are the common components pre-made?
TIA,
Jaz
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brad & TN, do you guys like the input jack in the left (fender) or right side (marshall) i have a preliminary chassis layout using fender tremolux/pro-reverb iron. nice fitment on a 17.5" x 8" x 2.5" unnamed-tube-store blank .09" thick aluminum chassis. it may fit into 5E8 combo style chassis by weber. it would be nice to have the chassis without panel engraving.
have the PS built and filaments strapped on BB. should be lighting it up on friday at the latest.
--DL
Nice drawing, what program did you use to create it, are the common components pre-made?
TIA,
Jaz
Visio 2010. i prefer Visio 2007; if you can get an unregistered legit copy of 2007, that's what i'd use. stencils of components were created by many including, BNWITT and SLUCKY who are 2 major contributors to the stencil libraries. the sockets, can caps, transformers, and switches are from my stencil library. the octal sockets are modern belton pattern and the 9-pin sockets are cinch/eby pattern.
--DL
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brad & TN, some bad news: :help:
1) don't have a working 2M pot. have one i salvaged from a PA i gutted, but it's reading over 4meg. i'll build the GA-70 and leave room for the 2ML pot when it arrives next week.
2) don't have a 500V 47u for the second filter - i'm using a 30uF until it arrives next week.
3) the choke is not the one i'd like to use. i spec'd a 5H300mA - the hammond 193L; thought i had one, i don't; it's on order too.
4) i built a pseudo fender bias PS - will tweak the divider values to meet the needs of the 6L6 bias at B+ we end up with. i may use a 5AR4 if needed to get the B+ up to 430V as called for in the original plan.
still on track though... :icon_biggrin:
--DL
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Well that's a drag, never enough parts.
Sounds like you've made good progress though.
Brad :icon_biggrin:
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Anyway, you build the GA-70 and you'll be all set to play at Bob's Country Bunker.
"Where gonna do a favorite of the band right now, Stand by your Man".
I meant you'd be all set to play both kinds of music: Country and Western! :icon_biggrin:
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Oh yeah they got both kinds of music there, Country and Western. :l3:
Hey come on you should join the band HBP. You got that Standel with the light up face control panel. You can play lead.
We just need a good name for the band. Hmmm.....
Brad :icon_biggrin:
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done! all strapped in... now to go get a late dinner then fire it up.
http://s2.beta.photobucket.com/user/pmitchel/library/Gibson-GA-7x (http://s2.beta.photobucket.com/user/pmitchel/library/Gibson-GA-7x)
power tube coupling caps are 47nF - not 22nF as specified - i'm out of 22nF!! sheesh! :BangHead:
--DL
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done! all strapped in... now to go get a late dinner then fire it up.
Man are you fast with that breadboard. :laugh:
power tube coupling caps are 47nF - not 22nF as specified - i'm out of 22nF!! sheesh! :BangHead:
They'll be fine.
Brad :icon_biggrin:
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They'll be fine.
i kno... :wink:
fired it up and light hum but no guitar - forgot the link from summing resistors to v2a g1. plan on at least one wiring error, eh?... :dontknow:
some telemetry taken and schema updated and attached in PDF. 7 .SCH formats.
i'm using a pair of RCA 6L6GC black plates that are matched within 1mA for V4 & V5, V1 is a GE 12AY7, V2 is a RCA 12AX7, and V3 is Raytheon 12AU7. all the iron is hoffam iron as stated in the schema, the choke is hammong 193H - i'm going to suggest we leave it as is - even on the breadboard the PS is quiet - very quiet. the only PS mod was the added filter for the power tube screens and the bias supply. i'll fiddle with the bias supply tomorrow afternoon if time permits - i'd like to get the idle plate dissipation up on the power tubes to 16W - currently they're running at about 13W.
this thing sounds really nice - i'm impressed, but i couldn't take it all the way up - buttery is crashed and has to work tamalli. on the breadboard i bonded both input channels and turning up the second channel fattened up the tone and growl considerably - should make for some really nice blues grit/crunch. playing clean at low to moderate volumes is still bright as you'd like and bass is still strong.
i'll see if can't acquire the services of a respectable guitar player and record some clips. this amp has a very nice chime and good bottom end even with the 35W fender pro/tremolux output iron. my only complaint so far is that the bass control is kind of weak, however, the treble controls works well.
after i get the circuit sorted and as close to the original as possible, my first mod will likely be a cross-line MV and give a 12DW7 a spin in the V3 position - the lower mu triode will be the concertina.
--DL
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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
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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
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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
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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
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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
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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.
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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:
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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:
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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
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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:
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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.
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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.
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Both of those drawings put dc on all the pots and into the grid of V3A.
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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
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Ok thanks Sluckey.
I think I figured it out. How about this one?
Brad :think1:
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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:
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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).
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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:
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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
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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 (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:
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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
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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:
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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.
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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
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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:
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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:
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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
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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:
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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:
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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
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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:
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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:
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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 (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:
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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 (http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0)
Brad :dontknow:
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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
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A JBL K120, aren't those a very bright sounding speaker compared to an old Jensen?
Brad :think1:
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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
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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:
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bump!
any progress?
:icon_biggrin:
--DL
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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:
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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:
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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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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:
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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:
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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:
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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:
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i think this will work - you'll need a 3PDT switch.
--DL
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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:
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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:
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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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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 :icon_biggrin:
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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 :icon_biggrin:
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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 :icon_biggrin:
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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. :BangHead: :cussing:
Should have them by Fri. or Sat.
Brad :w2:
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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.
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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 :icon_biggrin:
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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
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I bought it to build with.
And sure I'll post the measurements you want when I get them. Maybe today. :blob8:
Did you ever get that 2M pot?
Brad :icon_biggrin:
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I bought it to build with.
And sure I'll post the measurements you want when I get them. Maybe today. :blob8:
Did you ever get that 2M pot?
Brad :icon_biggrin:
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
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i'll get the tone network installed on the breadboard amp and report back tonight or tomorrow morning.
Cool.
Brad :icon_biggrin:
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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. :BangHead:
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 :icon_biggrin:
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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
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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 :icon_biggrin:
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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
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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 :think1:
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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 :think1:
i'm a dumbass. i missed transposing the numbers. 29.8sq * 8
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Close enough, probable put's it closer to 6.6K?
Just thought I missed something with the math.
Brad :icon_biggrin:
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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 :icon_biggrin:
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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 :icon_biggrin:
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
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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 :icon_biggrin:
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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
(https://www.circuitlab.com/circuit/4tdnuk/screenshot/540x405/) (https://www.circuitlab.com/circuit/4tdnuk/ga-77-ts/)
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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 :icon_biggrin:
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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 :think1:
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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...
:icon_biggrin:
regards,
--pete
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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 :blob8: and report what I encounter. :laugh:
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 :icon_biggrin:
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You've traveled down the road far enough not to be swayed by the run of the mill.
i'm extremely hard headed. :BangHead:
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
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bump!
any progress brad?
--pete
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Yes. :icon_biggrin:
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 :icon_biggrin:
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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 :icon_biggrin:
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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 :icon_biggrin:
that would be a nice cabinet for this amp. if you would, please let us know what kind $$ they'd want for a duplicate.
keep smokin' that rosin...
--pete
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that would be a nice cabinet for this amp. if you would, please let us know what kind $$ 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. :dontknow:
Brad :think1:
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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
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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 :icon_biggrin:
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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... :icon_biggrin:
but then again... 1 x 12" + 2 x 10" might be nice... or, 3 x 10".... or... :BangHead: :icon_biggrin:
--pete
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but then again... 1 x 12" + 2 x 10" might be nice... or, 3 x 10".... or... :BangHead: :icon_biggrin:
Will you stop already? I'm confused enough as it is!!!!!
Brad :l2:
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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.
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Yeah Ed, that's an option too. Plus then I would have the tubes/chassis right side up.
Brad :think1:
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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. :dontknow:
Second drawing shows the iron set placment.
If anybody sees any mistakes please let me know.
Brad :icon_biggrin:
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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. :BangHead: All it says is it's 100% polyurethane. They show a couch covered with it.
I really like this pattern and color.
Brad :icon_biggrin:
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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 :icon_biggrin:
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reads like a nice finished product. please keep us posted.
--pete
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Update;
Well I just got back from my friends house. :happy1:
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 :icon_biggrin:
Edit; They look bad because I cropped them in Windows 7 paint.
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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 (http://www.el34world.com/Forum/index.php?topic=10681.msg148641#msg148641)
Brad :icon_biggrin:
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Face/back plate update;
http://www.el34world.com/Forum/index.php?topic=10681.msg148666#msg148666 (http://www.el34world.com/Forum/index.php?topic=10681.msg148666#msg148666)
Brad :icon_biggrin:
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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.
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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.
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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.