'54 Gibson LP GA-40 Tremolo

[Jack_Hester]

This is my first attempt to repair a vintage guitar amp.  

Back in the Spring ('11), we had a 145 day outage on our Unit 2 (power plant), so long hours with occasional night shift was my life.  Work wasn't bad, just long.  Picked up this amp as a non-working project, very cheap.  It's been stored away since Spring.  Rainy day today (12Oct), and I have it off.  So, all morning was spent cranking out some long overdue bike work.  This afternoon, I decide to go get the amp from the house and begin the process of cleanup and troubleshooting.  

Cab needed a handle, as none was there.  Very obvious that it has had many handles, by the many holes.  I removed the back panel and lay the amp down on it's face.  Cab is in very good condition, so cleaning and new handle are all I intended for it.  Observed that one speaker wire is broken in the middle, between amp and speaker.  The other is almost broken.  I install quick disconnects on both.  Removed tubes and lay them out in the order that they go on the amp.  Carefully removed the amp chassis and lay it aside.  Next the speaker, 2204?? (I forget the week numbers), and lay it aside.  Thoroughly dusted and cleaned the cabinet, and same for the speaker.  Installed a new leather handle with machine screws, by splitting a large fender washer, marking, drilling and tapping to 4-40 threads, each washer half giving a new surface inside the cab to attach the handle.  Looks nice, though a bit longer than original.  Reinstalled the speaker.

[Jack_Hester]

Next I cleaned the inside of the chassis with compressed air, though it has had some amount of cleaning in it's past as the caps are not original to the amp.  Tightened the OT, as it was loose.  The power cord was unsafe and I removed it.  I have many computer cords from work, as one of our departments seem to acquire a large number of them, and throw them out when they run out of storage.  They throw them my way.  I cut the end off of one of these, and it slipped nicely into the grommeted hole in the chassis.  Wrapped two large cable ties around it and clipped the ends.  This secured the cord inside the chassis.  Soldered the hot to the switch and the neutral to the fuse.  Lugged the ground wire and secured it to one of the PT 10-32 mounting screws with a nut.  The tremolo cord was not in bad condition, so I left it alone.  I dressed up the wiring a bit by moving it all so that very few touched each other.  One had been pinched for many years, but was intact.  Made sure it was not damaged and resting away from the pinch point.

Reinstalled the chassis and reconnected the speaker wires.  Got out my little Eico Tube Tester and warmed it up.  Began the process of testing the tubes.  I've included a picture of the Tremolo schematic for this amp, with tube layout.  V6 had a ground in two positions on the tester, so I put it aside and dug out another 6V6 from my stash.  As I tested each, I installed them in the chassis.  V3 had several grounds, so it was put aside and another installed.  The rest tested good.  Amp was now ready for power.

[Jack_Hester]

Hooked up the lamp limiter and began with a 25W bulb.  Glowed and went dim.  As I had other things to keep me occupied, I left it alone for awhile.  I don't know how long is appropriate.  I may have left it for 20-30 minutes.  Swapped it out with a 40W and left it.  Did the same with a 60W, and later a 100W.  Finally, I plugged it straight up to full line voltage.  Listened and looked.  No smoke or smell.  Turned the Instrument volume full up and did it make an awful squeal and feedback.  Turned the volume back up slowly and it was clear with some ringing.  Tapped the chassis and it howled.  Powered down and replaced V2.  Powered it back up and waited a minute.  Turned the volume full up and amp was quiet with almost no hum.  Unbelievably quiet.  Turned it down and turned the Microphone volume full up.  Quiet with a slight ringing (not sure if that's the correct word for what I heard).  Tapped the chassis and got a louder ringing.  Turned it off and replaced.  Back on and same happened again.  Neither time was the ringing loud.  More like background.  So, I'm thinking the first V1 was good and I need to check elsewhere.  Left that for now.

Got out the cheap guitar that I store in the shop, for testing the other amp tinkering.  Connected and strummed a few chords through each of the Instrument inputs, and the Microphone input.  What a clear, clean amp, and the 'voicing' pot gave a very nice range of tone.  And, loud at full volume.  This is one really nice 'non-working' amp project.

Now, that I've been so wordy with the first part, I'll be brief with my question.  The tremolo does not work at all.  It plumb slipped my mind to jumper the connections in the chassis, while I had the back off.  Circuit must be completed with the switch, to tie the cathode of V7 to the cathodes of V5 and V6.  Have not checked the switch to see if it is working.  Sorry.  Should have before asking for help.  I'll do that tomorrow afternoon.  Assuming that it is good, I need some guidance as to troubleshooting, and where to start.  It looks like the tremolo circuit wiggles the screen grids of V5, V6, and V7.  And, V7 gets some amount of signal from V4b to it's control grid.  I don't have a scope, just a DVM and VTVM.

So, that's where I am.  Pleased that it is such a fine amp, and I really want to complete the task by having a fine working tremolo.  I am fortunate to have a little more leisure time to spend on it this week.  Then, troubleshooting will be brief moments at a time.  Thanks for any and all help.

Jack

Note:  Had to break my post down to multi-sections, as it exceeded the character limit as one post.

[Jack_Hester]

Tubenit -

Saw your reply as 10thtx about creating a schematic.  I got the email notice, but it never showed up here.  Anyway, I found a schematic by you, for a later GA-40.  No selector switch on yours, for the tremolo, like this earlier one has.  Thanks for responding.



Went out to the shop real early this morning, with a Gibson schematic in hand, and a blowup drawing of the tremolo.  Disconnected the switch wiring, when I discovered that it was wired wrong.  Hand-traced the tremolo cathode-side wiring and found it wired wrong.  Whoever did the cap job, in the past, did not follow the schematic.  The two 470K resistors (R25,R26) were landed in the wrong place.  The 20uf cap (C15) was missing.  The 1K resistor (R30) was landed in the wrong place.  Took me a bit to figure out the grounding on the tubes.  Gibson used pin 1 of the octals as a ground junction.  

So, I landed R25 & R26 in their correct spots.  Connected R30 to the cathode of V7 and left the other leg swinging, while I searched for a cap.  No 20uf caps on hand, but I had two small 10uf's.  Paralleled them and connected ends between the junction of R25,R26 and the swinging lead of R30.  Connected the switch between the cathode of V7 and the already joined cathodes of V5 & V6.  Checked the switch to verify that it worked.

Brought the amp up on the lamp limiter, as before.  No smoke or smell.  Reconnected it to full line voltage and let it simmer.  Turned the volume up on the Instrument channel and noticed hiss that I didn't notice before, plus the hum was a bit louder though still nice and low.  Same for the Microphone channel, except hiss was not there like on the Instrument.  Operated the tremolo switch and immediately heard the oscillation. Got the guitar and connected, and am much pleased with the very nice tremolo.  Very pronounced (to me, that is) difference between the three positions.  

Side note: Tubes V1 and V2 both ring when thumped.  Is this considered microphonic?  I tried several, staying away from the original bad one from V2.  No noticeable ringing if I don't move or thump the amp.  I'm thinking that they may be a problem at full volume, while amp is being used for playing.  Will deal with that later.  Most of the hiss disappears when the tremolo is on.

Another side note: Both channels are obviously louder than before, as they break up above about 3/4 on the Instrument and somewhere above half on the Microphone.

SO, when time allows, I'll hand-trace the the entire amp and do more cleanup and corrections as needed.  Time to find someone local, and give it a workout.  I may take it to Church, Sunday, and let my friend Don take it home.  Will post the results.

Jack

[tubenit]

Yeah, I was trying to help out & posted the SCH schematic and then realized it was not the same amp. So I deleted the post because I was concerned I throw things off course with it. The GA-40 I drew is in the SCH library.

With respect, Tubenit

[overtone]

Thanks for the post. I think I have the same model GA-40. 1954 with a '56 speaker and a tool box handle.
These pictures are from last fall, just after I got it from Wales.
Put own tubes in and it sounded good, but break-up was at volume level 2. I went inside in December and traced a bad 6,3V winding. The filaments run on their own transformer now. Managed to keep the original PT. I replaced some resistors that had drifted up x5 their value.
I still have not got the tremolo to work right, it is only very faint.
To my mind my recorded Voltages on the pre-amp look to be way, way too low, but she sounds good. When I get some time I will try to sort the tremolo and take another look at the preamp voltages.
Best, tony

[Jack_Hester]

Tony -

I saved your voltage chart to a folder that I have GA-40 LP info stored in.  I noticed that your line voltage is 109vAC.  Mine is typically 119-120vAC, so when time permits, I'll record the same voltage points from this one and make a chart.  Thanks for something to compare this one to.

Jack

[overtone]

Jack, I am on 230V here, so I was trying several drop-down transformers.
I have a chart with a 128V mains supply which will give you another reference point.
Treat it with caution, if I remember correctly it was way to hot (the Rk on mine is 140R.) and I didn't like the 5.5V on the rectifier.
I made a chart for each variation as I went. In this one below, it seems that I tested 100R screen grid resistors.
The other charts are in the same ball park, except for the preamp plate voltages when I first got it: V1=53V V2=117,5V, V3=133
It would be great to know how yours are. Pretty sure my preamp voltages are now too low and would like to go back inside later in winter.
Best, tony

[Jack_Hester]

Tony -

A bit off topic.  I lived on a U.S. Army Post (Camp Pieri), on a ridge above the city of Wiesbaden, for 18 months.  That was in '73-'74.  So, I visited Frankfurt quite a few times.  Loved the train transportation in Germany.  Wish we were that smart to have such, here in the U.S.. 

Jack

[Jack_Hester]

As part of my wiring/component verification of this amp, I need to have a clear schematic.  The best that I've been able to find are very poor scans.  I'm grateful that I have them to work from.  Little by little, I've been re-creating this schematic and have my first draft finally finished.  I've gone over it several times, and cross-referenced the components between the circuit and the component list.  I think that I've gone as far as I can, and find that the values for a couple components are odd.  But, not knowing standard values, I wrote them down as accurately as I could.  I'm not done with it, but what I have will surely help me verify the wiring in the amp that I have.  I'll pay close attention to each component, and update the drawing with what I find.

Jack

Note: 16Oct11 - Updated drawing to add wire/component junctions, and to make corrections to B+ connections on V7.
        17Oct11 - Created larger signal wire jumpers across caps C2(V1) & C5(V2).  Easier to see on a printout.
        18Oct11 - Corrected wiring on amp and drawing, to reflect the junctions described by Overtone and SLuckey.  Also, increased the size of the jumpers on the drawing.
        19Oct11 - Increased the size of the component I.D.'s.
        20Oct11 - Made corrections to values of R27 and R29 in the listing, as R29 was shown twice, with no R27.  Also, corrected wiring for R27 to Choke L2.  Thanks to PRR for seeing them.

[Willabe]

Hi Jack,

I have the book that's out on Gibson amps and it came with a disc with the entire Gibson service manual but it's not in there.

A friend of mine told me he heard that Gibson had a flood of some type at one time and lost some of their schematics because of it.      


            Sorry,   Brad    

[Jack_Hester]

Hi Jack,

I have the book that's out on Gibson amps and it came with a disc with the entire Gibson service manual but it's not in there.

A friend of mine told me he heard that Gibson had a flood of some type at one time and lost some of their schematics because of it.      


            Sorry,   Brad    

Thanks for looking, Brad.  I may be ok with the drawing that I made, once I identify the couple of questionable components.  I don't remember which ones they were, right off hand.  But, the values should be the give away for those who know what typical values are for these old amps. 

I'm interested in the book that you have.  Is it something still in print?  Please let me know the name.  Thanks.

Jack

[Willabe]

I'm interested in the book that you have.  Is it something still in print?  Please let me know the name.

Gibson Amplifers 1933 - 2008, 75 Years of the Golden Tone, By Wallace Marx Jr.

I thought I bought it from Antique Electronic Supply, but I don't see it on their web site.

On the back of the book is this;    Blue Book Publications, Inc.     www.bluebookinc.com       1-800-877-4867

It's 192 pages (with 86-91 in color) comes with disc of (PDF) schemo's. I like it, have'nt seen any thing else out there just on Gibby amps.

Hope this help's.


                     Brad        

Found it!      www.elderly.com        

It's not in books, you have to do a search there, go into books, then search for amplifer history. You'ld think they would just put it in with books?            

[Jack_Hester]

Thanks, man.  Found it on Amazon immediately, just by copying and pasting the info you provided.  Got my copy on the way.  You're the best!  Have a good one.

Jack

[Willabe]

Hey Jack, let me/us know what you think after you get it in your hands. Like I said I'm happy with it, good info in it.

    
               Brad          

[overtone]

Hi Jack,
probably there are kids playing on the old parade grounds of the base in Wiesbaden these days. I think most of that site was converted into to residential use.
I took a look at your schematic and wonder if the grid leak resistors are shown correctly? Shouldn't R25 & R26 be connected to ground? The old schematic scribble is not easy to read and I don't have the time right now to crack mine open and check. Please correct me if I am wrong, also I think the Ck C15 should be in parallel to the Rk R28.
Which values did you think were odd?
Mine still has the waxed caps in the tone section and when I drew the preamp I did not have a half decent cap tester to I guessed them. Maybe this weekend I can check their values to cross check with yours.
Best, tony

[Jack_Hester]

I took a look at your schematic and wonder if the grid leak resistors are shown correctly? Shouldn't R25 & R26 be connected to ground? The old schematic scribble is not easy to read and I don't have the time right now to crack mine open and check. Please correct me if I am wrong, also I think the Ck C15 should be in parallel to the Rk R28.

Which values did you think were odd?

Mine still has the waxed caps in the tone section and when I drew the preamp I did not have a half decent cap tester to I guessed them. Maybe this weekend I can check their values to cross check with yours.

The best that I can tell from the schematic, I see no connection to ground for R25 & R26.  And, for C15, I can only see it in series with R30, with C15 being tied between R25 & R26, and R30 tied directly to the lead going between the footswitch and the cathode of V7.  I see R28 connected between ground and a junction of the other side of the footswitch and the cathodes of V5/V6.  No cap paralleled with it. 

At present, I have it wired like I described and the tremolo began to work very nicely.  I sure wish I had another means to check it.  Your time sounds like mine.  We catch things on the fly.  Whenever you go back into yours, please check yours against the original drawing, and mine.  I know that I will have my hands full, hand-tracing to verify the rest of the amp.  Maybe a bit this upcoming Saturday (22?Oct).  And then, only a bit at a time.  Much work to get done in the shop.  This will be my break from the other projects.  I'll post all that I find, as I check. 

R37 & R38 were odd only because I couldn't read them or cross reference them with the listing to make them out.  So, I put a value as best as I could read.

As a side note:  I was looking at the 'voicing' circuit on the drawing, and it looks just like the one that some are calling 'Big Muff'.  Haven't compared component values, though.  Anyway, I'll post more later.  Have a good one.

Jack

[sluckey]

The best that I can tell from the schematic, I see no connection to ground for R25 & R26.  And, for C15, I can only see it in series with R30, with C15 being tied between R25 & R26, and R30 tied directly to the lead going between the footswitch and the cathode of V7.  I see R28 connected between ground and a junction of the other side of the footswitch and the cathodes of V5/V6.  No cap paralleled with it.

Overtone is correct about the grid resistors and the cathode cap. Look at this crop of the original schematic and once you realize the 'dots' are connections, it'll make sense.

[Jack_Hester]

Overtone is correct about the grid resistors and the cathode cap. Look at this crop of the original schematic and once you realize the 'dots' are connections, it'll make sense.

The both of you really have my curiosity up, now.  If time permits, tomorrow evening, I'm going in with jumpers.  I'll jumper my C15/R30 junction to the cathodes of V5/V6.  Then, I'll jumper the junctions of R25/R26 to to where R28 is grounded.  That should get me to the point of being wired like you have described.  A buddy is coming over, anyway, to show me his new Les Paul guitar.  Hopefully, I'll be done and ready for a test.  I'll post the results as soon as I can.  Thanks for you guy taking a closer look.  I need the additional eyes on this.  Back soon.  Have a good one.

Jack

[Jack_Hester]

Made a copy of the drawing with junction dots marked, and headed for the shop, late this afternoon.  Rather than using temporary alligator jumpers, I rewired to reflect your suggested connections.  Gave it a good once over to be sure that all was in place, including my paralleled 10µf caps, until I can get my hands on a 20µf.  Then again, I may just leave them.  Fired the amp up and switched on the Tremolo, with the volumes down.  I immediately heard the quiet thumping of the Tremolo.  Switched it through each position, and noted the change in speed.  Got out the guitar and plugged it into the Instrument channel.  Volume set at about halfway, and the Tremolo was much more pronounced (would this be considered Depth?).  Sounded really good.  Still some hiss in the Instrument channel, though not present in the Microphone channel.  The hiss was there the last time, but maybe a bit quieter.  I'll troubleshoot that when I give the amp a thorough tracing to compare against the rest of the schematic.  I probably won't rush to do that as the amp needs much usage, now, after an unknown time of neglect (from it's previous life).  

Thanks a bunch, guys, for taking the time to follow my progress (or maybe lack of), so that I can get this fine amp in good working order.  My drawing above now reflects the wiring corrections, plus I closed ranks a bit, in the pre-amp and phase inverter/tremolo section, so that it wouldn't be quite as spread out.  I also increased the size of the wire jumpers, for ease in seeing.  Sometime later on, I'll increase the size of the component labels, as they are not clear when printed on an inkjet printer.

I'll make an effort tomorrow to snap a couple pictures of the now cleaned up cab, and new handle.

Now, to do some head scratching to figure out the values for R37 and R38.  Thanks to all who have commented and made suggestions to get me back on the right path.

[sluckey]

Now, to do some head scratching to figure out the values for R37 and R38.

R37 = 9.1K (white/brown/red if cc)
R38 = 51K (green/brown/orange if cc)

Got any hi rez pics?

[Jack_Hester]

SLuckey -

Should have checked them yesterday, but I was tired by the time I fired it up.  Plus, the fact that it worked so well was enough to close up shop and head for the house for a break.  I get kinda focused on the fix, so I needed the rest.  I'll try to get in there and check color codes and have the DVM close by for a confirmation.  I have noticed that some of the color bands are somewhat smudged and faded.  The meter should tell the tale, as they are not paralleled with anything in the schematic. 

No pics.  This wiring is kinda trashy.  I expect the last work done in this amp was on the clock, so no attention to neatness was taken.  Plus, part of my hand-tracing later will be to get rid of some of the huge solder globs.  I've already cleaned out no telling how many of them laying in the chassis, loose.  The rest are just on the joints. 

I'll be back this evening with more results.  Sure am glad to get a working schematic closer to finish.  Thanks again, for the help.

Jack

[Jack_Hester]

In the process of drawing the schematic, I've been thinking about the Tremolo selector and how it could be done with pots.  But, only in a scratch-built project.  I'm not one to mod something vintage and working.  Anyway, how do you think it would work to substitute a pot for R31/R32 and one for R35/R36?  Or, should they be stacked and work together?  Or maybe, a pot for each of the three resistors R31/R32/R33 and R34/R35/R36?

I don't have this circuit clear in my head, especially how the oscillation starts just by closing the switch.  But after it starts, it appears that C18 passes the oscillation to S3a (I really don't know if this is the starting point.  It's just where I began looking), down through the resistors to the center tap, to the grid of V4b.  This sets up one in V4b that is passed by C19 to the control grid of V7.  R34/R35/R36 look like selectable grid resistors for V4b, but I can't picture that effect, yet.  Would the correct terminology for these be a selectable grid stopper?  So now, it looks like there is an oscillation going on to the screen grids of V5 and V6 with cathodes being tied to the cathode of V7 by the footswitch.  Now, it's even less clear.  But sure is interesting.

Jack

[sluckey]

The meter should tell the tale, as they are not paralleled with anything in the schematic.

Expect the value to be higher. CCs tend to increase in value as the universe expands.

I can help if you need replacements but can't find them.

[sluckey]

In the process of drawing the schematic, I've been thinking about the Tremolo selector and how it could be done with pots.  But, only in a scratch-built project.  I'm not one to mod something vintage and working.  Anyway, how do you think it would work to substitute a pot for R31/R32 and one for R35/R36?  Or, should they be stacked and work together?  Or maybe, a pot for each of the three resistors R31/R32/R33 and R34/R35/R36?

That's a real odd ball trem circuit. I'd never choose that crazy circuit for a scratch build. Rather than modify it, I'd just toss it, and use a more conventional phase shift oscillator.

I'd love to here some theory of operation on this trem circuit. Are you listening PRR?

[Jack_Hester]

R37 = 9.1K (white/brown/red if cc)
R38 = 51K (green/brown/orange if cc)

Made a quick check when I got home and the values that I have are correct, just as you stated.  I wiped a film off of each with my finger and the color bands were very clear on these two.  So, no update necessary to the component list on the drawing.  I did go back and increase the size of the component I.D.'s, so that they will stand out better when printed.  I posted the latest update (rev C) above, and commented.  As it stands now, the drawing should be fairly accurate in it's layout. 

I did snap a picture of the cab with new handle.  I'll try to do better with a 'before' picture, next time.

Jack

[PRR]

> Are you listening PRR?

Sigh.

Assume the trem on switch is closed.



Follow the solid purple line. The 6SN7 and 6V6 are a 2-stage amplifier, much like 2/3rd of an AA-Champ.

Except the 6V6 is triode-wired and the OT is a choke with a capacitor.

Is that a tuned circuit? A wild guess is 10 Henry against 40uFd which is 8Hz which is indeed in the trem-rate zone.

To oscillate it needs positive feedback. Follow the dotty line, goes back to the input. Feedback of some kind.

If the R-C coupling networks were super large, we have invert invert so the feedback is positive.

In fact 0.02uFd against 2Meg is only 3.6Hz, we have additional phase-shifts. Will it actually oscillate? Who wants to draw Nyquist diagrams for a week?

Gibson presumably had it oscillating. Jack says it does (right?).

A further guess is that any dinking-around with the R-C networks changes phase shift slope and thus the frequency where phase-shift is exactly 0.0 degrees, the frequency of oscillation. The L and 40uFd dominate the frequency but the R-C shifts it some.

This 6V6 will slam from zero current to large current.

If you un-snarl the wiring, this 6V6 is strapped across the main audio 6V6es screen-to-cathode. When it conducts hard, it steals all the cathode current and dips the screen voltage of the main tubes: signal is reduced. When it swings hardly-conducting, the main tubes do their job, signal passes.

It still looks wonked to me. That 6SN7 grid network sure looks like a Wein Bridge except it is missing a cap. Maybe it WAS a Wein, but the H-P patent was still in effect, and Gibson did not realize the H-P patent was not on the Wein (it was on the lamp stabilizer). Or maybe when they tried to get enough output to flog the signal tube K-G2 path, they had to beef-up with the choke then tune it with a cap, and now no need for a major tuning-network at the grid. You'd wonder why they didn't flog the output tube grids like Fender did... maybe they didn't want to be too close to Fender's patents.

[sluckey]

Since this amp uses all octals, I think it's really a '52-'53 model. Leo may not have been tremoloing yet. I picked up on the tuned circuit right away but I just didn't get how the frequency was made to change using the switch. Thanks for the explanation PRR.

[Jack_Hester]

Well, I've got to go back serveral times and read it, but I appreciate having the explanation to study.  Thanks a lot.

Jack

[PRR]

Gibson GA-40 schematic - Early - rev C.dwg Model (1) 10/19/2011

No value for R27.

R29 value is given twice: as 10K 1W and 470K 1/2W.

Are you sure R27 comes off V7 Plate, not the other end of the choke? The connection shown in the PDF significantly limits V4b plate swing. The connection shown in the hand-drawing shows this resistor from the hot end of the choke.

[Jack_Hester]

Gibson GA-40 schematic - Early - rev C.dwg Model (1) 10/19/2011

No value for R27.

R29 value is given twice: as 10K 1W and 470K 1/2W.

Are you sure R27 comes off V7 Plate, not the other end of the choke? The connection shown in the PDF significantly limits V4b plate swing. The connection shown in the hand-drawing shows this resistor from the hot end of the choke.

Thank you for critiquing.  Your observations are correct.  I'm making this drawing along with other chores, so I'm proving not to be multitasking very well.  I made the corrections to the drawing, and labeled rev D.  Even if I think it is correct, when I hand-trace the whole circuit, I plan to have the original close by to compare.

R27 is now the 10K, 1 Watt, in the listing.  There is only one R29 listed and it is a 470K, 1/2 Watt.  R27 is now connected to the opposite side of the choke (L2), from that of the V7 plate.

Thanks, again.

Jack

[PRR]

> I picked up on the tuned circuit right away

Then you may follow this next bit.

In simulation, realistic series resistance in the choke shifts the frequency up. I bet a shunt resistance (the output 6V6 G2-K circuit) also skews the freq, though I did not sim that.

There is some phase-shift in C18 R31-R36, but the frequency shift seems small, even with 1K series resistance in the choke (to limit Q).

1.0Meg 7.2Hz
1.33Meg 6.7Hz
1.66meg 6.4Hz

The unloaded output is about 90V peak, with around 15% 2nd harmonic distortion.

Circuit details are non-intuitive. Changes lead to small distorted output or long start-up time. This surely was not "designed", musta just-happened on the breadboard.

[spacelabstudio]

Hi Jack,
probably there are kids playing on the old parade grounds of the base in Wiesbaden these days. I think most of that site was converted into to residential use.

Funny.  My mother was born on that base in Wiesbaden.  That was 1949.

Chris

[Jack_Hester]

Funny.  My mother was born on that base in Wiesbaden.  That was 1949.

Chris

Then, that makes me real close to your Mother's age. 

I was homesick, but really enjoyed the country. 

JacK

[Jack_Hester]

Expect the value to be higher. CCs tend to increase in value as the universe expands.

I can help if you need replacements but can't find them.

That will be much appreciated!  A goodly number of the old resistors in this amp have very discolored bands on them.  Even with a now deciphered schematic, Some are just not readable (color bands, that is). 

I took the amp to Church this morning, to let my friend Don take home to give a workout.  He said there may be a problem keeping his son away from it (another very active musician).  I know that he'll keep it home and under close watch.  I don't mind who get's to play it, otherwise. 

Anyway, the help that you, and all the people who responded to this thread, is much appreciated.  I follow most of the topics here, but am never sure as to what I can contribute.  Maybe my efforts in redrawing the schematic will help someone else.  And, if any other discrepancies are found on it, please post.  I will update as I see them.  I'm hoping to make a physical layout for this early GA-40.  Got to hone those drawing skills, first. 

Jack

[Jack_Hester]

Well, friend Don has had the amp for a week, now.  He has it sitting next to a Gibson Falcon, and some other brand that I'm not familiar with.  The Falcon is one of his favorites.  I saw him at Church, yesterday, and he said the LP amp has a really wonderful sound and that the single Voicing pot gave it a very nice range of tone.  Made his Falcon sound 'thin'.  He doesn't use tremolo much, but was very pleased with this one.  So, I'm pleased with the outcome.  There's still some hiss in the Microphone channel, that becomes noticeable above 1/2 (no numbers on these settings) volume.  He's going to keep it awhile so that he and his son can put it through it's paces.

I've been working on a layout, but instead of building a symbols library for the components to represent their actual size, I used schematic symbols and placed them as close as possible to their actual position in the chassis.  The chassis is drawn to scale, with tubes, jacks, pots, and breaching holes in their correct position.  The chassis sides are drawn flat.  I've included the schematic and the layout in the same .pdf file.  I'm not an artist, so the drawings have something of a stickman appearance. 

Jack

[Willabe]

I'm not an artist, so the drawings have something of a stickman appearance.

No, that's a very good drawing, imo. Very nice.


                  Brad     

[Jack_Hester]

Brad -

Got my Gibson book and CD.  Think I'll rest from working on drawings and take a peek at it.  Thanks, again, for the info.  Have a good one.

Jack

[Deuce 225]

I wanted to thank everyone who contributed to this thread. I am restoring a '54 GA-40 and have found this thread enormously helpful.
Thank you.

Tim Cochran

P.S. I am trying to figure out the best solution for a replacement handle.  Any help would be appreciated.

[Jack_Hester]

I am trying to figure out the best solution for a replacement handle.  Any help would be appreciated.

I do not remember the source of the handle for this amp, other than ebay.  It requires 3 screws on each end, but overall the handle matches an original on another old GA-40 that I have.  This one had so many holes in it, from all the handles that have been used on it.  I cut a large machine washer in half.  Marked, drilled, and tapped each to match the handle mounts.  And, installed it with long 4-40 machine screws on each end.  This handle is a bit longer than the stock one, but is the same in appearace, otherwise.  There are covers that snap over the end mounts, and these finish the look of original. 

I'll try to remember to look out in my shop for one.  I think that I have another handle, though I don't remember if it has gold (like I used) or silver hardware.  I will PM or email you with what I find.

Jack

[Deuce 225]

Jack,
Thanks very much for the reply and tips for replacing my handle. I would be glad to buy your second handle if you can locate it.
Best Regards,
Tim Cochran