Weak tremolo in Gibson GA-40

[Tremolux55]

Hoping someone can give me some guide here.

I recently picked up a tweed 60-61 GA-40. Awesome amp but the tremolo is a bit "weak". Is it normally a subtle effect or should I look further, I'm using my Tremolux for comparison though I realise they are not the same circuit.

The tube tests fine and strong so if there's an issue I'm guessing it may a resistor and/or capacior type problem.

Any help much appreciated.

[Robert M. Martinelli]

Hi,
Is your amp a GA-40 or a GA-40T? I' m asking this because the two trem circuits differ, the first has a 6SQ7 with no LDR, the second has an 12AU7 with an LDR/lamp assembly.
If you're positive the tubes are OK ( not only they should light up but their efficiency should be good as well ) check the components around the trem section, one of them could have gone off-spec weakening the trem ( e.g. resistors usually drift on the high side ). Also, if you have a GA-40T check the LDR, as the trem volume/intensity is greatly influenced by its efficiency.
Hope this helps
Best regards
Bob

[Tremolux55]

Hi Bob,

Mine uses the 6SQ7 valve. I've checked it on my valve tester so I'm confident that it's fine.

Looks like I get to have some fun with the mutli-meter. Actually this works out well as I was thinking about making the groundswitch a stand-by instead as It is redundnat with a step down transformer so I was planning a bit of work anyway.

Thanks for your response.

Cheers,

Mat

[Robert M. Martinelli]

Hi Mat,
If you' re confident your 6SQ7 is OK ( does your valve meter measure efficiency-trans-conductance etc. or only the emission ? ), and the components are within their tolerance, I would try to clean/re-tension the tube socket(s), after all your amp is almost 50 years old, so the chances for a bad/oxidized socket are high....oxidized sockets add their resistance to the circuit so this could be the reason why the tremolo is weak.
Hope this helps
Best regards
Bob

[Tremolux55]

Hi Bob,

Good point about the sockets, I'll do all of them when I have the chassis out. A friend of mine currently has the amp to "test" it! So I'll go and wrest it back on the weekend and start working my way through it.

Thanks again

Cheers,

Mat

[Marc]

I'd suspect the resistors and capacitors of age related drift. Very common for tremolo problems...If the resistors are much more that 10% off change them and I'd change any electrolytics just because, unless you have a way to test them. If any of the coupoling caps are paper in beeswax they're suspect and likely bad. If they are later 50's production then check them for DC leakage-even a little will throw off subsequent stages....
Got pics? I love seeing old amps naked.

Marc

[Tremolux55]

Hi Marc,

Once I get it back I'll try and post some amp porn for you

[Diablo]

I'd like to see the circuit board and layout too. I'm thinking about scratch building one of these cause I can't find anyone who makes a kit.

[capnjuan]

Hi Diablo; no kit that I know of and if the GA40's board is laid out like its little brother the GA20T Ranger (5879 in channel 2 with 6SQ7 oscillator), you'll need pics of both sides of the board. Channel 2 in the GA40 is laid out like channel 2 in the GA20 including a filtering array of Rs/Cs some / many of which are on the underside of the circuit board including several of the tremolo caps; like this -




This GA20T matches the public domain schematic; when compared to the GA40LP, except for having a 12AY7 in the channel 1 preamp and minor component value deviations (ex; .0047 caps v. .005) and the location of the channel 1 volume control, the two schematics are identical. Anyway, if you're looking to leverage off the GA40 .... this pic will give you some idea of what you might be in for. Good luck and best wishes, CJ

[Diablo]

Thanks CJ for the photo. I'm just going to have to draw up a layout when I get some time.

[capnjuan]

Hi Diablo; as Rod Serling used to say on Twilight Zone ... submitted for your consideration. Below is a comparison between the preamp of the GA40LP and the GA20T Ranger. As you can see, the only major difference is the 5879 in channel one and the 12AY7 in the GA20T. The LP's channel 2 has the same 6SQ7 tremolo tube and a filter of Rc/Csin the red box.




Below is a mod to bypass the R/C filter in channel 2 of the GA20T.



I don't know whether the two amps share the same transformer set; probably the power transformer (same rectf, same outputs) but maybe not the output transformer. My point is that unless you just have to have a 2nd channel but you really want to hear the 5879, considering a single channel 5879 / 6SQ7 / 12AX7 / 6V6s / 5Y3 amp; it's all you need. Channel 2 in my GA20T, except for maybe the output transformer, is electrically identical to channel 1 in a GA40LP ... a poor man's LP ... unless you really want 2 channels, it would be easier to do without the 2nd preamp tube. Good luck on your project. CJ

[Mike62Jazz]

capnjuan, Don't want to hijack, but do you have pics of both sides of a stock circuit board for a GA20T? I have a friends' '56 here at the house that has had some work done in the last 20 years. There are some new filter caps and the tremelo footswitch is long gone. Actually, a pic of the whole chassis as you see it with the back removed would be great. The pic in this thread is a great help for the underside.

Thanks, Mike

[capnjuan]

Hi Mike: here you go!

Underside of board; the two large bumblebees in the middle now gone replaced with Mallory .047s; trem is perfect.




Top of board; original except: a) the 680ohm PI cathode resistor in the center has been bypassed with a 25uf/25V cap and b) the tone-killing R/C filter in channel 2 has been bypassed. In the upper left-center, there's a .01 cap tapping the signal from the plate of the 5879 and directly feeding the channel 2 volume control. The wiring was fed down an unused hole in the cb, across and up and 3) several resistors changed during trem hum chase. Otherwise, as original.




Full view of chassis; fresh ps caps racked in wiring strips and grounded separately. Because of the grid leak bias scheme in the preamp, these will hum like crazy if the center tap is left on pin 1. In the upper left-center, you can see the red/yellow heading to ground with the filter caps.




Showing off; the amp had been painted and between the surface prep for the paint and the stuff used to get it off, the outer layer of vinyl on the tolex gone. What was left was the cotton 'tweed' backing but the paint remover pulled the pigment out; left it like bleached cotton. The color is from diluted amber shellac used as 'sizing' and two coats of varnish ... "Buckaroo...."





Hope this helps! John

[Mike62Jazz]

John,
Thank you very much! I hav'nt fired it up yet, so don't know if there are problems. There is/was some kind of waxy, dried, tan goo puddled on the rectifier V7, and 6v6 V6tube sockets. I've been trying to clean it up before checkout. some of the wiring for V7 that is laying in the chassis is stuck in this dried substance. I suspect some of the old caps will need to be replaced also. The goo that was on V6 had seeped through the socket and effectively "welded" the tube to the socket. I had to pry the tube loose!
Thanks again,
Mike

[capnjuan]

Hi Mike; before pic of my amp - it's the waxy potting around the outside of power supply filter caps dripping on the 6V6/5Y3 sockets at left. Because there was so little ventilation in these models, the transformers get hot and their potting oozes out too Alcohol will thin that stuff; Q-tips, dental picks ... yech ... this one wasn't so bad ...



Cheers! John

[Mike62Jazz]

John,
I thought that might be what it was, but the only original cap left in that area is the orange Astron 20mf/25vdc with the resistor piggybacked. The others have been changed with (2) 20uf/500vdc Sprague atoms and (1) 10uf/450vdc from V5 to the 10k resistor (far left on Circuit board) with the yellow wire in your picture. It seemed strange to me that whomever did the cap job wouldn't clean that up! Any harm in leaving it pooled in the bottom of the chassis with wiring stuck in it? Does it conduct?

[capnjuan]

Hi Mike; I can only tell you about a GA8 I have. When I got it, alot of the transformer potting had melted out, pooled, and maybe with some DC leakage, contributed to some bad rust at the rim of the 5Y3 tube socket and chassis. It was easier to take the old socket out and replace it. When I did, I did the scrub-a-dub; picking at the crud chunk by chunk, mopping with Q-tips in alcohol in between. It took a while but it got done. I can't say I'm sure it doesn't conduct.

It won't bother an insulated wire but I think if it's in contact with a socket connection with B+ and the chassis steel, I think that's a problem. I'm maybe a little more obsessive-compulsive than others but I often sell my rehab projects. Because of that, I try to get them as clean as I can ... even if it means crud-picking. The only thing that would hold me back would be the certainty of damaging wiring insulation. Your call. John

[capnjuan]

Bump: Hi Mike - this is my '59 parallel-6V6 GA8. Consequences of melted potting and maybe a little B+. See old socket at left of pic; intervention was necessary. I cleaned to bare metal and applied something like phosphoric acid to damp down further corrosion.




I'd want to know what was under the crud ... J

[Mike62Jazz]

John, Fired it up! Played through it for about an hour. It's quiet and sounds great! Tremelo is subtle, but nice. Do you know if the channels are interactive, like a 5E3 Deluxe? It sounds like it even if it's not jumpered. Plugged into channel 2, if you turn up the volume on channel 1, the sound changes. Am I hearin' things? If jumpered, will it make a big difference or can this even be done?

Thanks again, Mike

[capnjuan]

Hi Mike; no, you're not hearing things ... the channels are joined at the 'Voicing'/tone control hip:



The red circle at the bottom is the channel 2 volume control. The red dot to it's left shows the 'Y' where the signal splits to the volume control and to the tone control. The blue circle is the channel 1 volume control and the green circle is the 'Voicing' / tone control common to both channels.

When the channel 2 signal reaches the line at the top, it backfeeds the channel 1 volume control and loops through the tone control. If the channel 1 volume control is full off - presents the highest resistance to ground - it shunts the signal back though the tone control but doesn't effect volume. If the channel 1 volume control is turned up, it can vary the apparent volume or tone.

I wasn't there back in the day but I'd bet a set of new guitar strings this is just the accidental outcome of saving the cost of a second tone control for channel 2; with respect, Gibson never met a cost it couldn't avoid.

When you say jumpering, if you're referring to channels, I wouldn't recommend it. Channel 1 was designed as a mic channel refer to Owner's Manual (http://www.harpamps.com/gibson/index.html) scroll down. It's a 'high' gain channel using grid-leak bias.

If it's gain you seek, unleash the channel 2 5879 this way:



Bypassing the filter section. Gotta go now, will finish this later. John

[capnjuan]

Hi Mike: finishing my thought here. This is the GA20T;s channel 1 preamp:



Each input jack of channel 1 feeds one half of the 12AY7, the tube relies on grid-leak biasing, and the cathodes of the 12AY7 are jumpered together. If you want to experiment with the amp, it might make more sense to un-jumper the cathodes, add cathode bypass resistors and caps, get rid of the 10meg resistors and input caps, and use a more reasonable value like 470K-1meg.

The front end of channel 1 bogs down pretty badly; if you wanted something out of the ordinary, the trick would be to create separately biased tube halves and take the output of the 1st half of V1 and feed it to the other half.

Anyway, good luck with your amp! John

[Mike62Jazz]

Thanks again John! It's not my amp, so I won't be doing to much experimenting. It's my guitar players amp. I just like poking around with these circuits and trying to learn something. I am currently learning how to read schematics and am reading books on tube amps maintenance and repair. I am actually a Bass player who likes to plink around on guitar and fool with tube guitar amps. Other than the nine Basses and a couple of Bass rigs, I have a Fender Blues Junior, Hot Rod Deluxe, and John Mayer Sig Strat. I have done some modding to both the Blues Junior and Hot Rod. Another friend of mine brought over an original '51 5B3 tweed deluxe that was his dads. All original except for 2 coupling caps. It is making some crackling and popping. I recomended he have all the electrolytics replaced.
Anyway, thanks again! You have a lot of knowledge with regard to these amps and I am going to keep these pics and schematics handy to refer to.

Mike

[capnjuan]

Happy to help; good luck! John

[Diablo]

Thanks Capnjuan for all the good info. I'll have to incorporate that RC filter lift mod in the tremelo channel. Looking over the GA-40 schematic, I have a couple of questions that maybe you can answer. There's a 1M pot in the post PI position. I assume this is the tone control, although I've never seen one before like that. Should I get a linear or audio taper pot? The two volume pots, I assume audio taper is best? How about for the tremelo frequency and depth - audio or linear taper? I'm thinking of using a PT from Ted Weber's Deluxe Reverb #25130. It's got 540V taps / 150 mA, that I figure should give 378V DC before the 5Y3 rectifer drops it down to around 320-330V. Does this sound reasonable? What kind of voltages are you seeing on your GA20? I'll probably build this as a head to do it on the cheap.

[capnjuan]

Hi Diablo; some time ago, when Victoria Amps rolled out their re-issue of the GA40, Tonequest published an extensive review of the original GA40 and Victoria's version. It's well worth reading ... sometimes the articles can be found on Tonequest's site. Anyway, the article mentioned the post-PI position of the volume control and that's what it is, volume - not tone. The Tonequests had never seen one there either.

I would just be guessing but I think it would be audio, not linear taper. In the GA20T schematic; both the trem depth and frequency controls are 500K Reverse Audio ... and the circuit is identical to the GA40 schematic; Gibson was pretty sloppy with those kinds of details. Not completely certain but it's my understanding that if the connections are reversed, a convential audio taper pot can be converted to a reverse audio taper ... there are smarter people than I here who can confirm this but, if it were me, I'd probably try RAs first ... at least there's a drawing out there to hang my hat on.

Can't be certain but in the GA40LP schematic, it doesn't look like there's quite that much voltage drop across the recitifier. If not, then you might need a dropping resistor of ? value to get you where you want to go. I don't have voltages on the GA20T to offer ... been so long since I fired it up. CJ

[Diablo]

Thanks CJ. So does the GA-40 have a tone control? I've seen pictures of the faceplate showing a tone control, but I can't find one on the circuit diagram. Maybe I have bad eyes...

[capnjuan]

Originally Posted by Diablo

Thanks CJ. So does the GA-40 have a tone control? I've seen pictures of the faceplate showing a tone control, but I can't find one on the circuit diagram. Maybe I have bad eyes...

Hmmm ... no ... not your eyes ... maybe my memory; GA40LP schematic from Schematic Heaven ...

http://www.schematicheaven.com/gibsonamps/ga40.pdf

The Tonequest article specifically talked about a post-PI volume control but in looking at this schematic, it looks like what's post-PI is the tone control ... My apologies ... I wish I had a better answer ... CJ

[Diablo]

Thanks Capnjuan. I don't have anywhere near the expertise of a lot of you guys on the forum, and I appreciate all the help I get here. So far, I've built a tweed bassman, deluxe reverb, and Tore-Vibe from kits, and they all sound great. Without the internet, this just wouldn't have been possible. I remember the first electric guitar I tried in the mid 1960s, played through a new Gibson combo amp with reverb. Boy, did that thing sound fantastic. I don't remember what the guitar was....I think Supro. But, I remember that Gibson reverb sound. I wish I could remember which model amp that was.

[capnjuan]

Hi Diablo; I've been messing with early 60s Gibson amps for the last few years - I like their vintage tone and feel and Gibsons are cheaper than similar make/model Fenders from the same time frame. Agree that the web has made it a lot easier to learn and build good projects. Back in the day, if you screwed up, you had to pay someone to straighten it out. These days, you can get some pretty knowledgeable people to help you out on-line. Good luck with your project! CJ

[roughcut studios]

I have repaired a "55 GA40 with the tremolo circuit using a 6sq7, caps, and resistors, and have built a clone of the same amp. I would suspect the caps are bad, and would go ahead and replace them, and while your'e in there check the resistors in the trem circuit for drift. The board is double sided and a pain in the neck to work on, so you only want to pull it one time, that is, replace all suspect parts at one time.
Great sounding amps!
Be sure to use a nice vintage alnico Jensen p12p for original tone.
I am able to supply board layout drawings if needed, as I documented everything when I did my clone.
Good luck, Jon

[tubeswell]

Welcome to the forum Jon

(This thread is a few months old now, and I think he fixed it a while back)

[roughcut studios]

Nothing like a timely response. I guess I should have looked at the date!
Just got so juiced by seeing a GA-40 post, I jumped before looking.
I have recently built a lite version of the GA-40, without the tremolo channel,
with true point to point construction,(no board). Nice and simple.
Sounds great.
...Jon

[Diablo]

Originally Posted by roughcut studios

I have repaired a "55 GA40 with the tremolo circuit using a 6sq7, caps, and resistors, and have built a clone of the same amp. I would suspect the caps are bad, and would go ahead and replace them, and while your'e in there check the resistors in the trem circuit for drift. The board is double sided and a pain in the neck to work on, so you only want to pull it one time, that is, replace all suspect parts at one time.
Great sounding amps!
Be sure to use a nice vintage alnico Jensen p12p for original tone.
I am able to supply board layout drawings if needed, as I documented everything when I did my clone.
Good luck, Jon

Say Jon, can you post up the board layout drawings of the GA40?
I want to build a clone and it would make it a lot easier on me if I didn't have to draw up a layout.
Thanks.....

[roughcut studios]

I dug out my files on my GA-40 build. I don't have a scanner, so here's a
quick photo of my drawing.
Great amp. I have recently built a prototype of a lite version with no trem channel. Simple and sweet! Have fun with your build!
.......Jon

[capnjuan]

Originally Posted by roughcut studios

I dug out my files on my GA-40 build.

Hi Jon; I see you replicated the R/C filter in channel 2; did you ever consider just leaving it out or did you just start with the idea of straight reproduction? I have 2 GA20Ts; except for a 12AY7 in the channel 1 preamp, the location of the tone control PPI, and a smaller OT, it's the same amp. I have bypassed that filter in channel 2 in both GA20Ts to duplicate - except as noted - channel 1 in a GA40. One bypass is a cap tapping the plate of the 5879 and going to the ch 2 volume control; the other is the same thing except switched via a set of contacts on the back of 1 meg volume control on channel 1. I don't have anyway to compare the results to a GA40 but what comes out makes a 12AX7 seem tame. The gents on the Hoffman BB ID'd the filter as knocking back low frequency oscillation but nobody had a good answer to how come so many. Congratuations on your GA40 work.! CJ