Half Of A Gibson GA40 Les Paul Amp Is ...

capnjuan

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1960-1962 but a couple of people on other BBs have gotten me going on this.

Reply #1; from the Weber BB as follows:

"As it stands now, your preamp tube is "grid leak biased", these typically don't like big signals. Convert triode B to cathode bias. Disconnect the ground reference from cathode B and add a 2.2K resistor to ground, bypassed by a 10-20uf cap. Relace the 10meg grid leak resistor with 1meg, replace the 0.01uf input cap with bare wire, you plate voltages may raise by 10-15% on this triode, it just comes with the territory." But the reply didn't include a reference to A, I prompted, and the reply was: "try a 1K to 1.5K cathode resistor at A? 20-50uf bypass cap, no bypass cap at all tends to make things cleaner. As with triode B, lose the input cap & use a 470K to 1Meg grid load resistor."

Reply #2; from the Hoffman BB as follows:

I said: The cathodes are currently jumpered together as shown but I intend to separate them, add 1.2K cathode resistors to both and bypass the cathode resistor in A with a 25uf/25V cap and a .47 in B (chosen because I happen to have one) but only if .47 is large enough to make a noticeable difference compared to nothing at all.

They said: "Connect your cathodes as you stated. Connect the grids like this..."
GA20Tfrontend.jpg


The most significant difference in the replies are the one suggest a 'straight wire' in where the other says a 33K. Nearly every other Gibson has a 56K-82K resistor there; the rest of it is (unfortunately) elective. Different values giving subtly different flavors.

Based on what these gents had to say, this is what I think I'll be trying:

preampstudiesA.jpg


I want a distinction between bassy, browner, darker, GA8/GA18 tone in 'Juan' and brighter, edgier, more sizzle in 'Coastie' ... his Silvertones use 2.2K cathode resistors w/o bypass caps. Don't want to re-invent anything, just get different basic tones. Both amps did not have cathode bypass resistors on the final 1/2 gain stage, now they do ... they are getting there.
 

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Nothing to add, except the T-32 has gridleak bias on the first gain stage. In this instance, it's a 4.7 meg resistor.
 

valcotone

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capnjuan said:
3. Would .47uf as a cathode resistor bypass cap do enough to justify its presence?


I would suggest it would make a very audible difference. I have a tweed deluxe clone I built with a switch between .68uF and no bypass cap in the second stage, the volume boost and increase in brightness/aggressiveness is very obvious when the cap is engaged. I don't know if it would be as obvious in the first stage of the amp, but I'd give it a try and see if you like it.

You could also try another toggle or push pull switch to jump the channels together internally... can be tricky if you have shorting input jacks, but another cool mod that provides thicker tone and more flexibility. I have that on my Deluxe too. Or just use an external A/B/Y box.

Great thread, btw. Thanks for sharing your project.
 

capnjuan

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Valcotone said:
capnjuan said:
3. Would .47uf as a cathode resistor bypass cap do enough to justify its presence?
I would suggest it would make a very audible difference ... tweed deluxe clone ... with a switch between .68uF and no bypass cap in the second stage, the volume boost and increase in brightness/aggressiveness is very obvious when the cap is engaged. I don't know if it would be as obvious in the first stage of the amp, but I'd give it a try and see if you like it. You could also try another toggle or push pull switch to jump the channels together internally... can be tricky if you have shorting input jacks, but another cool mod that provides thicker tone and more flexibility. I have that on my Deluxe too. Or just use an external A/B/Y box.
Hi VT; thanks for the compliment and the suggestions. Before launching into a reply, let me back up a little. Last summer, I noticed that the only other affordable amp in the Kingdom of Amps that used the magic 5879 like the GA40 was the GA20. Based on Michael's and your comments, my interested in Gibson amps, the fact that Victoria decided to update and re-issue it, and their value on the used market, I figured there must be something to the GA40 Les Paul. I bought a GA20T (#1) on eBay and didn't realize that it was based on 6EU7s in the preamp - something of a one-off model; it didn't match the schematic. A few weeks later, I bought another (painted) GA20T (#2) that although it matched the schematic, it was a basket case.

My goal was to acquire and enjoy the performance of a 5879-driven GA40 ... if only in one 5879-driven channel; whatever was in the other channel ... well, that would be a bonus. I started through the usual progression; get it working, get it working well, and piddle with the tone via tubes and speakers. Both got new power / cathode bypass / signal caps, trem running in both, grounding squared away, new-to-amp reconed speakers, and new more better gooder tubes.

The early results with both amps were disappointing; both channels flat, bright, over-drive proof ... insipid, dull, tasteless. At the same time, Mr. Marx book on Gibson amps came out and I finally got around to reading the Gibson GA20T owners manual. According to Mr. Marx, Ted McCarty and Seth Lover referred to amp distortion as 'wolf tones' (page 47); they were 'hi fi' guys. :shock: Because (like Guild?) Gibson was a guitar company, nobody considered the amp business to be of any particular interest or profit. The GA20T manual says that channel 1 was intended as a mic channel :shock: and all of a sudden, the lack of vitality in these amps started to make sense; the GA20Ts weren't supposed to sound snappy, alive, vivid ... :evil: Can you imagine?

To get to the 5879 tone, I bypassed the filter in channel 2 as shown above; in one amp with a cap and switch and in the other, hard-wired until the switch arrives. The GA20T has the same phase inverter that the GA40 has ... except for channel 1, they are virtual dupes ... phase inverter study below:

2040cathoderesistorsB.jpg


.... and now finally getting around to your suggestion ... you'll notice that the GA40 PI has a 20uf bypass cap and the stock GA20T none. Both of my amps, because their PI cathodes (6EU7 and 12AX7 versions) are tied together like the GA40, now have a 25uf/25V cathode resistor bypass caps to get the meaty thing going. This is the position of your tweed-clone switchable bypass caps. Coastie and I did some experimenting with one of his Silvertones; we put in the switchable bypass cap mod; .68uf/nothing/25uf and he reported a limited distinction between the two caps. Because I want a noticeable difference in the two halves of channel 1, since there doesn't seem to be too much difference between a .68 and a 25uf, and because I want to force one channel as close to a GA18 as I can get it (the GA18 having bypass caps in both stages ) one of the channel 1 halves gets a 25uf and the other probably nothing.

Unlike your tweed and most similar amps where the tube halves are in series, these halves are separate from each other providing this opportunity to have separately voiced first stages. So, if the two halves produce different tones, then 'switching' is done by putting the cable in the other channel 1 input jack ... like I say, it's the fact that the preamp tube halves aren't daisy-chained that makes this seem odd.

GA20Tfrontend.jpg



So ... the channel 1 cathodes that are now jumpered together will be separated and one tube half will 1.2K/25uf a la the GA18 and the other half a 2.2K resistor and nothing. If the added 25uf in the channel 1 preamp makes it too boomy, then I'll probably remove the 25uf on the PI because ... the 5879 section already has a bypass cap of its own as shown below; that is, it's less likely to be diminished by the removal of the PI bypass cap. Pic below done to study the cathode resistors; on the right you can see the 20uf bypass caps on the 5879s in both channels of your Les Paul and a 20uf (now 25uf) bypass cap on the channel 2 5879 in the GA20T:

2040cathoderesistorsA.jpg



These amps are roaring to life; not sure I'll ever know whether I'm getting close to a GA40 or not; the power supplies are the same but the Les Paul might have a better output transformer; haven't been able to find out. I will say that the 5879 is gainy but without the overheated quality, meaty, thick, and hot. Getting something interesting out of channel 1 will be icing on the cake. Considering that Gibson intentionally dumbed this amp down, I don't feel too bad about making changes to it; especially when the standards I want to hit are Gibson's own; not looking for Tweed Deluxe, not looking for Plexi, not looking for something these amps can't do ... but I do want some fire coming out of them. One will stay and one will go ... so I push to get them both to be as good as I can get them. Thanks for your interest. John

Notes:

Other Gibson models using the 5879 other than the GA20T
GA80 Varitone Schematic
GA90 Schematic
 

valcotone

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John,

Ha, I love your amp posts... I see one and feel the need to grab some coffee and dig in. Thanks again.

Sounds like you're on the track to improving your amps tone, that's what counts. I guess changing values of cathode-bypass caps make more of an impact further on in the circuit (like on the 2nd stage of my deluxe clone).

FWIW, there are a few other amps that used the 5978 that I've come across:

The late 50's Gibson GA-80 Vari-Tone used 2 of them, but not as the first tubes hit by the signal, but further on in the circuit. I've never heard one of these but there was one on ebay recently... the circuit is very interesting.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220353381221

bb7f_1.JPG
bb97_1.JPG


Also, the mid-50's National Stage Star with the 4 oval Rola speakers used a 5879 as input tube in one of the channels... it is a fiery beast as well:

56stagestar1.jpg
56stagestar2.jpg


Fun stuff!
 

valcotone

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BTW, the output transformer in my GA-40 is marked "GA-40-02, 166933".

Can you check the OTs in your two GA-20 amps and see what they are?
 

capnjuan

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Will do; sent email re/ your P12N. J
 

capnjuan

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Not the same ... these amps have GA-23-02, source code 166933. Wouldn't have expected them to be ... would have narrowed the cost gap between the two models.
 

capnjuan

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Hi Sean; dunno how I missed your post :? ... the GA80 in the auction link sold for $1,200. Don't know whether you post or lurk on the Hoffman BB or not but there's a thread Linked Here being led by a very bright guy who is discussing the 5879 and its variant the EF86 as an input pentode. The thread includes links to designs and tweaks for EF86-driven preamps; amazing stuff from hard-core ampistas who know what they're doing. I wonder if the gentleman's fascination is driven in part by the won't-go-away appreciation of the 5879-driven GA40 LP .... and its little bro GA20T.

Right before I bought your P12N, I put reconed P12Qs in both of the GA20s ... now spending time trying to break them in so I'm not yet getting what I want from them and holding off the preamp mods until I think I have at least one of the amps/speakers happy with what they're doing. ... and from the 'Substitute Cash for Design/Benchwork' Department, I recently sunk some money into a quad of matched Haltron 6V6s; as ever, looking for the darker side of the sound equation. Best. J
 

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Cap'n,

Now that you've been playing through the amp for awhile, what are your impressions of the 5879 input tube? Do you find that you gravitate more towards that channel or the 12AX7 channel?

One reason that I ask is that I recently got a mid-50's RCA tape recorder and the input tube is none other than a 5879. This amp is a single-ended 6AQ5 so from a power standpoint it won't sound much like your GA-20. But if you think it is a worthwhile endeavor, I might have to use this front-end and build me a Gibson like yours.

RW
 

capnjuan

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Hi Richard; I play mostly through the 5879 / channel 2 in these amps for two reasons; the tone quality of the 5879 and the fact that channel 1 was intended for mics ... something I'll be getting after shortly. Links to two 5879/EF86 threads on the Hoffman BB; do not know if you need to be registered to view or not. You can also Search for 5879 or EF86, a Euro variant.

http://www.el34world.com/Forum/index.php?topic=5901.0
http://www.el34world.com/Forum/index.php?topic=6150.0

If you slogged through this thread, you know that I've bypassed the R/C filter on the output side of the 5879; hard-wired in one amp, switchable in the other. Apart from the better grade OT in the GA40LP which can be seen hanging vertically in chassis pics, my amps have, in the switchable version, the same circuits as a GA40; when the bypass is switched out, it's the same as a GA40 channel 2; switched in, channel 2 is the same as the GA40 channel 1.

I posted the R/C filter on the Hoffman BB and was advised that they thought it was to diminish low frequency oscillation. When the bypass is switched in, the tone is hot, almost gainy, furry (LFO?), a little dark, edgy, and snarly. When switched out (normal), the 5879 is flat, a little nasal, and ... well ... boring. I have not tried running the volume way up and kicking in the tremolo; I think that's what the filter was there for. In these amps, the 5879 effects are more pronounced in the standard lineup (non-6EU7 / hard-wired bypass version) of the amp; the 6EU7 version / switchable is a little cleaner and with a brighter signal generally.

If you've come this far, you know the 5879 takes the same 9-pin socket as a 12A_7 so if you aren't pleased, there's a work-around available. Good luck with your project, hope all is well with you. John
 

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capnjuan said:
Hi Richard; I play mostly through the 5879 / channel 2 in these amps for two reasons; the tone quality of the 5879 and the fact that channel 1 was intended for mics ... something I'll be getting after shortly. Links to two 5879/EF86 threads on the Hoffman BB; do not know if you need to be registered to view or not. You can also Search for 5879 or EF86, a Euro variant.

http://www.el34world.com/Forum/index.php?topic=5901.0
http://www.el34world.com/Forum/index.php?topic=6150.0

If you slogged through this thread, you know that I've bypassed the R/C filter on the output side of the 5879; hard-wired in one amp, switchable in the other. Apart from the better grade OT in the GA40LP which can be seen hanging vertically in chassis pics, my amps have, in the switchable version, the same circuits as a GA40; when the bypass is switched out, it's the same as a GA40 channel 2; switched in, channel 2 is the same as the GA40 channel 1.

I posted the R/C filter on the Hoffman BB and was advised that they thought it was to diminish low frequency oscillation. When the bypass is switched in, the tone is hot, almost gainy, furry (LFO?), a little dark, edgy, and snarly. When switched out (normal), the 5879 is flat, a little nasal, and ... well ... boring. I have not tried running the volume way up and kicking in the tremolo; I think that's what the filter was there for. In these amps, the 5879 effects are more pronounced in the standard lineup (non-6EU7 / hard-wired bypass version) of the amp; the 6EU7 version / switchable is a little cleaner and with a brighter signal generally.

If you've come this far, you know the 5879 takes the same 9-pin socket as a 12A_7 so if you aren't pleased, there's a work-around available. Good luck with your project, hope all is well with you. John

capn,

After reading some of your previous posts, I know you are multi-lingual and seemingly fluent in several languages. However in this thread, what language might you be speaking? It's Greek to me...Amazing!

PS- Of course I'm somewhat challenged when I do an oil change... :oops: :mrgreen:
 

capnjuan

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Hi Fab; the language is Sapaah (semi-amateur player and amp hack) I've threaded a number of my amps here:

GA30 viewtopic.php?f=9&t=10155
GA8 viewtopic.php?f=9&t=7005
Univox U305R viewtopic.php?f=9&t=9488 (stalled)
Guild Thunderbass viewtopic.php?f=15&t=4781

And several others; a Guild T1 RVT, a Gibson GA18, and a Gibson GA19RVT that I'm too lazy to go find the links to. I have background in vacuum tube stereo that I've adapted to guitar amps. Just before we moved down here, I broke up my system that included a Marantz 7C, a Marantz 10b, and an AR D-76 (4X6550). These days, my interests run to amps that produce that glorious, tweed-flavored, crunchy brown tone of the late 50s and early 60s ... with all of its warts and flaws ... flubby bass, too thick in the mids, not enough of the creamy thing ... whatever.

This thread was mostly me following my interest in the 5879 tube; used in several Gibson models and by nearly no one else. In it's own way, it's making a comeback of sorts because old; Fender, Gibson, Ampeg or new: Mesa, Soldano, Divided by 13 ... nearly all of them depend on the respectable 12A_7s ... so the spread of available tones, IMO, is necessarily limited. Anyway, more Gibson amp porn and techno-jabber Here. Best wishes, J
 

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capnjuan said:
Hi Fab; the language is Sapaah (semi-amateur player and amp hack) I've threaded a number of my amps here:

GA30 http://letstalkguild.com/phpBB2/viewtop ... =9&t=10155
GA8 http://letstalkguild.com/phpBB2/viewtop ... f=9&t=7005
Univox U305R http://letstalkguild.com/phpBB2/viewtop ... f=9&t=9488 (stalled)
Guild Thunderbass http://letstalkguild.com/phpBB2/viewtop ... =15&t=4781

And several others; a Guild T1 RVT, a Gibson GA18, and a Gibson GA19RVT that I'm too lazy to go find the links to. I have background in vacuum tube stereo that I've adapted to guitar amps. Just before we moved down here, I broke up my system that included a Marantz 7C, a Marantz 10b, and an AR D-76 (4X6550). These days, my interests run to amps that produce that glorious, tweed-flavored, crunchy brown tone of the late 50s and early 60s ... with all of its warts and flaws ... flubby bass, too thick in the mids, not enough of the creamy thing ... whatever.

This thread was mostly me following my interest in the 5879 tube; used in several Gibson models and by nearly no one else. In it's own way, it's making a comeback of sorts because old; Fender, Gibson, Ampeg or new: Mesa, Soldano, Divided by 13 ... nearly all of them depend on the respectable 12A_7s ... so the spread of available tones, IMO, is necessarily limited. Anyway, more Gibson amp porn and techno-jabber Here. Best wishes, J

Hi capn,

Thanks for posting your amp threads. Because of my lack of knowledge (read dummy), they left me in a state of :shock: and somewhat :?.Guess it all comes down to how and why we view things as we do, and taking different approaches to life: namely the Classical vs.the Romantic view. The point is made in the excellent book Zen and the Art of Motorcycle Maintenance (which you might be familiar with and is reviewed here). For example, when viewing a motorcycle,the Classical POV sees it in all its "component part" glory and through trial and error, learns how and why it functions as it does; in addition, he learns to fix and maintain the machine as needed. The Romantic, on the other hand, sees the same motorcycle as a means to end, and doesn't necessarily care how and why it runs as long as it gets him where he's going. If it breaks down, why, that's what garages are for...

In actuality, both enjoy the ride but in the end they just look at things a little differently. Not better or worse, just differently. You can probably guess into which catergory I fit: the hopeless (and helpless) Romantic. But somehow, I still manage to enjoy the ride... :D

IMHO - you seem to exhibit characteristics of the Classical approach. And maybe most of us have some of both philosophies inherent in our lives... :)
 

capnjuan

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Hi Fab; allow me to duck your thoughtful characterization for the moment and just say there are lots of people with my interests - including me - who obsess over whether carbon composition resistors 'sound' better than carbon film resistors ... much in the same way that acoustic players agonize over whether they want tusq or petrified whale bone bridge pins or which gauge of strings best fits their style of play.

A good-running tube amp, with no instrument plugged in and turned up to say 8 or 9 will hiss a little. That hiss is the amplified sound of millions of electrons streaming across the tube from the cathode to the plate of the power tube(s). If you inject a musical signal, sixty times a second, that stream of electrons is so powerful that it will imprint an exact copy of the AC wave form onto the plate ... except it will be louder ...

The last time I was on a motorcycle, I was hitch-hiking from Baton Rouge to San Antonio in 1969; a guy on one of those old 160cc Hondas picked me up ... took what must have been 7 or 8 hours at no more than 55 mph or so ... I was burned to a crisp ... pretty much killed off any interest in motorcylces I might have ever had. Best wishes, CJ
 

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Hi capn - after reading about your experience on that Honda, I wouldn't be too eager to climb back on either...eight hours on the back of a bike on a hot, sunny day = toast!
 

capnjuan

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Rock the House !!! First two quarters of last year, the earlier chocolate over tans were regularly selling in the +/- $1,500 range ... Other than Buffalo Rob's that was a re-cover, I can't say I remember seeing a tweed GA40LP all last year ... must be some AIG bonus money ... or something ... Wow!
 

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That is an awful lot money for an amp listed as "needs servicing" and "as is" from a seller whose other items for sale include a WWI doughboy helmet and a soda bottle........
 

capnjuan

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In the first and second quarters of last year, VGC Gibson tweeds went for crazy money ... get a look at a $1,500 version of your GA18; page 1 of the LTG Tonequest thread :shock: No way to be completely sure but those brown twin-cigar caps in the auction amp could be original and the Jensen is the type originally fitted in the model. The GA40 has gotten a lot of ink ... I guess that explains why this one could bring such a price ... holy moly :( :wink:
 
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