Old Gibson amp fun

valcotone

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capnjuan said:
Just my $.02 but I think it's the 5879s in the GA40 preamp that gives it such a distinct tone ... and what Victoria is hoping to capitalize on it its re-issue.


FWIW, I definitely think the 5879 tubes contribute to the somewhat unique GA-40 tone, but I've recently built a dual-EL84 amp with a mixable parallel pre-amp setup, one side has a 12AX7 and the other has a 5879. It sounds nothing like a GA-40 of course, so I think the GA-40 is due to the some of it's parts, not the pre-amp tube alone (but I agree that contributes some of the flavour). I guess my point is that it's not only due to the 5879 pentode.
 

capnjuan

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valcotone said:
capnjuan said:
Just my $.02 but I think it's the 5879s in the GA40 preamp that gives it such a distinct tone ... and what Victoria is hoping to capitalize on it its re-issue.
FWIW, I definitely think the 5879 tubes contribute to the somewhat unique GA-40 tone, but I've recently built a dual-EL84 amp with a mixable parallel pre-amp setup, one side has a 12AX7 and the other has a 5879. It sounds nothing like a GA-40 of course, so I think the GA-40 is due to the some of it's parts, not the pre-amp tube alone (but I agree that contributes some of the flavour). I guess my point is that it's not only due to the 5879 pentode.
Hi Sean; so ..... you're saying that if you build a different amp with a 5879 in the front end, that it doesn't sound like a GA40?
 

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capnjuan said:
Hi Sean; so ..... you're saying that if you build a different amp with a 5879 in the front end, that it doesn't sound like a GA40?

Yeah... haha... :lol: No big surprise there I guess. I half-expected to get some of the fat and dark grunty flavour of the GA-40, but it wasn't present in the new amp build... lots of other variables involved.

It's interesting to compare the two pre-amps in this build though. Other than the biasing around the pre-amp tube, both pre-amps have a similar topology... the 12AX7 has a lot more gain, volume, and is brigher. The 5879 in comparison is quieter and more polite, but it could be the way I have the tube biased (but that was borrowed from the GA-40 pre-amp). Check it out:

hammond-organ-l100-18w_ga40-input-sml.jpg
 

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Hi Sean; well ..... I see your point. You've duped the GA40 channel 1 ... btw - is there a switch that you didn't draw in ... to shunt the input from one preamp tube to the other? Before going further, I see on your schematic the notation 'P10'. In this LTG thread; Tone Capacitors, 43jimfear said: "I have kicked around the idea of getting PIO's. Someone wise warned me that NOS PIO caps can have PCBs, which I understand can cause cancer and nerve damage." I think he was referring to 'oil and paper' caps ... but he used the P10 reference; I guess my question is, what type of cap are you ID'ing?

My earlier comment about the 5879 was about it's distinction from the more widely-adopted 12A_7 family of preamp tubes; disregarding better or worse, that it would sound materially different from a 12A_7. As you know, Gibson had other pentode/preamp designs built around the 6SJ7 pentode. Can you post the phase inverter? I have a nickle here that says you didn't dupe the Gibson phase inverter ... or go out of your way to find a cheesey transformer. There is some fair criticism of Gibson's 'spare every expense' attitude towards parts ... and you probably didn't shop around the web for the nastiest, cheapest resistors. I guess I'm saying you point is well made; it isn't just the alleged magic of the 5879 that provides the desirable GA40 'grunge' we'll call it ... if you put it in a better thought-out circuit with better parts, a GA40 you do not get ... 6V6s v. EL84s notwithstanding. I notice over on the Hoffman Board that among the threaded new builds over there, if they are pentode/preamp projects, the 5879 and EF86 are about 50/50. So ... what about your amp; works good? John
 

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capnjuan said:
Hi Sean; well ..... I see your point. You've duped the GA40 channel 1 ... btw - is there a switch that you didn't draw in ... to shunt the input from one preamp tube to the other? Before going further, I see on your schematic the notation 'P10'. In this LTG thread; Tone Capacitors, 43jimfear said: "I have kicked around the idea of getting PIO's. Someone wise warned me that NOS PIO caps can have PCBs, which I understand can cause cancer and nerve damage." I think he was referring to 'oil and paper' caps ... but he used the P10 reference; I guess my question is, what type of cap are you ID'ing?

I believe PIO is the common abbreviation for Paper-In-Oil. Those particular ones I used are the metal-enclosed "K40Y-9" PIO caps made in Russia, you can see them on ebay in various values. That's where I got them from. I've used them in a couple of amps now, and as tone caps in a Les Paul, and like the results. I don't know whether they contain PCBs or not... :lol:

There's no switch to direct the input to either pre-amp... the input signal hits both pre-amps all the time, then you mix them together using the two volume controls. Sort of like jumping the inputs on a Fender tweed (except in this case the jumper is hard-wired inside the amp). It might be better to mix the channels via two legs of the phase inverter instead of using the 470K mix resistors, I'll experiment with that later on.


capnjuan said:
My earlier comment about the 5879 was about it's distinction from the more widely-adopted 12A_7 family of preamp tubes; disregarding better or worse, that it would sound materially different from a 12A_7. As you know, Gibson had other pentode/preamp designs built around the 6SJ7 pentode. Can you post the phase inverter? I have a nickle here that says you didn't dupe the Gibson phase inverter ... or go out of your way to find a cheesey transformer. There is some fair criticism of Gibson's 'spare every expense' attitude towards parts ... and you probably didn't shop around the web for the nastiest, cheapest resistors. I guess I'm saying you point is well made; it isn't just the alleged magic of the 5879 that provides the desirable GA40 'grunge' we'll call it ... if you put it in a better thought-out circuit with better parts, a GA40 you do not get ... 6V6s v. EL84s notwithstanding. I notice over on the Hoffman Board that among the threaded new builds over there, if they are pentode/preamp projects, the 5879 and EF86 are about 50/50. So ... what about your amp; works good? John


I'm building this project into a 1966 Hammond L100 amp chassis... it was a working amp before I started except it used different pre-amp tubes. The rectifier is a 5U4GB and the transformers are of reasonable size. The phase inverter and power amp are still the old Hammond design which is quite complicated ... I haven't converted them to the Marshal 18W style yet, which I plan to do when I get some time.

Currently I'm enjoying the amp when it's set to on the verge of breaking up, with both channels mixed about 50/50... turned up louder it gets a little raspy and hard on the ears, and lower than that it sounds sort of dull. So it has a sweet spot that's pretty narrow. I've never ever liked EL84s before as amps with those tubes always sound sort of hard and unforgiving, and harsh, to me... so this is an experiment (and an excuse to finally do something with that Hammond chassis that's been on my bench for over 10 years!). I'll continue to tweak it as I have time and hope it develops further.
 

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Sheesh ... :oops: I was reading 'PI0' as 'P10' ... Pee-ten .. not Pee Eye Oh ... okay ... no, don't about pcbs either. Got it; use each channel's vol control to shut the channel off ... who needs the clicks and pops. Some of those old organ chassis that come and go on eBay have respectable-looking iron in them. If you get a chance to trace out the PI, it'd be an interesting comparison between it and the GA40. I have the two versions of the GA20T; one of them has the same PI as the GA40 ... I'll never know how close it gets to GA40 'grunge' (the good grunge) ... but 1/2-way there is probably reasonable and it's a very agreeable tone. J
 

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No problem Sean. Another of Gibson's stereo amps; a GA83S on eBay ... Scroll for schematic, pages 2 and 3

Seller says it's a '59 ... so is this Chrysler New Yorker ... just for the mood of this post ... :wink:

59chrysler.jpg



About as clean as they come; not a re-cover, the original tolex and grills in remarkable condition.

ga83a.jpg




Two pairs of EL84s; seller says "... 35 watts total." Well ... not really ... at least not in stereo mode; it's 15 watts or so L and 15 watts or so R ... stereo amps aren't rated by the sum of the channels' outputs but by watts/channel. But in mono mode, the channels' outputs are tied together and fed to the very original-looking P12R which might get closer to the 30ish watts but also at the extreme limit of the P12R's power handling ... :? Unlike the GA78s/GA79s that have ss rectifiers, the GA83 has a GZ34. Further, the GA78/GA79 speakers were at an oblique angle to a listener sitting face on; the GA83 8" speakers are 90 degs to straight on and 180 degs from each other; unless the amp were sitting in a small enough space so the sound would be reflected off side walls, I wonder how this arrangement creates the stereo effect of listening to L and R at the same time? I wish Gibson had spent a little more on OTs in its student amps ... something closer to the OTs in this amp.

ga83b.jpg
 
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That's pretty cool, but I too wonder about the application of those "stereo" speakers. The angle on the 79RVT seems totally usable, especially with an open back cabinet, but yeah, having them go straight out the side might not be that awesome.
 

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Guild Beginner said:
... but yeah, having them go straight out the side might not be that awesome.
Hi Caleb; maybe I missed something but it isn't clear how this thing is supposed to work ... maybe that's why they sold so few of them? :wink:
 
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Oh, I was just comparing how the "stereo" application of the 79RVT has an angled style that seem useful for a stereo sound, whereas this one with the speakers on completely opposite sides doesn't seem like it would produce that awesome of a sound. It would be like hearing an amp from an adjacent room. I was guessing (I'm not sure about this thing either), that this one can switch from the right to the left independently, or solely use the middle 12" for mono. Maybe you can get them all going at once, but I'm not sure.

Man, it's really clean though.
 

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Guild Beginner said:
... doesn't seem like it would produce that awesome of a sound. It would be like hearing an amp from an adjacent room. I was guessing (I'm not sure about this thing either), that this one can switch from the right to the left independently, or solely use the middle 12" for mono. Maybe you can get them all going at once, but I'm not sure. Man, it's really clean though.
Hi Caleb: the GA79 shown above and below has a stereo input jack that will send L and R signals down each amp 1/2 and to the bridging switch. Left open, the amp produces L and R stereo. If a mono guitar were plugged into either channel, throwing the switch would put the signal out of both L and R ... dual mono.

GA83stereovib07.jpg



By contrast, the GA83 doesn't have a stereo input jack. To get true stereo, must have a L signal in one side and an R signal in the other ... the signals are separately amplified until they reach a selector switch on the output side of the output transformers; switch as shown in the schematic at the left. In the middle, the switch is in the 'stereo' position; each amp 1/2 supplying signal to its pair of 8" - sideways facing - speakers. On the right, the switch is thrown tying in the 12" speaker which is being fed L and R. The amp is functioning as a stereo system while the 12" speaker provides mono output.

GA83stereovib08.jpg



Wally Marx' Gibson Amplifiers 1933 - 2008 talks about Ted McCarty's and Seth Lover's interest in 'hi-fi' as it was known back in the day. If you look at the GA78 and GA79, even their cosmetics look better-suited to a drawing room with people sipping wine while listening to some jazz than to almost any gigging environment you can think of ... like I said, maybe i missed something but sending signals off side walls ... I don't know what they were trying to do.

However, while McCarty/Lover were messing with this stuff, they weren't messing with either high-power, R&R-oriented gigging amps like Fender's 4X6L6 100 watt models or exploiting the EL84 designs they were working with towards to the same purpose. Wouldn't have taken a whole lot to re-arrange the speakers on the GA83, get rid of the two output transformers and replace them with one ... the result would have been an early 4XEL84 Marshall ... but that wasn't what Gibson wanted to produce.
 
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To get true stereo, must have a L signal in one side and an R signal in the other ...

That's why I'm using the quotes around "stereo", since we learned the definitions last week, which was helpful.

Yeah, these designs definitely seem more geared toward a different sound. I wonder how much of that was in response to Fender basically cornering the market on what players wanted from a R&R amp. Still, these strange Gibson models do offer a nice alternative for someone that likes a versatile, option-rich amp without just the option of a ton of knobs like a Soldano or something.
 

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Guild Beginner said:
... Still, these strange Gibson models do offer a nice alternative for someone that likes a versatile, option-rich amp without just the option of a ton of knobs like a Soldano or something.
Yup; that would be the deal. Project amp I bought several years ago ... '80/'82 Rivera/Jahns Fender 75 1X12... lots of knobs ... lots of power ... not for me:

fender75.jpg




Late in '07, a whole batch of GA79RVTs popped on eBay ... threaded here with prices I dumped the pics out of my photobucket account ... got where I couldn't find anything.
 
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