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gibson gss100 low output/oscillation

Started by fredrum, May 03, 2011, 09:59:35 PM

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fredrum


so i got this amp at a flea market a few years back for $12 or $14 or something like that, and it pretty much came straight home and onto the "broken" pile. the seller had said that all that was wrong was that the power switch had been bypassed (why someone would do that i have no clue), and i didnt feel like messing with it at the time so on the pile it went.

i finally decided that i have way too much crap laying around, and that coupled with the fact that i need to trade in some gear to pay off a sweet hiwatt dr504 finally motivated me to stop being lazy and go through the pile and fix up some of the "junk" (and on top of that, any little bit of repair knowledge i can teach myself tends to prove invaluable in my job as a guitar repair tech in a music store). im not exactly a stranger to the inside of an amp, ive done basic repairs on several of my tube amps, but im not all that familliar with solid state.

well, first things first, i wired in a new power switch and plugged it in(with the test lamp in series). flipped the switch and......no power at all. checked the fuse.....blown. replaced the fuse, flipped the switch and....on goes the test lamp. from prior experience with tube amps, first thing i checked was the power transformer (good) and the rectifier diodes (good). at that point i was a bit stumped, not knowing what else to do, so i did some research and came across this site and read that blown fuses in ss amps are usually a result of bad output transistors. so i pulled them out and sure enough, they were both shorted out. the only markings on them were "p 4091", which after a few hours of searching led me to pretty much nothing, and being as this is a pretty old ss amp (1966 or so), i wasnt exactly optimistic about finding replacements, and i couldnt find a schematic anywhere. i finally tracked one down by downloading the gibson master manual, and looking at the parts list it listed the output transistors as 2n3055, and upon searching that i was shocked to find out that they actually carry them at radio shack (something that quite possibly has never happened to me before-radio shack actually having what i needed.......). so i went in and picked up two of them and installed them. i held my breath, flipped the switch and.....on goes the amp. victory! or so i thought.......which finally brings
me to my problem.

i hooked it up to a cab, and plugged in a guitar. first the bass inputs. i got sound, but extremely low output, and a bit of a hum. then came the guitar inputs. super low output again, and on top of that an oscillation which seem to be activated by the treble knob, but changes pitch when any knob in the guitar preamp or the effects section is turned (actually sounds really cool, but.........). could bad electrolytic capacitors be the cause of both the low output and the oscillation? im also guessing i probably need to bias? any other suggestions?


tonyharker

Usually when the output transistors fail, the drivers and sometimes the VAS transistors go as well.  I suggest you check those as well otherwise you will need some more 2N3055s. :)

Tony

teemuk

#2
I can't remember details of that amp but am I on the right track that it was an interstage transformer coupled thingie...? If it was then likely there was no bias adjustment either, probably just some NTC/PTC resistors or perhaps a few diodes, if even that. If there is an adjustment the schematic will surely show it.

But, improper bias will not cause oscillation that you can alter with preamps knobs.

Yeah, I'd start from the electrolytics. It may not be the problem but at least you get that one crossed out from the list. They'd cause some issues sooner or later anyways. Secondly I would perform tests for all the preamplifiers transistors.

I guess I need to dig up my archives to refresh my memory. I know I have the Master Service manual on one of the CD-ROMs in that huge pile...

teemuk

Ok, found it. Which one's the revision you have, set A or set B? Something else?

Anyway, as you see, those tone controls are more or less "active", meaning they are integrated to a gain stage and the controls adjust the gain of the stage at specific range of frequencies. This is a good place to start: Check the condition of the transistors within the gain stages that are implemented to tone controls.

It also seems there is a bias adjustment for the output: two trimmers for each half. The manual gives two guides for bias adjustment, by scope or by ear. Here's a third one: the final output stage is powered by dual supply rails so the output node feeding speaker should idle at zero volts. For class A/B bias you want to keep the output transistors and their drivers barely conducting when the amp idles so you'll adjust the bias so that each has 0.5V across base-emitter junction. The manual also provides a list of voltage readings for transistors, check that they correspond to this scheme.

fredrum

thanks for the input

im not entirely sure whether its set a or set b just yet......its kind of hard to compare it to the schematics.....with the way this thing is put together you have to take half of the chassis apart to get to anything. first there are three vertical pieces of sheet metal, one that comprises the front with the preamp and effect boards mounted to it, then one after that seperating the preamp section from the power section, and then one making up the back with two big heat sinks and the speaker jacks mounted to it. all of the transformers and can caps are mounted on a horizontal piece situated between the divider and the back, along with the power amp board mounted vertically through a slit in this piece with a couple of L brackets. then hold the whole mess is held together with some end caps and sheet metal screws. ive never seen anything else like it.......

another thing that drives me crazy is that the part numbers for some of the transistors on the schematics dont match the part numbers on the parts list (although i am figuring it out, it just takes longer...)


anyway, my next step is going to be replacing the electrolytics, like you said, theyll cause problems eventually, and if i know theyre all good i can rule them out as a problem. being as this is a 45 year old amp and probably hasnt been used in years im sure theyre pretty dried out. the tough part is going to be finding some 4500uf 50v caps that dont cost an arm and a leg.....

joecool85

Quote from: fredrum on May 04, 2011, 11:17:19 PM
the tough part is going to be finding some 4500uf 50v caps that dont cost an arm and a leg.....

Power caps?  If so, just upgrade to 10,000uF 50v caps like these for $4/each: http://www.mouser.com/ProductDetail/Nichicon/UVZ1H103MRD/?qs=sGAEpiMZZMtZ1n0r9vR22Sj%252bosZ9KAFwnpRlnfV4qUM%3d
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

teemuk

#6
I think one good distinction between the versions should be that the "set B" has a "rock-n-roll" switch right where the "set A" had a "polarity" switch. So, if your amp has switch labelled R-R (On/Off) in between footswitch jack and the power indicator light then it's most likely "set B" version. If not, then it's likely set A. ...But who knows, theyre may have been many other versions too that just were not documented by the schematic collection that Master Service .pdf is.

JHow

Quote from: teemuk on May 05, 2011, 01:13:43 PMI think one good distinction between the versions should be that the "set B" has a "rock-n-roll" switch ...

Wow!  A "Rock-n-roll" switch.  Now if I could just find and amp with a "Play Better!" switch.

teemuk

That would be the on/off button. When the bandmates / wife / girlfriend / whoever operate it you know your playing sucks.  ;D

fredrum

due to the presence of the "rock and roll" switch, im going to assume set b....


sadly however, at the moment the rock and roll switch seems to neither rock nor roll, just mess with the effects a bit and make the humming louder.......

freddawilia

#10
Quote from: tonyharker on May 04, 2011, 05:53:31 AM
Usually when the output transistors fail, the drivers and sometimes the VAS transistors go as well.  I suggest you check those as well otherwise you will need some more 2N3055s. :)

Tony

I was having the same issue as the OP. Thanks a bunch for your suggestions; they helped me a lot.  ;D






Professor Fredda Williamson Bodenhamer

joecool85

Quote from: freddawilia on May 17, 2011, 11:19:17 AM
Quote from: tonyharker on May 04, 2011, 05:53:31 AM
Usually when the output transistors fail, the drivers and sometimes the VAS transistors go as well.  I suggest you check those as well otherwise you will need some more 2N3055s. :)

Tony

I was having the same issue as the OP. Thanks a bunch for your suggestions; they helped me a lot.  ;D

Glad you got the help you needed, and thanks for signing up to SSGuitar.com!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Billythekid

The set a is for Gibson gss 100. Set b is for Epiphone ea600rvt the sister amp. You can tell by the speaker outputs the gss has 2 and Epiphone has 3 for the 4x10 can and the 50 watt slave amp you could add on to the amp.

Saransk

while it may be a general "rule of thumb" some Gibson GSS had the "B" series of circuits
I believe that only the "B" version had the "Rock & Roll" switch on the front which makes that a pretty good indicator.
This switch bypasses the Tremolo/Vibration board, but like a pedal it isn't a true bypass, only the outputs of the Treble preamp and the Vibrato board are switched
I have a GSS-100 which has the "B" boards
It is very similar in design the American Thompson VOX Beatle amp (114/1141) amplifier of the same era.
On the Geofex web page there is a lot of information about fixing the output amp of the Beatle which is an interstage/totem pole amp as well.
I would go ahead and replace the Power Amp resistors and the bias trimmers.  I've had go luck with several Thompson VOX amps by adding precisions resistors and new transistor sockets