Found a 50's Gibson GA20 need advice for fixing up.?

Rob-Brown · 2012-11-12T01:34:04+00:00

Visiting at my friends house yesterday I happened to notice this somewhat decayed but beautiful amp in the corner. Turns out a lady he knows husband had been playing it for years but past away and asked my friend to try sell it. Anyhow my friend suggested I offer her R1000 for it since its in pretty bad shape and currently not working. I've been wanting a retro kinda low wattage amp and have been wanting to learn my way round some amp circuitry. But being that I'm quite new to working on electronics would this be a worth while investment for experience and a fun low watter to jam at home? The circuit looks extremely simple so i think i could manage. my main worries is that the transformers and speaker might be shot if you look at the pics you'll see some major rust spots.

makepeace

yeah. my point was, it wouldn't be a bad idea to check how well its working as is - and hear it for the first time. then you can get a better idea if there are any other demons lurking therein that you can fix nice at the same time.

RJN

Geepers dude nice find !

Rob-Brown

@makepeace: I see what you mean who knows might be a certain hidden quality to those old parts.

AlanRatcliffe

makepeace wrote yeah. my point was, it wouldn't be a bad idea to check how well its working as is - and hear it for the first time. then you can get a better idea if there are any other demons lurking therein that you can fix nice at the same time.

No! See Karel's post:

Karel wrote Some tips: Take care not to power it up in the state it is in now. It is quite easy to check the transformers and speaker without powering the unit up. Those capacitors looks like they've crossed over a while ago ???

Karel knows wherof he speaks...

makepeace

Alan Ratcliffe wrote
makepeace wrote yeah. my point was, it wouldn't be a bad idea to check how well its working as is - and hear it for the first time. then you can get a better idea if there are any other demons lurking therein that you can fix nice at the same time.
No! See Karel's post:
Karel wrote Some tips: Take care not to power it up in the state it is in now. It is quite easy to check the transformers and speaker without powering the unit up. Those capacitors looks like they've crossed over a while ago ???
Karel knows wherof he speaks...

Alan, there's no reason why the process outlined in my first post shouldn't be followed. Think you might have missed it.

Rob-Brown

Ok well I'd feel more comfortable knowing all the caps are new any way so I'll start by re-capping and then go over all the resistors with the multimeter and then proceed with checking voltages before putting the tubes in as mentioned.

Just out of interest what would the risks of damaged caps be? would certain ones be overloaded?

Oh and one other thing is it safe to undo the PT secondary output and check the voltages with a multimeter or how would one go about testing transformers?

vic

Rob Brown wrote Ok well I'd feel more comfortable knowing all the caps are new any way so I'll start by re-capping and then go over all the resistors with the multimeter and then proceed with checking voltages before putting the tubes in as mentioned.

Just out of interest what would the risks of damaged caps be? would certain ones be overloaded?

Oh and one other thing is it safe to undo the PT secondary output and check the voltages with a multimeter or how would one go about testing transformers?

Rather take it to a Tech for an assessment.....that'll be the quickest way to get to the bottom of it .

makepeace

Rob Brown wrote Ok well I'd feel more comfortable knowing all the caps are new any way so I'll start by re-capping and then go over all the resistors with the multimeter and then proceed with checking voltages before putting the tubes in as mentioned.

Just out of interest what would the risks of damaged caps be? would certain ones be overloaded?

Oh and one other thing is it safe to undo the PT secondary output and check the voltages with a multimeter or how would one go about testing transformers?

yep, that would be the safest. get a line of connector blocks and stick all of the secondaries in there, making sure that you know which pairs/triplets are which. then you can turn on the power and measure the voltages across each with the multimeter set on AC. be careful!

damaged signal caps will primarily cause a bit of DC leakage between stages, which isn't a massive problem. to be honest i'm not sure first hand what the dangers associated with bad PS caps are, but their failure is quite a common occurrence and from what i've read its not a huge issue. just be careful and always use common sense. wear protective equipment etc if you are unsure of what will happen or if there is any risk of injury.

read these, and do some googling about anything that worries you. there's tons of info on the net, and the best way to learn is to read and do.

http://www.aikenamps.com/SafetyTips.html

http://www.geofex.com/tubeampfaq/tube_amp.htm

http://www.analogbros.com/tech/articles/goodgobad.html

AlanRatcliffe

makepeace wrote Alan, there's no reason why the process outlined in my first post shouldn't be followed. Think you might have missed it.

You know more than I, but my thinking is, caps have a 15 year lifespan (so I've heard), so the originals will [definitely/i] be problematic by now. So wouldn't it make sense to just replace all the caps as a matter of course? And then run through the procedure you outline so nicely?

makepeace

Alan Ratcliffe wrote
makepeace wrote Alan, there's no reason why the process outlined in my first post shouldn't be followed. Think you might have missed it.
caps have a 15 year lifespan (so I've heard), so the originals will [definitely/i] be problematic by now.

not necessarily in fact. it depends on a lot of things - like the actual rated life of the caps, at what temperature you're running them vs the temp at which they're rated, voltage rating vs applied voltage, loads of other electrical characteristics, storage conditions etc. also, in engineering the design life will always be a conservative figure.

i've pulled some old caps from a really old tube radio - both signal and ps - that i put in my little reverb and they seem perfect.

obviously its good practice to replace them as a matter of course - so Rob maybe you should just do that. i would say start with the PSU caps. if you can get away with it - i'd leave the signal caps in there.

Rob-Brown

well guys its really no big deal my multimeter reads capacitance as well so checking for inaccurate caps would be no worries. Is it safe to assume caps that read under they're rating would change the amps dynamics greatly? What would be an acceptable tolerance?

don't worry Al I'm generally a very cautious person I wouldn't cut corners or do anything without decent research on how to!

thanx makepeace for all the info. What amp projects have you done before? Can restoration be mission without guidance from a tech?

makepeace

signal caps over or under their capacitance value by a normal amount will cause VERY slight differences in the frequency filtering of the signal, not nearly enough to be noticeable by normal people. if a value differs from the rated value+tolerance by too much then its probably safe to assume that it should be replaced. you should be able to read the tolerance on the cap, but a normal value would be +-20%.

i've built a 5E3 with a little bit of help from Karel, loads of effects pedals and a standalone tube driven spring reverb unit - all of them constantly evolving. but i found the internet to be an absolutely amazing source of info. create an account on music-electronics-forum.com and potentially wattkins.com. have learned loads on those two forums. tdpri is also a good one. got a couple of plans on the go: AC15, TW style amp, plexi style amp, hiwatt style amp and then a couple of amps of my own design; a multi-power tube single ended emp, a multi-tube high power amp and an amp using only octal tubes. i'd say if you spend enough time on the internet and you're interested enough, yes. but its nice to have a tech that you can comfortably ask questions - via e-mail or whatever.

Rob-Brown

Ok I know I'm probably gonna get into some trouble for this one but. I've been doing some research and people seem to keep comparing the GA-20 with the 5E3 saying that the main differences in tone and overall sound is from the output transformer. They were saying that the GA-20's OT was smaller than the 5E3's and caused it to become more saturated and lose more low end and because of this there would be very low clean headroom...

Soooo :-[ I know I said I wouldn't "mod" but wouldn't it be cool to put two different OT's on a switch loosely at the back (discreetly) to try and get both tones more usably, then it would solve my ridiculous thought of fx loop by giving me more clean for the times I need it (not having to transport an extra amp). WHILE still only being 4 snips and a quick re-solder back to stock. :-\

(awaiting annoyed posts back ?)

Rob-Brown

@makepeace Wow long list ahead keen to know how they go! Thanx for your constant help and advice!

is the 5E3 a tough build for a first timer?

makepeace

i would say anything is a tough build for someone that knows nothing about it. its a steep learning curve, and it required lots of reading and fiddling. its far from doable though, mine worked the first flick of the switch.

i would suggest that you hear it as it is first. the 5E3 has often been criticized of having far too much low end, and there is a camp that believes coupling and cathode bypass caps should be made smaller. that being said, if you feel that its necessary, it would be pretty easy to do the switchable OT thing, you could just strap the other one to the other side of the speaker.

another thing to note is that you'll need to do much more than the OT to get this to sound like a 5E3. the phase inverter is different (paraphase vs cathodyne - that's where a lot of the 5E3's sound is), the GA has pentode input stages, different signal filter circuitry, different coupling cap value all around.

Rob-Brown

Ok I see I'll need to develop some kind of test amp somewhere along the line where I can just throw different component, circuits etc and make some recordings to understand the tonal differences.

Quite keen to hear the dynamic of those 6v6gt's. I've only had an amp before with 6l6's

makepeace

6v6s are great.. blues bottles.

ezietsman

Yeah don't be afraid of building an amp of your own. Quite of few of the forumites have done so already and there are plenty of places that will provide great kits with instructions which is more than enough for a first-time build. Makepeace was one of the first with that beautiful 5E3 of his which convinced me to do the same (I built a Marshall 18 watt clone as a head) and Shaunf just recently finished a spectacular Trainwreck Express Clone (His first amp build). Nicholas L has built a couple (including a 100 watt soldano clone) and Atilla built an amp. Others include Karel, Gearhead and MikeM each having built multiple amps. There are probably some others I'm forgetting. All in all a great experience, as long as you follow the safety rules very carefully.

Rob-Brown

Oh nice! ? I had my eyes one that 18watt kit for a while they look superb especially with the white tolex. But with the GA-20 having a similar overdriven tone I'd probably try build something either fender, vox or orange voiced depending on the cost.

Would love to build a plain three knob amp head (with fx loop ?) +-15watt but more for clean "jangly" tone

Yoh much respect to Nicholas L can't imagine the work that goes into building a 100w high gain amp like that

ezietsman

Rob Brown wrote Oh nice! ? I had my eyes one that 18watt kit for a while they look superb especially with the white tolex. But with the GA-20 having a similar overdriven tone I'd probably try build something either fender, vox or orange voiced depending on the cost.

Would love to build a plain three knob amp head (with fx loop ?) +-15watt but more for clean "jangly" tone

Yoh much respect to Nicholas L can't imagine the work that goes into building a 100w high gain amp like that

The 18 watt is a different beast altogether. Tight bass, lots of drive (extremely loud without power scaling), its a little bit more abrupt and I have some clips I recorded of mine. The 18 watt is jangly but I wouldn't say its best side is its clean side, although it does that better than most because its always a little bit dirty. If you play blues or classic rock, its perfect. It is not "nice". It sounds approximately like the tones Eric Clapton got on the Beano album with a Les Paul. That is the ballpark. Its a great great amp but it is the kind of amp that wants to be cranked and you ride the overdrive with your guitar knobs. If you don't want to do this, this is not the amp for you.

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