Gibson GA 15 Amp Repair

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Gibson GA 15 Amp Repair

My old college chum, John Bickersteth (Lazy Lester, Incredible String Band) brought me his 15W valve amp, as it has a blown EL84.
It’s an opportunity to get a look at another design of amp for me, and do him a favour also. I got it complete with cobwebs and dust – you can see the white dome on the EL84 indicating the tube has been compromised and possibly moisture has entered.

This – normally silver – compound is called a “getter”.
“A getter is a deposit of reactive material that is placed inside a vacuum
system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. Thus the getter removes small amounts of gas from the evacuated space”.

http://en.wikipedia.org/wiki/Getter
It adsorbs (a surface only absorption process) or reacts with unwanted gases inside the tube to maintain a vacuum.
http://en.wikipedia.org/wiki/Adsorption
To not waste a new EL84 on an unknown amp, I removed the duff valve and powered up without it first. This at least helps indicate there are no other major failures if all the other heaters glow OK etc., and there is no smoke or noises, which would mean removing the chassis to check it out.
At this stage, without a circuit diagram, I’m assuming the two EL84s are a symmetrical pair in a “concertina” or similar balanced design output, not a parallel output power stage.
I will remove the chassis anyway at some point for a look, and to maybe change a duff volume pot also at some stage, but for now I just want to hear it if possible.
Ok, NOT good news for John – after fitting another EL84, not even mains hum at full volume – I’m hoping for his sake it’s not the output transformer also – a 40 quid repair for OT and valve.
My Frequency Response curve will have to wait…chassis removal time and search for a schematic.
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Testing resistance across the Celestion Vintage G10, 60W speaker I get 1.2 Ohms. By not getting close to 4, 8 or 16 ohms I know the speaker is still in parallel with the OT secondary, so that is probably still OK, but those I’ve measured so far in my limited experience, are much lower than this – appearing like a short of 0.3 ohms on a DDM, so I’m not sure yet.
Ok, that’s disconnected from the speaker now, and now reads 1.1, 1.3 and 1.6 ohms across the whole centre tapped windings so should be OK. At least it’s not a burnt out winding. Same for the center tapped primaries reading 230 and 215 ohms each side, and 445 across both.
The volume pot must have taken a knock – it was just rubbing hard on the chassis.
Ok, speaker good at 7 ohms.
Maybe it’s the DC to the board?
Nope.
After a clean of all the plug type connections, with IPA switch cleaner, she fired up and sounds great! A lovely tone, as you would expect from the Trace Elliot designers. This one has 15/6/99 on the TE label.
The only major complaint would be the bright switch capacitor value – way too small. It makes it so trebly, like the Marshall on Hi input, that for me, it’s unusable. John doesn’t like it either.
I’ll remember 470pF as a value now, to not use in a Boost circuit in future!
Ah, SHIT! Not good now.
After a switch off for an hour, I returned to start the FR curves for cut and boost and it gave a bad hum surge through the speakers, and gave some white lightning inside the other EL84 this time, and seems to have blown that one too, and a fuse in the process also.
I assumed it was just that the other valve had finally given its last breathe also, as the amp has been left unused for a while, according to John, so I guess it had its last burn when I played it before it died.
It didn’t seem to cause any other damage in blowing out, and I checked the new JJ was still OK by putting it in the Marshall. I thought I would risk another valve from an old radio that I used only recently.
Yes! All fired up and sounding great again!
This would be a lovely amp to copy, component for component with hand wiring, but with 60 onboard components or so, it would be difficult.
It is REALLY loud – way too much for a bedroom and ridiculously dynamic. The overdrive comes in late on the volume at about 8 before getting really crunchy, but it’s too loud to bear close up.
I did the usual Frequency Response – turning up to visible deformation of the FLS sine wave on the scope, then winding the volume back to half that voltage, at 1kHz and 200mV input.
Here’s the response:
GA15FR.jpg
There is the 12AX7 100-600Hz dip again. I tested this through the 16 ohm cab, and it was flapping the speakers at 20Hz! It has a massive boost here. I didn’t bother with putting 20 Hz on the other amp tests Excel sheet, as it is inaudible and/or barely measurable on the scope. This was twice the amplitude than at 50 Hz!
You would think it is a lot of effort by the designers to get a high response for 20 HZ as the lowest fundamental on a guitar is bottom E at 82 Hz, so why bother?
The difference in playing this amp is apparent though, because it is like all the strings have the same volume level across a low chord like a 6 note F, say. They all sit in a clarity space of their own it seems.
Note how much flatter the curve is also, compared to all my valve amps.
It reminds me a bit of how jangly and clean the Aston sounded when I built the original Tone pot only version, not the modified Treble and Bass version I have now, which has lost some character, overdrive and tone it seems. I should have left it as it was, and changed the front panel.
It will be interesting to play both back to back through the 16 ohm cab, and record them. The Gibson has a 16 ohm speaker out jack also. I won’t be able to hit distortion volumes though as its too powerful for my cab.
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Note – I take all that back re the Aston – goes to show how fickle memory is – I have just been playing both for a few hours back to back through my cab, and although the Aston kit amp is much thinner and less bassy with EQs set flat, it’s clean tone sounds as good OR BETTER by boosting the Bass to 9. It retains it’s “Strattiness” like this also, instead of having the mids and highs thickened too, like the Gibson does – makes that a little more Humbuckery. You have to A/B amps together to know how they sound – memory can’t be trusted in a comparison.
What a great little amp the Aston is – still one of my nicest clean sounding amps overall, especially now the ground hum is fixed, by having only one ground as per the Grounding PDF. (It may have been noisy due to my original messy wiring, not the wiring schematic – but 1 chassis ground is still better than 3 in a build if you can do it.
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If I saw one of these Gibsons on EBay I would definitely bid on it, but I doubt anyone would let theirs go once they have one.
Again, it shows what can happen if amps are left too long un-played. This was just a case of a bad contact somewhere – apart from the obvious blown valve – that just required a wipe and look over, and connectors and jack sockets spraying with Isopropyl Alcohol.

There is one major design flaw in my view – the controls at floor level at the rear – not good at all. I think the volume pot got a drunken boot to it at some point at a pub gig or similar – (probably John! Haha – he wears massive steel toe capped boots), the mains lead is here also so a trip hazard to get to the rear, and kneeling down to floor level or just reaching behind the cab for tone or volume during a gig…? No.
Other than that – one lovely amp, back on the road!

John’s got a 70s Marshall in need of assistance next…that will be interesting! I hope it’s not too much for my skill level.