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bartelby

Resurrecting a 60's Guyatone amp

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So several years ago an older workmate was clearing out his attic and mentioned he'd unearthed an old guitar amp, and asked if I'd like it. He'd bought it new, sometime in the early or mid 60s.

He said it wasn't anything special and he didn't want anything for it. So I ummed and arred for a few seconds and said yes.

The next day he brought in a Guyatone GA-620, so I went straight on to the web to do some research but couldn't find any info at all on this specific model.

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I took the amp to a guy in Bristol to give it a once over before I switched it on, he replaced some of the large capacitors that had leaked every where, and cleaned it up a bit. He had fired it up but not looked at anything else. When I tried it you could hear guitar quite loudly but none of the controls had any effect. 

Wizz on a few years and I took the amp to a professional amp servicing place in Cardiff. After a few days I called them and they said they were just about to look at it. 10 mins later they call to say the amp can't be repaired without spending more than it's worth. So I went and collected it. 

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After a bit of poking about here and there I found that all the paper in oil capacitors were dead. So just to see if I could get it working I bought some modern caps, all the ones I needed came to £19. After some soldering work and some contact cleaner in the pots and valve sockets I'm pleased to say that it's 75% working.

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The remaining issues are:

1) the volume doesn't go all the way to silent.

2) the minimum tremolo speed is still quite fast, this may just be how it's meant to be. But I want to see if I can slow it down some more.

3, and the biggie) the reverb doesn't work. This could be down to the fact that the link cable between the driver and the tank was shot. The inner insulation had turned in to a sticky tar like mess.

There are a coupe of resistors outside of their value tolerances, so I'll replace them soon.

But it sounds so nice. It goes from quiet-ish and clean to loud with a lovely crisp crunch.I was thinking of replacing the original speaker, but I don't think I need to.

 

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Well done on persevering on this amp.  I love to read about people deciding to get suck in with repairing things instead of disposing.  Glad it turned out well for you so far.

Re: he issues you describe, have you confirmed the reverb tank is operational?  I mean in terms of a basic continuity test.

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2 hours ago, Tonteee said:

Well done on persevering on this amp.  I love to read about people deciding to get suck in with repairing things instead of disposing.  Glad it turned out well for you so far.

Re: he issues you describe, have you confirmed the reverb tank is operational?  I mean in terms of a basic continuity test.

My plan for tomorrow is go through everything again. There was a broken wire in the tank that I’ve sorted and I’ve made a new link cable.

I haven’t looked into testing the tank itself yet.

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2 hours ago, skankdelvar said:

Slightly iffy upside down image of a GA620 schematic here

and a better one here:

https://imgur.com/GYC295B

Cheers Skank. 

I found the second one today on an Reverb.com ad for a ga-620 that sold last month.

So I’ll be studying that tomorrow too.

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21 minutes ago, skankdelvar said:

Good luck with your project, chap. It's a handsome little amp; love the Strat vol knob :)

All the knobs will be replaced I’m afraid. Once I find some decent vintage style ones that go from 0-10...

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Nice work, fingers crossed you get the minor problems sorted out. Sounds like you've got the hard stuff done tho.

 

 

 

Edited by ahpook

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On 12/10/2017 at 16:40, bartelby said:

All the knobs will be replaced I’m afraid. Once I find some decent vintage style ones that go from 0-10...

 Quick google image search of other Ga-620's suggests the knobs are all original apart from the Strat knob and in comparatively good nick. If you decide to remove them there's probably a Guyatone obsessive in Japan who'll sell a kidney to buy them off you. :)

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56 minutes ago, skankdelvar said:

 Quick google image search of other Ga-620's suggests the knobs are all original apart from the Strat knob and in comparatively good nick. If you decide to remove them there's probably a Guyatone obsessive in Japan who'll sell a kidney to buy them off you. :)

Now there's an idea!

Or I could see if I could source 1 knob. 

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I had a chance to have another bench session with yesterday evening.

Still have the same 3 issues. The trem speed isn't really an issue, more a preference. But the reverb not working is irritating and as the amp gets warmer it starts making rustling noises. I've reflowed all the solder on the valve sockets and any dodgy looking points on the boards. I'm going to order some new power stage valves after Christmas. Researching replacement resistors at the moment.

I have 2 more days in work then I have 2 weeks off. 1 week I'm away but then I'll have time to spend more than 90 minutes at a time looking at this amp. My to do list now looks like:

Replace capacitors on the bass control as they're also knackered.

Replace all the wiring as the insulation has degraded and hardened to the point of it cracking.

Decide whether to replace all or just some of the resistors.

then some cosmetic stuff.

 

so basically this could end up being a total rebuild. But it'll still work out cheaper than buying a new amp. And I'll have the added bonus of knowing exactly how it works, so maintaining it will be mush easier.

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38 minutes ago, gary mac said:

Don't forget to discharge the capacitors 😀

Yep, not electrocuted myself yet

  • Haha 1

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Sounds like you're almost there.  Slowing down the tremolo speed should just be a matter of increasing the value of those three .02uF capacitors in the tremolo circuit.  It may take some trial and error because if you go too large on the value the oscillator can stall and stop working.  I'd perhaps try .033uF and see how that goes.  The volume control issue may just be a worn out pot. 

Regarding the reverb, I'd check the DC resistance of the tank itself first - if the amp is the same as the schematic Mr Del Var posted, it should be roughly equivalent to an Accutronics AB type tank - the input coil should measure very low DC resistance (like 1-2 ohms) and the output coil in the hundreds of ohms.  If the tank turns out to be OK, I'd then try to isolate the problem to the driver or recovery stage.  If you can hear crashing through the amp when the reverb is turned up and the tank is tapped, the recovery stage is OK.  If the recovery stage is OK, I'd first check the driver transformer wasn't shorted or open, and check that the cathode resistor in the driver stage is OK (reverb drivers pull quite a lot of current, so can be hard on parts).  If all of that is OK, try a new valve in the driver stage. 

Edited by Beer of the Bass
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Cheers for the tips BotB!

 

My progress has been slightly hampered by:

Christmas

Being ill

Multimeter crapping out

and suppliers being closed over the holidays...

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After a couple of weeks off this ordering bits and recovering from flu, I've managed to spend a couple of hours with the amp again. With the aim of looking into the issue of the reverb. I did change one of the tremolo caps for a 0.033uF, which made no appreciable difference. I'll swap the other two next time.

As for the reverb I've checked the tanks transformers and they seem fine. I had managed to solder a signal wire to the ground :dash1:. I thought that may have sorted it but no. But I am getting the crashing noise, if I tap the tank, now. So the return part seems to be working.

Next week I should receive a new pair of output valves, some caps for the tone controls and a couple of other caps.  

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Quick update.

Found a mini jack to phono lead so I could use my iPod to plug straight into the reverb tank's input. I now know the tank and recovery stage work fine. Continuity of the cable between the amp and the tank is also good.

 

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Another update.

I checked the reverb driver transformer: Secondary coil was fine and 8 Ohms as expected. Primary was dead. No continuity, no nothing... :|

Chances of finding an off the shelf transformer 12k:8Ohm look pretty remote...

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I suspect a standard Fender reverb driver transformer (22K:8ohms) would work acceptably in that circuit, though not identical to the original.  Worth a shot, anyway. 

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Posted (edited)
3 hours ago, Beer of the Bass said:

I suspect a standard Fender reverb driver transformer (22K:8ohms) would work acceptably in that circuit, though not identical to the original.  Worth a shot, anyway. 

I can find a 8k:8Ohm would that be any better?

 

Edited by bartelby

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