bartelby Posted December 9, 2017 Share Posted December 9, 2017 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. 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. 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. 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. Link to comment Share on other sites More sharing options...
Tonteee Posted December 9, 2017 Share Posted December 9, 2017 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. 1 Link to comment Share on other sites More sharing options...
skankdelvar Posted December 9, 2017 Share Posted December 9, 2017 (edited) Slightly iffy upside down image of a GA620 schematic here and a better one here: https://imgur.com/GYC295B Edited December 9, 2017 by skankdelvar Link to comment Share on other sites More sharing options...
bartelby Posted December 9, 2017 Author Share Posted December 9, 2017 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. Link to comment Share on other sites More sharing options...
bartelby Posted December 9, 2017 Author Share Posted December 9, 2017 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. Link to comment Share on other sites More sharing options...
skankdelvar Posted December 10, 2017 Share Posted December 10, 2017 Good luck with your project, chap. It's a handsome little amp; love the Strat vol knob Link to comment Share on other sites More sharing options...
bartelby Posted December 10, 2017 Author Share Posted December 10, 2017 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... Link to comment Share on other sites More sharing options...
gary mac Posted December 18, 2017 Share Posted December 18, 2017 What a great, challenging project. Good to see that you are getting there with it. Link to comment Share on other sites More sharing options...
ahpook Posted December 18, 2017 Share Posted December 18, 2017 (edited) Nice work, fingers crossed you get the minor problems sorted out. Sounds like you've got the hard stuff done tho. Edited December 18, 2017 by ahpook Link to comment Share on other sites More sharing options...
skankdelvar Posted December 18, 2017 Share Posted December 18, 2017 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. Link to comment Share on other sites More sharing options...
bartelby Posted December 18, 2017 Author Share Posted December 18, 2017 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. Link to comment Share on other sites More sharing options...
bartelby Posted December 19, 2017 Author Share Posted December 19, 2017 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. Link to comment Share on other sites More sharing options...
gary mac Posted December 19, 2017 Share Posted December 19, 2017 Don't forget to discharge the capacitors 😀 Link to comment Share on other sites More sharing options...
bartelby Posted December 19, 2017 Author Share Posted December 19, 2017 38 minutes ago, gary mac said: Don't forget to discharge the capacitors 😀 Yep, not electrocuted myself yet 1 Link to comment Share on other sites More sharing options...
Beer of the Bass Posted December 31, 2017 Share Posted December 31, 2017 (edited) 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 December 31, 2017 by Beer of the Bass 1 Link to comment Share on other sites More sharing options...
bartelby Posted December 31, 2017 Author Share Posted December 31, 2017 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... Link to comment Share on other sites More sharing options...
bartelby Posted January 6, 2018 Author Share Posted January 6, 2018 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 . 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. Link to comment Share on other sites More sharing options...
bartelby Posted January 7, 2018 Author Share Posted January 7, 2018 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. Link to comment Share on other sites More sharing options...
bartelby Posted January 7, 2018 Author Share Posted January 7, 2018 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... Link to comment Share on other sites More sharing options...
Beer of the Bass Posted January 8, 2018 Share Posted January 8, 2018 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. 1 Link to comment Share on other sites More sharing options...
bartelby Posted January 8, 2018 Author Share Posted January 8, 2018 (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 January 8, 2018 by bartelby Link to comment Share on other sites More sharing options...
bartelby Posted February 23, 2018 Author Share Posted February 23, 2018 Progress on this has slowed down a little. Replaced the output valves and bases yesterday. List of issues remains largely unchanged: 1) rumbly/rustly noise once the amp has been on for 20-30 minutes 2) bass tone control is very subtle. 3) deciding what transformer to use for reverb driver. Link to comment Share on other sites More sharing options...
slyraccoon Posted November 7, 2019 Share Posted November 7, 2019 If you are still working on this amp, I have repaired perhaps 6 of these and I currently own 3 of them. My experience was that the cable between the reverb transformer and the tank almost always disinitegrated/degraded to the point where it had to be replace. As to the reverb transformer, if you check Antique Electronics or Hoffman you will likely be able to find a 12K:8ohm transformer. These are really nice little amps that break up nicely at a low volume. The alligator tolex is damn near indestructible and if you replace the old speaker with a more efficient one, this amp has plenty of power/volume for a a small club gig. These are perhaps one of the most underrated/underappreciated/unknown vintage Japanese tube amps out there. They are simply too cool for words. Link to comment Share on other sites More sharing options...
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