Those diodes protect tubes and output TX when no load is connected to output TX (faulty cable, faulty speaker jack...) If someone push power section hard without load, TX as a giant coil produces high voltage with no way to go out safely. Those diodes have to "catch" those HV spikes and protect transformer. They should be around 2kV or 2x1kV in series. You can find diodes like that in peavey or bugera amps. They often fail to short, so blown fuse and repair fee is cheaper than new output TX. Cheers!
Forgotten Gear Restorations on RU-vid just posted a video about using MOV's instead of diodes, wired from transformer primary B+ to output tube plate(s), to protect the output transformer from an inductive spike in case of a no-load situation. The MOV's tend to be self-healing and are unlikely to short circuit, which means that you probably won't have to take the amp in for repair if you accidentally try to run it without a speaker load. PS, Stuart apparently plugged the primary-side power connector of the output transformer back into the board with the unit turned on and powered up, which isn't a good idea, especially if the chassis is out of the amp and the speaker is disconnected. Perhaps not as bad if output tubes aren't installed, but still something to be avoided.
Fantastic information Stuart! I'm always excited to see new content from you. This is rich in diagnostic tricks. I'm working on one now that seems to have a similar issueissue but it seems more complicated. I'm going to try the alcohol trick to make sure. Thanks again, Mike
Brilliant Stuart.. I think you might start a trend here on replacing SMD's with normal components . You have proved it can be done and the chances are that the amp ( and it's owner), will thank you for "amp tech's fix". As an aside, I used Laney amps in the early 1970's as did Slade & Black Sabbath of course. They were all valve hand made jobs with great sound and reliability . Shame they had to go down the route of producing SMD internals.
I've had loads of Laney amps over the years, but didn't like their move to more and more surface mount technology, so I've replaced them with hand built kit amps, built by me, so I can trouble shoot and do all the repairs myself in years to come (well that's the plan). Then again I started building my valve hi-fi phono-stage, pre & power amps over 25 years ago and they've not given me a moments trouble since they were finished and I've completely lost the upgrade bug....🙂
I believe that the diodes are there to protect the output TX from transients leading to breakdown of the winding insulation,this could happen if the external speaker lost connection even though plugged in to the external jack and the amp was full tilt on volume with an unloaded secondary.
I used to have a Valvestate VS100R MK3 combo that developed a fault where it started motorboating and it came through the speaker on start up. I junked the amp years ago but I've always wondered what caused it.
Very interesting. I recently bought a 1999 Laney LC30 II, and it sounds excellent, but I expect it could do with being checked over properly. Certainly not a job I could do. The alcohol trick reminds me of a problem I had many years ago, with a Vauxhall Chevette: it had a slight misfire, which only happened when the car was moving, and which I'd tried and failed to pin down. It never misfired when stationary, with the bonnet up. Anyway, I diligently changed the anti-freeze ready for winter, and the next time the car misfired, I looked under the bonnet and found blue streaks running back from the water pump (which was leaking), over the spark plugs.
Great idea...I have a Bugera 1960 infinity 150 watt amp. .problem when i switch it on it makes thud sounds that are quite loud them after some time it stops. What would be the problem with this amp.. regards for any help.
Great job, Stuart. Would an IR thermometer be better than using the back of your hand or lip to find the hot region on the board? May still need alcohol (great tip!), but IR thermometer might get you close
@@stuartukguitarampguy5830 Many, many years ago I worked for a semiconductor company determining why broken chips failed, which involved microscopic examination, etc, to find the failing component. We used liquid crystal in a syringe to find hot spots (as you know, that's almost always an indication of the failure site), but alcohol cleverly does the same thing on a board level.
I recently saw on a YT channel (sorry can't remember which one) where someone had a thermal camera that connected up to a smartphone, I think using the usb connection. It may be a cheaper alternative than buying a purpose built thermal camera.
What does Snubber Diodes D5 & D6 do to the output transformer and output power tubes? I would think rubbing alcohol while the amplifier is turned on would create a short circuit because even 90% alcohol has water and water is a conductor that will create a short circuit while the amplifier is turned on.
H wayne. Those diodes protect the output transformer against very high voltage transients. Although tbh I'm not sure under what conditions those voltage transients arise. Obviously you don;t want to be slopping alcohol all over the board if turned on, but you'll notice I was very precise and just put a small drop onto the component.
@@stuartukguitarampguy5830 Yes I noticed it but its Risky with an amplifier its high voltage +400vdc to +600vdc put high current so that is why I'm scared of the risk to short out something use Alcohol because it has water.
Hi Wayve Yes you've got to know what you're doing, like anything really. I guess one option would be to unplug the amo and then do the alcohol if you were ubsure. The faulty component would still be hot.
@@stuartukguitarampguy5830 It's a sad thing that partly because of an deluge of Chinese fake components in Brazil, I'm closing my service at the end of the month (my repairs are no longer reliable). I will be returning to the UK in June and eventually set up a service centre again there, but focused on Reel to reel tape machines, Vintage Roland equipment and other stuff.
@@stuartukguitarampguy5830, Forgotten Gear Restorations recently posted a video about using MOV'S instead of diodes to protect the transformer; the MOV'S are usually self-healing and can shunt the spike without shorting out. (Circuit placement would be slightly different compared to using diodes: from transformer B+ to output tube plates).
Thanks for your reply - I have been an amp repairer/technician for 50+ years. High end manufacturers started to fit these fly back diodes post mid 80’s. They were fitted mainly to high powered valve amps 100w + to protect the valves from transient voltages produced at various frequencies by the audio signal. In my experience I had many failures due to shorting even when some manufacturers had put 3 diodes in series. The amount of valves that failed due to these transients was a lot less and if the HT fuse was always of the correct value no serious damage would occur. This is what I have experienced as a repair tech. Hope this helps - regards Mark
@@stuartukguitarampguy5830 , the transformer needs to see the reflected speaker load on the secondary winding. Trying to play through the amp without a speaker load, due to a speaker that is completely blown or accidentally disconnected, can blow the output tranny or cause arcing inbetween the tube pins or tube socket terminals. The higher the voltage and the more powerful the amp, the more likely that just one power chord without a speaker load could cause major damage. (Dust and grunge on the sockets or between the tube pins can make arcing more likely to occur, which is another reason why oily or greasy cleaner residue on the tube sockets should be avoided).
Hi Philip. Sorry I don;t think you're right on this one. They're reverse biased diodes there to protect against very high voltage transients which can occur ubder certain circumstances. I'm unclear what those circumstances are!
Brilliant Stuart.. I think you might start a trend here on replacing SMD's with normal components . You have proved it can be done and the chances are that the amp ( and it's owner), will thank you for "amp tech's fix". As an aside, I used Laney amps in the early 1970's as did Slade & Black Sabbath of course. They were all valve hand made jobs with great sound and reliability . Shame they had to go down the route of producing SMD internals.
