Laney CUB 10 valve amp barely audible - Page 1
Forum section: electronics / repair

author: pvennberg
author_rank: Newbie
timestamp: August 25, 2023, 06:12:05 am
content: Hello!I have a Laney CUB 10 valve amp that's barely audible.When I crank the gain and volume it's just loud enough to be heard over the guitar itself.There is no hum or other sounds so everything is quiet, not just the guitar.The guitar sound coming from the amp is quite clean, not much distortion going on.Not sure if this aids in the troubleshooting but the power led is not lighting up.What I have tried/checked:Tried the guitar and cable in another amp. Nothing wrong there.Replaced all the tubes and got the same symptoms.Checked the output transformer for shorts or any other issues, none could be found.Checked the voltages of the power transformer, got 504 V, 45 V and 7 V.Visual inspection of the circuit boards.Measured the "larger format" diodes and resistors while in circuit.Schematics can be found here:https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-10%20(2009)-Schematic.pdfNew to electronics and don't really know where to go from here.Any input would be greatly appreciated!Best regards

author: Audiorepair
author_rank: Frequent Contributor
timestamp: August 25, 2023, 06:26:03 am
content: Check the External Speaker jack socket. This incorporates a switch that is closed with nothing plugged in.If this is not making a good contact you will get little or no sound from the speaker.Clean it with switch cleaner or plug a jack in a and out of it a few times, or one with shorted tip/sleeve.One other thing is you can check you have HT on pins 1 & 6 of each pre-amp valve, it is safer and easier to do this by removing the valves and measuring at the pin in the sockets with the other meter lead on chassis.. It is surprising how often the Anode resistor supplying this HT goes open circuit in valve amps.

author: pvennberg
author_rank: Newbie
timestamp: August 25, 2023, 09:58:45 am
content: Thanks for the inputForgot to mention that i checked the connector for external speakers, both with a multimeter and by connecting an external cab.Exactly the same, very low volume.Checked the voltage on all pins on all the tubes.I disconnected all the tubes during the measuring.These are the results:Pre-amp had pretty much the same values on each of the two tubes:1. 353 V DC2. 0 V3. 0 V4. 5 V AC5. 5 V AC6. 353 V DC7. 0 V8. 0 V9. 2 V ACPower-amp had the pretty much the same on each tube as well.1. 0 V2. 2 V AC3. 360 V DC4. 360 V DC5. -22 V DC6. 0 V7. 5 V AC8. 0 VI guess this answers the question regarding pin 1 and 6 on the pre-amp tubes.Are these the values one should expect?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: August 25, 2023, 10:23:16 am
content: Just double check that there is 6.3V AC or thereabouts BETWEEN pins 4/5 and pin 9 on the pre-amp tubes.Your readings seem referenced to ground which is not how the heater circuit works.Otherwise everything else looks OK.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: August 25, 2023, 10:38:58 am
content: If you have a scope, my next step would be to hook it to the Volume pot and see if there is any signal either in or out.It should be quite large.That would at least divide the problem into one of two halves of the circuit, and is quite safe and easy to do.You really don't want to be scoping the wired side of the tube sockets, it is far too easy to slip and then have a far bigger problem on your hands, or a nasty shock or worse.In particular you really shouldn't ever scope the Power tubes or Output Transformer.I petty much never do that if I can help it, and mostly you don't need to.

author: pvennberg
author_rank: Newbie
timestamp: August 25, 2023, 12:41:01 pm
content: Checked the voltage between pin 4/5 and 9.Got between 6.3 and 6.6V AC on both tubes.Don't have a scope unfortunately, will try to get a hold of one.Will let you know how it goes.Thanks for the help, great pointers

author: floobydust
author_rank: Super Contributor
timestamp: August 25, 2023, 07:34:24 pm
content: Quote from: pvennberg on August 25, 2023, 09:58:45 amUh, "disconnected all the tubes during the measuring"- no don't pull them all out. Measure the tube voltages with them in-circuit and warmed up.At least narrow down which stage is dead. I just touch grids here and there for hum to see if there is gain.A really quiet amp I have seen due to a coupling cap crack in two, go open circuit.