Tube Guitar Amplifier, simple question with attached schematic - Page 1
Forum section: electronics / repair

author: Zyan
author_rank: Newbie
timestamp: January 14, 2017, 07:24:10 pm
content: Hello, I got one question, look at attached schematic. I wonder if I can fix my amplifier by replacing a tube.Can V3 (located at F4 on the first page) that is a phase inverting tube be a bad tube that makes a humming in the speaker that is not affected by Master volume, but the noise is effected by speed and depth (located at G1 on first page)?I will break the question down for you.I got a hum when master volume (located at G1 on first page) is turned down. Therefor my conclusion is that it cant be a bad V1 or V2, cause the volumeknob would not let the humming go to the speaker.I can control the humming with the tremolo knobs speed and depth (located at G1 on first page), then I know the hum is created infront of the Tremolo, then it cant be the four big powertubes on the second page that makes the hum cause then it would not be affected of the tremolo knobs.Please confirm these 2 asumptions so I know that V1, V2, V4, V5, V6, V7 is not humming. And please help me to know if a broken V3 can make this hum.

author: Shock
author_rank: Super Contributor
timestamp: January 14, 2017, 08:58:39 pm
content: Yeah.. I can't really understand your reasoning there but if the speed and depth are turned fully down and the tremolo turned off at the foot-switch and the hum totally goes, it's likely going to be the tremolo circuit that is the problem.To troubleshoot V3, it looks the same tube as V1 or V2, you can normally plug in the guitar via a foot pedal straight into the effects loop return (only). You may also need switch the bypass off (read the manual if you are unsure where any of this is).The idea is to isolate the front end of the amp from the recovery/power amp end via that effects loop. Then if you still have the problem you could swap V1-V3 and see if replacing V3 resolves it.If all this sounds confusing and you have never replaced tubes before etc then you should take it in for warranty/servicing.

author: Zyan
author_rank: Newbie
timestamp: January 15, 2017, 04:31:36 pm
content: Quote from: Shock on January 14, 2017, 08:58:39 pmThank you for the reply good sir.The state is:Humming is always on and doesnt care about the volume knob (the humming is at the same volume even when I turn the volume up). So so my reasoning it cant be the first two valves V1 and V2, cause you cant hear them when volumes is turned down. I doens matter if I disconnect evrything, bypass the loop(of course the amp also has the normal tube amp noise when volume turned up, but that is ignored here because that is the normal function, and I also forgot to mention, the amp works well and sound good except this added hum)The humming is effected by the tremolo, so the humming must be infront of the tremolo. The powertubes are after the tremolo so the hum cant start in those tubes.I have no problem with changing the tubes, I dont have any spares and would, but a rotate of V1, V2 and V3 is a smart option if we know that it can be one of these tubes that makes this hum.I tried the thing with the pedal, I dont have a pedal but I made substitute with a stereo tele plug and two switches. The humming is still on even when I turned the tremolo off with the foot switch jack.I will take it to the shop if we can show that all the tubes are fine, tubes are replacement part that is not covered by warranty.And dont worry, Im smart enough to not take any chances, I know that the filtercaps can contain at most 400V even when it is turned off and unplugged.

author: Cubdriver
author_rank: Unknown
timestamp: January 15, 2017, 08:40:54 pm
content: How old is the amp? Is the hum at mains frequency or twice mains frequency (50 or 100Hz for you)? I'd suspect defective power supply filter caps based on what you're describing.-Pat

author: Shock
author_rank: Super Contributor
timestamp: January 16, 2017, 09:53:59 am
content: Yeah take back to the shop especially if it's new, just don't get suckered into paying for a tube or set if the amp and tubes have a few hours on it. A hundred hours is a different matter and more likely natural causes.If the amp is old get them to service it, if all the tubes are old and due to be swapped, keep functional tubes as spares.

author: oldway
author_rank: Super Contributor
timestamp: January 16, 2017, 05:02:53 pm
content: Looking at the schematic, it seems difficult for a faulty V3 tube to cause a hum.The only possibility would be a lack of insulation between cathode and filament but even in this case, I do not think this is possible because the 6.3V circuit is not grounded. (Only 2 resistances of 1M to ground)The insulation fault between cathode and filament is rather rare.To make a diagnosis, it would be necessary to measure the ripple of the power supply and to check that there is no alternating signal on C17 with a multimeter in ACV range.

author: Zyan
author_rank: Newbie
timestamp: January 19, 2017, 08:30:26 pm
content: Thank you for your replies.The amp has a couple of hundred hours but is still in the warranty, after some arguing the dealer is willing to take it back, fix it and pay for all shipping. But I will be without it for 3 weeksso I was hoping to be a faulty tube.Dont have a scope so I cant measure the hum, but the hum has a low note, it is about G2, and that is 98Hz (take or give cause I dont have perfect pitch hearing), here in Sweden the ac is at 50hz so it is twice the main frequency is very possible.All evidence is saying that it is not a faulty tube and I have to send it back.

author: Cubdriver
author_rank: Unknown
timestamp: January 19, 2017, 09:16:27 pm
content: That sounds a lot like a dead filter cap in a full wave rectified power supply, then. Hopefully they fix it quickly and without too much fuss.-Pat