Guitar tube amp Laney VC30 212 - Page 1
Forum section: electronics / repair

author: AndreyS
author_rank: Regular Contributor
timestamp: January 09, 2022, 11:24:27 am
content: Hi there,while I was measuring tube voltage, I accidentally touched plate lead (green arrow in the image) with my mm (the other lead was on ground), the amp then made poping sound, I immediately turned it off. When it cooled I turned it on and it works, but it now makes low while noise even when all volume knobs are on 0. Its not a deal breaker, but it used to be completely silent.Previous owner looks like soldered 1 Ohm resistors to the tube plates. Tubes are 6P14P.It is class A amplifier.Before the accident, I took a time and measured current across those resistors (according to the image from left to right): 53 33 49 31 mV.After the incident I measured voltage in point P51 to ground = 315 V (according to schematic it should be 290 V).What caused that noise?

author: AndreyS
author_rank: Regular Contributor
timestamp: January 09, 2022, 02:46:40 pm
content: I think I've found plausible culprit: resistor r12 measured 170kOhm at 1% tolerance (while it should be 150k)

author: AndreyS
author_rank: Regular Contributor
timestamp: January 09, 2022, 02:49:22 pm
content: in fact when I disconnected all small tubes, there was no noise (or so little, thats ok)

author: CaptDon
author_rank: Super Contributor
timestamp: January 10, 2022, 02:34:08 am
content: So what if you touched the plate lead with your multi-meter set to volts??? No harm done. You just measured the plate voltage. As for the pop, yes that happens when the transient imbalance is picked up by the pre-amplifier section. Class-A you say??? What moron builds a push-pull power amp and biases it to class A?Maybe AB1 or AB2??? Class A would be stupid for that design. No sounds when you take out the small tubes??? No Shit?? That kind of trouble shooting is like taking the engine out of a car and saying it isn't as noisy as it used be. Let us know when the smoke starts rolling out.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 10, 2022, 09:54:46 am
content: hum the gray wire next to the 2x reds with your added arrow on them, where does it go ? doesn't seem soldred at allNormally you could do a torough check, carefully remove the tubes, mark their positions, measure all the resistors just to be sure, measure all the voltages, dont slip the probes this time lollEven a ''short period'' short could cause some damage, normally tubes can accept some beating before dying outsockets contactsIt is not an pure class A, since it has an push pull outputBUTin theses times the pre-amp section would be a class A driven section.

author: Circlotron
author_rank: Super Contributor
timestamp: January 10, 2022, 11:09:44 am
content: Quote from: coromonadalix on January 10, 2022, 09:54:46 amClass A means the tube has a conduction angle of 360 deg, so you can have both single ended and push pull class A.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 10, 2022, 12:22:28 pm
content: A pure class A will take as you say the entirety of the signal 360 degree as you say, and will drive the output xformer directly, meaning only one tube lamp as the main output.Once you add an push pull section, the ''pure class A'' die for the output section, search class A tube amp you'll see what i mean ...but i wont start a feud here / dilute the OP thread

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 10, 2022, 12:43:58 pm
content: Be very wary with this amp when changing the output valves, as Pin 1 is connected to HT, the screen grid resistor then jumps this to Screen grid pin 9.Some EL84 valves including Mullard, EI, RCA and Golden Dragon have pins 1 and 2 shorted together internally.Plug in one of these valves and your amp is going to go bang in spectacular fashion.

author: strawberry
author_rank: Super Contributor
timestamp: January 10, 2022, 01:06:17 pm
content: poor tubes

author: CaptDon
author_rank: Super Contributor
timestamp: January 10, 2022, 03:26:12 pm
content: Yes, you can have push pull class A, but it would be stupid. So much heat being generated by the output devices with no power gain (in fact a bit of power loss) over AB1, AB2 or B push pull. Sure it can be done but I can't think of any time I have ever seen it used. Well, then there are those audiophools who have probably built one selling at $100,000 USD and claiming .00000001% THD because it is 'distortionless class-A'. You can build a car with square wheels and Lord Corporation in Cary N.C. will design and build a suspension system for you to make it ride smooth. But I haven't seen that yet either. Cheers!!! More snowfall coming here on the north coast. Great time for finishing those projects laying on the bench!!

author: commongrounder
author_rank: Frequent Contributor
timestamp: January 10, 2022, 03:31:21 pm
content: This amp employs a cathode-bias class AB circuit. You can see the 9-volt test point at the cathode resistor/capacitor. The previous owner added those 1-ohm resistors to monitor the four output tube currents individually to aid in matching. Based on your measurements, those output tubes are not well matched at all. I also see a discrepancy in the screen resistor values. The amp has 82-ohm parts, and the schematic calls out 100-ohm. It could be a mid-production run change.

author: strawberry
author_rank: Super Contributor
timestamp: January 11, 2022, 04:41:15 am
content: Quote from: commongrounder on January 10, 2022, 03:31:21 pmand never be, four tubes cathodes form constant voltage source, one bad tube can send good tubes in thermal runaway (redplating)Quoteor run out of 100ohms or dont care

author: floobydust
author_rank: Super Contributor
timestamp: January 11, 2022, 04:55:06 am
content: Slipping with the probe can damage things - i.e. short pins 6,7 here and nail the control grid with plate voltage. Nobody likes that. Put a heatshrink sleeve on the probe and generally stay away from the output tube plates, they can kill multimeters due to their input capacitance and the readings are useless anyhow.Your cathode-resistor measurements show one pair is much weaker 31/33mA vs 49/33mA than the other. So there might be an older or junkier batch of 6P14 in there. Just give them a gentle tap to flush out the noisy one.Guitar amps can be heavily biased into Class A for a "fatter" and warmer sound. Basically to the limits of the output tube's dissipation.

author: AndreyS
author_rank: Regular Contributor
timestamp: January 15, 2022, 08:26:46 am
content: Quote from: CaptDon on January 10, 2022, 02:34:08 amThank you for your input, very inspiring indeed and helpful.As for Class A, I just cited what is said in the manual.

author: AndreyS
author_rank: Regular Contributor
timestamp: January 15, 2022, 08:30:17 am
content: Quote from: coromonadalix on January 10, 2022, 09:54:46 amIt's the previous amplification stage plate output.It is soldered to P33 (seen in schematic) ob pcb

author: AndreyS
author_rank: Regular Contributor
timestamp: January 15, 2022, 08:35:58 am
content: Quote from: Audiorepair on January 10, 2022, 12:43:58 pmYeah, man. I've seen a guy on youtube who burnt Mullard tubes that way. Nasty. And I did some investigation - its not a mod, it was soldered that way during manufacture

author: AndreyS
author_rank: Regular Contributor
timestamp: January 15, 2022, 08:41:47 am
content: Quote from: commongrounder on January 10, 2022, 03:31:21 pmI was concerned about those resistors as well. The fact that everywhere I see the guts of this amp having those particular resistor (with greenish looks) proves they are soldered during manufacturing process.

author: AndreyS
author_rank: Regular Contributor
timestamp: January 15, 2022, 08:56:50 am
content: In conclusion to the starting issue, the noise faded away over less than a day, I mean I left the amp untouched until the next day I tested it again.Somehow isn't not noisy now, or at least the noise floor is similar to what it used to be. Especially the clean channel is clear as it used to be.This is my first endeavor into tubes. I don't have any ee qualifications, although I'd love to have them.Also, some master replaced original reverb tank with noname with higher input resistance, I was wondering why I don't hear reverb effect. Found suitable accutronics.Almost everything I use used to be damaged/with defect: monitors, mouse, speakers, keyboard, PSU and other stuff I hardly remember now.I feel very grateful that I can breathe a new life to broken things.What killing me the most are those human vultures that ripping off all the copper from electronic home appliances people just threw on the sidewalk. Many of those things can be easily repaired.I think there should be some charges vs those vultures, which would make their doing unprofitable.Also it makes recycling organizations less profitable, and maybe that's the reason why we don't have such an organization in every town

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 15, 2022, 12:09:16 pm
content: Quote from: AndreyS on January 15, 2022, 08:35:58 amFunny story, the WEM Dominator amp was wired with the grid signal on pin 1 of the EL84's, as they used Mullards with pins 1 and 2 connected internally.So if you come across one with the original Mullards, (I have twice), changing the EL84's to JJ's or whatever means the amp won't work.Took me a bit of head scratching on the first one I saw to work out what they had done.