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As I understand, it was normal in the day to sell them separately so pairs weren't sold with consecutive serial numbers but this pair's numbers are extremely different. It could be that someone who could afford the amplifier back in the day might have been older and wasn't sold on the new-fangled stereo sound fad and saved themselves the extra cost! The second one was added once the owner (not necessarily the original owner) wanted another for stereo sound.

Amazing how you struggle with this beast 🙂Your videos show expertise and experience and demontsrate how much you know about valve amplifiers. Respect! The loud crash sound in this video reminds me of a repair of a guitar amplifier many years ago I did for a friend. He had a guitar amplifier with a MOSFET output stage which did not give any sound at all. But all the time when I turned the volume knob (and only then!) extremely load crashes could be heard in the loudspeaker, exactly sounding like in your video. I sat hours in front of the amplifier and couldn't find the reason. Then I noticed that the wiring from the output stage to the loudspeaker was very close to the input signal routing. They were only a couple of centimeters apart from each other. And when I increased the distance between them considerably, the amplifier awoke from its "sleeping beauty" slumber... So, my idea (just an idea...) is that HF oscillation could be the cause of the loud crash sound. The unshielded ribbon cables (especially when folded and bent close to the pins of valves !) create some unwanted feedback via stray capacitance and make the valve amplifier walking on the edge of instability. And because the shielding backplate is removed, whenever you come the ribbon cables close with your fingers, hands or your face, you increase the unwanted capacitive coupling from the amplifier's output (loudspeaker wires, power valves, etc.) to the ribbon cables or other internal circuitry and initiate oscillation. Moving away the ribbon cables from the valves' pins towards the grounded backplate would eventually help to prevent HF instability, provided the corrosion is removed from the backplate connections. Warm greetings from the romantic Black Forrest in South Germany 🙂

Yeah, 15:00 is the mark. No, the owner of the chair, I think you did a very good job of explaining the things that you wanted to explain. I quickly skimmed over a lot of it but I can tell, as a chem/physics teacher, that you were doing a very good job of explaining how a potentiometer works. It’s just not why a lot of people came here.

STUART UK, How can you measure the tubes Transconductance in-circuit? The transconductance is the tubes GAIN at a certain Bias voltage. Do the transconductance formula would be Plate Current / divided by the Grids Bias Voltage = Transconducance?

Hello Stuart, when measuring and tuning the frequency response, how was the setting of the presence-control and how does it affect the scope-measurement?

I missed the last little bit first time around, so what happened? Should you have put a new rectifier tube in ?

Stuart does it again..Great job and great video as always.😊

I love all those little jokes...

Nice to watch but I wouldnt like to have fixed it. Everything is throwaway, none fixable. Cheers

Cheers Stuart very handy little piece of equipment Ill make .

Excelent!👏👏👏

Very nice repair. Your pretty good at these repairs..cheers

Hi Stuart, I'm enjoying your video and have a quick question. I did not see you discharge the capacitors and was wondering if these are safe to service without doing so? I believe tube amps have much larger capacitors? Thanks

Why have you different bias meters ? You had the LCD screen ones before ? Cheers

My dad used to have something similar and was always splicing magnetic tape to make the loop. But it worked fairly well.

Is that hum at 100Hz ? ie 2 x 50Hz mains ?

That was a good well thoughtout fix. Well done matey. cheers

that old tape loop looks a bit short, the red arm shouldn’t be angled in that far. i remember talking to Charlie Watkins about tape loops and he said they should ideally be changed after a few weeks use! mine usually last at least a year! 😂

That was great.. cheers

As always an interesting video Stuart. The odd heater wire arrangement might have been something to do with lowering the current through the PCB tracks. Great fix that's bound to please your customer.

Not very often we see a time served electronics engineer that not only repairs the amplifier but plays guitar too!

There are 2 connectors to halve the current going through the connectors. I've had problems with heater connectors on PCB's before. If the connection is not perfect they heat up and can damage the PCB or the connector itself. It looked like the cables on the faulty connector were showing heat damage implying a bad connection.

Great video! I have a guitar that has an issue with the volume pot when turned full up the sound cuts out slightly with sort of slight drop in volume and gain only when FULL up. I’m wondering if I pull it and try some contact cleaner if that would fix it. The problem is only in the same spot every time at full up position and because of this I can only play with the volume pot at about 9.5! Others have said that it’s just a short in the pot and needs to be replaced.

The great DeOxit controversy solved for the world!

Stuart, Another interesting dive into an amp with issues. It seems to me that some manufacturers want to find ways to complicate things. This heater supply could have been done much easier. What do i know! Seemed like an easy fix.

Hello Stuart. What a bazaar way to wire the heaters. What were they thinking when the designed that one. . That's an impressive stash of control knobs. Take care Sturat.

I know you keep saying you are boring us with the schematic, but your defo NOT boring me and I would think most people watching this. So explain as much as you can or all your knowledge will disappear. Cheers

Nicely spotted and good fix..cheers

How did you know you had the right polarity in the heaters?

great vid, did you ensure that the two heater branch's have the same end of the winding to pin 9 of the ECC valves, this can add hum if they are out of 0hase with each other. grwat vid.

I have known heater pcb tracks to fail. They are not up to the current carrying capacity over a long period of time or they blow if a heater fails. Never had this issue with the old point to point wiring.

That's an interesting failure. I suspect they may have done the parallel tube heater wiring to reduce hum or something along those lines. Question is how/why did that fail?

Hi Stuart.. Excellent catch and repair..😊 Ed..UK..😊

Thanks! Great work, Sir.

One of my always look for new uploads

I truly enjoy your program and I’m learning so much. Do you have a thanks button. $ ?

STUART, What are grid leakage and grid stopper resistors used for? the grid leakage is leaking the AC peak to peak voltage to ground which I'm guessing the term leakage means a voltage drop? and the grid stopper is stopping what? the AC peak to peak voltage from going into the tubes grid. The terms Stopper and Leakage I'm confused about why they are calling them stoppers and leakage and what they mean by them

STUART, What do you mean by the primary magnetic field is "CUTTING" the turns of the secondary windings? What do you mean by "cutting"

Hello Stuart. I can't ever remember seeing the can completely empty like that one. Still it's in keeping with the time of the year; fireworks night. Great video. take care Stuart.

I think the bad design is made on purpose. An human trait is to make life difficult for others... cheers matey good video as always.

STUART< What is an electrode? the cathode, anode and Grid are considered 3 separate electrodes?

Blackstar actually repair all of the products they sell. Any component on the board can be replaced by the guys at Northampton and the charges are very reasonable.

You didn’t need to drop the chassis out. Just take the bottom back panel off and turn it upside down.

Nice video, have two JCM800s, a 2210 and 4210. Probably will get the Eurotubes probe but please clarify me one thing: is one probe enough with a matched pair (or quad), and if so, does it matter under which tube the probe is mounted?

Hi Steve,always interesting see you troubleshooting ,and i hope see the next video in short time..... Cheers Roby

STUART, If an amplifier has 4 output tubes and each power tube is NOT matched, how can you take the Average bias current? How would you take the Average of the bias current on 4 unmatched power tubes

Thanks for the wire stripping tip. I've been wrestling with the wire strippers for years! Got a Blackstar with an intermittent rotary encoder dial here. Board out. Boo. Have fun. Thank you.

always love watching your videos it's like a nice jam being of a similar age I don't get many valve amp to Work on So I watch your videos the young seem to go for transistor amps😊 I work like you for the love of music and not the money

Thanks to you I fixed the vibrato in my 1972 Twin Reverb tonight. I forgot or never knew that the vibrato foot switch needs to be closed for the vibrato to work. It's counterintuitive that when the foot switch is closed, a node in the vibrato circuit becomes grounded to enable the vibrato. The fault was the vibrato foot switch plug and the mating jack on the back apron of the chassis were tarnished to the extent that there was no electrical connection. As you showed in your video the shield of the reverb foot switch cable accomplishes the ground for the reverb and vibrato switches on the chassis back apron via the shell of the reverb foot switch plug. So I cleaned all the phono plugs and jacks for good measure. Fender should have provided shielded cable for the vibrato foot switch as well. I then noticed the Fender schematics depict the vibrato foot switch closed and the reverb foot switch open. Evidently, these are the switch states to enable vibrato and reverb. Live and learn.

I come here just to hear Stuart butcher Ted Weber's name lol ruclips.net/user/results?search_query=ted+webber