Marshall JCM800 2204S ???

I think you're right, with a 12" speaker there's just more margin for error. With these 10" speakers, not only is the "sweet" area smaller, but I have a hard time seeing their boundaries through the grille cloth, so I'm flying a little blind when I've miked them so far.

One of the last couple of clips I've posted had a kick mic mixed with an i5, on different speakers. The other one was just with the i5. I'll try an i5 with a 57 soon, and see how that goes. I like the tone you get with that setup. I've definitely got much more experimentation ahead of me. Plus having that 2nd cab will add some extra options. It's gonna be a loud couple of weeks over here :)
 
So the "matching" B cab isn't really a matching B cab. No tweed front or gold edging. And the tolex is a much darker green, almost black. It is actually a 1965B cab, with those same G12L-35 speakers in it. But it's of a different vintage. I grabbed it anyways because the price was OK, but I think I might sell it off. It has a little plaque screwed to the back of it, citing some now-defunct Kansas City guitar shop. So I think it was probably recovered in a not-quite-matching tolex to try and match the A cab. The tolex is split on the top side of it, so it wasn't exactly a top-notch job. It's disappointing, but I think I might be able to make a buck or two off of it.
 
Yeah, they're both 1965 cabs. The A means slant/top, the B means straight/bottom.

Don't sell it though, unless you need money. That B cab might have the bottom end you're looking for. In most cases, the larger interior space of a B cab results in more low end.
 
I worded my post poorly...I knew it was a 1965B/straight cab going into the deal, I just thought that it cosmetically matched the A cab. I was looking forward to seeing a full tweed-faced stack. I didn't pay any attention to it when I was at the shop, and when I actually bought it, it was through GC's website. They didn't have a picture up since it was used and apparently they're a little slow on the uptake at this new store.

A little buyer's remorse at the moment. I'm sure it'll take a few days/weeks to really decide if I want to keep it.

Good point about the increased low end due to the increased air volume inside the straight cab. I played through it for about 30 minutes earlier tonight, and my ears are still ringing. 8x10 can put out some serious SPL! It really shook the house...it was pretty awesome, I must admit.
 
ocnor said:
Just FYI that's not "tweed" it's checkerboard grill cloth. It looks like it might be small checker which is far more rare than large checker.
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Ah, you're right. And I really do like the checkerboard, it's just a classy look.

In other news, I ordered new tubes and a bias probe so I could retube it with a known commodity and check the plate current. It had Groove Tube EL34M power tubes, and unbranded ECC83 in the preamp section. I went with JJ gold pin ECC83S in the preamp section and JJ EL34L in the power section.

Watched and read a ton of instruction about biasing an amp, and fortunately the JCM800 2204 has the handy bias pot that sits perpendicular to the circuit board. Cleaned the tube sockets with some DeOxit D5, gave a few squirts to the front panel knobs and the fuse contacts as well. Fired it up with the new tubes and the bias probe in place. The plate voltage registered about 460V, and the plate current was a whopping 65mA!!! I did the calculations with the dissipation for my new tubes (25W) and the plate voltage. Max allowable plate current was 57mA. Jeez, no wonder this thing has a sagging board and had already browned the paint on one of the Groove Tube power tubes. It's a wonder that I didn't fry the poor thing, considering that I've had both volumes dimed most of the time. 70% of max current works out to about 38mA, so I set it there and I'm letting it settle now. It already crept up to 47mA once...not sure if I should be concerned about that or not. Set it back to 38mA, letting it settle more as I type.

Maybe the dissipation of an EL34M is different than an EL34L, but damn...65mA is a shit-hot bias setting! Curious how this fella will sound with the new tubes and bias. Will report back in the tone thread at some point with my findings.
 
I like the checkerboard too. What's left of the grillcloth on my 1982A cab is small checkerboard. I left the cab at my brother's house and his cats tore the shit out of it.

Left at 65ma and those tubes would have burned out in no time. I usually bias my EL34 tubes to about 42ma because they just seem to sound better at that setting.

I don't care for the JJ ECC83s because they are too compressed sounding. They may work fine in your amp though.
 
So far, I really like the amp's new attitude. All of the fiddling definitely made a difference. No more crackling when I adjust any of the knobs, no more "fuzzy" sound when the preamp is cranked, and the low input channel is certainly brighter. And it has a ton more preamp gain on the low input. It sounds tight and bad ass.

BUT...the high input has been totally neutered. It's quieter than the low input, and very noisy. I don't know if this is indicative of a bum preamp tube or what. I have no idea what the gain staging is for the hi vs. low inputs. But something is definitely wrong. Both inputs worked before the tube swap, and I didn't jack with any of the circuitry. I don't recall bumping any of the components while I was in there. All I can think is that one of the preamp tubes didn't survive the trek from Portland to my house intact. I'll try some tube swapping tomorrow to see if I can narrow it down.
 
I swapped out all of the preamp tubes one by one, and the high sensitivity channel never came to life. But damn its amazing how different the amp sounded when all 3 preamp tubes were the old generics versus the new JJs. The JJs hands-down have more gain, and the amp is simply louder when they're in there. It was nice to do a comparison as I was replacing each JJ ECC38S with an old ECC83M. No idea what the differences are in specs, but I don't think that anybody would argue when doing a side-by-side comparison which one was better.

Anyways, I might go and swap out the power tubes just to make sure, but I kinda doubt that they're the culprit since the amp operates flawlessly on the low sensitivity input. And it sounds freaking amazing on that input.

On the high sensitivity input, it's much more quiet and flat sounding, with very very little preamp gain. Except when I hit a low A, G# or G, then the amp lets out a fuzzed-out square wave version of that note that's much much louder. It sounds pretty terrible. I think it might be time to schedule a doctor visit for this thing, because I have about zero interest in dicking around with the electronics inside.
 
My guess is that the jack itself isn't making proper contact or, you have a bad solder joint on or near the jack.
 
ocnor said:
My guess is that the jack itself isn't making proper contact or, you have a bad solder joint on or near the jack.
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True. Just looked at the pics again, I forgot even they're soldered straight to the board. I bet something is goofy in in that area.
 
Hey thanks again fellas.

I've been feeling crappy the last few days so I haven't felt like pulling my amp chassis out again. Finally got to it tonight. Swapped out the power tubes, just to see (since they were in there the last time it worked, and I've already tried swapping out the preamp tubes). Still the same...an awful zapping, electrical, square wave kind of sound on the high sensitivity input when I dig into a note loudly. Frightening.

Here are some gut shots. Not really much to see since the inputs are pretty self-contained (sorry about the size, I'm feeling too lazy to resize them and this thread only gets limited traffic):

003.jpg

Here's where the preamp tubes and ginormous capacitor #1 connect:

004.jpg

Here's where the power tubes connect:

005.jpg

Here's where ginormous capacitors #2 & #3 connect:

006.jpg

And here's my best attempt at getting a different angle on the inputs:

009.jpg
 
I don't see anything that looks burnt. I still think that it's a bad solder joint. You should take the board out and check the tracing on the other side. You also might want to check the 12ax7 sockets to make sure that all of the pins fit tightly with no arcing. If they are loose you can carefully crimp them with a tiny screwdriver or pointed tool.
 
Okay, here's what you do....and do it very carefully. Pull the chassis and flip it upside down or stand it on edge. Hook it up to a cab and turn it on, ready to play. With a long pencil, chopstick, something non-conductive, using one hand, with your other hand touching nothing, keep it behind your back, probe around with the utensil. Gently wiggle wires, wiggle the input jacks, tap on the tubes, caps, resistors, etc with the utensil and see if any of that probing around yields the noise you hear. A bad solder, bad cap, bad resistor, bad tube, or loose connection can reveal itself very quickly this way.

Do not use both hands. Don't hold the amp with your free hand. Do not allow any part of your sweet supple flesh to contact the amp. You do not want all that power crossing your chest if you stumble onto something live. Use one hand, and keep the other away. There's a lot of voltage up in that bitch.


IMO, YMMV, etc.
 
Greg_L said:
There's a lot of voltage up in that bitch.
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It's exactly that which makes me think it's totally worth it to pay a professional to mess with it at this point. I'm a total noob layman fraidy-cat when it comes to electronics. I'm pretty proud of myself to have biased it, but when it comes to diagnostics on high voltage equipment, it'll be worth it to me to hand it off to the doctor.

I'm headed out on vacation next week anyways, so that'd make a good opportunity for it to spend a week in the amp hospital.

It sounds VERY unhappy on that high input. I'm afraid to play it on the low input because I can hear that same noise feintly in the background there as well. I don't want to do any irreparable damage to the poor thing. It sounds like a cartoon interpretation of what it sounds like when somebody is getting electrocuted.
 
I wussed out and dropped it off at the repair shop this afternoon. That shop is a solid 45 minute drive from my house, so I had to burn some vacation time to make the round trip. But I trust the guy, he did some warranty work on my Mesa several years ago and did a good job. Nice, honest fellow. And amp repair guys are in extremely short supply here in the KC metro area. So, I'll have it back in a couple of weeks. I timed it like this so most of that time I'll be on vacation and won't miss the amp. Even if it's something simple that I could've fixed myself, I'm really OK with having a pro do the work. I want this amp to work for decades to come, and to not stop my heart with a jolt of electricity in the process.
 
Not everyone is comfortable working on amps and that is understandable as it can be dangerous. Getting it fixed correctly is really all that matters.
 
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