Picked up a 1982 JCM800 Combo…bias questions on page 2

[On the Rocks]

Hi All,

New here and just to say this up front, I am familiar with soldering irons and schematics but I am in no way a professional amp tech. I know how to be safe working on these things but I’m also not too proud to call a pro when I’m over my head. Now with that out of the way, here are some pics of my new amp.

I used to have a JMP50 non master volume 1/2 stack and it was amazing. Sadly I had to sell it along with most of my gear many years ago. I just kept one guitar and a lonely little Fender Champ.

I found this JCM800 locally and came to deal with the owner. It works, but has a lot of hiss and hum. I figure it certainly deserves a once over so that’s what I plan to do. I want to run EL34’s so I want to finish the budget tube conversion previously done. Then I plan to replace the filter caps, and the two bias supply caps. Clean the pots and tube sockets. After that I can get some better tubes and see where it’s at.

My first round of questions:

- What has been changed or modified? I can see the lone orange drop cap, and the one resistor next to the diodes (for the bias supply). Those definitely are not original. What about the little cap and resistor coming off the input jack? Original? The Teapo caps, as well as the big ITT filter caps? Original? I can see the resistors added to the tube sockets to ground. Added to quickly check bias?

- what brand caps are you guys using to replace out of spec caps on the board? That orange drop is definitely going. lol.

- speaker cones are both 1777 so that looks good. But have you ever seen a 8 ohm label on a 16 ohm speaker? How weird it that? Explanations?

- the knobs have pointers. I don’t recall seeing JCMs with those before.

- sticker on chassis shows August 1981 build date. But serial number identifies it as a 1982 (ends in P). Why?

- tubes are all JJ and will be changed. Any current production EL34 you guys like or is NOS/vintage the only way to go? Any brands you guys like?


- and of course, anything you feel like commenting on.

Thanks!

 

[On the Rocks]

More pics

 

[On the Rocks]

Even more

 

[On the Rocks]

Yup more

 

[shredless]

Thats a great amp! Did you check the actual ohm rating on the speakers? I hope it hasnt been run on the wrong setting. Those are killer speakers but l think someone wrote that for god knows why.

The 1982s had chrome cap pointer knobs…lookem up…lol hopefully they arent cracked but are still in demand
l sold all mine and replaced em with new regular marshall knobs

That odd resistor is 220k right spec for el34

The orange drop doesnt need to be 600v thats why its huge…l cant see its value but it should be 2n2 or .022uf just like the square red one to the right of it on the other side of the 2 resistors.

 

[shredless]

I do see now its a .022 uf cap… who knows why the changed it.

 

[shredless]

The input cap and resistor look stock

Explain the budget tube conversion?

 

[shredless]

I vaguely recall some mislabeling of celestion speakers….

Oh welcome to the forum!

 

[shredless]

Double check fuses…mine was marked for half of what they should be… l kept popping fuses l guess this was a issue around that time. Should have been four amp but marked as 2 l think. Its been a while since l had mine

 

[TheKman76]

I'm not expert on these, but it certainly looks largely original and unmolested.
Someone had cludged the bias circuit, I'd fix that properly.
I'd be inclined to check the impedance of those Celestions individually too.
It's entirely possible the chassis and the cab were not originally mated at the factory. Might explain why the speakers have different labels too.

 

[syscokid]

Congrats on a killer amp combo!

Speakers are original and look to be still wired in parallel. Measure the speaker cable’s resistance between the plug’s ground shaft and its hot tip. If correct, you should see a reading of slightly less than 8 ohms. Amp should be set to 8 ohms of course.

Orange Drop cap stands out like a big orange drop, ha ha! I think that OD is a polypropylene dielectric. Polypropylenes do not belong in these Marshall circuits. If sourcing an authentic/original cap is not your thing, a modern Mallory 150 .022/400v will be excellent. If you’re mostly into very high gain, a .0022 (2n2) would make for a more articulate choice.

Those carbon comp 1ohm, current sensing resistors at the power tubes could be a source of noise and need to be replaced with precision 1 ohm metal film resistors (1 or 2 watts) that are at least 1% tolerance or less. Or better yet, just remove those carbon comps and don’t replace them with anything. I guess this would depend on what method you use to bias an amp like this.

When operating on EL34’s, and if you care to follow proper Marshall schematics, the pair of 150k grid resistors between the phase inverter and power tubes should be replace with a pair of 220k’s.

Replacing the original 42 year old filter caps is probably a good idea. I like the ARS caps for this purpose.

All your other maintenance procedures that you posted are spot on.

 

[On the Rocks]

Thank you to everybody for the reply’s. I think this gives me all the info I need to proceed. Of course I’ll follow up with more posts as work progresses.

 

[GIBSON67]

Unless reconed, one speaker is 8 ohms and the other is 16 ohms based on the T numbers. I'd check the speakers separately.

 

[VERVEHAMMER]

SCORE!!!

Acquire an original JCM800 Lead Series 1960B to stack or 1960A to place besides.

Unleash the Low End.... : )

 

[Blake F]

Great purchase! That thing can sound killer. Neil Geraldo (Pat Benatar) played one of those back in the days when they had their hits on the radio. His may have been a 100W I can't remember. Everything on those Pat Benatar recordings came out of the exact amp you just bought. 1) If the amp has 150k power bias resistors, it was made for 6550's and the previous comment that you'll need 220Ks for EL34's is correct. Be ready to check the bias on those immediately when you power it up if it had 6550s cause you won't know where the bias pot is and it could run them way too hot. 2) You can run mismatched speakers but they would have matched when installed so it could be one of them is defective. Those G12-65 speakers can sound great. I have a cab of them. 3) Those red box capacitors are Iskra (took me a while to find that symbol on the internet) and my '83 2203 is full of them. Amazingly, they sound really good. You won't find one of them to replace that orange drop but as stated by another commentor, Mallory 150s sound good. 4) As for output tubes you will be incredibly happy if you find a pair of NOS Siemens/RFT EL34's for your amp. Yes, new manufacture tubes can sound ok, but IMO once you stand in front of the right Marshall with the right tubes, right speakers and right guitar, it will flat-out floor you and new tubes just don't do that. 5) I've done a lot of work on what makes great tone and was surprised to find that the values and brands of filtering and filter caps in Marshalls makes a substantial difference in tone. I believe the old can caps add something that new ones don't so I always re-form can caps and reuse them unless they won't come back to life. If you want quick, and aren't a tone perfectionist, ARS are fine; F&T are the closest sounding to the old caps that I've tested and ARS are really just repackaged caps I believe anyway (I don't think they're manufacturing capacitors). If you find the topic of re-forming your existing can caps interesting you can find articles on the internet on how to do it fairly cheaply but it's easier to get a capacitor leakage detector like the HeathKit IT-28 (the one I use) that will re-form capacitors really well as an unintended feature.

In case you like the Neil Geraldo tidbits, Neil said in an interview that Ken Schaffer (the guy behind the now-famous Schaffer wireless system) modded a couple things so Neil could hook in the very high-end studio effects processor he was using so he could get that tone live. Seems like he even said Ken was able to mount the unit in the combo cab under the chassis.

 

[On the Rocks]

Hi gang!

Just thought I’d post a few progress photos. First stage of my repairs completed. Changed filter caps, bias caps and some random resistors. Happy enough with the results.

Next up, cleaning the pots and jackets.

 

[VERVEHAMMER]

OUTSTANDING!!!!!

 

[PelliX]

Those carbon comp 1ohm, current sensing resistors at the power tubes could be a source of noise and need to be replaced with precision 1 ohm metal film resistors (1 or 2 watts) that are at least 1% tolerance or less. Or better yet, just remove those carbon comps and don’t replace them with anything. I guess this would depend on what method you use to bias an amp like this.

I'm curious as to why you suggest 1 or 2 Watt rated resistors for measuring current? Surely, 1/2W would even be overkill, as you're dealing with <0.1W?

 

[syscokid]

I'm curious as to why you suggest 1 or 2 Watt rated resistors for measuring current? Surely, 1/2W would even be overkill, as you're dealing with <0.1W?

My suggestion is based on my personal lack of electronic engineering knowledge and relying on what I’ve seen other amp kit vendors like Metro Amps, Allen Amps, and Ceriatone set up their kits with. Looking at the simple fact that there’s a minuscule amount of current involved at the current sensing resistor, a 1/2 watt resistor is more than enough to safely deal with the electricity going through there. But why are some amp builder/designers installing higher rated resistors here? I highly respect the forum members who offer sound knowledge, and you’re one of them. In this situation, is it possible that a power overrated resistor could lead to more catastrophic damage to the amp if the tube failed?

 

[PelliX]

In this situation, is it possible that a power overrated resistor could lead to more catastrophic damage to the amp if the tube failed?

No, in a scenario where that resistor starts witnessing more than a 1/4W (standard resistor value) there are bigger problems and fuses and/or other things are popping. Hopefully the former, of course.

But why are some amp builder/designers installing higher rated resistors here?

That's a very good question. Perhaps it's a case of the cheapest option being to buy more low tolerance 1W 1Ohm resistors, or they already have them for another purpose and use them for that because they're already on the BOM. I honestly don't know, haha.