Marshall JCM800 2205 help needed

[gibson17]

Hello all,

My friend brought me a JCM800 2205 that was having issues producing any sound. I took the amp in and disassembled it. There was a bunch of cold solder joints on the board that I reflowed. The resistor on R59 was burned and failed open. I replaced that with a new resistor and also replaced R58 since it was the same resistor with the same value while I was there. The R58 resistor was still in operable condition. All the tubes are fairly new and test well on my TV7. The power tubes are within 1 GM of each other.

I am now able to get volume and sound from the amp but I am having a few issues.

1) Even though the tubes are closely matched to each other the tubes are more than 10mA apart. One side at 44mA and the other at 32mA. The bias pot is turned all the way down and that's as low as it will go. I'm getting a fair amount of noise which I am assuming is from the power stages being that far apart. Shouldn't the tubes be somewhere around the same current, especially of they're closely matched? What should I check?

2) I know I need to change the resistor that feeds the bias pot so I can gain better adjustment. I've had to do this on other amps... Which resistor should I be targeting? I do have a decade resistor that I can put in place to find an appropriate value.

Thank you in advance for your time and knowledge.

 

[Pete Farrington]

Even though the tubes are closely matched to each other

What causes you to think that?

the tubes are more than 10mA apart. One side at 44mA and the other at 32mA

How did you measure those currents, detailed procedure please?

 

[gibson17]

Hello Pete,

The tubes were bought as a matched set. I measured the tubes on a calibrated Hickok TV-7 D/U. The GM was one digit apart at 46/47.

I measured the currents with a dual bias probes from SRS into a Fluke 115.

My plate voltage is 464, so I'm aiming for 37mA but the pot is bottomed out at 44mA.

 

[RadioAD5GB]

No man. A class AB1 circuit really isn't that complicated. Don't over-think things. First of all, what's with the huge disparity in OT current? Got a 'scope? Signal generator? All the junk you need to safely probe around in a live tube circuit? ( these things CAN KILL YOU ) If so, OHM's law - and associated math, will be your friend! E = I/R and all of its permutations. Seems to me you're already on the path to success. Don't discount however,, the odd and - off - chance of RF parasitic oscillation. I myself, have never seen it in Marshalls, or decently wired clones, but it's been known to happen. Current sucking tube like you described , with no other reason...... that's when I jump into A/C diags and don't forget the possibility of RF parasitics........

Have Fun!

 

[danfrank]

Hello Pete,

The tubes were bought as a matched set. I measured the tubes on a calibrated Hickok TV-7 D/U. The GM was one digit apart at 46/47.

I measured the currents with a dual bias probes from SRS into a Fluke 115.

My plate voltage is 464, so I'm aiming for 37mA but the pot is bottomed out at 44mA.

Hi,
You said "bought as a matched set". Where did you buy? The reason I ask is because some places sell "matched sets" that aren't matched... Like Tube Depot, their matching sux! Lol! CE/Antique Electronic Supply does well at matching and it is worth it to pay the extra few $$ for "burned in and matched" sets... This weeds out most failures before you buy the tubes.
Also, matched by Gm is only half the equation... Tubes need to be matched by Gm AND current, these are two different tube parameters... The original Groove Tubes used to do it this way. Think of it this way: You can consider Gm matching as "dynamic matching" and matching for current as "static matching". Both need to be done for a truely matched set of tubes.
Hope this helps

 

[gibson17]

No man. A class AB1 circuit really isn't that complicated. Don't over-think things. First of all, what's with the huge disparity in OT current? Got a 'scope? Signal generator? All the junk you need to safely probe around in a live tube circuit? ( these things CAN KILL YOU ) If so, OHM's law - and associated math, will be your friend! E = I/R and all of its permutations. Seems to me you're already on the path to success. Don't discount however,, the odd and - off - chance of RF parasitic oscillation. I myself, have never seen it in Marshalls, or decently wired clones, but it's been known to happen. Current sucking tube like you described , with no other reason...... that's when I jump into A/C diags and don't forget the possibility of RF parasitics........

Have Fun!

Hello Radio,

I do have a scope. I have no operable signal generator, the one I have is broken. I do have all the junk I need to safely work inside an amp. I do understand that the voltages are lethal. I have decent mechanical ability but very limited electronics theory. Thank you for the vote of confidence. I'll keep on driving towards my goal, hopefully with some guidance and maybe I'll learn something along the way. What do you mean by huge disparity with OT current? Are you referring to the imbalanced mA per tube?

 

[gibson17]

Hi,
You said "bought as a matched set". Where did you buy? The reason I ask is because some places sell "matched sets" that aren't matched... Like Tube Depot, their matching sux! Lol! CE/Antique Electronic Supply does well at matching and it is worth it to pay the extra few $$ for "burned in and matched" sets... This weeds out most failures before you buy the tubes.
Also, matched by Gm is only half the equation... Tubes need to be matched by Gm AND current, these are two different tube parameters... The original Groove Tubes used to do it this way. Think of it this way: You can consider Gm matching as "dynamic matching" and matching for current as "static matching". Both need to be done for a truely matched set of tubes.
Hope this helps

Hello DanFrank,

The tubes were bought from Viva tubes as matched and burned. Perhaps I've fallen victim to poor matching? I realize that the measurement of GM is really theory only. My understanding is that tubes are in inexact science and that the only real measurement was the theoretical GM measurement of transconductance. Thank you for the information and detailed explanation of what tube matching really is. I do have a TV-2 tester that puts the tube into real voltage conditions but it is not calibrated. Is there a way for me to voltage test the tubes so I can have the complete equation of tube matching with both dynamic and static matching?

 

[Pete Farrington]

I'm getting a fair amount of noise which I am assuming is from the power stages being that far apart

What sort of noise?
I run valves at a greater disparity in idle currents that that, with absolutely no issues.

JCM800 2205 … R59 … R58

Schematic link? It’s just I recall there being 2 very different versions of the 2205. Without that, there doesn’t much point referring to R58 etc, better to explain its function in the circuit, or what it connects to.

 

[myersbw]

Here's a very simple A/B comparison to do regarding the mismatch. Swap the tubes regarding their current sockets. Does the "mismatch" follow the tube or stay with the socket? If the former...you have a tube mismatch. If the latter, start measuring components surrounding the tube sockets to see if values have extremely drifted, etc. From personal experience, I've seen new tubes "matched" that are that far off (sadly enough). Won't hurt to verify that bias circuit and give us the measured bias voltages, too. All the best!

 

[Gblev]

Gm matching doesn't really matter with tubes. It might make you feel better about the possible longevity of them when purchasing some tubes if the Seller list it, but it's not going to help your amp run more balanced. I laugh every time I see someone selling power tubes as being "Matched" on eBay, and they are basing that off of their tube tester's Gm reading, or Emissions reading for the pair, or quad, they are selling.

As far as having mismatched tubes go, you will hear circuit hum if your tubes are severely mismatched, but this doesn't come through the speakers. There's a lot of players out there that like the sound of mismatched tubes, as they will have an increase in the 2nd order harmonics (overtones) of the sound. If you play loud enough you won't hear the hum. The downside of having mismatched tubes is that means some tubes are working harder than others, and will possibly have a shorter life. As such, you don't want your tubes to be mismatched by too much or it might cost you more money.

I'm pretty sure not too many people worried about mismatched tubes back when they could just go to the corner hardware store, or local Radio Shack, and grab a new tube when one blew. You went there, got what you needed, went home, swapped it in, and was done with it. If it hummed really loud when idling, you thought you got a bad one, so you'd take it back and swapped it for another one. Rinse and repeat until satisfied. If it sounded good, then it was good!

 

[mickeydg5]

@gibson17

46 and 47 are close. Mutual conductance does matter and power tubes are best when matched to a certain degree. This provides maximum output power aiding in cleaner more balanced output from the push-pull arrangement and does cut idle hum noise.

Why don't you start with providing DC and AC voltages at the power socket lugs?
This is provided you are experienced and comfortable with fooling around in an open chassis which has high voltages.

No one starts with the basics.

Also, did you clean all power tube socket prongs and the power tube pins with a good electronics contact cleaner/lubricator?