Help with 2204 please!

[Matty_V]

Trying to iron out this jcm800 2204 and I can't figure it out. It sounds better if you pull the second output tube. When you plop it back in it gets really brown and mushy, plus it gets quieter. I've swapped tubes around, changed the PI tube, checked for any dc leakage on the PI coupling caps, disconnected nfb. Voltages look ok, bias is ok. What gives?!? 

[LooseChange]

Is this a 50 or 100 watter?
No red plating?
Check the screen resistor.

[Matty_V]

The 2204 is the 50w version. Its actually a jmp, but I like saying jcm800. No redplating, voltages are equal at both screens. I'll check the screen resistor anyway.

[Leevi]

Have you measured the DC resistance of the OT primary (both sides)?
/Leevi

[Matty_V]

Across the primary ends was around 86 ohms or so IIRC, and around 42 ohms per side. Not sure if thats where it should be but its equal on both sides.

[phsyconoodler]

Input jack issues can cause those symptoms.Is this a production amp or one you built?

 Sounds like one gain stage is being muted.

[Matty_V]

Production JMP that I rewired a long while ago with a turret board. It was my first step into amp building/modding. How could a flubbed input jack cause the amp to sound better with one output tube pulled? Just so I can understand that better so I know what to look/listen for.

[Leevi]

Are you sure there is no parasitic oscillation after you have added the second tube?
To find that use chopstick method.
/Leevi

[Ritchie200]

Does it matter which side you pull/add the tube from?  Do you have another set of output tubes to put in?

Jim

[Matty_V]

I haven't fooled around with this amp yet, haven't had time. Anyway, the problem stays with the sockets, not the tubes. You can swap the tubes and you get the same thing. I threw in a spare EL34 to test with and still the same. Pull the first and it sounds awful (as you would expect), pull the second and it sounds louder and way less muddy.

[Ritchie200]

Ok, I'm cornfuzed.....  It's a 50watt version with two EL34's.  If you pull the first one it sounds bad and when you pull the second it sounds better and louder?  So you are saying you pull ALL the output tubes and it sounds better?  What am I missing here...?  Are you saying that one socket sounds better than the other when you go to one tube?

Jim

[Matty_V]

Pulling them one at a time with the other in. Sorry for the confusion. Or cornfuzion rather 

V4 + V5 = muddy, not very loud
V4 only = a bit louder, less muddy
V5 only = very tinny and ugly

[phsyconoodler]

Yes,if the input jacks are wonky it will definitely cause an issue.The first stage is bypassed when plugging into the low input so the gain is much lower.When you plug into the high input it cascades into the third gain stage.
  Make sure the jacks are not sprung,all contacts are grounding on the unused input.
Is this a problem that just happened or has it been there since you rewired the amp?If it's after the rewiring,then I suggest you recheck those jacks very carefully.If it just suddenly happened then it could be the same thing or you may have a wonky ground on one of V2's cathodes.
  Something to check.

[Matty_V]

The amp was very sterile when I got it years ago. I had new tubes put in it and when I got it back home it didn't work. That was when my interest in tube circuits started. I ordered a kit from metroamp and redid it. So, I'm not sure what this thing is supposed to sound like from the factory. I've re-re done it a few times as my skill and knowledge improved. I don't think I've replaced the input jacks, only rewired them.

I've got some good ideas to try out this weekend. Word of the day: Wonky 

[Ritchie200]

Ah, cornfuzion no more!     DId this condition start with a fuse-a-poppin'?  Check V5 socket for any signs of arcing, carbon tracking, top and bottom.  Simple stuff first!

Jim

[Matty_V]

No blown fuses in this guy. I've only blown a fuse once in my 18w when a heater wire touched the [way too] long socket bolt. Sparks and smoke all over the place. I don't recall any carbon tracing but I'll double check it when I finally convince myself to go work on it. Thanks for the help.

WONKY!

[Matty_V]

Just to close this thread out I figured out the problem. And feel quite stupid at what the culprit was. I had the grid and cathode wires switch on the non-inverting side of the PI. Sounds GREAT now. Goes from mean to clean. I dig.

The listening amp is super helpful.