Volume drop on modded JCM 800

[BeëlzeM]

Alright, I've done quite some work to my JCM 800 2205 head. I've modded it quite a bit. According to this thread:
http://music-electronics-forum.com/s...ad.php?t=10329

Everything went pretty well. But right after I installed a 10uF/50V capacitor across R25 (1k5) in this schematic, my amps started to drop in volume. Polarity of the installed cap is correct.

http://www.schematicheaven.com/marsh...h_50w_2205.pdf

When I turn it on, all the tubes light up, and when I turn standby off so I'm ready to play most of the time the volume of the amp is much lower than it should be. Sometimes the volume bursts up and I can play for a few minutes on proper volume but most of the time it's pretty quiet. Is it the cap I used? 10uF/50volts seems right to me...

Any help? Is this the tubes? My solders look good to me, but I'm not sure what the problem is.

[Bruce / Mission Amps]

Alright, I've done quite some work to my JCM 800 2205 head. I've modded it quite a bit. According to this thread:
http://music-electronics-forum.com/s...ad.php?t=10329

Everything went pretty well. But right after I installed a 10uF/50V capacitor across R25 (1k5) in this schematic, my amps started to drop in volume. Polarity of the installed cap is correct.

http://www.schematicheaven.com/marsh...h_50w_2205.pdf

When I turn it on, all the tubes light up, and when I turn standby off so I'm ready to play most of the time the volume of the amp is much lower than it should be. Sometimes the volume bursts up and I can play for a few minutes on proper volume but most of the time it's pretty quiet. Is it the cap I used? 10uF/50volts seems right to me...

Any help? Is this the tubes? My solders look good to me, but I'm not sure what the problem is.

Sounds like you have actually unsoldered the parts from either the lead, 12AX7 socket or the parts.
I'm betting the tube's biasing resistor is not grounded properly when it is not working right.

[BeëlzeM]

Sounds like you have actually unsoldered the parts from either the lead, 12AX7 socket or the parts.
I'm betting the tube's biasing resistor is not grounded properly when it is not working right.

What should I do? It's now at a point even where the amp doesn't hum anymore and the sound is very, very weak. All tubes glow.

I've also clipped out R46 on the schematic by the way, to take the DI-output out of the circuit. I haven't unsoldered any other parts.

What should I do?

[BeëlzeM]

I can't seem to find any wrong parts or unwanted unsoldered components. And everthing went just fine for maybe 15 minutes then it started to drop volume and now it won't go back to proper volume.

My sound is there, but it is very quiet.

Can't these be just my powertubes? They both glow, but can these be the problem nonetheless? Could this be a fuse problem as well, considering it makes a sound but it's very quiet or would that lead to no sound at all?

My preamp tubes glow as well, and I've swapped those out with some I had left to try it out, same results.

[BeëlzeM]

Sorry for posting again in this thread but I tried some stuff already, and measured some resistors.

When measuring the screen grid resistors, the 5 watt 1k ohm ones, it doesn't measure any value, it just stays on as if you measure nothing. It doesn't even measure 0 ohm. Certainly no 1k ohm.

Could it be the screen grid resistors? I know that blown screen grid resistors can cause weak output, what I'm having. They don't show any wear or bad color on the outside though, can it be on the inside?

I've ordered a new set of powertubes as well, they needed to be replaced anyway.

Any idea's? Screen grid resistors?

[Jag]

Before you go looking at things that are probably not involved, check the things you changed. If it was fine before you installed the cap, remove it and see if the problem goes away. Unless something else happened that you haven't mentioned, I would'nt start looking for bad output tubes, etc. You most likely made an error somewhere around where you were working. Be sure that the connections on both ends of the resistor are reflowed, at the least. Double check the things Bruce already suggested.

If I run out of gas my first thought is not "My God, I need new tires"!

[Jag]

Another thing to check if the other things don't help is, since you're working close to the FX send/return, try jumpering send to return with a known good patch cable. You might have buggered up something there by accident or the contacts could just be dirty.

[BeëlzeM]

Hey man, thanks for the reply. I've removed the cap I added and soldered a resistor that I clipped out back in place (though I'm sure this wouldn't really be a problem, since it only clips out the DI-output on the amp). It still remains the same, low volume and somewhat grittier. It worked just fine before I added the cap and clipped the resistor, so theoretically it should work like this, except when something is bad or broken?

I've measured the 5 watt 1k ohm cement screen grid resistors across the tube sockets and those give infinite value (as if you measure nothing, really). I think that shouldn't be. These can be broken on the inside right, without showing any wear on the outside?

I'm almost sure these are bad, but I don't know if these kill my volume.