5150 standby doesn't work

[MitchK]

So if anyone has read any of the other threads I've made recently I have a 5150 I've been making some changes to.

Recently I changed the screen grid resistors to 1K and I also connected a small computer fan up to the heater tap on the PT.

Problem is now I turn the amp on and its automatically fully on the standby switch seems to be bypassed somehow? I've checked the switch itself, its fine, I've even disconnected it and the amp still comes on fully. I've double and triple checked everything, measured resistance everywhere on the small PCB board for the power section and I can't see it, I'm thinking its happening somewhere else in the circuit but I just can't figure out where to begin looking or what I could have done while I was making those changes that would do that?

I tried playing the amp and it was ok at lower volumes, but didn't quite sound right and when I switched to the distortion channel or turned up a bit I heard a large snap. took a look and on the PCB where it connects to the tube sockets pin 3 had arced to pin 4.

[Enzo]

Well that is the problem. Look at the schematic. The standby switch opens the path between plate and screen supplies. And since the preamp nodes all follow off the screens, the switch kills everyting but the OT plates.

Your arc between pins 3 and 4 means the amp has made a new path from plate to screen, which neatly bypasses the standby switch and the 400 ohm resistor . Replace the arcing socket. ANd if the pc board has arced, grind away ALL blackened material.

[MitchK]

So I took the board out and resoldered all my connections, grinded away the pc material that was charred and now my problem is low volume. hmm can't seem to get it right.

http://www.geofex.com/ampdbug/lowpower.htm

I've looked at this and I've checked all the preamp tubes just to be safe as well as the power tubes, they are all fine. I'm really assuming this has to be something in the power section since thats what has been played around with so much. I checked the B+ and bias voltage, both look fine, I made sure none of the new screen grid resistors had gone open. I'm going to go resolder all my connections yet again but any general advice? I just don't know what would cause that at all?

both the pre and post gain controls work but of course even with the amp totally dimed its about as loud as it would be on less than 1 post gain.

[Amp Kat]

Is your standby switch working now ? Check your screen voltage vs your plate voltage. The screen voltage should be about 5 to 10 volts below the plate voltage. You can also check across that power resistor which isolates the plate and screen supply I think it's somewhere around 400 ohms and see if it drops across that resistor. If it doesn't you still have the same problem. You really need a scope to insert a signal and follow it from preamp to power amp to see where it drops off. The 5150 has a preamp out so use it to the front of another amp or to the return of another amp to verify the preamp is not also the problem. Remember that the Post volume henders the preamp so make sure it's at least past 3 when doing the preamp out test.

[MitchK]

Alright so yes the standby switch works now, no sound at all when its on standby, when the standby is off I can get a bit a volume out of it by cranking up the pre and post gains.

screen and B+ differ by about 12 v, there is a 8 v drop across the choke resistor.

when I run the preamp out into my marshal I get sound but to be honest I'm not sure what I should b expecting I've never used the preamp out before.

[Amp Kat]

When using the preamp out your basically bypassing the power section of the 5150. The preamp out is like a send out. Most preamp outs are not effected by the post control but the 5150 is so the post does need to be cranked a tad. Using the pregain control on the 5150 will give it more gain and in case of the 5150 will also add distortion,noise and hiss. Use the Marshall Volume control to increase the volume. Now take the send out of the Marshall and plug it into the 5150 return and see if you get high volume by using the Post control of the channel of the 5150 your in. If it's the Rhythm use the Rhythm post control or lead use the lead post. Your Marshall preamp will now control the gain but not volume. If it gets loud your problem is the loop jacks but if it's still low we have to dig somemore.

[MitchK]

When using the preamp out your basically bypassing the power section of the 5150. The preamp out is like a send out. Most preamp outs are not effected by the post control but the 5150 is so the post does need to be cranked a tad. Using the pregain control on the 5150 will give it more gain and in case of the 5150 will also add distortion,noise and hiss. Use the Marshall Volume control to increase the volume. Now take the send out of the Marshall and plug it into the 5150 return and see if you get high volume by using the Post control of the channel of the 5150 your in. If it's the Rhythm use the Rhythm post control or lead use the lead post. Your Marshall preamp will now control the gain but not volume. If it gets loud your problem is the loop jacks but if it's still low we have to dig somemore.

hmm unfortunately the marshall has no preamp out so I won't be able to do that unless i can borrow something but that won't be for a while.

I'm quite sure that the problem is going to be in the power section somewhere. the preamp section has not been touched, and I've checked all the tubes and they are fine. The power section on the other hand I've just been downright abusing (jokes!) messing around trying different things.

Most recently I replaced the screen grid resistors to 1K and put a computer fan (12v) on the heaters, this wasn't a great idea. When I hooked it all up I had my standby switch problem, but I was getting volume out of it, however first time I turned it up I got a blue flash from one of my tubes and my mains fuse blew.

Later on I took the fan out thinking it was drawing too much current since it was a 12v fan running off of the heater voltage, and tried it again this time when I turned up a bit I got the arc that I described earlier.

So I took the pcb for the power section out again and just resoldered all the screen grid resistors (this is what I had changed before the problem so i figured this is where the problem lays) and that seemed to fix the standby problem, however I now I have my volume problem.

So once again I feel pretty confident that its something in the power section, but I don't know what parts failing could cause this type of problem?

[Enzo]

Anything.

PLug a guitar into the effects return jack. Is the sound out the speaker weak now? Obviously not as strong as through the preamp, but you should hear whether it has "the problem" or not.

[Amp Kat]

Does the Marshall have send and return jacks? If so use the send or anything like a keyboard,tuner or even a headphone out might work or the gutar as Enzo suggested.

[MitchK]

Alright so I tried my guitar direct into the effects return on the 5150 and it is significantly louder than if I run it directly into the input...

[Amp Kat]

Then that means you have a problem with your loop jacks. Did you patch a cable from send to return jack on the 5150 ?

[MitchK]

I'm confused now, you want me to run a patch from the 5150's effects send to the 5150's effects return? Once again if you mean the marshal the marshall is a jcm800 2203 no efx loop no pre out or anything if thats what you were thinking of.

I ran my guitar directly into the effects return of the 5150 and I got more volume than when i ran it into the regular front of the amp input.

So now that I'm getting more volume using the efx, I'm bypassing the preamp right? so this tells me my problem may be the preamp? I don't understand what the loop jacks have to do with this, I'm even surprised it would be the preamp it just seems like it has to be the power amp, thats the only think i changed.

[Enzo]

That is exactly what we want you to do, connect those two jacks together with a cord. The send and return on the 5150.

You verified the power amp works by plugging the guitar into the return.

You verified the preamp works by sending the signal to the Marshall.

What is left is the effects loop. The signal from the preamp flows through those jacks on its way to the power amp. If the jacks are not right, the signal does not make it past them.

Trust us, this is a valid test, and it has zero risk to your amp, in fact it is exactly the same to the amp as having an effect connected there and set to bypass.

DOn't assume the problem MUST be where you worked. usually that is the case, but sometimes things just coincidently fail at the same time.

[MitchK]

Ok so I've done the send to the return, and plugged my guitar in the front and tried it and I have the same reduced volume as usual...

[Amp Kat]

It is still possible that the jacks are bad. We were hoping they were just making bad contact but you may need to replace them. There is also one posibility that V3 is bad on 1/2 of the triode (12ax7) so you may want to change that also. Other than that without the amp in front of me and a scope it may be something else were overlooking but there isn't much there.