1987x loss of volume

[Jaymz77]

Hi all.
I'm having some issues with my 1987x I'm hoping for some input on.
Late last year I put a turret board in it loaded with sozo's and did a switchable 2204 mod. After some initial teething issues the thing has been absolutely singing for the last few months.
I was using it last night and it was fine, but tonight I turn it on and it is very low in volume. The tone is fine, no weird noises or anything. I tried the obvious and swapped output tubes, no change. I tried a different Pi tube, no change. I am using a Rivera Rock crusher so I unplugged it and went direct to the cab, no change.


I'm fearing it could be the OT. What does everyone think? I've hear attenuators can be hard on tubes and OT's if you crank the amp all the time but I only run it on 5-6.


Any input is appreciated. This is so frustrating cos I had finally got everything working so damn well together: new Derek Trucks SG (Ang-ified), new Solo Dallas Storm pedal (wow)... now the amp is giving me grief. AARRGGHH😡

[Jaymz77]

Anyone?

[sluckey]

Loss of volume can be almost anything. Dirty jacks, bad tube(s), failed component, loose/broken wire, bad solder job, etc. Post voltages for all your filter caps. Post voltages for all pins on all tubes, even if the reading is zero. There may be a clue in the voltage readings.

[Jaymz77]

Thanks, I'll do that.
Just the way that it happened is weird. Turn off one night, it's fine. Turn on the next night and it's very low. Nothing had been unplugged or changed. Just off to on.
Cheers

[Jaymz77]

I do know that pin 3 on the output tubes has 423V because I had to bias the new tubes I tried. So the primary of the OT is seeing the same voltage.

[Blind Lemon]

Same speaker box? Try another if you have one


BL

[Jaymz77]

Yeah same box, the only one I have

[Jaymz77]

Sluckey, I'll do what you suggested but I can't help but think...
Jacks - unlikely because the amp is 4 months old.
Bad tubes - already eliminated
Loose/ broken wire - possible I guess but how that happen between uses I don't know.
Bad solder - as above
Filter caps - possible I guess


I'll get voltages tonight.

[Ritchie200]

Sounds like you made some significant changes.  Did you happen to add an effects loop?  If so, check your shorting jacks.  Also check your "switchable 2204 mod" parts and wiring.  Might be time to get out the chopsticks.  Do the easy stuff first.


Jim


edit - Sorry I didnt see it was the reissue.  For sure check your loop jacks as well as the bypass switch.

[Blind Lemon]

Solder joint ? because of thermal cycles, removing tubes and putting them back in and generally just moving the amp around.

Did you build it? I see above you did, that's not to question your work just to ask if you feel safe moving around in an amp chassis that is on.

If so chop stick it and move everything (components/wires) around and listen for the trouble spot.

Is it a head? Make sure your amp to speaker cable is good and all the way plugged in.

BL

[Jaymz77]

Thanks for the replys gents.
Jim: I had an effects loop from the factory. The jacks aren't switching jacks I do t think. It just has a bypass switch for the loop. I don't even use it. All my effects are up front. I'll look at the 2204 bits but all they really consist of is a push pull pot. I sacrificed the plexi  normal input to make that half of v1 the "cold clipper" for the 2204 mode. I have the same problem in both modes.


Lemon: yes I built it. I only undid the necessary parts to do the job. Meaning all the OT connections are as they left the Marshall factory, untouched by me.
I am an electrician so I'm quite comfortable poking around while the amp is on.
Yes it's a head. Speaker cables are brand new greyhounds. But again, the only thing that was changed or unplugged between when it worked on Wednesday night to when it didn't on Thursday night was me unplugging the guitar. Nothing else was touched. Only turned off.


Thanks again for the input so far.

[sluckey]

Sluckey, I'll do what you suggested but I can't help but think...

I threw out all that stuff because you didn't provide any info other than you lost some volume.

If you have been abusing the amp and fear the OT is bad, then put a new one in and see if anything improves.

[Jaymz77]

Sluckey. Don't get me wrong I appreciate your input. I was just expressing my thought on those things. Doesn't mean I am right. I wouldn't say I've been abusing it. It's just the way that it happened that has me puzzled. Fine one night but not the next. I can't understand how it can happen when all I did was turn off, unplug, replug, turn on. And the only new part of the equation is the rock crusher.
I just had a thought though... the storm pedal is a boost. Most of my drive is coming from it as I only have the gain at 10 o'clock (2204 mode). Perhaps something has been stressed by driving the front end hard with the storm pedal.


Cheers

[Willabe]

It's just the way that it happened that has me puzzled. Fine one night but not the next. I can't understand how it can happen when all I did was turn off, unplug, replug, turn on.

It happens, don't get hung up on it.

Sometimes it can be explained, sometimes it can't.

[Ritchie200]

I just had a thought though... the storm pedal is a boost. Most of my drive is coming from it as I only have the gain at 10 o'clock (2204 mode). Perhaps something has been stressed by driving the front end hard with the storm pedal.

That would be a first.  You can bang the front ends of these or any tube amp pretty hard.  Your not going to hurt it with a pedal. 


When you put the new board in did you leave the effects loop card in there? I know it's true bypass when switched out.  Just wanted to make sure nothing was changed or removed.


Jim

[Jaymz77]

Hopefully this is one of the times it can be explained.


Since tone hasnt changed and there are no weird noises I thought it must be the output section.
How can I test if it's the OT? If it is and is only failing under load, measuring the dc unloaded resistance won't really tell me much.
Otherwise what order should I be looking at thing? Remembering I have ruled out Pi and output tubes.

Jim, yes I left the board in. I just gave it shielded wire connections. I never use it though

Cheers

[Jaymz77]

I do remember having to fart around with the bias resistor supplying it though. I can't recall what value I used but the factory schematic says 30v supply. I made sure that whatever value I ended up using it was getting the correct voltage.

[Blind Lemon]

Their may be others with different experiences, I have either had an OPT work or not, dadgum things act like a fuse that have had too much current run through them and they open.

Is your amp in a perpetual state of low value, broke right now? or get hot and lose volume? I know before you said worked one night and the next was down on volume. Still that way.

Try this if its still down on volume. Turn volume all the way down, look at your schematic and and with your DMM probe start at V1 and get all the voltages of the tubes and post the voltages as Mr. sluckey ask. As you are doing that and after you write down the readings, again starting at V1, lightly tap the pin(s) to the plate(s) of that tube. Listen to see if you get a difference in sound quality between the tubes as you move to the power tubes. If the sound goes from muffed to bright and clear, between the muffle and the clear is the problem.

Just a quick question, after the amp has been on for about 5 minutes are both power tubes hot? Very cautiously feel for heat on each one.

BL

[Jaymz77]

yeah its stuck at low volume ATM.
i haven't noticed any heat or tube heat issues. They aren't red plating.


I just chilled last night after work. Today is Saturday here (Sydney) so ill get those readings today and have a poke around as you said and post what i find.


Cheers mate

[Jaymz77]

Ok, I've got some voltages.


V1     V2     V3     V4     V5
260    168   215   .04    .04
0       0       24     0       0
2       2       39     434    433
0       0       0       427    426
0       0       0       -40     -40
216   300   208    432    432
0      168    25      0       0
2       36     39     .4       .4
0       0       0

[sluckey]

V2 pin 8 is not right. It should be about the same as pin 7.

[Jaymz77]

Ok let me double check that one

[Jaymz77]

Yep i quoted correctly.
But now they are 170 on 7 and 37 on 8

[Jaymz77]

Bad half a tube in V2?

[Jaymz77]

Yep, bad half a tube in V2.
Bloody thing. That was a known good tube too. not anymore i guess.
I think I'm right now, thanks to everyone who helped.

[Jaymz77]

The volume is definitely back.
But i have this kind of trailing static sort of sound when i hit a big power chord. Don't really notice it with the higher notes, mainly the bass strings.
A bit more fiddling is required.

[Blind Lemon]

12AX7 is a lot cheaper than an OPT. Glad you figured it out.


BL

[Jaymz77]

It is. Thanks for you help. Everyone else too.
I think the crackling is the tube socket connections or the tube.