Sudden loss of volume in 18w clone

[jennings]

I was playing my 18w Marshall clone at a gig on Sat night, and suddenly there was nothing sound-wise!  Upon closer listening (when I got it home) the amp powers up and plays, but only has much reduced volume coming out.  The tone (although quiet!) remains kind of how it should.  Occasionally full volume pops back for a second, then we're back to a whisper.  Can anyone figure out what might have happened here to give me a rough ball park to start investigating?  I've tried the usual valve swapping with known good tubes.  I've tried checking for dry joints to and from the output tranny (I've re-heated each just to reseal them).  That's not fixed it so far.  I've cleaned all the inputs and outputs, and tried both channels.  All the same.  Could this be a short or loose connection in the output tranny?  I'm confused as to why the overall tone's not affected really, just the massive volume drop.

[rzenc]

I don't know if it will help but... have you tried to reflow solder joints on the P.I. and other sockets? Checked voltages?
Maybe a pot wiper losing contact? there are a bunch of things to investigate, but I would start from the basics first...Check voltages and post them...maybe someone can sniff it out by looking to a voltage chart... I myself would first look into the P.I. since you say things are the same with both channels...

Hope this helps

With Respect

Best Regards

Rzenc

[jennings]

Cheers Rzenc...I'll look into that.  I'll try and get some time toward the weekend to have the amp open again, so I'll take some measurements etc.  I've tried to reflow most of the solder joints on the power valves, and the PI, but I'll take another look too.  I initially suspected one of the vol pots until I found the same effect on both channels.  I tend to run the amp at full vol at 8 ohms into an 8 ohm cab, so whether I over strained the tranny or not, I'm not sure.  The amp's a second hand Ceriatone clone.

[Dynaflow]

 Give all the tubes a jiggle (with it on, play a chord and hold it and give them a bit of a jiggle) could be a loose pin in one of the sockets, had that happen on a 18 watter before.

Regards,

Dyna

[duke of earl]

Jennings,

I have a Hoffman AB763 circuit 50 watt combo that I built and have been breaking in for a while. After playing the amp loud for a while, one channel will fade out and then I switch to the other channel which works for a bit and then it fades away. I have replaced tubes but the problem still exists. I
pulled the chassis out and did some checking to find that the output tube bias had fallen to 15ma. I guess that some components have broken in and caused the change in bias current. I rebiased the amp to 35ma and played it a while with no problem. I will report in a week or so to see if the problem still exists.

[jennings]

Hi Duke!

That's an interesting problem...let me know how it pans out.  I've so far tried reflowing the solder joints, and using known good tubes.  That doesn't work.  I've replaced the output tube sockets (as they seemed a little loose anyway for my liking...though i might as well while I was about it!), and that hasn't worked.  The amp's very quiet, on both channels, and very occasionally jumps back to proper volume for a second, then reverts to being quiet.  No change to the tone though.  Weird!  Almost like there's suddenly an attenuator switched on! 

I've taken values for all the resistors, and all seem to read close to what their colour codes say they should be.  Except for the two 68K resistors, which seem to read out at 35k each.  Could this be a problem?  And isn't it a bit weird that they both come out just as low?!  I'm not convinced it's these that's causing my problem.

[jennings]

Ah, and some extra info I remember...at the gig where the amp went down, at the start of the night there was a little microphony and feedback which I at first attributed to a tube.  It seemed to be picking up the rattle of the spring retainers that used to hold my EL84 tubes in place I reckon.  I say used to, as I've replaced those sockets now.  Could that actually have been a component going bad, or the output tranny?  I mean, I've tested all the tubes anow, and replaced them with known good, and that hasn't cured the amp.

Also, if people want to know voltages, what voltages do I need to measure?  And how?!

[labb]

That amp probably has shielded cable going to the grids of the pre amp tubes...Do a good check and make sure that the shield is not touching the core wire. All it takes is one little hair..How do I know this? Had it happen to me. like to have never found it..Found it by injecting a signal at each stage untill I lost it..

[bigdaddy]

It happened to me also. I use plain yellow solid core wire now unless it's a higher gain amp. It doesn't hum more with solid core 22 gauge on a Champ amp though.

[bnwitt]

Except for the two 68K resistors, which seem to read out at 35k each.  Could this be a problem?  And

Unless you have disconnecting them to take the ohm reading, you are reading them in parallel through the jack which is half of one's resistance. 

You obviously have a loose connection somewhere.  I'd start with an unplugged denergized amp and take ohm readings to the chassis from every point that is connected to ground.  Like volume and tone pots, triode and power tube cathodes (should show resistance of cathode resistor on meter between the tube pin and the chassis).  Remove, clean and retighten that PT ground point which has all of the ground wire connections.  Chopstick the amp while playing it to see if you can find the loose joint.   If none of that works, fill out a voltage chart for the amp.

[jennings]

I've addressed the ground contacts...doesn't seem to be that, unfortunately.  I'm convinced it's a contact somewhere though, so the hunt's still on!

[bnwitt]

Well this is a situation where the signal generator and injector come in very handy.  By injecting a signal into your amplifier's input jack and then checking for that signal at the first stage through all subsequent stages until you find where it disappears, you can narrow down the problem area.   If you don't have those units you can make do with a cd player for your generator and a home made listening amplifier like the one Doug shows on his cool tools page for your signal tracer:

http://www.el34world.com/Hoffman/tools.htm

You probably would have been done by now.

[Heinz]

I'd also check for broken components. Tap the components on your board with the chopsticks while the amp is on. Sometimes a leg breaks off but is held in place by the outer component material so you don't see it.

[Heinz]

With the symptoms you described (loss of volume but 'correct' tone) I'd look at

• the cathode resistor of the power stage
• the supply voltage that feeds the OT and its components
• the OT secondary wiring & sockets

[duke of earl]

Alright I said I would report back about my AB 763 build. Since the re-bias the amp has performed well with no loss of volume. This was too easy.

[jennings]

Duke, I’m glad things worked out for you there!  Well done.  Sadly I’m still at a loss so far!

I’ve tried all the following:
1.   New known good valves and clean the sockets – no change
2.   Replaced the output tube sockets with tighter ones – no change
3.   Tightened the star ground lugs and cleaned…plus re-routed some of the grounding cables – no change
4.   New speaker sockets – no change
5.   Bypassing the output ohm selector knob – no change
6.   chopstick the amp – nothing happened, and no poking etc could reveal anything untoward
7.   visual checks of all joints, and reflowing all joints – no change
8.   Measuring all resistors in the circuit – all read as expected
9.   constructed a voltage chart – looked ball park normal to me (see below), although I’m no expert in the slightest!

I’m at a complete loss now…the amp sounds normal, but with a loss of volume, and occasional flickers back to higher or full volume (very occasional!).  Sounds to me like something’s not quite connecting properly.  I just can’t find or see what it is!

[bnwitt]

Your voltage chart looks fine to me also.  It may be your OT.

[Dave]

By reading your list and voltage chart, it looks like it leaves 3 possibilities.
1. A bad pot.
2. A bad OT.
3. Bad coupling cap.

But to me, it sounds like your signal is grounded somewhere.

Dave

[Baguette]

This sort of things may happen with something loose around the speaker / OT sec.
Did you try another speaker cab?

[jennings]

I was wondering about those coupling caps...might try swapping them out just to see, as it'll only take a short while.  Would a bad pot really have this effect...both channels have their own set of pots, yet both have the same volume drop.  I could try removing one channel at a time from the driver perhaps and see whether the issue persists.  Would that be a quick one wire type test to see if it's coming from one of these channels?

I'm erring now towards thinking it's possibly the OT, as I can't see any unexpected groundings anywhere, despite lots of checking.  I was also playing with the amp dimed (as I usually do), so I guess that puts more pressure on the OT.  How do I check the OT?  Those wires linking to the EL84 tubes clearly have the right voltages present at the pins, but how do I check for the right levels at the OT output?!  Never done this before, you see!

Just as a point, the amp behaves the same with both my 2x12 cabs, and my other heads all run fine through these cabs, so I'm sure it's not them.  Good suggestion though!

[jennings]

Hi Folks!

Well, I checked everything else on the amp out, concluded it must be the output transformer, and took a chance on sourcing one here in the UK.  I've literally just finished fitting it and fired the amp up...works fine now!  Back up and running!  Must have blown the old transformer at the last gig.  I didn't replace it with a Ceriatone OEM unit, I used one that I found via Ebay, just to save on costs and to try something new.  Seems like a good unit, and the amp seems to have a touch more treble. Or maybe that's just me!  Anyhow, thanks for all the guidance while I was doing this folks!

Cheers!!!

[Dynaflow]

 Great ya got it working!  :hello2: Thats the problem with some of the Asian rim transformers, they'll often work forever without a problem or they don't last real long kind of a shot in the dark (not unlike they're tubes some you can't kill, some you can't keep alive). I think you made a good decision on trying something else. I've had good luck with hammonds so I keep using them, but lately I use what I have I'm hoping the weber power supply I'm getting ready to use with my high powered princeton will hold up ok its been through two experiments which I didn't like (a tmb 18 watter which I redid in another chassis and I'm not sure it'll stay in that one either, I think I'm moving away from el84 amps towards something warmer and mellower sounding i.e. getting old and a deluxe 5e3 clone head which was ok, but not as good as my dumb old green juke 5e3 amp with cheap parts in it) but is still hanging tough at the moment. Anyway good on you for getting it working!

Regards,

Dyna

[John]

Just got done reading this thread. My Harmony that I'm working on had very similar symptoms, except that it also had very nasty buzzing/distortion too. It's why I opened it up to start with. I knew if I kept looking long enough someone would have an amp that had a similar problem to mine! :)