Problem with 5F4

[Bluefinger]

Hi,
I have a problem with my main amp, a 5F4 Tweed Super. after turning it on everything is ok but after a couple of minutes of playing it starts to crackle and then after a short while the volume drops dramatically and it sounds thin and distorted at a fraction of the original volume. If I turn it off and let it cool down it's ok again and the whole thing starts all over again. Then I tried to turn it on and let it idle for an hour. It still was ok but after a couple of minutes of playing the trouble started again.

I need to repair this amp as quick as possible but I never had this kind of problem. Any ideas?

thanks!

[Jazz P Bass]

Got a spare set of tubes?

[g1]

Do both channels have the problem?

[Chuck H]

Indeed... We'll need an out of chassis test with voltage reading for this one. This amp has apparently been someones standard for a time and is now starting to misbehave! I love a good cause!

Bluefinger, get the amp out of the chassis now so that any further commentary can be quickly tested. It's probably just a bad tube or tubes, but being prepared with answers may be a good thing now,

[Bluefinger]

Indeed... We'll need an out of chassis test with voltage reading for this one. This amp has apparently been someones standard for a time and is now starting to misbehave! I love a good cause!

Bluefinger, get the amp out of the chassis now so that any further commentary can be quickly tested. It's probably just a bad tube or tubes, but being prepared with answers may be a good thing now,

Yes, I swapped all tubes with my 5E5A and it still does the same thing on both channels. I'll try to write down all the voltages later

thanks!

[Bluefinger]

There is one thing that I totally forgot ... I don't know why I didn't think of this earlier. It all started after I replaced a non working presence cap. Can a leaking presence cap (the one that goes from the presence pot to ground) cause these symptoms?

[Jazz P Bass]

Short answer: maybe.

It is common to both channels.

And shorted (leaky) it could kill the output signal.

[Chuck H]

There are some questions that need answers. First, if the problem started after a repair why wouldn't it be assumed that the work performed may have been the cause? That's a real no brainer.

I'd like to know what the amp was (or wasn't) doing that implied the presence cap needed replacement.?.

It's possible there is a cold solder joint or an intermittent short that is affecting the cathode resistor on the PI driver stage. Maybe the presence pot was already compromised and the work damaged it further. Pots do go bad. Especially when soldering.

[Bluefinger]

ok, here's an update:

I replaced the presence cap because the presence control was not working anymore. It was like it was turned off all the time. After the cap change it worked again. And yes, it was a no brainer to check the repaired area first so I replaced the cap again and the pot as well. That did not change anything. I still get the heavy crackling but the power loss did not occur anymore since yesterday (without changing anything inside the amp).

Just to make sure I replaced the tubes one by one again but nothing changed. It's not the tubes.

With the volume control durned down there is no crackling as well but it gets more obvious the more I bring up the amp (yes I tried different guitars and different cables). But it's not like there is something that is coming from before the volume controll. It seems like the more signal I send in, the more crackle it causes at some stage later. Here's why I think it's happening after the volume control. At very low volume (set at 1) there is no crackling. At 3 or so I get a short crackle after I hit a short note (like an echo t the played note). The more I bring up the volume, the louder the crackle gets but it also gets longer. If I really crank the amp and hit a short note I get a crackle afterwards that lasts almost two seconds until it stops. SO the crackling gets longer, the louder I play ... and It doesn't make a difference if I controll the volume at the amp, at the guitar or use a linear booster in front of it.

Time to change all the caps??? The solder joints look ok and I tried taping the wires with a chopstick ... no results. The symptoms don't sound like a mechanical problem to me anyway. Any ideas?

Oh, yes ... and I have used only carbon comp resistors in this amp. But a bad resistor should give me a constant crackle, right?

[Jazz P Bass]

Do you have another speaker that you can try?
As a test.

[Chuck H]

Did you sub the preamp tubes AND the power tubes? It may be microphonic power tubes.

What happens if you turn the amp up and whack it?

When you poked around with the chopstick, was the amp on? it should have been. And you would need to poke EVERYTHING. Components sometimes break internally. Tube pin slots can become worn, loose or dirty.

Looking at a solder joint won't tell you if it's good. A bad one may take years to start causing problems. I've seen a few that didn't cause problems until the amp warmed up.

The problem must be after the volume control because it affects both channels.

If your filter caps are old there is a slim possibility that it is an oscillation caused by poor decoupling.

It could be a failing plate load resistor. They usually cause the problem you describe and the issue can be heat sensitive, so, after the amp warms up.

[Bluefinger]

Do you have another speaker that you can try?
As a test.

I do and I tried it ... nope

[Bluefinger]

Did you sub the preamp tubes AND the power tubes? It may be microphonic power tubes.

Yes, even the rectifyer. It's not the tubes

What happens if you turn the amp up and whack it?

My hand hurts after a while ... nothing else

When you poked around with the chopstick, was the amp on? it should have been. And you would need to poke EVERYTHING. Components sometimes break internally. Tube pin slots can become worn, loose or dirty.

Yes, the amp was on of course and I poked around on everything ... resistors, caps, wires, solder joints, the board, pots, turned the pots, wiggled the tubes, ... it does not seem to have a mechanical reason

Looking at a solder joint won't tell you if it's good. A bad one may take years to start causing problems. I've seen a few that didn't cause problems until the amp warmed up.

see above ... I did the chopstick test

The problem must be after the volume control because it affects both channels.

That'S what I would guess as well ...

If your filter caps are old there is a slim possibility that it is an oscillation caused by poor decoupling.

I don't have filter caps with the correct value at home. I'll have to order some and try it.

[/QUOTE]

It could be a failing plate load resistor. They usually cause the problem you describe and the issue can be heat sensitive, so, after the amp warms up.

I'll try replacing them ...

I'll also try to replace some coupling caps. I should have some at home ...

Thanks a lot for the detailed feedback. I'll try some more and get back to you all ...

[Bluefinger]

ok, now I have replaced all the coupling caps on the board and the crackling is gone ... I'm not sure if that was it because sometimes it takes quite a while until it starts ... I'll keep an eye on it.

[Bluefinger]

Oh man, now crackling is back but this time only on the normal channel. It seems like I had a combination of more than one issue. It did not start to crackle if I played it for a couple opf minutes like it did before. The crackling occurs half an hour to an hour after the amp is switched on. This time it should be easy ... tubes, plate resistors and coupling caps are already replaced and ruled out so there aren't many components left ... Hope I'll be able to fix it this weekend