Cap job?

[seeker]

Hey all. First post.

I recently picked up an old silverface Fender twin reverb. After half a dozen times of playing through it I started to notice an occasional light pop sound, which was just a single pop once in a while. Then I started noticing that the output had dropped pretty significantly, where the amp was earbleeding loud on 5 before the output dropped, the output is now fairly tolerable at that volume. I also noticed, even before the output drop began, that when pushed up to 4-5 there is some nasty distortion that sounds like it is coming from the bass end of things. Turning down the amp's bass control to 3 or so seems to take care of it. And last, with the amp turned up a bit I noticed, before the output drop began, a distortion which sounds something like a ring modulator on the high notes when the amp is up to 4-5. Does all of this sound like possible issues with the filter caps? I am not noticing any unusual noise of any kind.

When I bought the amp I was told by the owner that he was told that the filter caps had been replaced when he bought it a few years back. I was also told that the amp had sat in the garage for the last year unplayed.

Today I discharged the amp and opened it up to have a look. The filter caps and other electrolytics look original to me (filter caps are the big brown mallory's), and I'm thinking that none of the caps have ever been replaced. Other than a 3-prong power cord, the amp looks untouched inside (and in great condition outside) including having all RCA tubes which seem to also be in good condition. I'm thinking that the amp is most likely in need of a cap job. Any recommendations on good electrolytic caps at a good price?

While I have the amp out of it's cabinet, I will do some poking around checking voltages and currents and clean the pots. Is there anything else that I should check? I am not an amp tech, but years ago I was able to tinker around with a diy 18 watt build without killing myself. So at this point I have some refreshing of memory to do on some basics, but I'm not in any hurry. The amp will be there when I'm ready to tackle it.

I should also mention that I did some swapping around with using pairs of output tubes (rather than a quad), and I didn't notice any change in the lowered output. I think the output tubes are ok, but I don't have a tube tester to make sure. And I tried both channels of the amp including swapping around some preamp tubes and the phase inverter tube with no real change in output. All of the tubes seem quiet, not microphonic, and the output tubes have a nice blue glow about them.

If pics and sound files would be of any help or of any interest, I can post that sort of stuff.

[m1989jmp]

Hey all. First post.

I recently picked up an old silverface Fender twin reverb. After half a dozen times of playing through it I started to notice an occasional light pop sound, which was just a single pop once in a while. Then I started noticing that the output had dropped pretty significantly, where the amp was earbleeding loud on 5 before the output dropped, the output is now fairly tolerable at that volume. I also noticed, even before the output drop began, that when pushed up to 4-5 there is some nasty distortion that sounds like it is coming from the bass end of things. Turning down the amp's bass control to 3 or so seems to take care of it. And last, with the amp turned up a bit I noticed, before the output drop began, a distortion which sounds something like a ring modulator on the high notes when the amp is up to 4-5. Does all of this sound like possible issues with the filter caps? I am not noticing any unusual noise of any kind.

When I bought the amp I was told by the owner that he was told that the filter caps had been replaced when he bought it a few years back. I was also told that the amp had sat in the garage for the last year unplayed.

Today I discharged the amp and opened it up to have a look. The filter caps and other electrolytics look original to me (filter caps are the big brown mallory's), and I'm thinking that none of the caps have ever been replaced. Other than a 3-prong power cord, the amp looks untouched inside (and in great condition outside) including having all RCA tubes which seem to also be in good condition. I'm thinking that the amp is most likely in need of a cap job. Any recommendations on good electrolytic caps at a good price?

While I have the amp out of it's cabinet, I will do some poking around checking voltages and currents and clean the pots. Is there anything else that I should check? I am not an amp tech, but years ago I was able to tinker around with a diy 18 watt build without killing myself. So at this point I have some refreshing of memory to do on some basics, but I'm not in any hurry. The amp will be there when I'm ready to tackle it.

I should also mention that I did some swapping around with using pairs of output tubes (rather than a quad), and I didn't notice any change in the lowered output. I think the output tubes are ok, but I don't have a tube tester to make sure. And I tried both channels of the amp including swapping around some preamp tubes and the phase inverter tube with no real change in output. All of the tubes seem quiet, not microphonic, and the output tubes have a nice blue glow about them.

If pics and sound files would be of any help or of any interest, I can post that sort of stuff.

I'd definitely change electrolytic caps that old, especially when the amp hasn't been used in a year.

Is there a nasty 100Hz hum in the background?

ESR meter could give you an indication of their current function. I use F&T filter caps, they're in the business for a long time.

[seeker]

I'd definitely change electrolytic caps that old, especially when the amp hasn't been used in a year.

Is there a nasty 100Hz hum in the background?

ESR meter could give you an indication of their current function. I use F&T filter caps, they're in the business for a long time.

There is no nasty hum or other noises that I can hear. The only real noise is some interference from a nearby computer being picked up by a guitar which mostly goes away when changing the angle of the guitar in relation to the computer. I did mention above that I was hearing an occasional light pop, but that seems to have gone away. I wonder if that might have been caused by a tube pin having intermittent contact, and maybe the process of swapping tubes around took care of it. All the caps look fine visually, with no bulging, leaking, or anything of that sort. But they also look to be original to the amp. I guess the only way to really know is to test them with an ESR meter as you suggest.

Looking up information on using an ESR meter to check caps I came across a video by Uncle Doug on Youtube. He seems to have a different mindset about changing out capacitors than what I have read elsewhere, being, only replace what is broken. In general I agree with that approach, but on electrolytic capacitors specifically, the common wisdom seems to be that old caps are ticking time bombs. If it's of any interest, here is the mentioned video. https://www.youtube.com/watch?v=nat1YYNMW5A

Doing a little reading on how an ESR meter works, it seems that what is going on is generating a high frequency low voltage signal and measuring the resulting voltage across the capacitor in order to determine the resistance of the capacitor. And that got me thinking. Since I am not an amp tech, an ESR meter wouldn't see frequent use in my hands. But I do have a sound card and audio software which can sample and output at 96k hz, which would allow for passing a 48k hz signal of any shape. So I'm wondering if that might be useful in place of an ESR meter.

Also, thanks for the recommendation on capacitor brand.

[Enzo]

I am in between. I think you should solve any problems the amp has, and THEN after it works, you can think about replacing aging caps or other maintenance work.

Throw a scope on it, is the thing oscillating at above audio freqs? When an amp does that, it makes it sound weak because most of the amp energy is going into making RF.

Also possible anywhere along the signal path the signal is interrupted. ANy inoperative stages?

Find out WHERE in the signal path the signal is diminished, and the problem will probably become evident.

[Mick Bailey]

I like ESR meters, but I did repairs for many years before I got one. ESR affects the ability of a cap to charge and discharge. With a PSU once it gets high enough this results in more hum than usual and with a coupling cap you can get signal reduction. I rarely find an ESR issue with Silverface amps and those Mallorys usually read as low as a new cap. But that isn't the entire picture; these meters test at a few volts and this isn't representative of the loads and voltages encountered in an amp. If your amp doesn't hum any more than usual then your ESR is going to be within operational tolerance. Not saying the caps are good, just that I don't think this is your problem. If you measure the AC ripple on your first and last B+ nodes this will show you how much ripple you have - usually about 3 or 4 v on the first node and just a few mV on the last.

If you do want a really cheap ESR meter then take a look at this;

http://music-electronics-forum.com/t44998/

[pdf64]

...a distortion which sounds something like a ring modulator on the high notes when the amp is up to 4-5. Does all of this sound like possible issues with the filter caps? ...

That often indicates excessive ripple (eg due to HT caps not doing their job, perhaps high ESR) at the main reservoir or screen grid HT nodes.
The '2 x mains freq' ripple acts to modulate the guitar signal, creating the ring modulator effect.
It may not manifest as '2 x mains freq hum' because such a common mode noise, when affecting the balanced stages of the power amp, will tend to get cancelled out.

[bob p]

I'd definitely change electrolytic caps that old, especially when the amp hasn't been used in a year.

Maybe I'm overthinking what you said, but I think that declaring caps bad after a year of non use is a little over the top. Bad caps need to be replaced but good caps don't go bad in 12 months on a shelf. If I had to change out E caps on every amp that I haven't used in a year, I'd be doing so much recapping that I'd never get to use any of my amps.

I prefer to test the caps to see if they're performing well rather than just doing blanket replacements. If an amp has sat for a really long time then I'll reform it's caps rather than just yanking them out. I've got some original caps in vintage amps that work fine, without any problems. I can't see the point in replacing perfectly good parts.

Like Enzo says, replace what needs replacing, don't replace what doesn't.

[bob p]

http://music-electronics-forum.com/t44998/

thanks for the reminder -- i ordered one of those and because it hasn't arrived yet i had forgotten about it.

[m1989jmp]

Maybe I'm overthinking what you said, but I think that declaring caps bad after a year of non use is a little over the top. Bad caps need to be replaced but good caps don't go bad in 12 months on a shelf. If I had to change out E caps on every amp that I haven't used in a year, I'd be doing so much recapping that I'd never get to use any of my amps.

I prefer to test the caps to see if they're performing well rather than just doing blanket replacements. If an amp has sat for a really long time then I'll reform it's caps rather than just yanking them out. I've got some original caps in vintage amps that work fine, without any problems. I can't see the point in replacing perfectly good parts.

Like Enzo says, replace what needs replacing, don't replace what doesn't.

If the caps are 30+ years old I tend to change them if the amp will be used regularly and fairly loud.

They're a critical part of the amp, and it's not a pretty sight when they explode.

That being said, I always save the old caps and try to reform them; But I don't think old, tired caps belong in a well used amp.

[seeker]

I am in between. I think you should solve any problems the amp has, and THEN after it works, you can think about replacing aging caps or other maintenance work.

Throw a scope on it, is the thing oscillating at above audio freqs? When an amp does that, it makes it sound weak because most of the amp energy is going into making RF.

Also possible anywhere along the signal path the signal is interrupted. ANy inoperative stages?

Find out WHERE in the signal path the signal is diminished, and the problem will probably become evident.

I think that is a logical approach.

I don't have a scope here. For as little tinkering that I do I never could justify the cost of one. Although I would like to have one around, just because.

I'm probably pushing my luck, but I spent some time this weekend playing the amp more. I thought that it had no microphonic tubes, but apparently I was wrong. Both V1 and V2 are microphonic after warming up for a good while. I was playing the amp and flipped it into standby for a few minutes, which turned into an hour. When I came back, I did some pecking around on components and wires in the amp to see if I hear any odd noises, which lead to pecking on the tubes. V2 gave some light pops when pecking on it. So I likely found the source of the light popping that had seemed to go away.

All stages are working fine as far as I can tell, although there is a light click with the vibrato on, which I hadn't noticed until yesterday. Maybe it was already there, but I hand't noticed it before.

I like ESR meters, but I did repairs for many years before I got one. ESR affects the ability of a cap to charge and discharge. With a PSU once it gets high enough this results in more hum than usual and with a coupling cap you can get signal reduction. I rarely find an ESR issue with Silverface amps and those Mallorys usually read as low as a new cap. But that isn't the entire picture; these meters test at a few volts and this isn't representative of the loads and voltages encountered in an amp. If your amp doesn't hum any more than usual then your ESR is going to be within operational tolerance. Not saying the caps are good, just that I don't think this is your problem. If you measure the AC ripple on your first and last B+ nodes this will show you how much ripple you have - usually about 3 or 4 v on the first node and just a few mV on the last.

If you do want a really cheap ESR meter then take a look at this;

http://music-electronics-forum.com/t44998/

Thanks for the tip on the meter. I think I will pick one of those up. It looks like I'm going to have to pick up a new digital meter, too. I haven't touched mine in a while, and it has been loaned out a couple of times in that time. It looks like it is flaking out. I couldn't even get a reliable resistance reading from it. And my analog meter seems to be missing in action. It's probably right in front of my face, under a book or something. Time to do some picking up and reorganizing.

Still doing some catchup reading here inbetween work and doing other things that need to be done, so really getting to the amp will likely be slow going. I'll report back after I get time to dig in a bit. On the plus side of the catchup reading, the advancement of online learning resources available today in comparison to even 5 years ago is amazing. Kids should know how lucky they are to have so many older guys sharing their knowledge and experience with the world.

[Justin Thomas]

When I came back, I did some pecking around on components and wires in the amp to see if I hear any odd noises, which lead to pecking on the tubes. V2 gave some light pops when pecking on it. So I likely found the source of the light popping that had seemed to go away.
...

Thanks for the tip on the meter. I think I will pick one of those up. It looks like I'm going to have to pick up a new digital meter, too. I haven't touched mine in a while, and it has been loaned out a couple of times in that time. It looks like it is flaking out. I couldn't even get a reliable resistance reading from it.

Don't throw your tubes out yet. Did you retension and/or clean your tube sockets? They may have been dirty and/or loose. Hey, it's worth the time and it's free...

As for your meter, when is the last time it got a fresh battery

Justin