1977 Fender Twin, buzz then fuse blowing

[asd123321]

2 times a prominent buzz showed up after 5 minutes which was not changed by the
volume knobs. Opening it up and wiggling parts it ran OK but blew the pilot
light. Next time it blew a fuse after 20 minutes. Then another one quick
with a loud snap.

Is there something to check with it unable to run?

Would it be good to simply replace all 5 of the big capacitors since I an
not great at diagnosing these?

I have replaced 3 capacitors in others successfully.

[Chuck H]

There are several things it could be and it may be a combination of them, even intermittently as well. Consider old power tubes, old electrolytic caps and oxidized contacts. The amp is old enough that if any electrolytic caps are original, they should be replaced. THIS DOESN'T JUST MEAN THE 5 BIG CAPS!!! There will be smaller caps on the board. Those should also be replaced since they suffer the same age consequences as the big cans do and some correspond to the bias supply, which is a critical circuit. Those bias supply electrolytics are in reverse polarity to the other electrolytics and this must be observed. Clean all non soldered contacts (pots, jacks, pilot socket, fuse socket, tube pin slots and switches) and see about the age and usefulness of the existing power tubes. Always see that the amp is biased properly. It's usually common practice to also check carbon composition resistors for drift out of spec and preamp tubes could also be old, weak, noisy or microphonic and should also be checked.

If you have to replace all the electrolytics, the power tubes, a pilot lamp, a fuse and a couple of resistors you're into it for something like $150 to $200 in parts. But if you fail to snuff it up now you may blow up a transformer or burn up components on the board and need to replace the whole thing. WRT tube amp maintenance it's expensive to go cheap. $200 might seem like a lot to cherry a not so collectible amp, but it's a lot better than selling it for that amount and buying a new amp if that one blows up. It doesn't sound like there's any problem that can't be easily (if not cheaply) fixed right now.

[Randall]

I'm with Chuck H on this. If the power supply caps are original, I would replace them at 40 years old. A failing HV cap could cause your symptoms. While I was at it, I would replace all the little electrolytics in it, as they are cheap. Then I would see where it as at. Pay close attention to the big power supply caps, one of them is reversed polarity as place on the board to the others.

[lowell]

Yes I agree with Chuck and Randall.

To me it sounds like a bad power tube. If the bias is hot, that could cause thermal runaway hence the HUM (buzz?) And popping the fuse. If the pilot blew that's typically a shorted power tube. Be sure and check the 100ohm resistors on the pilot if they exist. Those usually blow in such an instance.

First Thing I'd do is pull power tubes and turn it on. If the fuse doesnt blow after some time...you most likely simply need new power tubes.

That said...as the other guys have said...if those caps are all original it'd be in your best interest to replace all of them.

[bob p]

2 times a prominent buzz showed up after 5 minutes which was not changed by the volume knobs.

That the noise is not changed by the volume knobs is a useful piece of information. As a confirmation, please tell us the frequency of the noise and that will help to confirm it's source. The frequency of noise is a very helpful datum that many people tend to overlook.

Opening it up and wiggling parts it ran OK but blew the pilot light. Next time it blew a fuse after 20 minutes. Then another one quick with a loud snap.

IMO the best method of diagnosing problems when you don't know what's causing the problem is to divide and conquer. You might consider pulling out all of the tubes out and creating a voltage table at idle, and repeating the chart with the tubes in place. If the amp draws excessive current when the tubes are in, then it would be helpful to determine whether or not it also does it when the tubes are out.

Would it be good to simply replace all 5 of the big capacitors since I an
not great at diagnosing these?

Blindly replacing caps may or may not solve your problem, though it's certain to drive up your repair costs.

On the subject of doing the shotgun method of cap replacement because the amp is 40 years old, I would ask a few questions:

Are you fixing this amp for yourself, or are you fixing it for a customer?

Is this an amp that will end up in a museum, where preservation is important? In a basement? Or is it going to end up on the road?

Are there budget constraints, such that you need to get the amp up and running first, and then work on bulletproofing it later on?

Are you interested more in a learning experience of a fast repair?

One school of thought says that it's best to use the shotgun method of replacing old caps because old caps are known troublemakers. Another school of thought advises against taking blind stabs in the dark, and only replacing components that are defective and actually need replacing. That second method requires actually testing the old caps while the first one doesn't.

As a testament to the quality of the old caps used in Fender amps, it's not at all uncommon to find old amps that still have their original PSU caps that are still in good operating condition, still meet their rated frequency response and capacitance specs, and work as they are supposed to in-circuit. Dare I say it, when I come across a clean, unmodded vintage Fender amp that still has perfectly functioning original caps under the doghouse I prefer not to replace them if they pass a battery of tests and I can avoid replacing them, because most of the time I'm working on museum grade amp preservations / restorations for Vintage Anal Homos. I wouldn't hesitate to replace them on a gigging amp though, if I wanted to be sure about bulletproofing an amp for stage use.

To address your question about diagnosing PSU caps -- the first steps diagnosing PSU caps is easy. So easy that you should get out your VOM and take some voltage readings off of the amp before you go making decisions.

IMO to determine if the amp has bad electrolytic caps, the first step is to create a voltage chart for the amp, measuring the voltage to ground for every pin on every tube. Compare them to the voltages on the schematic. Post the results here if you need help interpreting them. If you've never done this then it would be a good learning experience.

If the voltages in your table match the voltages on the schematic during idle conditions, the next step is to repeat the test under load. If the PSU caps aren't causing trouble then I'd start looking elsewhere to find the problem in the amp. Basically, this is another divide and conquer method. IMO it's better to learn how to isolate problems through diagnosis than it is to do a shotgun parts replacement that would forego the learning experience.

If the PSU caps test OK and you're inclined to keep original caps for a collector type amp then there are other tests that are helpful, such as sweeping each stage in the circuit to measure it's frequency response to verify the cap's in-circuit performance, but most people who work on gigging amps for customers aren't interested in operating at that level. It's too time consuming and time is money. More often than not techs are inclined to replace them.

As a first step in checking out an amp that's having problem, I always verify the PSU voltages with tubes out before doing anything else. That information helps to guide you into where to look next. Once the PSU is determined to be operating properly the next step is to inject signal into the amp and trace it from one stage to the next. If you've never done this, it's another learning experience that's worth the investment in time that it takes to learn it.

Back to the subject of bad caps, my experience with Fenders of that era is that more often than not, it's those little cathode bypass caps that tend to be the troublemakers. I can't tell you how many dead Fenders I've picked up for a song because of bad Ck. IME the big caps under the doghouse can also cause problems, but most of the time the little caps tend to fail first. Those are famous for failing in ways that are difficult for most users to diagnose. When I do an on-bench stress test of the amp, more often than not the Ck in various stages tend to fail in a domino fashion: when I find one signal stage that has gone bad due to a bad Ck and replace it's cap, then another Ck in another stage will often fail shortly thereafter. It's not at all uncommon to play whack-a-mole chasing Ck failures through these old amps. Those Ck are inexpensive and subject to old-age failures, and they are definitely a case where I'm on-board with shotgunning all of them once one is determined to be bad, because if one is bad then they're all likely to be on their last legs. Like Chuck said pay attention to the bias caps. Bias caps are important, and when a Fender bias circuit fails the results can be catastrophic. But IME the big PSU caps aren't typically the first ones to cause problems. Because they're expensive you might want to verify that they're bad before replacing them, if you're on a budget.

[Randall]

While I certainly am not going to disagree with anything bob mentions in the above post, my take is somewhat different. Pretty much all the amps I see are working amps. I tell people that E-caps are usually thought of as having a working lifespan of anywhere from 25 to 40 years, depending on who you listen to. Sure some last longer, but at 40 years old you should expect one or more of these to fail before too long, and that will probably happen on stage, and when that does happen, bad things usually happen. It's usually worth the cheap insurance and peace of mind to my customers to replace them now and not have to worry about them for another couple of decades. So for me, that outweighs by a long shot leaving a 40 year old HV cap in place because of it's originality, which BTW I find silly. Not to mention probably saving the customer from an onstage gig killing failure.

I replaced the cans in a very early $5K Marshall JTM45 head last year that still measured OK, but with one having a slight bulge. The owner makes quite a handsome living playing guitar, and he needs reliability over all else, and didn't balk for a second over the idea of it no longer being completely original or the possibility of some cork sniffer somewhere down the road scoffing at the reduced value of it since it had been serviced. Better than waiting for the actual failure and possibility of an transformer replacement.

[asd123321]

I had it on a long time without the 4 6l6. I tried it with 2 at a time and it
plays ok with no buzz now. It is just for playing quietly at home. I don't know
what a cathode bypass cap is or bias caps. Also this has had a little static
type noise for a long time that is not changed by the volume knobs. Changing
all capacitors in the Hammond, Leslie, 65 Bandmaster, Twin and 1936 Radio
is way too much. In the Bandmaster I replaced 2 of the biggest capacitors
so I suspect those have more problems. Thanks for everyone's advice.

[bob p]

Like Randall mentioned, once a cap starts venting it's time for that one to go. If you see any caps in your amp that are bulging or are leaking liquid, those must be replaced.

so how many amps are we working on? You mentioned a Hammond, Leslie, Bandmaster, Twin and an AM radio. That's an awful lot to attack in one bite. I think we'd be better off dealing with them one at a time and focusing on the Twin.

I had it on a long time without the 4 6l6.

did you take voltage measurements? until we have voltage table we're taking blind shots in the dark.

I tried it with 2 at a time and it plays ok with no buzz now.

which two? did you try substituting all of them as randomized pairs? unfortunately you haven't provided enough information to let us know if any of the tubes are bad.

with the information you've provided it's not possible to draw any conclusions. that voltage table that i mentioned earlier would really be helpful, along with a more complete description of your tube swapping experiment.

I don't know what a cathode bypass cap is or bias caps.

sorry if i was at all unclear in making assumptions about some of the lingo. cathode bypass caps are the caps that go from a tube's cathode pin to ground. we abbreviate them as "Ck" where "C" indicates that the part being mentioned is a capacitor and the "k" subscript indicates it's position on a cathode. Similarly, Rk would be an abbreviation for a cathode resistor, "Rp" for a plate resistor, etc.

In a cathode biased gain stage, the cathode cap is the bias cap. In a fixed bias stage, such as your Fender Twin, the bias is fixed and the bias cap is part of the circuit that supplies the negative bias supply voltage.

Also this has had a little static type noise for a long time that is not changed by the volume knobs.

In terms of "static type noise" that isn't changed by the volume knobs, does this occur only after the amp has been on for a while and has been heated up? if that's the case, take a look at the screen resistors on the power tubes. fender mounts their tubes beneath the sockets, which tends to cook parts like an easy-bake oven. it's not at all uncommon for old screen resistors to suffer thermal damage, to drift in value, to crack, or to produce thermal noise, which sounds like intermittent crackles, after the amp has heated up. this is a very common maintenance issue on old fenders.

this is a good resource: Tube Amplifier Debugging Page