The Steel Guitar Forum

So my Seymour Duncan 84-40 is coming back from a very reputable tube repair shop. They've given up on it. There is a hum. I think its from transformers. Seems to be the fatal flaw for these amps.

When do you give up on an amplifier? Geez I've only owned it since my thirties.

When is worn out worn out?

I see these amps historically have transformer issues. And they jerry rigged the effects loop ( of no value to me ) into the power supply in a non standard seemingly irreplaceable way.

It sure sounded sweet with my Sho~Bud in a church playing I Love You Because for a wedding. Could never get the thing to overdrive for my steel though.

Never had to give up om an amp before. Not a good day for the Seymour Duncan name in my house.

If I were you, and I had more than 36 hours off a week, I'd get tempted to try learning to fix 'em myself. After all, look at all the others here that seem to do their own.
And I'd think even knackered transformers can get replaced

its another case of PCB mounted tube sockets, pots and jacks - similar to Fender reissue amps - here is the amp circuit:



amps like this eventually will wear out - its possible that there is something as simple as an open trace on the circuit board. But there are probably many small problems causing one large problem.

Unlikely that a transformer would cause hum unless its a mechanical hum coming from the transformer itself. You can isolate that by putting the amp in standby - hum still there? Then you have transformer induction or mechanical vibration.

Unfortunately amps built in this manner do have a lifetime that is much shorter than their hand wired brethren. They are not the same animals, and are much more susceptible to heat and vibration related failures. They are also a lot harder to fix - your tech may have called Uncle on this one. It would probably be more expensive to fix it than to replace it with something that already works.

Transformers and chokes need to be mounted 'orthogonal' to each other... or they'll talk to each other magnetically. This means that each one should be oriented 90degrees from the other... this gives three possible orientations, so a PT, a choke, and an OT can all be mounted this way. The poor little reverb drive transformer will just have to do as best it can... I often see it mounted at a 45degree angle to the others, and further apart as well.

Tim Marcus wrote:its another case of PCB mounted tube sockets, pots and jacks - similar to Fender reissue amps - here is the amp circuit:



amps like this eventually will wear out - its possible that there is something as simple as an open trace on the circuit board. But there are probably many small problems causing one large problem.

Unlikely that a transformer would cause hum unless its a mechanical hum coming from the transformer itself. You can isolate that by putting the amp in standby - hum still there? Then you have transformer induction or mechanical vibration.


Unfortunately amps built in this manner do have a lifetime that is much shorter than their hand wired brethren. They are not the same animals, and are much more susceptible to heat and vibration related failures. They are also a lot harder to fix - your tech may have called Uncle on this one. It would probably be more expensive to fix it than to replace it with something that already works.

Precisely why I'm paying big bucks to have my 59RI Bassman chassis rebuilt to original spec... however, wives don't seem to appreciate the value in this somehow...
....sigh..

I've been in electronics since 1955, when I went into the Air Force. I was also an amp tech in Nashville (and the Ampeg factory repair tech at the time). I've never heard of a printed circuit "wearing out".

PCB's can have bad solder joints, just like the old Fender tube amps. I can't tell you how many Fender amps that I fixed by resoldering connections.

Jack Stoner wrote:I've been in electronics since 1955, when I went into the Air Force. I was also an amp tech in Nashville (and the Ampeg factory repair tech at the time). I've never heard of a printed circuit "wearing out".

PCB's can have bad solder joints, just like the old Fender tube amps. I can't tell you how many Fender amps that I fixed by resoldering connections.

Jack I undesrstand where you are coming from. The experts say that the electrical current has no idea if its flowing through a wire or a trace on a PC board, and it should sound the same.. However most of us don't buy it and like the sound of hard wired amps... Same thing with reliability.. A good PC board amp in theory IS repairable, and the trace has no business "wearing out". However in practice, I must agree with the premise of the OP..
You very rarely see original hand wired amps tossed aside because they can't be fixed. Yet you DO see this happen with PC board amps on a pretty regular basis.. There must be reasons for this other than the fact that hand wired amps are more "collectable"..
I don't think PC board amps are anywhere near as forgiving of heat, vibration, current fluctuations etc as PTP wired amps are, but I defer to those that are more experienced.. I will never buy a tube amp that isn't PTP unless its practically given to me.. I simply don't trust PC board/tube amps... bob

i give up on an amp as soon as i have something i like better next to it. lots of times the 'better' amp is also cheaper and simpler.

Well, I've repaired a lot of point to point wired amps. I've also repaired solid state amps and later computers.

Many PCB types are replaced because its easy to do. I will concede that the older PCB's were not very well built or reliable, but as PCB technology has evolved it has gotten much better and reliable.

One of the other well respected amp techs in Nashville, when I was there, was "Carl" (I don't remember his last name) who had a shop across the street from the original Gruhn Guitars. Carl was an old time tube tech and when he got a solid state amp to repair, he would either send them to me or all he would do is call the amp manufacturer and get a new board as he refused to learn solid state or work on a solid state or tube PCB.

In mid 1968, when I went to work for Bendix Field Engineering on their NASA Apollo tracking station project, I had a couple of weeks wait before going to my assignment at the Ascension Island Apollo Tracking Station. They put me to work at their PCB repair depot temporarily. All the high precision and high reliability NASA tracking station equipment, such as the Telemetry Processors, Collins S-Band equipment (except the transmitter tubes, obviously) were all solid state, PCB's. Power supplies were conventional supplies, NASA would not allow a switching power supply on a tracking station.

When pots, jacks, and/or other surface-mounted components are also connected directly to a PC board, as you see in the 84/40 pic Tim posted, there are going to be problems eventually. It's much worse if the PC board is thin with small traces. Sure, you can jump the traces, you can even rebuild the amp, but the issue is time and cost. Some PC board amps even have the tube sockets mounted to the board, which is a total disaster as far as I'm concerned. But on these Duncans, the tube sockets and transformers are mounted to the chassis, not the board, see here -



My take is that these Duncan amps had a lot of reliability issues. It's too bad, they designed some pretty cool amps in the 80s, but execution was, IMHO, flawed. The Convertibles were a very cool idea, but everyone I knew who had one had problems with it. I would have bought one except for that. Vibrations caused intermittent issues, PC boards developed cracks, on the Convertibles, there were contact problems with the preamp modules, there are a host of documented issues.

My guess is that the amp is fixable, but the issue is finding someone who will do it and also not charge more than the amp is worth. I see them for $200-300 occasionally, here's one that sold on ebay for $229 after not getting any bids at $249 - http://www.ebay.com/itm/SEYMOUR-DUNCAN- ... 0779214735

My experience with repairing PC boards is usually bad parts, bad solder joints, bad interconnection (usually bad connectors that were spec'ed wrong), and broken traces or bad plating on feed-thru holes.

Thanks for taking on this question. Great and informative answers and recounting of experience.

OK So this is a lesson of life. In the auto business they'd have recalled these things for "design flaw" maybe.

What do I do with the leftovers? The tubes, speaker, reverb pan. Use it as a 12in open back speaker cab?

Buy a homebrew kit to replace the amp while reusing the speaker cab?

I have two Velocette's and a stereo Gibson Goldtone. Gives me 4 channels of Class A 15 watts each. So I can accept the loss of the Duncan from the line. Was a great practice amp.

I thought only British amps could get knackered?


And yes, the notion of a scatter-wound, hand-built, point-to-point space shuttle... well, at least it'd be easy to fix when it comes back! If it comes back.

I would get a second opinion before trashing it. You can see there are a lot of different opinions about PCB tube amps. Find a shop that ain't skeered to work on them.

I would suggest sending the chassis back to the factory. They must have run across this problem before? It can't hurt as you have been this far or maybe they can forward you to someone that was involved with the design of the unit.

Hi Len and Earl,

Good advice. Mr. Duncan doesn't make amps anymore. Reading the user manual now, just some 80's hype with an overdrive channel and effects loop. Which makes me all the more disappointed.

The technicians have spent 6 weeks working with it. Ordering parts caused a wait. They are only charging $65.00 plus parts. If I take it to someone else I"ll spend more than it was worth in the first place.

I like the simpler designs with just a single tone knob.

I should get a kit board and mount it in the existing chassis. That would be "Sweet Mental Revenge"

I'm just trying to work with all my equipment to see what's ship shape and what's not. Finding "age" means a lot of worn out equipment not up to the task.

Thanks

I had an 84-40 that I used with an r&b horn band from approx. 1990 to 1996. Mine had 2-10's. I was told that there were some production runs in Korea that had numerous problems. Supposedly they moved production to the US to try to solve these problems, which it didn't.. The amp sounded incredible, for guitar, but it was extremely non-roadworthy. I had problems with tube sockets failing, pot stems breaking and components coming loose from the circuit board. The amp went down a couple of times a year, on a gig. I carried a bassman head with me to deal with these eventualities. After 6 years I gave up and bought a Victoria. My son still has this amp. Next time I visit him I'm going to fire it up.

As an aside, I tried using this amp for steel. It sounded awful.

Hi Howard,

In a band setting it was inappropriate for steel. I have a single 12 speaker. Couldn't get rock'n overdrive out of it with steel. But with a Strat - boom wonderful distortion.

I love how the amp warmly rolls over into overdrive as the steel notes sustain, so sweet. I played the 84-40 exclusively in Japan with a Sho~Bud. I fear the trip to Japan and back again was too much for it.

A lesson here is pro usable gear is not built by every company selling you guitar amps.

My problem was that I could not get it to play cleanly, with a steel. I used it on the clean setting and it sounded awful to me. I'm not big on overdriven steel guitar. The overdriven sound was not particularly good particularly good as overdriven steel sounds go. But with a strat or a 335 is was quite sweet.

Affects of the ROHS initiative is another issue I see popping up lately. I've fixed a lot of newer SS amps by re-soldering surface mount components.
Yep - led is bad for you but it sure made for a good solder joint.

I'll tell ya a story, for whatever it might be worth to you. About 20 years ago, I started having a major problem with my '65 Twin. I'd turn it on at a gig, and it worked great for about 5 minutes. Then it would literally start screaming. I took it to 5 different techs over a period of about 6 months. They tried all kinds of things, replaced all the tube sockets, re-soldered connections, etc. All said it should work fine, but the problem persisted. I'd lost confidence in the amp, which I'd owned and loved since I bought it in 1977 I think it was. I was ready to set it out by the curb.

I happened to run across another tech who worked on vintage tube amps only. He was a young kid, but he was really smart and completely obsessed with old tube amps. He was intrigued by the problem, and wanted to tackle it. I told him to keep it as long as he needed; it wasn't doing me any good in it's present condition. He called about a month later and said he was sure he'd nailed the problem. It was a loose ground strap. He said it wasn't a bad reflection on the other techs who worked on it, because checking that particular strap wasn't part of the standard troubleshooting procedure for that amp. I don't remember what he charged, but it wasn't very much.

The amp has worked perfectly ever since, and needless to say, I'm really glad I didn't set it out by the curb. The moral of the story is, if you like the amp, don't be in a huge hurry to ditch it. It doesn't eat, and you don't have to make mortgage payments on it. It could turn out to be an easy fix once the root cause is known. Obviously you'll need a reliable amp in the meantime, but you'll need one whether you trash it or not.

If it were me I'd hang on to it, keep investigating and see what happens. You have nothing to lose and everything to gain. JMHO, of course. Good luck.

Thanks Johnny. My Twin, I bought used in 1970. First time in the repair shop I noticed it came back with a different reverb pan in it. Seems mine was coveted and they tried to slip one by me.

I've had the amp at King Amplification and they have crawled over every nook and cranny and would like nothing more than to please me. Since they couldn't fix it, they won't bill me for their time invested total. Instead they only gave me their basic charge, plus all the parts. They guaranteed finding the problem or they only bill the minimum. Pretty Good! With all those new parts I can't "dispose" of it.

I am lucky they are local for me.

It is still true that I can take it elsewhere. And I may.

From their website:
http://www.kingamplification.com/repair.html#Tube

Warranty Service
We are a factory authorized service center for the world’s finest amplification products. Valid warranty repairs for these products are billed directly to the factory. A copy of your purchase receipt is required before work can begin. We do not accept warranty repairs via UPS or other third party carriers.

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A 'scope will tell you exactly where the noise is coming from. Correcting the problem, especially with PWB's, is another. "Giving up" on an amp or guitar is not a problem or sin, but thinking you need a specific guitar or amp for "your sound" certainly can be.

If it has definite problems - get rid of it! Life's too short to endure simple problems like replacing a piece of bad gear.

Thank you everyone. It has been a greater topic than just my own initial query.

I have acted. I am enrolled in an Amplifier Kit Class at King Amplification in Santa Clara California. I will be hand wiring a circuit board to replace the one I have. I am reusing the amp. It will likely be a Fender Tweed Deluxe circuit, but that is still TBD.

Over the course of 6 weeks I will twice a week will be trained and work on my board.

Thank goodness I know how to solder.

Great to hear that you're going to rebuild it, Peggy!