Resurrecting a Neglected DC-3

[jakep]

Greetings. My DC-3 has been packed in a crate in the garage for about 6 years. My son talked me into buying it new (for myself) years ago because he was learning guitar and was really into Metallica.

It is one loud little devil, but I never succeeded in getting tone that I like from it. At the time I bought it I was more into blues and jazz than metal. Anyway, I eventually started playing classical guitar exclusively, so the DC-3 went into storage.

However, recently I picked up my Strat again, and I want to make another attempt with the DC-3. I suppose I mostly like Fender clean tones. Even when I try the suggested clean settings in the manual, the amp sounds kind of, er, I dunno, "choked" or "compressed", a bit lifeless. Maybe it needs to be turned waaay up, even on the clean channel? (The neighbors probably would object to me cranking it.)

Now, Youtube did not even exist when I bought the DC-3. So, last night my search for "DC-3" (or DC-5) on Youtube only turned up demos of what I consider horrible, cheesy distorted tones. I know how to set the graphic EQ to get that chopped-out metal sound, but I would sooner chew on tinfoil than listen to that. (Just my taste, don't be offended ;-) The only demos I found of clean tones also had that kind of choked sound that mine gets.

So does anyone have suggestions for getting a kind of chimey, shimmery clean sound like, say, one of the medium sized Fender "xxxVerb" amps? Or is the DC-3 just the wrong amp for me?

Also, is it likely to need service after sitting idle? Do tubes "rot", or maybe the capacitors dry out?

Thanks for any help offered.

- J -

 

[bendo]

Tubes don't rot, but if it was ridden hard before being put away, and is still on stock tubes, new tubes are a really good idea.

Current Production JJ EL84s seem to be the only tubes that survive stock. You can change the bias resistor which will run the amp cooler, and prolong tube life. http://forum.grailtone.com/viewtopic.php?f=28&t=51785

If you are still on a V30 speaker, a C90 will help open up the low end a bit. When I had a V30 in my combo, the only word to describe it was farty. C90 sounds MUCH better. There are some blanket mods and tone stack mods that can be done to liven up the clean channel and add some sparkle. See this thread: http://forum.grailtone.com/viewtopic.php?f=28&t=58252

Reverb mods from above link will help a well.

Your caps should be fine. I would not worry about those.

Just for an example My clean channel:

Gain = 3.5-4
Treble = 6.5-7 <-- Where this is set GREATLY effects the response of the rest of the controls. Above 7 it adds more gain.
Mid = 2-3
Bass = 4 <--- this one is REALLY output dependent. At lower output settings, I have to turn it up to fill out the clean. At higher output settings, I have to roll the bass off or it gets really flubby.
Presence = 4
Reverb = 4
Master = 4

I have also changed a few resistors in V1A. None for the tone stack, just gain resistors to give a bit more clean headroom.

BTW. cheesy distorted tone is what the DC does It is a high gain amp.

 

[talltxguy]

I think two essential things to do with a DC-3 - especially if you're after clean tones - is to replace the dinky output transformer and use the coldest EL-84s you can find (or just do a bias mod!). I did both of these mods on my former DC-3 and was very happy with the clean channel.

 

[bendo]

I think two essential things to do with a DC-3 - especially if you're after clean tones - is to replace the dinky output transformer and use the coldest EL-84s you can find (or just do a bias mod!). I did both of these mods on my former DC-3 and was very happy with the clean channel.

Which OT did you use?

 

[jakep]

Thanks for the tips, guys. I have been offline for a while, so did not get a chance to reply.

It turns out that I have a couple of more serious things to get fixed on my recently resurrected DC-3.

First, I am getting a tremendous amount of hiss on the clean channel, even with the gain at zero. (The hiss does go away when the master is at zero.) This is pretty clearly something in the V4 circuitry; putting a brand new Mesa 12AX7 in that slot does not help at all. In fact, I swapped all 12AX7s out one by one, but it made no difference.

Next, the reverb does not work. The connectors at the tank and the amp both were badly corroded. I cleaned these up but it made no difference. Then I put in a brand new Mesa 12AX7 in the reverb tube socket (I forget the number as I write this) but that did not work. The tank had a lot of corrosion on the top of it. I think some moisture (condensation probably) affected the bottom part of the combo, so maybe the tank is shot. Are tanks reparable, or should I just order a new one? (I see that they are fairly cheap.)

Next, I am getting quite a lot of 60Hz hum even with the master volumes at zero. To me this says "PS caps"; correct me if I'm wrong. I may attach some new ones temporarily in parallel to see if it helps.

Finally, also with the master volumes at zero, I am getting an odd intermittent low rumble/thump effect. By intermittent I mean sometimes it goes away for an hour, and then starts up and goes for an hour. This noise has the "rhythm" of a bad connection or solder joint, but it is entirely low frequency in nature. I have no clue what it might be but when the amp is next on my bench I'll tap and wiggle connections (avoiding high voltage spots of course).

So, it appears that being in storage and neglected was a bit harder on the thing than I had hoped.

However, I am going to be patient and nurse it back to health. Many years ago during my misspent youth I took a year-long college electronics technician course. I then worked for a company that repaired a lot of tube PA amps, which were kind of obsolete even then, but which the boss preferred. So I think I can relearn this stuff. I may have to buy some equipment to diagnose the thing.

Also, knowing that some mods will enhance the clean sound (once the amp is basically back in shape) is encouraging. I'll use my amp modelling SW on my computer in the meanwhile.

- J -

 

[bendo]

Your power tubes are likely bad. Do not run the amp any more until you put a Matched quad of EL84s in. JJs will work fine, just ask them for the coldest set they have.

Though if you have a lot on internal corrosion, you have bigger problems.

 

[jakep]

Bendo:

Well, ok, I can try new power tubes. That would be simpler than fiddling with filter caps. That may solve the hum and rumble problem, but it seems unlikely to me that power tubes are the source of the white noise, since that is channel specific.

On the other hand, I am a bit puzzled about your comment that the caps are probably ok. From my recent reading online I gather that a largely unused amplifier that is over twenty years old may have dry electrolytics. Mind you, I'm not saying you're wrong. It is all just speculation until I try some more things!

So thanks for the help.

- J -

 

[bendo]

I've seen a lot of 20 year old amps, haven't seen a DC that needed new caps yet.

Electrolytic caps have come a long way since the stuff in the 50s and 60s that dried out. Most of the caps in our amp are polypropylene. They last worlds longer than the older electrolytic stuff. Plus later production electrolytic caps have much better sealing and packaging. I doubt they have dried out.

The behavior you are describing screams bad tubes. You can have ALL kinds of funky stuff going on when the tubes are wonky. Channel changes that you don't do, weird noises, and bumps in the night.

Retube the ENTIRE amp. It will cost a little money, but the piece of mind is worth it. I try and get all my tubes locally. I was using Doug's tubes, but, I have had a lot of failures lately with orders from him. He replaces them ASAP, but the hassle of having to ship them back is getting to be a PITA.

 

[jakep]

I have decided to take your advice. A complete re-tube will happen this afternoon.

It is worth the price just to avoid days and days of speculation. If it doesn't work, I am no worse off than before, and have a new set of tubes at least.

Thanks.

 

[jakep]

Sigh.

The amp now has new Mesa-branded tubes in every socket. All of the problems described in my earlier post are still there: hiss, line hum, intermittent rumbling, and of course a dead reverb.

So, having bought the tubes from my local guitar store here near Toronto (probably at premium prices) I am $200 poorer and no further ahead. Of course, the amp isn't any worse either.

At this point my options seem to be:

1. Patiently try to diagnose the amp myself; I'm not in a great rush to get it working, and I do have some long-ago experience with this kind of thing. If I choose this option I'll go after the 60Hz hum first. But trying to do it myself could be a huge waste of time if I don't get anywhere. The time might be better spent playing the guitar
2. Take it to a tech and get an estimate on having it fixed. This end up being expensive, I am not sure how much more money a 20 year old amp is to me.
3. I could cut my losses and just sell it for whatever I can get in this shape; that may be maybe very little.

Does anyone have further recommendations to make?

Thanks.

- J -

 

[bendo]

The voltages inside your amplifier can kill you. Make sure your filter caps are discharged. http://ampmaker.com/discharging-capacitors-946-0.html

Can you pull the chassis and take some clear photos of the board and all components? Is there any visible corrosion on the board or any leads. Are any of the caps swollen and the tops popped up? Are there any burnt resistors?

The first step to diagnosing an amplifier issue is tubes. In your case, being that is sat for an extended amount of time, there is no telling what environment it was sitting in. If there is corrosion on the reverb RCAs, there is likely corrosion on your board as well.

 

[jakep]

I'll take some photos and post them. At the moment the amp is assembled again, but I'll take it apart tomorrow. I don't recall any funky looking parts, but this time I'll use the fine-toothed comb.

The amp has been in storage in a sealed wooden crate that I built to ship it across the country. I am in southern Ontario. The humidity gets crazy high here, so my guess is that condensation was the culprit. I should have put a bunch of those little dessicant bags labelled "do not eat" in the crate with the amp. Instead, whenever I get ahold of those little bags I just eat 'em like popcorn :lol:

I have my own silly (and could have been worse) story about undischarged caps.

Early on during my time as a tech I was servicing a tube amp with some part or other that was overheating badly. I could smell it burning, but could not see anything that looked black etc. So, (with the amp unplugged) I stuck my nose down close to the circuitry to try to sniff out the hot component. Of course, I knew that the filter caps could hold a whopping high voltage charge, but I just got in too big a hurry and did not discharge them.

Anyway, I managed to get the tip of my nose between the high side and chassis ground, and a giant "zap" was heard. I got a fairly nasty burn right on the end of my nose, and looked like Rudolph for a few days. It scared the s**t out of me because I realized the discharge could have gone through my brain instead of the end of my nose.

(Or maybe it DID go through my brain - that would explain a lot about how my life turned out subsequetly :shock:

Anyway, all this to say that I am all too aware of the dangers.

So thanks again for your suggestions. I'll post photos soon.

- J -

 

[talltxguy]

Bendo, I used a Hammond OT and was very happy with it!

 

[jakep]

Bendo: BTW, on one of your earlier points, I watched the video comparing the V30 and the C90 speakers. I found the difference to be quite striking. If I can get the amp working, the C90 would go a long way toward making the DC-3 sound like I want. It does indeed seem to be at least partly responsible for what, to my ears anyway, is a "choked" sound on my DC-3.

I'll have to check into the cost. Of course I won't buy one if I cannot get rid of my noise problems.

- J -

 

[bendo]

The C90 sounds much better when only using one speaker. I have a 1x12 with a C90, and a 2x12 Recto cab with V30s. You should be able to pick one up around 140.00. Probably get a used one cheaper. There were some fakes rolling around, so it might be a better idea to get one from your local dealer.

A single V30 makes a DC3 sound farty.

 

[jakep]

Greetings.

A couple of days ago I stated that I would post some photos of the inside of my DC-3 chassis. I still intend to do that, but I decided to take a few days to set up a better environment for trouble-shooting the amp. To that end I am building a fold-out workbench in my garage, with good lighting etc. My house is small and jammed with stuff, so I had been "servicing" the DC-3 on the garage floor. Since this process is likely to take some time, I decided to make it easier to do.

I hope that you folks will tune back in and give me some advice once I am able to post the photos.

Thanks.

- J -

 

[jakep]

Below are some pics of the DC-3 mentioned in my original post. I don't see much that looks suspicious. Maybe someone with a keen eye can spot something.

I also posted a video at http://www.youtube.com/watch?v=lyWCPvyRRtA . This shows the hum and hiss problems, but I could not get the amp to exhibit the low frequency "sputtering" or rumble, which is intermittent.

The schematic for this amp is available online. I hesitate to repost it in case there is a copyright problem. However, by looking at the schematic I am guessing that the clean channel hiss originates in the region of the two V4 preamp stages.

Also, I notice that the power transformer gets very hot even when the amp is idling, i.e. switched on but not producing any sound. Is this normal? Should I try to measure the current being drawn at idle?

Pics:

 

[bendo]

Board looks good. None of the caps appear to be popped.

Current Mesa production EL84 tubes WILL NOT hold up in a DC3. They are Sovtek EL84 standard and your amp will eat them alive. Your video indicates exactly that. With the settings you have dialed in, you are only putting power to the power tubes. Your problem lies there. Once again, current production Mesa EL84s will NOT hold up in a DC3.

The heat issue can be resolved by replacing the 120K bais resistor with a 100K resistor. Some piggy back a 470K resistor on (which will make it 100K) but, I don't like that. The below photo points out the resistor to change. Be sure to use a 1/2watt resistor of good quality. Radio Shack resistors are OK.

However, the plate voltages on your power tubes will be around 400-450 volts depending on how hot or cold your tubes are. Key there is to make sure you get a REALLY cold set of tubes. I tried using the Mullard reissues and they did not last long. I have been using JJ EL84s rated at 20 idle current draw. These are holding up well and sound great. I have a set of Sovtek EL84M (military version) that are supposed to be designed to handle higher plate voltages. I am currently running a quad of JJs that test out @ -16.7-18.2ma draw with 436v on the PV and the tubes are holding up well. Amp is silent and sounds great.


You can also further resolve the heat issue by installing a 117V fan from Mesa. Call Mesa Customer Support, and they will be able to set you up with the right fan. After this, the amp will run MUCH cooler. I have mine mounted to the bottom front of the chassis. I have a DC3 head, but I do have a DC cabinet. I will check and see if it fits in there and clears the speaker. If it does not, you will have to do what I did on my last DC3 combo. I "modified" the tube guard to mount the fan to it. However, on a combo, you might have issues.

I did a ghetto mod on my old DC3 combo with a 12V PC power supply fan. I wired in a standard outlet plug from an extension cord to the amp power switch, and then used the smallest wall wart I could find to plug the fan into. Worked really well, and actually came out looking relatively clean.

Also, I have not had good luck with current production Mesa tubes in the V6 (P/I) position on my DC3. I have been using JJ 12AX7s there to great success. If you have a Mesa JJ (have to look at the plate structure to tell, they use JJ, Penta, and Sovtek) that will work as well.

 

[bendo]

They Mesa fan does not clear the speaker where i have it installed. So you will have to get creative.


Thanks
Chris

 

[jakep]

Would the new Mesa Boogie tubes cause these problems immediately? The symptoms did not change at all right after I stuck the new EL84s in. If the Mesa-branded tubes are the problem, I wonder why the amp ran for a couple of years after purchase without problems?

Thanks.

- J -