1964 Fender Concert

[Leo_Gnardo]

True, but can't cones get brittle with age? I would think playing brittle cones will degrade them.

Yes I'm sure the paper's gotten brittle but isn't that part of the "charm" of old speakers. Why "good used" P12R's sell for more than reconed ones for instance. And "tone connoisseurs" insist on the original item? Either use 'em or sell em off to someone who will appreciate them as-is, and I'll bet you'll find a willing buyer fast. Reconing is not cheap and I wouldn't do it to an otherwise working speaker.

The OT has been replaced with a 3-tap OT and I'm wondering if it's 4/8/16 with the 4 powering the current speakers. That would be a little concerning. It might be a 2/4/8, though. What makes me think it might be 4 is the presence sounds weird when it gets up to 7 or so.

I'll test all this out. If the OT is wrong, then I'll get a replacement. I'll look for a vintage Concert or Bassman one on e-bay. Otherwise I'll go with a Classictone or something.

I'll probably run an 8 ohm tap to the external speaker while I own it so I can use my other cabs with it as well.

DO test your OT so you'll be assured what its taps are, and make sure it's properly connected. It is an advantage to have multi taps as you apparently plan to run the amp thru different cabs. I think you can find an appropriate OT at Mojo and Antique/CE/AmplifiedParts also, at reasonable prices. I've used 'em and they work fine, no complaints.

[Pdavis68]

If you're running a 2-ohm load on the 4-ohm tap, just add another 4-ohm cab. Only thing better than a 4x10" Concert is a 4x10" Concert with a 1968 Fender 2x15" Trust me, it won't blow up.

Which specific OT is in it? If it's another Schumacher, get the specs. Probably no need to worry about it!
Justin

I'm not sure what the transformer is. I couldn't see any of the numbers from the angle I could get. I'll know tomorrow when I remove the chassis (I may even start tonight, we'll see). It's black like the original and it appears to be about as big, so I think power-wise, it's fine.

I don't have any 4 ohm cabs. Both of my external cabs are 8 ohms. One is a 1x12 and the other is a 2x12, so if I put them in parallel the power handling wouldn't be even. Of course, split among 7 speakers, that probably wouldn't be a big issue. Would be interesting: 4xP10Q, 1xP12Q, 1xWGS ET65, 1xWGS Veteran 30 (the latter 2 are the 2x12).

Really, I want to run it through my WGS speakers because the just sound so amazing with my other amps, but I've only run 5W amps through the cab and I don't know if that's enough to really break the speakers in, so I'd like to really hammer them a bit.

[Justin Thomas]

What specific OT is in it? Depending on what is, it may not matter much. And even if it blows up, it's already replaced, so...

I ran my Bassman on a half-load for years before I figured out it should have had an 8-ohm load instead of 4. Never a problem. I occasionally even ran two cabs with it. And even if your Concert's speakers <ARE> wired for 2R, on the 4R tap it probably won't matter <IF> it's got a good replacement tranny (say, Bassman head or Pro Reverb). If it's a good replacement, I'd run 2 or 8 ohms with no real concern.

Justin

[Justin Thomas]

Sorry, I made a glaring blunder in the post you quoted, so I erased it...

Justin

[Pdavis68]

Okay, so I got home a bit before my wife and daughter tonight which gave me a chance to really crank the amp. I had a sneaking suspicion, because the tubes had recently been replaced and now he was selling it, that there might be a problem that would show when I cranked it and sure enough, one of the tubes is red plating. Before it started red-plating the blue glow in the tubes would really shine when I the chords. About a minute later one of the tubes started to get that unhealthy red glow... So....

I've never worked on a fixed bias amp before. I'm guessing the first things I should check are the 220K grid leaks on the power tubes and the 470R screen grid resistors? Also, those screen grid resistors are only 1W. Should I use 3W or 5W there instead?

I'll test the tubes as well, though I don't have any way of checking to see how matched they are.

It doesn't look like there's any way to adjust the bias. Is that right? So what do I do if it's off? Speaking of which, how do I measure current without a cathode resistor?

[52 Bill]

...one of the tubes is red plating.

Start by swapping the position of the two tubes. If the same tube red plates, then it's the tube. If the same socket red plates, then it's the socket.

Just in general, check the bias setting to see if it's borderline low.

[Enzo]

No the first suspect is the tube. Swap places with that tube and one from the other end. Does the red plating move with the tube or stay with the socket?

[Pdavis68]

It was recently retubed which is why I suspect it's something else, but I will swap the tubes and see what happens. Doh, I just watched a video the other day on red-plating and that's the first thing he said to do: Swap the tubes and I completely forgot.

I've got it put up for the night. My work space is right next to my daughter's room and she's going to sleep now, so it'll have to wait until morning.

[Leo_Gnardo]

Red plating you'll also want to suspect the capacitor that brings signal to the control grid of that tube in case your tube swap reveals the tube is not the suspect. In old Fenders they seldom fail, but sometimes they do. FWIW I like to use Mallory 150 film caps for this function.

There's a resistor in parallel with the bias supply filter cap. That resistor loads the supply - draws some current - and by varying it you can alter the bias voltage. To experiment you can add resistors in series with the one that's already there. Also keep in mind, if you haven't yet installed a fresh bias filter cap, that the old one may be leaky and that's a problem that needs to be fixed before you start adjusting anything else.

One good way to measure bias current with an ordinary volt/ohm meter is measure the resistance of the OT primary from center tap to each of the output tube plates. Take notes, how many ohms from red CT to blue plate and CT to brown plate. Then warm up your prize and measure voltage from CT to each plate, should only be a couple volts at most. Ohms Law gives you the answer, I (current) = V divided by R. Calculate, and your plate current will be expressed in amps. Multiply x 1000 to express in milliamps.

You can loosen one end of the suspect possibly leaky signal cap and see whether the redplate tube settles down.

It does make good sense to have bias distribution resistors that are matched and in spec, and without a doubt leak-free signal capacitors.

[Pdavis68]

I'm going to crack it open tomorrow. I strongly suspect it's going to need to be recapped and I wouldn't be surprised if a handful of the resistors have drifted over time. Unfortunately, I don't have any appropriate electrolytics in stock, so I'm going to have to order those. So I may not be able to get this fixed until next weekend, but I can at least start getting it diagnosed.

Thanks for the info on measuring the current. That sounds easy enough...

And thanks for the info about the 56K resistor. I was wondering what it's purpose was and that makes a lot of sense.

[Pdavis68]

Step 1: Swapped the tubes. Red plating stayed with the tube. So maybe a bad set of tubes. He threw in the originals. I'll test them all, but I'm not really sure what to check for with red plating.

I have the boxes for the tubes: Ruby 6L6GC/5881-RI Matched tubes. 37mA, 3243 Transconductance, both tubes.

Update: Doh, now i see what's wrong. He swapped 1 of the two tubes... There's one Ruby inside the map, the other is unlabeled. In the two Ruby boxes there's one Ruby tube and one unlabeled.

If the tubes are terribly mismatched, couldn't that cause red-plating in the one doing most of the work?

[Leo_Gnardo]

Update: Doh, now i see what's wrong. He swapped 1 of the two tubes... There's one Ruby inside the map, the other is unlabeled. In the two Ruby boxes there's one Ruby tube and one unlabeled.

If the tubes are terribly mismatched, couldn't that cause red-plating in the one doing most of the work?

Let's find out now whether the presumably matched pair of Rubys will run satisfactorily.

Without a doubt the tubes that were in it were mismatched, and if you check them by the method I mentioned in #24 ^^^ you'll certainly find they are. Too much plate current is what makes the plate heat excessively. Heat (in watts) = voltage x current.

Plugging the tubes into a tube tester may not reveal much. Your red plate special may move the needle further than the good tubes showing once again that tube testers don't tell us everything we'd like to know. Testers can tell us a tube's OK, or even "better than average" then when you use the tube you tested in a circuit, you find out that's not the case. The ultimate tube tester is your amp. Mostly I use the tube tester to check for short circuits in tubes I suspect are dodgy, and today mine confirmed a rare short circuit in a 12AX7 that failed a customer mid-gig .

[Pdavis68]

Leo, I'll do that shortly. I wanted to do a little exploratory work first. Also, with regards to matching tubes, could I just set up a basic circuit on my breadboard (my amp breadboard, not the little low voltage ones) to measure current on the tubes? A simple Class A type thing? If I just set up a simple Class A circuit and measured the voltage across a 1R resistor off the cathode, couldn't I use that as a basis for matching the tubes? Not sure that would be any easier, but it might be and I could then use it as a way to test the rest of the tubes in my collection. I only have 2 actual matched pairs of tubes in my collection: Some a pair of EL844s and a pair of EL34s. Everything else i have is a mix of different tubes, so I'd like to come up with some way to try to match others, if I can...

But back to the topic at hand...

A few choice photos:

Mr. Death Cap, I presume, attached to the 3-prong cord and ground switch. We'll be saying goodbye to you...


Here's a shot of the whole board. Cathode electrolytics have been replaced and a couple of orange drops in there as well.


When they replaced the OT, it looks like one of the screws went right through the board... A few times...


Are these new bias caps? They're awfully clean if they're not...


New screen grid resistors. 2W, I believe. Good enough or should I go to 3 or 5?


The last 3:

Obviously it's been recapped despite the thick layer of oxide on the doghouse. Should I recap anyway? IC doesn't have the best reputation.

I think the PT is original On the underside it has 67233 on it which is a brownface transformer.

The OT, not original. It has 3 wires taped up on one side and 4 wires taped up on the other side. The only marking on it is A-2529. Can't seem to find any info on it. Anyone have any ideas?

[Leo_Gnardo]

A-2529 looks to be a Stancor number. Perfectly good transformers. I'm sure one of our MEFster friends will ring in with its specs & wire color / function chart.

Orange Sprague bias caps look 1970's, wouldn't be a bad thing to put in fresh replacements. They're a clue as well, likely the originals failed causing output tubes to draw lots of current which wrecked the original OT.

Hi voltage Illnoise, well they lasted this long right? No telling how long they've been in there, never saw a date code on an IC cap. Yes why not put in a fresh set then no worries for at least another 20 years. F&T are popular, good quality & reasonably priced.

SG resistors, optional. If they're measuring OK no worries. For replacements, 2 schools of thought. Some folks like to use 1W parts so they'll act as a fuse if there's a tube failure. Others would go 3 or 5 watts wirewound and hope the line fuse does what it should in case of a tube failure. Choices choices choices . . .

[Pdavis68]

A-2529 looks to be a Stancor number. Perfectly good transformers. I'm sure one of our MEFster friends will ring in with its specs & wire color / function chart.

Orange Sprague bias caps look 1970's, wouldn't be a bad thing to put in fresh replacements. They're a clue as well, likely the originals failed causing output tubes to draw lots of current which wrecked the original OT.

Hi voltage Illnoise, well they lasted this long right? No telling how long they've been in there, never saw a date code on an IC cap. Yes why not put in a fresh set then no worries for at least another 20 years. F&T are popular, good quality & reasonably priced.

SG resistors, optional. If they're measuring OK no worries. For replacements, 2 schools of thought. Some folks like to use 1W parts so they'll act as a fuse if there's a tube failure. Others would go 3 or 5 watts wirewound and hope the line fuse does what it should in case of a tube failure. Choices choices choices . . .

Awesome. I love Stancor!

Fixed the ground wire (was between a locking nut and flat washer wrapped around the transformer screw. Replaced it with something similar to this: http://mnats.net/images/chassis_grou..._nut_thumb.jpg

Yes, F&Ts are going to be ordered.

What makes you think the orange drops are old? The big orange drop looks almost exactly like one of my brand new ones (print is indistinguishable. Codes are identical, except for values) . The other might be older, but it looks pretty similar to some other new ones I have.

I guess i'll leave the 2W SG resistors in there.

Thanks for all the info.