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Hey man,
I have two questions about my Marshall Jubilee 2555:
1) Is it necessary to have my amp re-biased when it's time to switch out the tubes? If so, is there a way I can do it myself?
2) I'm thinking of getting a THD Hot Plate attenuator. Since I can switch between 4, 8 and 16 ohms on the amp, does it matter which version I get? (Obviously the cab will have to match, too.) Will one sound better or do a better job?
Thanks for your input, Amp Man!
Mike
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Hi, Mike.
The answer to your first question is a definite "yes." You should get your amp re-biased when you replace the output tubes. Now, I'm not saying the amp won't operate unless you re-bias, but it will operate more efficiently--and sound better--if the tubes are biased properly. It's okay, when you're in a pinch and it's absolutely necessary, to install a set of tubes without biasing. Over the years many customers have called me on Monday mornings and said, "Man, I blew a tube on a gig Saturday and I had to put in my spare set of tubes to get thru the night, but now I have to bring it in to get it re-biased." That's fine, but beware. The amp could suffer internal damage, depending on how the bad tube decides to fail. This can generally lead to three scenarios. First, the amp works fine and is simply in need of re-biasing; secondly, the new tubes don't work properly either, due to an internal component failure; or thirdly, you turn on the amp and the new tubes start celebrating their own little version of New Year's Eve (woo-hoo . . . fireworks!), again due to the failure of one or more internal components. The bottom line is, have your tubes replaced and biased before they fail. But about the possibility of doing it yourself, I really don't recommend that. You can purchase equipment that will let you read the bias (idle) current of a tube, but that's only part of the story. I never bias tubes solely on the basis of current draw. I also look to see what the output of the amp looks like on a scope, and I make sure I listen to the amp. Correct biasing is a combination of all three factors, so if you can find a qualified tech in your area, have that person do the biasing. And while we're on the subject of replacing tubes, let's refer back to the Monday morning phone conversation in which the caller mentioned a phrase that might be unfamiliar to many players: "spare set of tubes." Hey, you own a tube amp, so don't leave home without 'em! I don't care what kind of amp you play, even if it's a Budda! Carry some spares, esp ecially output tubes. Someday you'll thank me.
Now, on to the THD Hot Plate. There is a school of thought that says using the highest impedance setting on the transformer yields the best sound. I'm not really convinced of that. The secondary of a fixed-impedance output transformer is wound to a specific load impedance. Consider the case of a good-sounding 2x12 combo amp that uses two 8-ohm speakers for a total load of 4 ohms. Assuming this school of thought is correct, if I add additional windings to the secondary of the output transformer for an output of 8 ohms, and then install two identical (but 16-ohm) speakers in the amp for a total load of 8 ohms, the amp will now sound "better." I just don't buy that. So, to answer your question, go with the Hot Plate with the impedance that matches the speaker cabinet you'll be using.
Now, have your tubes replaced and biased, pick up a Hot Plate, and make that amp sing!
Amp Man...
I recently re-capped a friend's Silvertone model 1483 amp that had repeatedly blown fuses on him 15 years ago. I told him I'd take a shot at it. After re-capping and turning on the amp I got a fairly loud hum that didn't change with the volume-knob setting, and the 5Y3 tube seemed to get very hot. If left on for a couple minutes the output transformer would start to smell and make a bubbling sound like hot oil. The power transformer voltages checked out as "good" (no load), so I'm wondering what might be causing the output transformer to overheat even without any signal being applied. I've tested the tubes with an old SECO tube tester, and they all got "good" readings. Would a bad output transformer make the rectifier tube overheat?
Thanks for your input, Amp Man!
Mark
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Hi, Mark.
One word: yes. It sounds as if your output transformer has a short to ground somewhere on the primary side. That would cause the B+ (high) voltage to be pulled to ground through some degree of resistance in the transformer, causing a substantial increase in the amount of current being pulled through the rectifier tube. That, and the fact that the transformer "starts to smell and makes a bubbling sound like hot oil" are a pretty good indication that the output transformer needs to be replaced. You could try sourcing a replacement output transformer from the guys at Mercury Magnetics (www.mercurymagnetics.com). I know they manufacture some Silvertone "clones," but I'm not sure about that particular model. If not, you could always source a general replacement transformer for use with two 6L6's and an 8 ohm load. You should be able to find one at MoJo Musical Supply (www.mojotone.com) or New Sensor (www.newsensor.com). Since this is a relatively low-powered amp, you might even get away with using an output transformer from a Deluxe Reverb amp. It's likely that even that is physically larger than the stock transformer, but if you can fit it in, the amp will more than likely sound fuller and richer.
Like I always say, you can never have too much iron!
Jeff Bober
www.budda.com
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