Listen to your own advice

[Enzo]

Geez, I warn people about it, and yet...

Had a MArshall TSL100 in here and was having a hard time setting the bias. The controls were extremely interactive for the two sides. Blah blah blah. I forgot to check the mute switch. The recent Marshalls with AMP MUTE buttons on front mute the amp by shorting together the output tube grids. That kills the signal alright, but is also connects together the separate bias twiddles. SO you cannot alter the difference between the sides.

After wasting an hour measuring resistors etc, I remembered... sure enough the button was in. Pop it back out and voila, bias falls right into place.

SOmetimes you have to step back and think, "now what would I do at a time like this?"

[Mark Black]

I think it is WAY cool to talk about this kind of thing on this forum.

I'm guessing (based on the flavor of your usual posts) you posted this to drive home the idea of remembering to look at the simple stuff, as well as reminding us all of the particular issues with this amp.

Wish I had a buck for every time I forgot to put the connector back-shell on a cable before soldering. Maybe 5 bucks for every time I did it on a multi-pin. (Then again - maybe I should be docked 5 bucks for every one of those since it was extra stupid). Never-the-less I seem to keep on doing that on a periodic basis, keeping me quite humble...

[Steve Conner]

I design electronic equipment for a living (unfortunately lab instruments, not amps ) and rather than blaming Enzo, I'd blame the designer who made the mute button work in such a surprising way. It's a violation of the "Rule of least surprise".

If he wanted to save a 5 cent capacitor by having the mute button work that way, he should have had the front panel silkscreen changed to label the button "Mute and screw up bias"

[Enzo]

You're right, it shouldn't be that way in the first place, but it was something I was already aware of. I couldn't see the forest for the trees. I was pissed at myself for not realizing what was going on, and you guys would at least know what I was talking about.

I like to share such experience for its instructive value, and also to make the less experienced techs here understand that we ALL make mistakes, and that 30 years of experience doesn't make us infallible.

Even "Bias Adjust: Mute must be off to set" would be a nice reminder on the rear panel.

[Euthymia]

That is really awful design.

Some people might actually deliberately engage the "mute" button before setting bias thinking they could easily and thoroughly kill the signal that way.

Dang, I had some amp in here last year, mighta been a Marshall, and it had 4 different pots for setting bias and balance between its 4 output tubes. I think there was a pot for each pair that would set the "global" bias for the pair, and then another that would set the balance within the pair.

I couldn't get the "balance" pots to do much of anything, so I left them where they were when I started (which was about 50%) and set the "global" bias pots so that all 4 tubes were drawing within 1mA.

I got the desired result (I thought), but now I wonder if I had some random control engaged that messed with the bias readings.

Stuff like this is why I really detest when engineers get clever for the sake of being clever.

Put the mute circuit in the last section of the amp a tech would look for it (if perchance it breaks and someone has to fix it), with the side effect of also making it impossible to properly bias the amp with it engaged. Sheesh.

[Sweetfinger]

Feature creep!

Damn bells and whistles! If I had a buck for every Marshall head with that switch pushed in that I sent to warranty service because it "stopped Working", I'd have.....a buck. That's a mistake you hope you only make once. To be honest, I didn't actually check out the head myself, but two shop monkeys assured me the thing was broken. I've caught others that were about to be sent out, but thankfully, only ONE call from the warranty repair guy explaining the friggin' mute button.

[bob p]

... I'd blame the designer who made the mute button work in such a surprising way. It's a violation of the "Rule of least surprise".

If he wanted to save a 5 cent capacitor by having the mute button work that way, he should have had the front panel silkscreen changed to label the button "Mute and screw up bias"

Steve, since you are in the business, you probably know that the completed project that comes out of the design lab may never see production in the form in which it was created. After the design engineers create a circuit and make it do what its supposed to do, the prototype often goes to the "production" lab where someobody implements changes to make the design more efficient to produce. The production lab typically is the home of the guys who implement the changes that screw up the design to save $0.05. All too often, the production team ends up making changes that drive the design team crazy -- and there's nothing they can do about it. Maybe Enzo's case could be an example of this.

[Alex R]

TSL100 cathode 1ohms

Hiya. New to this forum, sorry to start with a request for help... but then maybe it's not that at all but an intriguing mystery. I run an amp repair shop part-time. Today I had in a TSL 100 with no output. The two 1 ohm cathode resistors they measure the bias over were burned open. I replaced them and it was fine. The guy had 4 new GTs in there, but they hadn't been biased. Supposed to be 90ma a side; they were doing 85 on one side and 105 on the other - not too scary but a bit too high for my liking. So I took it down to 80 a side and left it on. Runs fine.

So why would it pop those resistors? They're 1 watt; they only drop about 100mv each - what could have happened to burn them both?

There you go, newbie question.

[Enzo]

Imagine what happens in those resistors if the power tube above it shorts plate or screen to cathode? Or an arc somewhere. 480v 1 ohm 1 watt. Serious math for the resistor.

The guy had new tubes in it, but we never saw what those tubes replaced.

[Alex R]

TSL100 and Ashdown Fallen Angel

Good point - thank you. I was wondering why both sides went, but I guess if one side went pop then the change in loading might cause the other side to short or arc, esp if the bias, hot anyway, was even hotter than it should have been. Ok I'm a bit happier about it now - but you know the feeling - have I dealt with the symptom not the cause? Probably his old valves did it, yes, and the new ones never worked. They don't always give you the whole story do they?

Funny thing, I also have in an Ashdown 'Fallen Angel' 180w that has 10ohm cathode resistors for some inscrutable reason; that too had burned one of them. The guy said 'it sounds weak', and I kind of agree with him even with the new resistor in - high gain when you switch it in but weedy sustain even with everything on 10 (ok not the MV!), and a choked kind of sound. It does now put out 180W RMS though. Seems to me it just doesn't sound very good as an amp. Anybody had this issue?