Toroid Taps question

[Enzo]

I think the difference is lead free-ness. The two Vishay types are the same part, but the expensive one is the older non-RoHS (lead-free) one. The cheap one is the commodity RoHS version which contains no lead. I see no reason to spend $8 a pop.


Yes, when blown up like this, replace the whole lot of them, even the ones that test OK were no doubt stressed by the amp failure. Don;t forget to test all those 0.47 ohm 5w and the little 47 ohm resistors.

[ldervish]

Will do. I've got a little work ahead of me and then an order to place.

I get it, PBF = lead free

What do you think is the reason for the 'common failure' of the 12AU7's? Both of you have mentioned it at one time or another…is there something in the design that stresses them or puts them at the edge of their performance envelope?

[Enzo]

Dunno. Maybe modern 12AU7s are not very reliable.

I don't know that it is a common failure. More it is commonly what is wrong WHEN there is a failure. There is a difference. A common failure is one that occurs in most of the amps. I'd say the majority of these amps don;t kill that tube. But when certain problems DO arise, THEN I look to that tube.

[Jazz P Bass]

Huh, good point of view.

[ldervish]

+1

Tests show Q2 is bad, the trim pot is under spec as are a couple others. R32 and R34 read open and the rest are .6 or .7 so I'm going to replace all the .47R 5W source resistors.

Mouser didn't show me a trimmer like AP1, but there are quite a few multi-turn models. From what I understand about the bias procedure and the sensitivity of the OEM trimmer, wouldn't a multi-turn be an advantage? Is the 1A rating appropriate or something else?

I know they are pricey but may be worth it. I don't have a 'scope so when I get to biasing I'll be relying on my Wavetek DMM.

[Jazz P Bass]

I would simply replace those drivers, too.
Q3 & Q4.
If they where stressed (as it seems apparent) they will most likelyhood fail. (at some point in time)
If they do, poof goes all of the work that you did.

It's a maybe they will, maybe they won't call.


I would ere with 'maybe they will'.

[ldervish]

I'd been thinking along those lines also...

Your thoughts on the multi-turn trimmer and current rating needed?

[Enzo]

I think a ten turn pot would be complete overkill. The bias adjustment is not remotely that precise.

And 1 AMP??? Please explain that? Unless Q2 opens, the trimmer will always have the B-E diode of Q2 across it, so it ought to have a volt or less across it. If Q3,4 short out, then that little trimmer is the least of our worries. I am not aware of these amps burning up trim pots unless everything else is burning up, I don't see it as a concern.

[g1]

When you said the trim pots were "under spec" did you mean low resistance? Were you measuring them out of circuit?
It is fairly common for pots not to measure their exact value. Unless they are grossly off the value they are probably fine.

[Enzo]

Ah. I assumed he meant it needed to be of higher wattage, since he mentioned "1A", which I kinda thought might mean 1 watt.

[Jazz P Bass]

I dislike that term.

Under spec/ out of spec.

[Enzo]

I agree. If you use the term it needs to be in context, for example "the part is underspec'd at 1 watt" or "the part is underpec'd at 10k". Otherwise it is unclear what is meant.

[ldervish]

The trim pot read 9.09k out of circuit which is well outside the 5% tolerance named in the blurb, and the threads I've seen regarding this amp seem to indicate the bias circuit is very difficult to adjust finely, and that misadjustment is easy and potentially catastrophic. When asking about multi-turn trimmers I was looking for more control in the adjustment.

Your collective experiences are quite helpful and I appreciate hearing them.

[Enzo]

Well, it certainly won't hurt anything to be 10-turn. Other than you will not be able to make large changes quickly in case it is way off. SO if you feel more comfortable with the precision pot, then go ahead. I will be surprised if you find one that fits the footprint though.

[ldervish]

I'm waiting on parts & etc. so I'm going to close this thread and open a new one should the need arise.

Deepest appreciation to all who helped me get this far. Thank you.