TAD 60 blown fuse - twice

Fran604g

Just Call Me Junior
My apologies to Mr. Grzybek, but this is really rinky-dink. 😳

The pots are held in place by friction alone. I've always known I needed to use care while triming the pots for bias, but had no idea how cheesy this assembly was.
20190610_132842-1008x756.jpg
 

Fran604g

Just Call Me Junior
Im not sure how it got broken. Maybe the last time I biased them, the pot moved and just enough was left hanging on to keep working...until it wasn't?

Weird. A real head scratcher. 🤔

20190610_134543-1008x567.jpg
 

Fran604g

Just Call Me Junior
Now what the hell am I going to do? I can probably resolder the wire back onto the pot, but maybe I should think about a little better arrangement...
 

paulbottlehead

Active Member
That's not a head scratcher at all. Those pots are supposed to be mounted to a PC board, and yours are not. The little tiny leads on those pots are not intended to have wires connected to them, and you have to be extremely careful when you do that.

The ultimate solution would be to fabricate a new piece of metal that holds four panel mount potentiometers with solder lugs, or attempt to install some in the holes I see in the chassis that are already there. Another option is to make a proper PC board to hold pots that are made to be adjusted from the top when looking down on the PC board.

Whatever you do, I wouldn't just solder it back together as it is, as I'd expect that same thing to happen again.
 

Fran604g

Just Call Me Junior
That's not a head scratcher at all. Those pots are supposed to be mounted to a PC board, and yours are not. The little tiny leads on those pots are not intended to have wires connected to them, and you have to be extremely careful when you do that.

The ultimate solution would be to fabricate a new piece of metal that holds four panel mount potentiometers with solder lugs, or attempt to install some in the holes I see in the chassis that are already there. Another option is to make a proper PC board to hold pots that are made to be adjusted from the top when looking down on the PC board.

Whatever you do, I wouldn't just solder it back together as it is, as I'd expect that same thing to happen again.
I agree completely. I like the idea of using the holes already in the chassis, and want to keep the adjustment from the top for sure. Are there any special considerations for the pots themselves that I need to know about? Whatever the solution, I really have to thank you for the great tutorial, @paulbottlehead! I've wanted to get a handle on this amp for years, but had little reason.

The tube must've red plated before it blew the fuse, then. I've been streaming Radio Paradise a lot lately, and the signal gets dropped sometimes, and just reestablishes after a few seconds. I do recall that I noticed the signal had dropped, but I was doing something, and it didn't occur to me for a few minutes that the music hadn't come back on. That's when I went into the room and noticed the amp pilot light was out, checked the fuse, and replaced it, only to see it immediately blow again.
 

Fran604g

Just Call Me Junior
So, I'm guessing I should use 100K linear potentiometers, but what wattage is adequate? 1/2W, lesser, or greater?
 
So, I'm guessing I should use 100K linear potentiometers, but what wattage is adequate? 1/2W, lesser, or greater?
The math is pretty easy. With 78v across a 100K pot, the dissipated power is V^2 / R = 78x78/100K = 0.06W. Giving a 2x margin just to be safe, the calculation tells you that 1/8W or higher should be fine.
The pots that are currently in the amp look a lot like there Bourns 100K Cermet 15 turn pot, which are rated at 0.75W.
---Gary
 

Fran604g

Just Call Me Junior
This is what I'm thinking of for replacement of the pots, using existing holes in the top of the chassis. I want to upgrade the components, not just replace with the same cheap stuff.

I would need to enlarge the small holes ever so slightly (that were the access to the old trimpot wipers - from .192" to .201") to accept 2mm test probe sockets (like these), and mount new trimpots in the large (standard 3/8" thread, like these) holes that were used for the multimeter test probe to measure bias in mV.

Sure this solution is a bit pricey, but I'll be keeping this amp for the long haul because I'm really happy with it.

New trimpots and test sockets (Large).jpg
 
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Fran604g

Just Call Me Junior
For posterity, this image is from the 1st. page of the documentation that Paul G supplied with his TAD 60. I include it for anyone not familiar with his intentions using the access points topside. :smile:

Bias.jpg
 

paulbottlehead

Active Member
What impedance are the trim pots that are in there now? 100K seems very high for a bias trim pot, but I would certainly replace them with the same value.

Mouser has a few different sizes of Alpha pots that are available with solder lugs, I would encourage you to use them and probably to order both sizes just to see what fits properly.

Your plan to use the pots in the larger holes and test points in the smaller holes is exactly what I would do.
 

Fran604g

Just Call Me Junior
What impedance are the trim pots that are in there now? 100K seems very high for a bias trim pot, but I would certainly replace them with the same value.
The only reference I have is the schematic at 100K. Measured from tab 1 to 2 (wiper) while adjusting, it goes from 0-100K ohms. Measured at 1 and 3 it's 100K.
 

Fran604g

Just Call Me Junior
Mouser has a few different sizes of Alpha pots that are available with solder lugs, I would encourage you to use them and probably to order both sizes just to see what fits properly.
Thos Alpha's look like they'll fit well. I thought the outer diameter was larger than they show on the spec sheet (17mm = .667"). The only problem is the shaft isn't slotted for a screwdriver, but I could slot them myself, or Ideally I would secure an extension on them and finally get my fingers away from those hot output tubes! I have access to a lathe and could make something pretty.

Thanks, @paulbottlehead!
 

Fran604g

Just Call Me Junior
One thing I forgot to mention was the reason I didn't choose the Alphas in the first place, because they're single turn. only. Will adjustment be fine enough for my purposes? Probably I'm just being OCD.
 
Single turn is going to be "touchy"......google search for solder lug multi turn pot I found...


Now is it worth 20 bucks each, thats up to you
 

paulbottlehead

Active Member
Ten turn pots tend to be large and pretty expensive when you get the ones with solder lugs. As seen above, three of those options mean laying out $600+ where a couple of $3 pots will do.

Single turn pots are OK, but again a real compromise has been made in your amp because there aren't resistors restricting the range of negative voltage available to the pot. You might have negative voltage adjustable from 0V down to -100V, but what you really need is -30V to -60V, and you could get this with a pair of resistors and reduce the sensitivity.

If you need to slot the pot, you can do it by hand with a small hacksaw.

(Also: RV16AF-10-15S1-C100K-LA Alpha (Taiwan) | Mouser )
 

Fran604g

Just Call Me Junior
If you need to slot the pot, you can do it by hand with a small hacksaw.
Not a problem. I'm a journeyman toolmaker (retired). I could just turn something down on the lathe as an extension, too. I'll keep the adjustable bias so I can still roll tubes if I'd like. :)
 

Fran604g

Just Call Me Junior
Single turn is going to be "touchy"......google search for solder lug multi turn pot I found...


Now is it worth 20 bucks each, thats up to you
Yep. The 3590 series are what I mentioned in my earlier post. The amp is absolutely worth the 80+ dollars to me. :)
 

Fran604g

Just Call Me Junior
So, I further narrowed down the test probe sockets and pots to these:

3542-2 Pomona Electronics | Connectors, Interconnects | DigiKey
unnamed (2).jpg

3549S-1AA-104A Bourns Inc. | Potentiometers, Variable Resistors | DigiKey
unnamed.jpg


Also for posterity:

Through it all, the only ground I've discovered to chassis is the mains ground at the IEC socket. The entire circuit ground is otherwise separated completely from the chassis, and grounded only at the PT through the network of grey/white wires, as shown in my photo below. Just wanted to share. :)

20190604_073329~2-1008x756.jpg
 
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