TAD 60 blown fuse - twice

[Fran604g]

Hiya folks,

Yesterday my venerable TAD 60 popped the inline fuse at the power supply between the mains and PT. I replaced it to corroborate, and the fuse failed again. I removed the amp from the system for troubleshooting.

I checked all the tubes - as I suspected an output tube failure (4 ea. Tung-Sol reissue 6550). They all tested fine and relatively strong (incidentally, 1 from each channel tested stronger than the other 1 from the same channel) on my Precision Electronamic Tube Master Series 10-12 for shorts and "strength". I then tested the 3 small signal tubes for the same (1ea. JJ ECC803S and 2ea. Electro-Harmonix 12AU7). None failed the short test, but the the JJ ECC803S tested relatively low strength at 60 (out of 100) in both elements. The 2 12AU7's tested no shorts, and strong.

The output tubes have maybe a couple thousand hours on them (new a little over 2 years ago from UpscaleAudio - my 2nd quad of these - I really like them), and the signal tubes are of unknown hours, a few years old. I lost track when I was rolling tubes several years ago. Anyway, the JJ ECC803S was the worst of the bunch. I have several spares of this tube, and I checked them for relative readings and found them all to be less than respectable. Some appeared to be new in the box, one was shorted, and another had problems when moved slightly in the socket while testing. This left a nasty taste in my mouth (so to speak), so I suspect the ECC803S that was in the amp might be part of the failure.

I realize that testing tubes with only a dynamic tester is subjective at best, but at least it's a start.

So, because none of the tubes had an obvious failure, this morning I removed the bottom plate from the amp to have a look for anything obvious.

The first thing that caught my attention was the blackened and obviously overheated disc capacitor-looking thing at the fuse holder between the fuse and the power transformer. Definitely charred!

I Googled the part number (STM CL 80) on it and discovered it's a thermistor (similarly, an Amphenol Advanced Sensors Inrush current limiter per a web description).

A back story that may or may not be relevant: I bought the amp in 2007, it was built by Paul in November 2006, was less than a year old, and still retained one year of his 2-year warranty. It came with his stock supplied Electro-Harmonix 6CA7EH output tubes, but I wanted to roll me some tubes! When I contacted Paul Grzybek in regard to trying 6550's, he provided me with instructions for biasing them (and other types). I had corresponded with him many times over the first few years of owning this amp, and he provided me with a lot of great advice/perspective, but unfortunately I didn't save those emails. In retrospect, I really wish I had.

The first version of these tubes I used was a matched quad of the venerable (now unobtainium) Russian SED "Winged C" 6550C. Unfortunately, they caused the stock fuse (4a fast blow) to fail. I contacted Paul, and he told me because of the higher current draw of the 6550, I should replace the stock 4A fuse with a 5A fast blow. I did, and have lived happily ever after through 3 or 4 or 5(?) subsequent, different quads over the years.

Until yesterday that is.

At any rate, the Amphenol CL 80 Thermistor is rated for 3 amps 47 ohms, and given that Paul had me up the fuse to 5A, I'm wondering what I should do? It measures now at 55 ohms. Should I just replace it with the same value, or a different higher value? Or consider something else, altogether different?

Should I replace the tubes first, and see if the problem persists? I have an unused quad of 6CA7's stashed, and plenty of good signal tubes to use. I do need to buy more fuses, though.

I should also mention that I've noticed that the power transformer typically gets very warm since using the Tung-Sol 6550's - I'd guess about 130-150°F; hot to the touch, but not enough to burn the flesh (I was a Chef for 20+ years, so I know temps to the touch relatively well). I assume this is because of the additional current draw of the 6550's. They obviously bias much higher than the stock 6CA7's (375-400 mV vs. 600-620 mV for the 6550 - per his specifications).

What would y'all recommend?

Thanks,
Fran

 

[paul_b]

First and foremost, if the thermistor fails, the fuse won't blow. These get burning hot, and you're still able to read a DC resistance measurement, so I wouldn't focus on that at all. They actually don't work properly unless they get really hot, and the builder has located that part in an area where nothing will get harmed by the heat.

Every red resistor in there is a very low quality resistor from China encased in steel. If they get too hot, their failure mode is to decrease in resistance, which could blow your fuse. This is the opposite of every other resistor that I have every used, and for good reason! In another Chinese amp I worked on that was full of these, many of the 150K resistors bleeding off the B+ supply looked cooked and measured at 1/2 or less of their labeled value. In the center of your amp, I see one resistor that looks cooked like this, but I wouldn't expect this to blow a fuse either.

My strongest recommendation would be to disconnect the wires going to the HV pins on the power transformer, pull all the tubes out of the socket, then try to power up the amp. With no load on the heater windings and the HV winding disconnected, turning the amp on in these conditions and seeing the fuse blow will eliminate a whole lot of what's under the hood and leave only a few components. (Namely the power transformer and possibly some rectified low voltage supplies like a bias supply or a DC filament supply)

If you'd like more specific help about disconnecting the HV winding, you'd have to clip the zip ties and move the wires away from the bottom left corner of the PT so we can see exactly how it's hooked up.

If you fail this test, then there's a little more we can do to narrow it down. If you pass this test, then there's a separate direction to take to continue debugging.

 

[Fran604g]

Thanks Paul. I've ordered more fuses, once they arrive I'll move forward.

In your opinion, can an output tube fail under load but still test "okay" or within limits?

 

[paul_b]

This is remotely possible. You can safely power this amp up with no output tubes plugged into the octal sockets. I would bet that the fuse still blows under these conditions.

I do not believe any of the driver tubes could fail badly enough to pop your fuse, so I would leave them in for this test.

 

[Fran604g]

Paul, definitely several resistors out of spec, just like you predicted. Looks like I should replace them all. I'll check the caps, too I think.

I'll check the PT for continuity and/or shorts. Oy.

 

[paul_b]

It's too bad about those resistors because they look really nice.

On the PT, you can check for resistance between various windings. Most of the Chinese transformers that I've had go bad develop a short between the primary and another winding.

-PB

 

[Fran604g]

Black measures out of spec, silver within spec.



I think I might have to think really hard on rebuilding this amp. I only paid $800 for it in 2007, and have enjoyed the hell out of it. $800/12 years = $67/year. That's tough to beat, especially considering for the last 8 years it's been on for 6-8 hours a day 6-7 days a week. Yes, I spend a lot of time at home.

The thing is, if the rest of the components begin to fail, then what? Where do I draw the line? And those blue box capacitors...wth are they really?

 

[Fran604g]

@paulbottlehead I'm checking a 560K, and it measures low but keeps climbing in value. Quickly at first, then gradually slowing down to 223K+ and sloooowly increasing still after a couple of minutes.

Do you know why it might behave like this? There are 2 of them acting exactly the same.

 

[GaryB]

I'd suggest that you take a deep breath and not freak out over a few resistors that are out of spec. I definitely wouldn't trash the amp just because you blew a fuse and are getting nervous. When you say that the resistors are out of spec, exactly what do you mean? Do you have a schematic that you're comparing to? Or just comparing the value to what's printed on the resistor body? Tube amps are pretty forgiving and will work just fine even after some of the resistors drift a bit. Only a few resistors are really critical in terms of being exactly at the value shown in the schematic.
I'd follow Paul's advice and see if it's still blowing fuses once you pull all the tubes. If so, then that points you to a problem in the power supply such as a bad power supply capacitor. If the power supply is fine, then I'd be suspicious of the tubes or the tube bias circuitry. Just take it one step at a time. If you're not comfortable doing this then find a local friend who's done this kind of thing before.
Hope you can work it out.
---Gary

 

[GaryB]

@paulbottlehead I'm checking a 560K, and it measures low but keeps climbing in value. Quickly at first, then gradually slowing down to 223K+ and sloooowly increasing still after a couple of minutes.

Do you know why it might behave like this? There are 2 of them acting exactly the same.

That happens when a resistor is bypassed with an electrolytic capacitor. The capacitor is charging up and giving you a variable reading. The measured resistance will slowly asymptote to the correct value but it may take a while. It's really hard to measure parts values when they're wired in a circuit.
---Gary

 

[paul_b]

Yes, if there are caps on either side of the resistor, this can throw off your meter. For something like this, I would remove one end of any resistors that look iffy, then check the resistance.

But still, the resistor issue is unlikely to have anything to do with the fuse blowing when you try to turn the amp on. The tubes aren't conducting when you flip that switch, so very few resistors are actually doing anything.

 

[Fran604g]

Thanks guys. I now see where that resistor is bypassed by a capacitor.

@GaryB I'm not going to trash the amp, but I need to be pragmatic - that's all.

A few years ago (after Paul G's untimely death), me and a few others at AK were trying to find a schematic for these amps, and the closest we came was to the Chinese counterpart being sold overseas by BEZ, designated as the T8C-60. I discovered then, through comparing that schematic to my amp physically, there were a couple seemingly minor differences, but by and large I concluded they were the same amps. Thing is, Paul had years earlier said he made his own modifications to these. What that meant, I don't really know, so I'm trying to figure this all out.

Consequently, I'm comparing my measured values against the values printed on the parts while tracing out the schematic against the one I have for reference, and noting both the labeled and measured values on the schematic to identify any differences. There are a couple of items I noted that were different from the schematic, but I'll double check them before I feel confident I'm correct. Whether those differences might cause little changes or big, I'm not knowledgeable enough to understand.

For instance, there are 2 separate 9.1K resistors shown in the schematic. 1 is in the feedback circuit far right, and the other is in the +B2 at the top, left of center. I cannot find the latter one anywhere. And because of the "blue box" capacitor labeling, I'm having a terrible time understanding their values. I think I had a cheat sheet here at one point, maybe not.

I'm going to take the advice @paulbottlehead gave me to check the PS once more fuses arrive.

Onward! Albeit slowly.

Thanks again for your help.

 

[Fran604g]

Is this a tubular ceramic capacitor? At first I was thinking it was a really small wirewound resistor, but it just didn't add up.

 

[paul_b]

I would guess that is the 1500pF cap shown in the schematic for compensation. I would not blame the builder one bit for deviating significantly from what's drawn there!

 

[Fran604g]

There's 2 confusing cap/resistor combos coming off GT2 there. That cap in series with a 66.5K going to grd., and another 66.5K with a cap (.47uF?) in parallel, headed off to the voltage splitter (12AX7).

Am I right to believe the cap below is .47uF? I have troubles with these alpha-numeric codes.

 

[paul_b]

472 is 4700pF.

(You're doing a marvelous job at not testing the PT )

 

[Fran604g]

472 is 4700pF.

(You're doing a marvelous job at not testing the PT )

Yeah. Waitin' for fuses. Thought I might as well figure out a schematic. I need the learnin'.

Thank you, Paul!

 

[Gene_StL]

While you wait for fuses lift all the secondary connections from the power transformer. (one on each pair only)
It probably will continue to blow fuses because to fast blow a 6A fuse probably is the transformer itself rather than a reflected load which would probably take a little bit of time to blow the fuse. Unless something is a dead short which lifting the connections will unhook.

 

[paul_b]

I do wonder what kind of capacitors are hidden under those cardboard wraps too.

 

[Fran604g]

I do wonder what kind of capacitors are hidden under those cardboard wraps too.

Same here.