Dead PT...autopsy help requested

[johnnyreece]

So, I was running my 5e9-a-based amp on 6K6GT tubes into what I *thought* was a 16-ohm cabinet. Turns out, it was wired as 4 ohm (my bad; should always verify). After playing for a couple hours, the fuse popped while the amp was sitting at idle. I assumed maybe one of the old tubes I was running kicked the bucket. Nope. So, pulled tubes, put it on the dim bulb limiter with a new fuse installed. The bulb flickered, then went to full brightness (60w bulb). Bummer...disconnected all secondary connections on the PT, still full brightess. No voltage on any secondaries. Removed the fuse and put my meter in line to reveal over 400mA draw with nothing connected. So...bad PT. My question is, would the mismatch be enough to kill a PT? I know it risks the OT, but I wasn't sure about the PT. After this, I'm going to fuse the PT B+ secondary so this *hopefully* doesn't happen again. The inside of the amp is clean; no usual signs of arcing (have had plate to heater pins arc before, so I've seen what to look for). I'm assuming the procedure will be:

1: Make sure new PT isn't shorted out of the box (my mechanic friend likes to tell me NEW is an acronym for "Never Even Worked") using the limiter.
2: Install PT in amp. Fire up using limiter and no tubes.
3: Install rectifier, then preamp, then power tubes, making sure to check on limiter at each stage (and make sure speaker cab is plugged in).
4: If limiter test is passed, plug in direct and measure voltages to be sure it's within spec.

Sound reasonable? Usually, the stuff I build just works and I don't cook it. Not spent a lot of time on this end of things. If the impedence mismatch isn't a viable cause for what killed it, any other ideas? The heater wiring isn't shorted. I did check that much. I appreciate any ideas!

[Stevem]

Just so
I am clear on what you posted , so you were running an 4 ohm load on the 16 ohm amp tap?

[johnnyreece]

Just so
I am clear on what you posted , so you were running an 4 ohm load on the 16 ohm amp tap?

Correct.

[Stevem]

What did you use for a PT in that build?

Keep in mind that a tube is high impedance, so low current and high voltage.

A speaker is a low voltage high current item.

I would say your PT failure may or may not have been influenced by the load mismatch, but at this point it would take near distructive testing with another PT like you used to nail that down.

[johnnyreece]

https://www.tedweber.com/w022798int/

This is the transformer. May try one again. Price is decent enough, anyway. Although, it may be a case of you get what you pay for…

[nuke]

If your transformer died, what size fuse did you have in the AC line? I'd start there. That's kind of a big honker of a power transformer for that circuit, but we use what we have on hand.

6K6GT are going to be working real hard in the Tremolux 5E9A circuit, and prone to failure depending on the voltage and current you have them at.

That sets up a failure chain, which might take the rectifier with it, which in turn probably takes the power transformer.

Sometimes, if you're lucky, you might find a protection device inside the transformer, like a thermal cutout or fuse. Take off the end bells and see if there's anything in there. Other wise, it's a copper and iron brick for recycling.

[johnnyreece]

It was a 2A fuse. The amp is made to work with a variety of power tubes. As I was using it at home, I went with the lowest power option available (6K6). My favorite is KT66 with GZ34 rectifier. I've added a copy of the schematic, if that will help. I did do a partial teardown of the PT, and no luck with an internal fuse. Nor did I find any obvious issues immediately in the PT (not that I went through all the windings). As a side note, I did test the power and rectifier tubes afterwards, and all test as good. No leaks or shorts, and well within their "good" range of lifespan. I had kind of assumed at first that's what the issue would be; a dead tube. On an unrelated note, I did measure my wall voltage while testing, and it was 115. The summer heat must be pulling it down. I usually have 122-ish.

[Helmholtz]

If the PT had an open thermal fuse, there would be no primary/mains current.

[nuke]

It was a 2A fuse. The amp is made to work with a variety of power tubes. As I was using it at home, I went with the lowest power option available (6K6). My favorite is KT66 with GZ34 rectifier. I've added a copy of the schematic, if that will help. I did do a partial teardown of the PT, and no luck with an internal fuse. Nor did I find any obvious issues immediately in the PT (not that I went through all the windings). As a side note, I did test the power and rectifier tubes afterwards, and all test as good. No leaks or shorts, and well within their "good" range of lifespan. I had kind of assumed at first that's what the issue would be; a dead tube. On an unrelated note, I did measure my wall voltage while testing, and it was 115. The summer heat must be pulling it down. I usually have 122-ish.

Ahh, got it.

It would be interesting to see what winding on the PT bit the dust.

The part about looking for an internal protection device would apply if the primary was open somehow, perhaps not for this case, but for some future thread-reader.

FWIW, I have a new-old-stock NSC-022798 transformer I've been meaning to get rid of. It doesn't have the multiple secondary windings, just a straight up replacement for the 5F6A, Super Reverb, etc. PM me if interested.

[R.G.]

I tend to agree - the low impedance load on the speaker output probably was not the proximate cause. As noted, output tubes are high impedance devices and are not hugely strained by low impedance loads, even short circuits. It's solid state amps that are prone to failure on low impedance loads.

It was probably just a failure of the power trans all by itself. Internal temperature is the most stressing thing for a transformer. It ages the wire insulation and eventually lets an internal turn-to-turn short happen. Power line spikes can sometimes accumulate damage with tiny insulation punctures becoming big ones over time. Sticking a 150V MOV across the primary of a PT is a good idea on any amp that doesn't have one, just as cheap insurance.

For definitive testing on a transformer, use an ohmmeter to verify that
> no windings are completely open
> No separate windings connect when they're supposed to be separate
Once that is done, reading the magnetizing (no-load) current into the primary gives a clue. Actually, 400ma is not all that huge a no-load current. But if you get no secondary voltages, something is clearly wrong.

If you want to protect a new PT,
> fuse each independent section of winding; one for a single primary, two for each CT secondary. If the fuses are correct, no section of winding can conduct enough current for long enough to overheat and damage the insulation. If you do this, you're far better protected than with only a B+ fuse.
> AC line spikes
The B+ fuse protects the OT primary much more than the power trans. It does nothing at all if a rectifier or filter cap shorts. Per-winding fuses stop this.
The incoming AC fuse does not guarantee to protect the primary side. A short on the high voltage secondary usually will open the primary fuse, but a shorted heater winding may bery well not open the AC before burning out the secondary insulation.

[johnnyreece]

Thanks everyone for chiming in. I didn't think going too low on impedance was the issue, but I wouldn't consider myself an expert on magnetics, either. I suppose maybe it was just a coincidence. Makes me wary of buying one of the same make/model again, though. It only lasted two years of not heavy usage. Although, similar replacements are going to cost 3-4X the price. Ah, decisions, decisions!

I'm thinking strongly about fusing the HV secondary, at least. I plan on running anything from 6K6 to 6L6. Would .5A fast blow be an acceptable rating for those conditions? Any other suggestions of common areas to fuse/precautionary mods to protect other parts? I've attached a snip of the schematic circling where I would think the best point to insert fuses would be. Thanks again for those who've dropped in and taken the time to look at this!

[Guy77]

Thanks everyone for chiming in. I didn't think going too low on impedance was the issue, but I wouldn't consider myself an expert on magnetics, either. I suppose maybe it was just a coincidence. Makes me wary of buying one of the same make/model again, though. It only lasted two years of not heavy usage. Although, similar replacements are going to cost 3-4X the price. Ah, decisions, decisions!

I'm thinking strongly about fusing the HV secondary, at least. I plan on running anything from 6K6 to 6L6. Would .5A fast blow be an acceptable rating for those conditions? Any other suggestions of common areas to fuse/precautionary mods to protect other parts? I've attached a snip of the schematic circling where I would think the best point to insert fuses would be. Thanks again for those who've dropped in and taken the time to look at this!

Just chiming in, I understand fusing is important too . I think I wound start with a more expensive name brand transformer too, Pacific, Heyboer, Hammond especially the 300 series.

Cheers
G

[trobbins]

johnnyreece, did you progress the repair?

Did you further interogate the fault location in the PT, given highish primary magnetisation current, and zero secondary winding voltages? Eg. the primary resistance was lower than default (ie. you bought another PT of same type), and the secondary windings had nominal resistance.

Can you confirm you just tested the rectifier tube on a typical tube tester, as that wouldn't have applied anything like rated PIV or been able to measure leakage current at a reasonable PIV? Were you going to add ss diode protection to the rectifier tube anodes, as a way of avoiding rectifier tube arcing from being a possible fault scenario?

There is no bias adjustment - did you measure the bias voltage with the 6K6's and check if idle dissipation was ok and balanced?

Do you have a way to measure mains primary current with a meter? You may want to check what current can be reached, and then check if a lower valued fuse rating is reasonable.

[Roe]

https://www.tedweber.com/w022798int/

This is the transformer. May try one again. Price is decent enough, anyway. Although, it may be a case of you get what you pay for…

these weber international transformers tend to overheat and blow due to the 50hz ac. I've blown two

[flemingmras]

You say this amp is "made to run a variety of power tubes". Something tells me that you fail to consider the heater current differences (and I doubt any Fender style transformer is going to be wound for the heater current of many of them).

6K6 - 0.4 Amps (2x tubes = 0.8 Amps)
6L6 - 0.9 Amps (2x tubes = 1.8 Amps)
EL34/KT77/6550/KT88 - 1.6 Amps (2x tubes = 3.2 Amps)
KT66 - 1.27 Amps (2x tubes = 2.54 Amps)

This is in addition to the preamp heater draw, which is an additional 900mA, or 0.9A.

I doubt that transformer has the heater current to run many of the tubes outside of the 6K6/6L6/6V6 arena.

What output transformer are you using? These tubes also have very different load expectations.