Very weird reason for blowing heater fuse JTM 60

[torcamaniac]

Hi guys,

discovered very strange thing today.
Wanted to change the power tubes on my JTM 60.
The old ones were TUNG SOL EL34B and the new ones TT EL34B (Tube-Town).
After I installed them the heater fuse F101(T6.3A 250V) blew. At first I didn't suspect the power tubes. Also the rectifier was replaced by a better one as are the caps so I was hoping it was only old fuse.
Anyway, went and bought myself 10-packs of T6.3A and T10A (10 amperes just in case).
1) I installed the new 6.3A fuse with old tubes and everything worked fine. I then put the new tubes and the new fuse blew also...
2) Than I noticed that the Tung sol tubes didn't have Pin 6 on them and TT's did... But this can't be the reason I thought. As far as I know pin 6 is not used n EL34 even if its phisically there... So I thought what the hell, and removed the pin 6 from new tubes (cut them).
3) put another new 6.3 fuse with new tubes without pin 6 and it blew again!!!
Dunno why, but it must be some internal tube thing.
4) than I put 10Ampere fuse with new tubes and it was working fine...

What the hell? How can this be?
The pin 6 on PCB is not connected to anything in the circuit, just an empty hole, no solder pad or anything...
I'm wondering now if I should install only Power Tubes fabricated without pin 6 such as my old Tung Sols...

[Chuck H]

Well since you cut the 6 pin from the new tubes and the fuse blew anyway, that pretty much rules out pin 6's existence or lack of as being the cause. Whether or not a tube manufacturer decides to include pin 6 on the sockets wouldn't have anything to do with the current draw of the filament. But I'll guess that what you're saying is perhaps tubes manufactured without pin 6 are more likely to have other similar characteristics like a lower filament current. But I wouldn't count on that since the two features can be mutually exclusive and therefor arbitrary.

Something you probably SHOULD do is put just one of the old tubes back in and measure the current in the filament circuit. Write it down. Then put one of the new tubes in and note the current. Then put the other new tube in and note the current. If the new tubes are drawing current way over spec for an EL34 they should be returned. And certainly if only one of them is grossly high they should be returned.

Have you noticed any additional hum? What is the heater fuse spec for the amp? Perhaps you were using a lower than spec fuse and the older tubes just happen to actually be the anomalous ones drawing less current than usual.

Like Enzo said. Never invent a reason not to check something.

[Enzo]

I don't follow. You said the tubes WITH pin 6 blew the fuse, then the tubes WITHOUT pin six also blew the fuse. So clearly pin six didn;t matter.

6A fuses blow, 10A fuses don't. Seems to me you just needed the larger fuses.

Edit to add, I suspect it is the cold heater surge, not the steady hot operating current that was blowing the fuses.

[torcamaniac]

I don't follow. You said the tubes WITH pin 6 blew the fuse, then the tubes WITHOUT pin six also blew the fuse. So clearly pin six didn;t matter.

6A fuses blow, 10A fuses don't. Seems to me you just needed the larger fuses.

Edit to add, I suspect it is the cold heater surge, not the steady hot operating current that was blowing the fuses.

Yes Enzo I suspect it's the surge current too.
Chuck, the one that blows is original specs. T6.3A 250V.
You can also see it on the schematic were heaters are before the bridge rectifier (F101).
It doesn't blow with Tung Sols. It also didn't blow with original tubes from the 90's. If I remember correctly they were Sovtek EL34...
Chuck, don't have the means to measure current that easily, and installing a resistor to measure it indirectly would be a pain in this amp.
Even if I could it would be impossible to measure correctly if it's only an fast turn-on current...

[torcamaniac]

Also read somewhere on Ampage archives that a JCM 600 user (JCM 600 and JTM 60 are same amps basically) had similar problem and at some point he also said it happend after installing new power tubes which had Pin 6 on them whereas old ones didn't.
Anyway, would it be safe to just leave 10 Ampere fuse inside and be done with it?

[Chuck H]

If you have clip leads you could do it at the fuse holder.

Of course you'd also need a resistor that won't load the circuit and can take the current. Something like a .1R to .5R at 5 Watt-ish. No real good reason to have something like that in our practice. I happen to have some because I've used many off the shelf Hammond PT's with the 115V primary so I sometimes want to tame filament voltage.

Probably the surge current as noted. BUT, it's still anomalous behavior that you have elected to excuse from testing.

[torcamaniac]

If you have clip leads you could do it at the fuse holder.

Of course you'd also need a resistor that won't load the circuit and can take the current. Something like a .1R to .5R at 5 Watt-ish. No real good reason to have something like that in our practice. I happen to have some because I've used many off the shelf Hammond PT's with the 115V primary so I sometimes want to tame filament voltage.

Probably the surge current as noted. BUT, it's still anomalous behavior that you have elected to excuse from testing.

I have small aligator clip cables.
Unfortunaltely I have a 10 Ohm 2 Watt resistor and 10kOhm 5 Watt only

[torcamaniac]

Also checked the bias on new tubes over OT.
They were running at almost 46 mA with playes at 452V. Way too much. So over 80%.
I set them now at around 65% with plate voltage at 460V.
One is running at 35 mA and the other at 33mA.
Before, the Tung Sols had less than 1mA difference so they were somewhat better matched. But 2mA is also okay....

[Tom Phillips]

I have small aligator clip cables.
Unfortunaltely I have a 10 Ohm 2 Watt resistor and 10kOhm 5 Watt only

You don't always need to use a series resistor to measure current. You could...
1) Measure the heater voltage right at one of the power tubes with the old tubes installed and with the new tubes installed. Do the measurements one right after the other and verify that the line voltage is reasonably the same for each measurement. If the heater voltage drops with the new tubes installed then you have verified that they are drawing more current. You just won't know the absolute value. However, this simple test will get you one more clue & some verification regarding the situation.
2) Does your meter have an AC current measuring feature? If it does, then you could remove the heater fuse and measure the current across the fuse holder. BUT - to keep thinks simple and the current lower, remove ALL the tubes and then then install just one power tube at a time and record the current drawn by each of the four power tube heaters that you have. Note that, because of the lower current draw, the heater voltage will be a little higher than normal which will cause the heater current measurements to be a little higher than normal. However, the data collected will still be useful.

If you are unsure about using a meter to directly measure AC current then let's start by you telling us what meter you have. It would be good if you post a photo of the meter too. We need to verify the capability of the meter before you do the test.

Cheers,
Tom

[Enzo]

I'm with Tom, just measure current directly by replacing the fuse with a current meter.

I am perfectly willing to believe one brand of tube has a harder turn on surge than another.

[torcamaniac]

I have a pretty good meter. PeakTech 2010 DMM.
Offcourse it has an AC Current function.
It's fused for 12A on 20 Ampere setting (30 seconds only). And 200mA on 200mA setting.

[Tom Phillips]

I have a pretty good meter. PeakTech 2010 DMM. Offcourse it has an AC Current function.
It's fused for 12A on 20 Ampere setting (30 seconds only). And 200mA on 200mA setting.

Your all set then. For the first test just replace the fuse with your current meter on the 20A range . No need to remove the other tubes, just measure the static current with the old and new set of EL-34s. I realize that you won't be able to capture the current surge data. We will be interested in knowing the results. It's also worth verifying that your 6.3 A replacement fuses are really the "T" version. I.e. slo-blow.

This really just an exercise to rule out any weird problem with the new tubes. Not trying to re-design anything.

Tom

[torcamaniac]

Your all set then. For the first test just replace the fuse with your current meter on the 20A range . No need to remove the other tubes, just measure the static current with the old and new set of EL-34s. I realize that you won't be able to capture the current surge data. We will be interested in knowing the results. It's also worth verifying that your 6.3 A replacement fuses are really the "T" version. I.e. slo-blow.

This really just an exercise to rule out any weird problem with the new tubes. Not trying to re-design anything.

Tom

Tom I made sure the fuses are T slow-blow.
Anyway I measured AC current as you instructed. Also tried to note the highest shown at turn on.

New TT EL34B:
- At turn-on around 9,8 Amperes
- stabilized at around 4,4 Amperes

Old Tung Sols;El34B:
- turn-on at around 6,54 Amperes
- stabilized at 4,3 Amperes

So it seems there is too much surge current on new tubes. Also it was stabilizing (going down) somewhat slower than Tung Sols.
I guess I'll just leave the 10A fuse inside��

[Chuck H]

Nice!

Is it arranged as 9,8 means one tube was 9 and the other was 8? Just checking.

Five extra amps at turn on (and it's hard to know if you caught the worst of it) seems significant. I don't think it's unreasonable that a new tube has this characteristic, but my concern would be the traces in amps like Mesa products that don't always use burly enough copper or raise components off the board. Overall I'd probably shrug my shoulders, say "meh.?." and just see how it goes with the 10A fuse in place. Another example of new tubes being potentially problematic for older designs. I don't even know if there's a spec for heater surge current on any data sheets I've seen. This is probably NBD but it is something to be aware of in case it comes up. Thanks for doing the testing.

[torcamaniac]

Nice!

Is it arranged as 9,8 means one tube was 9 and the other was 8? Just checking.

Five extra amps at turn on (and it's hard to know if you caught the worst of it) seems significant. I don't think it's unreasonable that a new tube has this characteristic, but my concern would be the traces in amps like Mesa products that don't always use burly enough copper or raise components off the board. Overall I'd probably shrug my shoulders, say "meh.?." and just see how it goes with the 10A fuse in place. Another example of new tubes being potentially problematic for older designs. I don't even know if there's a spec for heater surge current on any data sheets I've seen. This is probably NBD but it is something to be aware of in case it comes up. Thanks for doing the testing.

Chuck, I measured the current with amp meter connected to fuse socket with both EL34 inside.
I guess it's 9,8 Amperes for both tubes installed.
The amp is 50 W with 2 power tubes.
I didn't write point because in europe we use , instead of .
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