OK, so what happened to my HT-40? Weirdness going on ...

cvogue

cvogue

Yes, that's Oolong. :)
So I was doing some preamp tube rolling (see my other thread, I'm a JJ convert now!) and something weird happened...

I have to preface this with the fact that I may well have played my amp with the speaker connected to the wrong impedance jack for a couple of hours the other night... thwap0

So I connected the internal speaker (16 ohm) to the correct jack and the amp hummed big time no matter what, guitar plugged in or not on both channels. One power tube was glowing brightly and the other didn't glow at all. I'm thinking... WTF... I damaged my amp. :mad:

So I thought, maybe it's that speaker output jack that's being weird so I plugged in the internal speaker and the extension cab to the (1x8 ohm or 2x16 ohms) jacks... worked great, no hum tubes were glowing like they should (faint orange glow). I thought "OK, must be that 1x16 ohm jack. So out of curiosity I unplugged the extension cab and plugged the internal speaker back into the 1x16 ohm jack...

And everything is fine. No hum, no bizzare one tube burning up while the other does nothing.

Any ideas on what may have happened? Could there be some smarts in the amp that detects when impedances don't match and they compensate somehow?

I may just take it into the tech anyway... going out of town for several days so it's a good time to have my amp out of commission.
 
That's different...? I'm stumped. Possibly dirty tube sockets could cause the one tube to not glow correctly. Couldn't hurt to spray some Deoxit on the pins (not into the socket).
But the noisy speaker not working and then working is definitely weird. I think if the impedance is mismatched, the output tranny is what takes the brunt of the damage. I've accidentally mid-matched impedances before and never damaged anything. I could be wrong but it seems like that type of damage would occur over a period of time or under harsh conditions such as playing for hours while really pushing the amp.
 
gtrjunior said:
That's different...? I'm stumped. Possibly dirty tube sockets could cause the one tube to not glow correctly. Couldn't hurt to spray some Deoxit on the pins (not into the socket).
But the noisy speaker not working and then working is definitely weird. I think if the impedance is mismatched, the output tranny is what takes the brunt of the damage. I've accidentally mid-matched impedances before and never damaged anything. I could be wrong but it seems like that type of damage would occur over a period of time or under harsh conditions such as playing for hours while really pushing the amp.
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Depends on how hard you were pushing it while the impedance was mismatched.

Funny thing is that there are so many amps out there with output transformers with different impedance ratios for the same tube configuration that I'm amazed that impedance matches are as big a deal as people make them out to be. Marshall has used OTs with 3-4 different impedances on the primary side for 100 watters (1.7K, 2.2K, etc.) and they haven't blown up because of that.

As far as cleaning the socket... Clean them all sexy-like. In and out and in and out and in and out and in and out...
 
Is it possible that the tube was already near end of life and pushing it harder with the mismatch just wore it out?
 
jp_nyc said:
Is it possible that the tube was already near end of life and pushing it harder with the mismatch just wore it out?
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Since it's working again I'd say no.

You may have had a random electrical gremlin that's since worked itself out. Play the amp some more and see if it happens again.
 
Just some general comments:

Tubes glowing means the heaters are getting power, not that the tube is working normally. Now, the heaters getting power is required for a tube to work, but just because a heater is glowing doesn't mean the tube is working.

Most output transformers can handle a mismatch upwards (i.e. 8 ohms into a 4 ohm tap) and not die a horrible death. Not all of them, but most quality transformers can. Some can handle mismatched loads in both directions.

The advice on cleaning the sockets by unplugging and plugging in the tube is probably the direction I'd go first.
 
GilmourD said:
Since it's working again I'd say no.

You may have had a random electrical gremlin that's since worked itself out. Play the amp some more and see if it happens again.
Click to expand...

Yeah that's what I'm thinking, plugging both speakers into the appropriate jacks seemed to fix it. Going to play this afternoon (jam with a prospective band) hopefully it will behave!
 
wagdog said:
Just some general comments:

Tubes glowing means the heaters are getting power, not that the tube is working normally. Now, the heaters getting power is required for a tube to work, but just because a heater is glowing doesn't mean the tube is working.

Most output transformers can handle a mismatch upwards (i.e. 8 ohms into a 4 ohm tap) and not die a horrible death. Not all of them, but most quality transformers can. Some can handle mismatched loads in both directions.

The advice on cleaning the sockets by unplugging and plugging in the tube is probably the direction I'd go first.
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I know tube glow is just a quick sanity check, but when it was humming/misbehaving one of them didn't glow at all and the other glowed a LOT more brightly than when it was behaving properly. Not good...

We'll see when I play it for a few hours later today.
 
cvogue said:
I know tube glow is just a quick sanity check, but when it was humming/misbehaving one of them didn't glow at all and the other glowed a LOT more brightly than when it was behaving properly. Not good...

We'll see when I play it for a few hours later today.
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Aside from the heater glow, most tubes have a faint glow (usually bluish) that can be seen in a darkened room. If one is WAY brighter, you may have a tube that is about to nuke itself. If turning the amp off for a bit cures this, it's not fixed - it's just temporarily working - it will go bad again - it's just a matter of time.
 
Btw, if/when your tube does go nuclear, it may take some surrounding components with it. Most popular is the screen resistor.

Not trying to scare you, but that tube being that much brighter is telling you something. I'd replace the output tubes ASAP.
 
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