Preamp Tubes? - Who, What, When, Where, Why, and How?

[mickeydg5]

I see what you mean about the old stock Svetlana markings on the EL34 and 12AX7.

The picture I spoke of earlier showing an Svetlana old stock 12AX7 has short plates, but not squatty, and with 3 holes on each side of the plates along with what looks like 3 horz ribs similar in looks to a GE. The getter is hidden behind the flashing and the glass looks like Russian with that bulgy tip.

 

[RiverRatt]

I learn something about every day or so in this thread when it's moving along like this. This is the kind of discussion that got us going in the first place.

 

[brp]

Pardon my ignorance guys but I need some advice.
As you know, I don't know a great deal about tube amps.

A fuse blew in my Vox Night Train while recording today. I didn't have it cranked or anything, master on around 1pm in triode mode, preamp at about 2pm.

I was tuning, and heard a little "klunk" sound, signal faded, then nothing, nothing out of the speaker at all, no hiss, dead quiet.
What would this fuse be and why did it blow?

Is this something I should be worried about or just something that can happen. In my experience, a fuse blowing indicates a larger problem.
What could have caused this?

No, I haven't put in another fuse as I don't have one. I'll get one tomorrow.

It's says "T160mAL 25OVP". Should I expect to just be up and running with a new fuse or is it likely there's a problem that the fuse is only an indicator of?
Any advice appreciated....

 

[MartyStrat54]

Which power tubes are you running in the NT? I hope it isn't the high dollar one's from Alan. I would take a hard look at the power tubes. Remove them from the amp, change the fuse and power the amp up. If the fuse is okay, turn off the amp and replace the power tubes. If the fuse pops, you have a bad power tube.

 

[brp]

Ya, the Sittard Hollands are in there.
I'll do that as soon as I get a fuse tomorrow, thanks man.

They weren't actually that high dollar, Alan gave me a great deal. Hopefully they're ok. They were still lit up if that is an indication...

 

[RiverRatt]

Damn, I hope it's not those Holland tubes either. Check in the fuse holder on the power cable socket - sometimes there's a spare fuse stored in there.

 

[brp]

Never thought to look there lol

The one in there says T125AL 250V.

Neither of the fuses in the amp are that rating

 

[MartyStrat54]

That's why when I buy a used amp, I always check the fuses. When I got my Twister, the fuse holder was actually cracked and could have shorted out. You never know. I then found out that the amp had been dropped from about five feet.

Funny thing is, I was able to trace all the prior owners down (on The GEAR PAGE), including the guy who dropped the amp. Several of the guys were real assholes towards me, but they both later apologized to me.

Just a little story. Keep us posted.

 

[hacksaw]

^ seems to happen a lot with twister affiliated folks Sorry.. <!!!! HAH



Couldn't resist that one

 

[RiverRatt]

Never thought to look there lol

The one in there says T125AL 250V.

Neither of the fuses in the amp are that rating

It's probably a T1.25A 250V fuse. 125A fuses are pretty heavy-duty. My Marshall uses a 4A mains fuse. 1.25A would be more in line with what an amplifier would use. The T means it's a slo-blo fuse. It's important that the replacement fuse matches the fuse spec'd for your amp. I'm not familiar with the NT and its fuses. Maybe Joe can enlighten us.

 

[solarburn]

I'd suspect a power tube but do the test Marty said and see. I play mine at volume all the time and not a peep so far. A tube can go bad any time though. Just say'n my NT has been solid excluding a bad power tube.

 

[RiverRatt]

Bill, let me know if you have a bad tube. I have one more from that set that matches the two you got. I'll send it to you if you need it. I've got it in my little AC4TV mini right now. It's not exactly the best amp for tone checks. I've got a half-dozen or so oddball EL84's right now and nothing to do with them. I need a Class 5 or something. I saw a used one in Nashville at GC for $299. I had my credit card with me, too. I also had my wife with me. She thinks I have enough amps and guitars.

 

[brp]

Heheh,

Ya, I meant 1.25, that's what it says on the back right by the fuse holder in the power cord compartment too.

So why'd they give me that in there if it doesn't match what the amp has in it. The other fuse (I mean the other fuse spot in the amp, not the one that blew) uses a T6.3AL and the one that blew is a T160mAL, so the extra doesn't match either one?


Also, there was no smell or anything when it stopped working so that's a good sign, and all 4 tubes were still glowing.

 

[RiverRatt]

Sometimes a power surge or spike can pop a fuse. It may turn out to be a non-issue. If there is only one fuse in the power cable socket, it's probably in the circuit and not a spare. Some have a storage space for a second fuse.

If the power tubes are lit up, it's most likely the HT fuse. When it blows, it would have the same effect as putting the amp on standby. If you know how to safely poke around in an amp, take each fuse out and check it for continuity. I've had fuses that looked perfectly normal that were burned out.

 

[RiverRatt]

.

 

[RiverRatt]

Hey all. I got my free Tung-Sol KT66's today. I put them in the VM and biased them up HOT. The lowest I could get them was around 39mV. The plate voltage was right at 444V. I started dialing them in for tone and I hit the sweet spot at around 48mV. I know that's high but they never broke a sweat. I've heard it said that the Tung-Sol KT66's are really KT88's relabeled. I don't know about that, but when I got these things dialed in they are the best I've tried so far. I compared them to a pair of Russian Gold Lion RI's and the stock Marshall Shuguang tubes. Not bad at all. They have that great percussive Marshall attack.

Since I had the power section speaking to me, I started messing with the preamp tubes. I left the chassis out of the box while I was working with the KT66's, so I moved some preamp tubes around. With a long plate Mullard in V1, a BEL in V2, a Raytheon black plate in V3 and a strong 1960's Amperex short plate for a PI. I'm now rethinking my Raytheon/V2 rule, at least for the Vintage Modern. If I hadn't remembered the BCS championship game, I'd still be back there playing. This amp is sounding GOOD.

 

[BluesRocker]

Hey Alan, by any chance with that Tweaker deal, did I get an RCA 5751 from you? I found one today and cant remember where I got it.

 

[brp]

So I picked up some fuses today (yes exact same spec as the blown one).

Came home and popped in the 2 BP Raytheon 84's that Marty gave me, replaced the fuse and the amp seems to be working fine.

I just went to put the Holland tubes back in after letting the BP's cool and when I picked up the first one I noticed right away that it has gone all milky white around the top and there's significantly less of the coating at the top that was there, now only about half of what is in the other one. I know this happens when vacuum is lost.

I assume there's no point putting them in to see if it works and that I've found the reason for my blown fuse.

 

[RiverRatt]

Hey Alan, by any chance with that Tweaker deal, did I get an RCA 5751 from you? I found one today and cant remember where I got it.

No, all the ones I have are GE or Sylvania. Does it have black plates and triple micas? The only primo tube I remember putting in the Tweaker was a Matsushita long plate in V1. V2 and V3 aren't gain stages so I used whatever was cheap and handy there. There's a chance that one of the GE or Sylvania 5751's ended up in there but it's not likely. I watch those things pretty closely. I don't have a lot left.

 

[RiverRatt]

So I picked up some fuses today (yes exact same spec as the blown one).

Came home and popped in the 2 BP Raytheon 84's that Marty gave me, replaced the fuse and the amp seems to be working fine.

I just went to put the Holland tubes back in after letting the BP's cool and when I picked up the first one I noticed right away that it has gone all milky white around the top and there's significantly less of the coating at the top that was there, now only about half of what is in the other one. I know this happens when vacuum is lost.

I assume there's no point putting them in to see if it works and that I've found the reason for my blown fuse.

I sent you a PM.