Blowing Filaments?

[apehead]

Hello all,

I recently purchased a Fender twin that is quite a mess. First, it is some sort of hodge-podge. The amplifier itself is a transitional 1977 100W master volume silverface, with a hum balance like the 135W models, but no line out. The grill cloth is the early silverface aluminum-bordered 1968-1969 style, and the cabinet sticker lists the amp as the "AB763" circuit, the coveted blackface design, so my guess is that the grill cloth and cabinet used to contain an extremely early, transitional silverface amp with a blackface circuit! The speakers are Dyna-Drive UES-92 speakers (never heard of them, look cheap). Now for the bad... The idiot who shipped the thing to me didn't take the tubes out of the amp before shipping! All of the power tube bases cracked or the key broke off, and at least one of the preamp tubes is shot (no getter). Against my better judgment, I was convinced to A/B the sound with a 1974 Quad Reverb. After a cursory look around the amp for any broken or burnt components, I installed all of the known good tubes from the Quad Reverb. Energize. A bright flash emitted from the base of one of the power tubes. Shut off power. checked for filament continuity... open. My question is this: Would the damage this amp sustained during shipping possibly cause it to start blowing filaments? I was told the amp was fully operational when I bought it. I did see any sign of physically broken parts near or around the filament circuit, and far as I can tell, only one tube kicked the bucket. What might be the cause of something like that?

[stokes]

In my opinion,shipping an amp that has the chassis secure in the cab,and nothing inside to bounce around to break the tubes should be okay with the tubes in.Unless they fall out and nothing pierces the carton to hit the tubes,how could they break?I suppose if the amp is dropped hard or thrown around the shock could crack a tube,but to break off the pin would mean the tube was sharply skewed to one side,but anything is possible.Sounds to me like he shipped it with broken tubes to begin with,but thats just my opinion.Are you sure the flash you saw was the filaments?You say you checked the fil's for continuity and found it open.Where did you check this?From one tube to the next,from the tranny tap to ???I would start by removing the tubes and checking the filaments for ac voltage at each tube,dont check from ground to each pin but from pin 2 to pin 7 on each power tube and then pin 9 to pins 4&5 on the preamps.The flash you saw in one power tube could be something besides the filament,so check the actual voltage before you assume its the fil's.

[apehead]

The guy used flattened cardboard for packing material, (not exactly the most impact absorbing material on the planet). When I first saw the tubes, one of the interior four was partially lifted out of its socket and leaning against the other tube, suggesting a big fall or impact against one side of the box. Also, the plastic keys were still in the sockets when I removed the tubes, they had just broken off. None of the pins were broken, and it doesn't seem like the vacuums were compromised. So I think the guy was honest, but really negligent. As for the filament, the flash was localized around the base of one tube, and what made me think fils right off the bat was how reminiscent it was of a light bulb burning out. I pulled out the tube and checked across pins two and seven with a DMM, and it was open. All of the other tubes filaments are between .6 and .1 Ohms.

[Enzo]

All of the heaters are wired in parallel. ANy voltage surge or spike would hit ALL of the tubes, not just one. Your tubes are beat to death, and instead of looking at that as the cause of your tube failure, you are leaping to some mysterious circuit problem. Pull the tubes out and stick a voltmeter down the holes. That will tell you if there ar any funny voltages. What usually kills heaters is a short to a high voltage element inside the tube. Just the sort of thing that happens to beat up tubes.

I would have to agree, your amp took a hit on the end they were leaning towards. When people send me amps for repair, I always specify the tubes be removed.

[apehead]

I installed all of the known good tubes from the Quad Reverb.

The tubes I put in to try the amp out have maybe 10 plate hours on them, tops. I wouldn't trust broken tubes in an amp. I checked the fil voltages with no tubes installed as Enzo recommended, and it was sitting about 7.08 VAC. I have a hard time believing that it such a new tube would just fail like that.

[Tom Phillips]

...I have a hard time believing that it such a new tube would just fail like that.

But is does happen.
Keep an open mind to all possibilities and remember that a failure can happen at any time. I get people all the time who bring in an amp and say. "The tubes are all OK. I just replaced them last week."

Regards,
Tom

[Enzo]

Never assume a tube is good. New tubes come out of the box bad ALL THE TIME. And NEVER EVER think up reasons NOT to test something.

[apehead]

Whew! Talk about an expensive hobby!

[stokes]

Okay,so its not the heaters.You say you used "known good tubes" from a Quad Reverb,is this the same Quad from the other post?Are you sure they are good?It sounds like the same problem you were having with the Quad.Are you sure you arent using the same bad tube that went from socket to socket in that amp?I doubt the damage done to the tubes in shipping would cause internal damage to the amp,but using a bad tube would,as you have seen in the case of the Quad.

[apehead]

Sorry, I meant I used power tubes from a Peavey Butcher, and the preamp tubes from the Quad.

[Sweetfinger]

... The idiot who shipped the thing to me didn't take the tubes out of the amp before shipping! .... I was told the amp was fully operational when I bought it.

Hey, the 'Death Star' was fully operational....before it blew up!

I consider ANYTHING (musical instrument/amp/effecs) that I buy off ebay to be broken. period. Rarely, I am disappointed to find that something actually works or plays as advertised I will generally make an offhand comment somewhere in correspondence with a seller about "..of course just pull and wrap the tubes so they don't get broken, I'll be able to figure out how they go back in". Vaccuuming glass shards and powder out of an amp chassis and cabinet is fun...once.

[Alex R]

Things do break in transit inside amps. Six foot drop off a loading bay on to concrete? Happens every day in the haulage trade, sorry, the logistics solutions business. The tell-tale sign is like in this case, everything leaning the same way, and broken. I had a transistor amp in once that had fallen off the back of the van - it was like a hurricane had blown across the board, everything leaning over and microscopic cracks spreading out from the mounting screws.

I don't knock Ebay, it keeps me in business!

[apehead]

As far as the amp goes, I had some 21 year old 6L6GC's laying around that I pulled out of my Peavey Butcher (they are the original stock Phillips). I decided to put them in, due to the fact that, frankly, if they went, I wouldn't care! Played through it for about an hour, no problems yet... but we will see. Thanks for the help, more than likely I'll be back!

And so I am! So the amp was doing fine, no problems at all. Last nigh, my buddy was playing guitar through it for about 45 minutes, when I happened to glance over... the plates of those poor old Peavey tubes were bright orange! I assumed bias was lost, but bias levels measured -38V, plates were 477V, and screens were 270V. No visual problems. Are these just two more to add to the pile? Do I have the worst luck with tubes, or what?

[stokes]

Are you checking the bias on the power tubes before you put them into use?If you arent checking/adjusting the bias of these tubes,its not a matter of having the worst luck with tubes,its just a matter of them being biased too hot.

[apehead]

I'm not entirely sure what you're asking stokes, but I've used the working (so far) Quad Reverb as a Baseline measurement. Here's how that one is running:

Bias Voltage: -48V
Cathode Current: 45mA
Plate Volts: 477V
Plate Watts: 21.5W

According to the data sheets, any typical 6L6GC maximum plate dissipation is 30W.

Here is how the Twin is set up, remember, it does not have an adjustable bias, only a balance pot:

Bias Voltage: -38V (the balance pot is set dead center)
Cathode Current: Unable to Determine (haven't installed bias test point mod)
Plate Volts: 508V
Plate Watts: Unable to Determine

I just thought that since the bias circuit in this amp has been untouched, that -38V would be acceptable. The tubes that cherry plated were installed in the Peavey Butcher before, and that thing totaled about 20.5 plate watts before I installed an adjustable bias, so I figured they could handle it.