I'm buying a used Fender vibrolux tube amp , and I'm not familiar at all with tubes technically , so any help about tubes generally is appreciated . (use as much scientefic/technical terminology as you want , I'm an elec engineering student , but unfortunately they don't teach anything about tubes in my college !)

I've read or heard before that an amp can be working with a blown tube , so I want to know if that's true (if a tube is blown , will the amp be still working with less volume or something, or what) , and how to check if a tube is working nicely or not.

Also , I tried the amp yesterday , and I noticed that as you go higher with the volume (over 5) , the sound gradually gets a little bit distorted (very very little crunch or overdrive) , puting in mind that's it's a one channel amp (no overdrive channel) , and there is no gain button at all .... Is that pretty normal from an amp like that , or does it mean that something not good is happening inside ?
I also noticed that the tubes doesn't have EXACTLY the same color when they're working (considering that the three of them are 6L6) , is that ok ?

Hey Sly, how you been, before we get to deep into this, is this amp a Vibrolux or a Vibrolux Reverb? Is it an older Silverface or a Re-issue? To answer some of your questions, yes an amp can operate with a blown power tube but not very well, a big loss of volume and a weird sounding distortion is a dead give away, some amps will not survive too long with a blown power tube, part of what the power tubes do is match the impedance of the output transformer and you can see why one tube being blown in a push-pull circuit would cause a problem, Fenders are fairly rugged in this area but this isn't something to be left unattended, tubes are cheaper than output transformers, Cranking the amp above 5 and getting some distortion is a good thing, you are pushing the power tubes into clipping and thats normal for a non-master volume amp, and by the way it's a two channel amp, should be 2 jacks for the normal channel and 2 for the vibrato channel, the Tubes in that amp should be (if it's non-reverb) 3-12ax7, 1-12AT7, 1-GZ34, and 2-6L6, as to what color the tubes appear, if the 6L6's are glowing bright red/orange their is a problem with the bias , probably just needs adjusted and the sooner the better, if they are glowing blue it's just some off gassing from the plates, no problem and very normal, well Sly keep me informed, if I was an E/Eng student I think I'd better pretty excited because old Fender amps like that are great for learning about tubes, and if you got the bug for doing some mods to it their are a ton of things you can do very cheap that are just a blast, I have a 67 Bassman that is an source never ending experimentation, quite fun for those so inclined.......

[Edited by pstring on 10-04-2003 at 03:42 PM]

Thanx man , I'm very glad you replied first , cuz I was like posting this thread looking for help from you (and/or Lordathestrings) particularly .

First , it's a Vibrolux reverb ... Not pretty sure wether it's pretty old or a re-issue , but it has a silver face .
Since you say it will lose lots of noticeable volume with a blown tube , I assume it's working well , cuz it's a hell of a loud amp for a 40watt combo , I was very impressed with it's big volume (I think that's because I'm used to Solid States , yeah ?)

Yeah , it's got 2 channels with 2 input jacks each , one for the reverb & vib , the other is clean.

As I recall , there were 3 6L6's , and there were 12ax7, & 12AT7 (not sure how many) ... I think one of the 6L6's had a little blue color , and the rest were orange ... Adjusting the bias would be like arranging the resistors , like biasing a transistor , right ?
If a tube is blown , does it glows in a certain color , or it shouldn't glow at all ?

Yes, I'm very excited to learn & experiment more about tubes , so I think I'm buying the amp this weekend , considering that I'm getting it for about $250 and in very good condition. (saw them on e-bay ranging from $500-2500 !).


One last stupid question , the tubes other than the 6L6's had like a metal cover on them ... I checked how the 12ax7 look online and they were uncovered glass , just like I would expect ... Is that ok ?

Huh?

The output tubes in a push-pull circuit are arranged in pairs! I'm not very familiar with Fender amps, and I can't find my book of schematics just now, but I'm pretty sure you should have 2 of those 6L6 tubes, for about 40 Watts output.

There should be a bit of an orange glow from the heater filament, deeep inside the tube. Usually, you can only see it at the very top or bottom of the tube. The whole metal structure should not be glowing!

The bias adjustment is usually a negative DC voltage applied to the screen grid G2. Some amps have a handy adjustable trimpot, but quite often the stock setup has fixed-value resistors in a voltage-divider string at the output of the bias supply.

Metal sleeves are sometimes placed over the preamp tubes to shield them from electrical noise. That's a good thing to see.

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$250! What a score!, sounds like that amp is a CBS era Fender, as far as I know Fender has never put out a re-issue with a silver-face, and you mentioned the 12ax7's still have the metal covers on, so it sounds like it is in very original condition, those metal covers are usally gone, so thats good, as Lordathestrings said the 6L6's are a push-pull pair, and the amp is designed to operate in Class AB, setting the bias makes the tubes operate efficently within that class. Your amp will have either have an adjustable bias circuit, ( Very Early Silverface ) or the typical CBS era Hum adjusting circuit which was a stupid idea but not a big problem since it is very easy to convert to the adjustable bias, you will need two resistors, it takes longer to get the amp out of the cabinent than it does to convert it to adjustable bias. Your amp should have the following tubes in this order, looking in the back of the amp from right to left, 2-12ax7, 1-12at7, 2-12ax7, 1-12at7, 2-6l6, 1-gz34, that GZ34 is a rectifier tube, I'm going to check a website to see if I can find you a schematic and a board layout.............

i want a tube amp.

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Go to the site below. Click on left side, Silver face Vibrolux reverb. You'll see a full description of the amp, and links to a schematic and chassis layout.

http://www.ampwares.com/ffg/

Silver face amps mostly used a 5U4GB rectifier as stock. A GZ34/5AR4 was stock in blackface amps, you can use a GZ34 to get closer to the blackface tone (i.e. less sag). pstring got the rest of the tubes correct. You can use other power tubes also (6L6GB, 5881, 7581) with different effects - always re-bias when changing power and/or rectifier tubes. You can also try other preamp tubes for subtle tone changes - but don't use anything but a 12AT7 in the reverb driver slot (3rd from right looking at the back of the amp). That overdrive you hear at 5 on the volume is a good thing.

Check this website for a description of the preamp tubes in a Fender amp.
http://www.kcanostubes.com/layout.htm
That's a great amp at an incredible price. I strongly recommend that you find out if the electrolytic capacitors have been changed (cap job) in the last 10 years. If not, get it done ASAP. You may want to "blackface" the amp at the same time if a tech already has it opened up.

I've had a '74 silver face Super Reverb for almost 30 years now. I blackfaced it 3 years ago and it made a great amp even better.

Any other ???'s you have, ask away.


[Edited by John O'Carroll on 10-08-2003 at 10:20 AM]