Tubes

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 ?

Originally posted by SLY
...(considering that the three of them are 6L6)...

Huh? :confused:

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 [u]not[/u] 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.

$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.............

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]

Just got the amp today ! :D

Thanx pstring & everybody else for the help .

Another stupid question , why do you have to stand-by the amp for a couple of min before switching on ... If not done , how can it affect the amp ?

The Standby switch allows the tube heater filaments to get the tubes up to proper operating temperature before you apply the high voltage to the plates. Failure to do this can quickly destroy your tubes. Always make sure that the Standby switch is in the OFF position before turning on the power.

Originally posted by Lordathestrings
Always make sure that the Standby switch is in the OFF position before turning on the power.

Oops !
The guy who sold the amp to me used to put both the stand-by & the on/off switches on ... Is that too wrong ?

The tubes are glowing orange & some blue around , so I guess the biasing is ok and everything's normal , yeah ?

Start up:

• Make sure [u]both[/u] switches are OFF, then plug in the power cord.
  


• Turn ON only the Power switch, leaving the Standby switch OFF. This keeps the high-voltage part of the power supply disabled, while the low-voltage section applies power to the tubes' heater filaments.
  


• [u]After[/u] a few minutes, you can turn ON the Standby switch also, (both switches ON), and start playing.
  

Standby:

• Turn the Standby switch OFF, leaving the Power switch ON. This keeps the tubes warm and ready to go, but there is no sound output because the high-voltage section of the power supply is disabled. The amp is quite happy to 'idle' like this for an hour or more between sets.
  


• Turn the Standby switch ON when you want to play again. This enables the high-voltage section of the power supply, allowing the amp to operate.
  

Shutdown (Option 1):

• Turn OFF the Standby switch. This disables the high-voltage section of the power supply.
  


• Turn OFF the Power switch. This kills everything.
  

Shutdown (Option 2):

• Turn OFF the power switch. The high-voltage section is still connected to the tubes, which are still hot and operating. This drains off the charge stored in the power supply filter capacitors. It also makes a 'pop' sound come out of your speakers. That pop can be annoying, but it doesn't harm anything, and the hot tubes are happy to drain off the stored voltage.
  


• Turn OFF the Standby switch. [u]Be very sure to do this[/u]. Otherwise, you will cause yourself some grief the next time you turn ON the Power switch!
  

[Edited by Lordathestrings on 10-13-2003 at 08:10 PM]

Thanx a million man , that cleared everything ! :)


So what happens if the stand-by is turned on before the power switch ?

Originally posted by SLY
... So what happens if the stand-by is turned on before the power switch ?

Tubes are able to conduct small currents because the heater 'boils off' some electrons from the Cathode. These charge carriers are not available if the tube is still cold.

The Standby switch connects the high-voltage section of the power supply to the plate circuits of the tubes. If this connection is in place when the Power switch is first turned on, there is a large voltage difference between the plate and the cathode of each tube. With the cathode still cold, there is a high probability of arcing between these electrodes, causing physical damage (called sputtering) to the structure. This literally tears chunks of metal out of the electrodes! :eek:

Whooa !
Will certainly take care of that ... Thanx again man ! :)

Hi again !

I think I have blown a preamp tube , the one that takes care of the vibrato , which is a 7025 tube.
The bass & treble controles in the vibrato channel aren't working , and there's hissing and humming everywhere that increases with the volume ... Also , the amp starts cranking from 2 or 3 !
I think these are all signs of a faulty preamp tube ,yeah ?

The good thing is that the normal channel is working pretty nice , but I the noise comming from the other channel can be heard slightly. (the noise increases with the vibrato's channels volume only )


My first question : If I carried on operating the amp with this blown tube for a while , is it possible that it may affect any other components or tubes ?

Second : In case I don't find an exact 7025 tube replacement , from some articles that I've found on the web, it seems that a 12Ax7 can fit in nicely ... So , am I going to have it rebiased or something ?

The problem in the vibrato channel may not be due to a faulty tube. You can check this by swapping some of the preamp tubes, to see if the trouble moves with the tube.

The 7025 is a military-rated 12AX7A. You can substitute one for the other without problems. If you want less gain, and more headroom, you can try 12AU7 or 5751 tubes as well.

The circuits around preamp tubes allow them to bias themselves. You don't need to adjust anything.

Is there anyway that I can test a tube with a digital multimeter ?

What do you mean by "military-rated" , are they the same , different gains or what ?
And yeah, I'll be happier with less gain ... To have this , should I replace all preamp tubes or just one will have a significant effect ?

As I understand it, the 7025 is built with slightly heavier internal structure to make it more rugged. The electrical specs are identical, but some folks claim that it sounds a bit sharper than a 12AX7A. Whatever...

You need to apply high voltages to a tube to check it out. That means testing it in-circuit. That means risking electrocution! :eek: I realise you have a lot of electronic training, but you need to do some research on the subject, and get some training on how to work on [u]high-voltage circuits[/u] before you do that.

The simplest troubleshooting technique is to swap some tubes around. You need a schematic to help you sort things out. Basically, if you swap a tube from a noisy channel for the same type of tube in a quiet channel, and the noisy channel gets quiet while the quiet channel becomes noisy, you know that the tube is the cause of the problem. If there is no change, the problem is not the tube you moved. Look for something else - either anther tube, or the circuit around the tube socket. The schematic should have some 'typical' DC voltage values for various points in the circuit. These values are for 'zero-signal' conditions, no audio signal applied.

Yeah ,I'm not familiar with high voltage circuits ... I'll search more on that topic .

I've got the schematics ,layouts and everything , and they show voltages up to 400VDC range ... Is there any special precautions you take other than not touching conducting terminals ?
Frankly, I'm not used to working on live circuits .... I never though that tubes would be that hard to deal with , considering that the schematics shows a little bit simple circuits.

Thanx a million man !



[Edited by SLY on 11-03-2003 at 07:33 AM]

The most important thing is something known as the "one Hand Rule". This is based on the concept that if your hands are touching two different points at very different voltage potentials, the resulting current path will be from one hand to the other [u]through your heart[/u]!!! :eek:

Do [u]not[/u] probe a circuit with more than one hand at a time, and be sure that you are not grounded.

Yeah , I know that one ... I try to keep my left hand in my pocket most of the time to make sure I don't touch anything with it.