Valve question

[9 volts]

Hey there, can a valve be blown and still light up? Eg can the heater supply still function even if another aspect is blown?

[John Lyons]

Usually the heater/filament is the first to go. It is possible for the heater filaments to work and something else to fail though.

John

[petemoore]

  Hey there, can a valve be blown and still light up? Eg can the heater supply still function even if another aspect is blown?
  They usually act wierd for a while when burning up, you may see the plates glow while this happens, when the plates 'melt enough to short out, they're done, and can have good filament function.
  Unless you don't have a tube tester, a replacement tube comparison is the easiest way to tell if your bottle is usable...but it's possible you have a tube socket which blows tubes..

[soulsonic]

There's all kinds of things that could be wrong with the valve and still have the heater function. I recommend what Pete said, try swapping another tube in there - if it still doesn't work, it's probably something wrong with the amp itself and it should be serviced.
I highly recommend picking up a cheap tube tester on eBay, they can be a big help when diagnosing problems with tube stuff. It's also handy to have an extra tube or two hanging about so you can swap in case something starts misbehaving.

[9 volts]

Thanks, I was in the process of moving around some valves and testing with another spare output tube. Turned it on and forgot to insert the phase inverter (sloppy I know). I've reverted everything back to normal (push pull) and I don't even get a hum. I fear I may have blown the output transformer but am hoping it's just the ouput tubes. Everything lights up and I did a resistance test on the output tranny and the primary and secondary are still separate. Thanks again, bit of a flat feeling going on here.

[soulsonic]

It's very rare for an output transformer to ever blow up. What were the circumstances behind the amp not working? Did it just not work one day, or were you in the middle of playing and it shut off? There's lots of things that can kill the sound that have nothing to do with the output transformer. Does the amp have a fuse for the high voltage supply (B+)? If it does, check it, it may be blown. If it is, replace it and put in a known good set of tubes, and if it blows again you'll know something is maybe wrong with the power supply. Has the amp been biased recently? I've known many amps to blow their B+ fuses if the bias is way off.

[9 volts]

Cool! It was the fuse that blow!!! I put my spare which is a 7027a in with a 7027. I didn't think this would have mattered.........(as mentioned above I also had the phase inverter removed.......). I was just trying to determine if I needed new tubes. The amp is cathode biased. Thanks Soul sonic. Certainly lifted the weight!
ps I've replaced all th caps and I'm trying to reduce a hum thats not related to the preamp section. Since its cathode biased I thought maybe the tube had developed problems.  (possible?)

[soulsonic]

Hmmm..... there's lots of things that can cause hum... Well, if the output is unbalanced (once side of the push-pull working harder than the other), that could maybe cause hum because the common-mode noises that normally get canceled-out in the output section might not be getting totally cancelled. Since it's cathode biased, that may be from the tubes not biasing up evenly, which could happen if they're not matched, or maybe they didn't age evenly - but it's hard for me to say for sure.

Is the hum a low, deep hum, or is it more of a buzzy hum?

Was the hum there before you replaced the caps? It may be a bad ground connection on one of the new caps.

What kind of amp is it? What you're describing kinda sounds like an old Ampeg, but I'm not sure.

[9 volts]

Low deep background hum. It's an old amp I've pretty much rewired not unlike an early bassman. Whats weird is that it seems to come in the longer the amp is on (eg getting hotter. I've read through RG's tips and pretty much changed most resistors and caps relating to power. I'm pretty show it's not heater hum. The volume doesn't change it so it's not preamp (tested preamp tubes). I figured my next option was output tubes.  There seems to be mixed opinions about wheither possible noise can be introduced by output tubes. ( which is why I've left them until last).
I understand it's a big ballpark went trying to determine a problem with an unknown quantity like a home made amp. I'm up for suggestions. Thanks again I looked at pretty much everything except the fuse. The out put tubes are original so maybe 50 yrs old.

[soulsonic]

If it's a low deep hum, it's most likely related to the filaments in some way. Since it seems to get louder as the amp gets hotter, that means something is changing value as it heats up that's allowing it to become sensitive to the hum. Does the cathode bypass resistor have a cap in parallel with it? Is the cap located closely to the resistor? If the cap is right on top of the resistor, the heat from the resistor could be stressing the cap and causing it to "change" which could possibly make it more susceptible to hum... maybe. Either way, if the cap is right on top of the resistor, it would be a good idea to move it  so it's not right next to it because the excess heat is bad for it.

Or it could maybe be happening in the tubes themselves. Since they are so old, there may be some sort of breakdown between the filament and cathode where the noise from the filament is getting into the signal through the cathode, this is something that does happen occasionally. It might be a good idea to try a brand new set of tubes in there - though it's pretty sweet that you have an original set of 7027s. JJ makes a new 7027 that's supposed to be as good as the old ones. What kind of voltages are in there? Maybe you could sub a set of 6L6s and see if that makes any difference.

There's some other things that could do it too, I'm just throwing ideas out there to see what sticks. And as I mentioned before, hum can be caused by unbalance in the output section, where the hum doesn't get canceled-out like it normally does. Using separate cathode resistors for each tube can help with this. Also, you could use a hefty rheostat to wire up a balance pot between the cathodes - if the hum is from unbalance, this control should be able to get rid of it. Having a hum balance control on the filament circuit itself could also help.

[9 volts]

Yeah I have to say I like the old tubes they look cool. They have a shared resistor (5W) which is well away from the bypass cap. Perhaps its the biase as I simply found an old RCA datasheet sheet and put in the recomended resistor value.
I'm toying with the idea of putting in a dc filament supply just to make sure this is not the cause. I'll do some internet research, if the is a schematic floating around or any ideas.....it would be cool. At present  I've just added a humdinger (two 100 ohm resistors to ground) to the filament circuit but this hasn't made a difference.
By memory the plate voltages aren't too high (380 I think) so maybe I could try some 6l6's. Though it's probably best to bite the bullet and just get a new set of 7027's at some stage.
It's a hum you can live with, but it'd be nice to work it out
Thanks again.
DC a good idea?

[petemoore]

Though it's probably best to bite the bullet and just get a new set of 7027's at some stage.
  Now's as good a time as any to figure out what you want in there.
  50yr tubes...definitely worth getting something newer, if only to compare with.
  I've seen really old tubes that work fine, I don't really subscribe to the '6 month' policy [I read that in a new amps manual once]...more like 5-10 years for me..I like to have my spares, I have like 3 spare new output tube sets for my 6v6 amps.
  How old do the capacitors look..worth it to shore those values and reliability with new caps, especially if they look like they went in with the tubes. New caps = confidence, old caps may mean hum, other wierdness, and major amp troubles.
  Caps and tubes for amps is like oil and tires for a car. Blowing a bald tire at speed can cause major damage, as can running without oil. If you're planning to do much driving, and you have really old tubes and caps, I'd recommend replacing all electrolytics and tubes with new ones....
  er..mainly the output tubes. Pre-amp tubes..it is probably sufficient to have the good spares to have something good to compare the old ones to.

[9 volts]

Thanks, I've already replaced all the electrolyic caps, I like the car analogy, i think your right about changing the valves, at least to hear comparison etc. I was under the impression that the pre amp tubes aged faster than the outputs. I'll keep you posted. I'm also going to look into the idea of dc filment supply.

[Gus]

If it is a dual triode like a 12ax7 you can burn out 1/2 of the fil when wired for 6.3V and still have the other half glow.

[soulsonic]

If it is a dual triode like a 12ax7 you can burn out 1/2 of the fil when wired for 6.3V and still have the other half glow.

It's not anything like a 12AX7 - it's a very large beam tetrode power valve.

9V,
You may have to use a larger resistor for the cathode - 5W may not be big enough. Measure the voltage across it and use that with the value of the resistor in this forumula:
E²/R
There's a good chance the number you get may be larger than 5 - which means the wattage of the resistor you have in there is being exceeded. Usually I see at least a 10W resistor being used in that spot, so that may be the problem. Also, you said you used a resistor value the RCA manual suggested - that can work fine, but you have to be sure it's the right value in relationship to the plate voltage you are using. If the value suggested was for 380 volts, then you're fine, but if it's not, then you'll have recalculate it to get the correct bias for the tubes.

Using a pair of 100ohm resistors on the heaters is a good idea, but you have to make sure that the center tap on the filament winding (if it has one) isn't grounded, or else it will screw it up.

[9 volts]

Two points, first off thanks Soulsonic. My voltage was lower than the one on the data sheet (I'm going to recheck it). My apologies I do have a 10w resistor in there but I am going to have to work out a more appropriate value. I went on the larger side to begin with for safety reasons. (get it up and running was my first objective). My heater wires aren't centre tapped, so I'm Ok there.

Gus on the pre amp front I am using 12ax7's, I understand that these valves have one joined filament for the two sides and a centre tap. So 6 or 12 volts can be connected to them. What are the implications of this? Can 12ax7's run on 6 volts, does it effect the sound? From what you are saying a lesser voltage on the filament may be less obtrusive to the audio path.
I bought some new fuses today and we are rocking again. (yeah!)
Thanks again I'll get the maths happening.

[soulsonic]

If the 12AX7s are running off the same filament line as the 7027s, they should already be at 6.3v because that's what the 7027 uses. For 6.3v operation one line of the heater wires should connect to pin 9 of the 12AX7, and the other line should go to pins 4&5 tied together.

Another thing that can help with hum is to make sure they're all connected the same on each tube, ie: the line that goes to 4&5 on one tube should be going to 4&5 on the next - you can mix it up and it will still work, but there will be increased noise. And be sure that the line that goes to 4&5 on all the 12AX7s goes to pin 2 on the 7027s, and line that goes to pin 9 on all the 12AX7s should go to pin 7 on the 7027s. If you do this, the maximum amount of heater hum will be canceled-out in the output stage. Doing something as simple as getting all the heater wires straight can sometimes help alot if there's problems with filament hum. And also make sure the heater wires are twisted closely together. Some amps don't use twisted wires, but for that kind of arrangement to work right, you have to have the wires run perfectly parallel to each other and be extremely close to one another, so most of the time it's easier just to have them twisted. I like to twist mine with a drill so it's nice and tight, and I use two different colors so it's easy to tell which line is which.

[Gus]

soulsonic

  I know what a 7027 is I have a old ampeg amp.   I was aswering the first post.

9V what brand caps did you use?  I have come across new caps that measured bad on a sencore LC102 meter and made an amp worse when used to replace the old ones.  Did the amp hum before changing the caps?

What brand amp and model?

HUM did you check the hum balance pot or hum balance resistors or made sure the fil supply CT was connected to ground.

I have had the 7199 cause hum and be microphonic in my ampeg.   I had to try a number of them to get one that worked right.  If it is an old ampeg it might have this in the PI section.

[soulsonic]

Gus,
What brand of caps did you get that were bad? I've never seen that happen before, even with cheap ones. I've seen new ones that had sat on the shelf too long and went bad, but that wouldn't be an issue if they came from a reputable supplier. I'd like to know so that I can avoid the same potential problem.

[9 volts]

Thanks again. I followed al those tips with the heater wiring (even the drill). I've just been reading the mesa boogie site page about noisy tubes. Both of my output tubes make a noise when they are tapped (tapping test?). So I think I may have found one problem at least. (microphonic)
Gus the caps are OK I think. The hum was there before I put them in. Soul sonic, I was under the impression that 6.3 volts connected to 4 and 5 of a 12ax7 put 6.3 through each triode (hense 12 volts). I did another voltage test wj=hich was 386V (the recomended is 180 R) I'm using 220 so I might drop it alittle. Keep you posted