Question about resistors.

[antieatingactivist]

This is something that I've been trying to figure out for myself for a while, but can't find a definite answer to.

Is is possible for a resistor to test fine with a multi-meter, but be very much out of spec when carrying 400+ volts?

what about diodes?

As always, thanks for the helpful answers.

[RicharD]

>Is is possible for a resistor to test fine with a multi-meter, but be very much out of spec when carrying 400+ volts?

Yes, especially if the resistor is not rated for that high of voltage.  Carbon comp resistors tend to be drifty too.  I'm not sure about diodes but it seems like either it works, fails open, or more often in my hands fails short.

[jjasilli]

Yes, also Watts, or power handling capacity.  Watts = Voltage Drop X Current.  So current is a major factor.  When a resistor resists it generates heat.  As the temperature of the resistor rises, the value of its actual resistance increases.  If it gets too hot it may go up in value permanently.  Of course at some point it will fail.  (You can use this to your advantage to permanently increase the value of a resistor by purposely heating it with your soldering iron.)

By using a resistor with a sufficient wattage rating for the intended application, it won't get hot, so its value should not change in operation. (But whatever you do may theoretically affect the tone of a warmed-up amp.)

[antieatingactivist]

Thanks for the info. I will proceed to shotgun every resistor in my Ampeg V2. Are there any resistors that are especially prone to this? I've already done the plate resistors and the power supply dropping resistors. Im getting too high voltages in the preamp, which I guess is causing low voltages in the power amp.

[jjasilli]

Im getting too high voltages in the preamp, which I guess is causing low voltages in the power amp.

That doesn't seem to make sense.  The B+ starts high (in the power amp section), then goes through dropping resistors and gets progressively lower.  Once the B+ voltage is dropped it can't go back up.  So if voltage is low in the power amp, there is a problem in the power amp, probably not on the B+ rail, which requires a scientific explanation.  Either a tube, cap or other component is bad; or there's a partial short somewhere.  (If the problem were low voltage on the B+ rail in the power amp, then all donwstream B+ voltages would be lower too, and couldn't get higher, absent a rather monumental wiring error, I guess.) 

[antieatingactivist]

I was actually just thinking about this.

I could probably blame the high preamp voltage on the high wall voltage. I guess I have a separate problem somewhere in my power section. The only thing that is any different from the schematic is the 1 ohm resistors on the cathode to ground for biasing. Any usual suspects for low plate voltage?

[sluckey]

Any usual suspects for low plate voltage?

Be more specific. Which amp? Got a schematic?

[jjasilli]

Yes, please post the correct schematic for YOUR amp, as there different schematic versions with Capital Letter designations, A through D, whatever.  The parts numbers on the correct schematic will match the parts numbers on your printed circuit board.  Some Ampeg schematics are posted in Doug's library section.

Then post a voltage chart of the actual readings in your amp. 

[antieatingactivist]

will do.

[antieatingactivist]

v1 - 12ax7
1-257
3-2.18
6-218
8-2.23

v2 - 12ax7
1-256
3-2.36
6-210
8-2.32

v3 - 12dw7
1-348
3-219
6-217
8-1.97

v4 - 12au7
1-226
3-10.65
6-227
8-10.65

v201 - 6k11
2-216
3-24.8
5-149
6-1.44
10-349
4-159

v202 - 6cg7
1-120
3-4.31
6-275
8-5.69

v203 - 12ax7
1-120
3-0.76

v5-6 - 6l6
3-567
4-566
5- -71
30ma per tube

B+ at rectifier - 574
B+ at screen supply - 572
B+ at reverb supply - 448
B+ at PI - 389
B+ preamp - 352

AC at power switch - 120

http://www.schematicheaven.com/ampegamps/v2preamp.pdf
http://www.schematicheaven.com/ampegamps/v2powamp.pdf

These match the schematic pasted on my reverb pan.

I guess nothing here is off by more than 10% or so except for pin 5 on the 6k11. It doesn't seem to be causing any problem though. I fitted the amp with a master volume to dump some of the signal off at the ext. amp jack. I turned it down until the PI stops showing strange wave forms (not yet experienced enough with the oscilloscope to describe what I am seeing). Now the power tube grids are seeing a more conservative (and even) amplitude and I am still getting as much volume as I would expect from this thing.

I am basically waiting for someone to tell me "Your amp is fine, this amp was not designed to be used with the volume turned all the way up unless your pickups are weak" I am using p-94's, so there is no shortage of output from my guitar.

[jjasilli]

Here's some service manuals that may help: 

http://www.el34world.com/charts/Schematics/ampeg/Ampegvt22manual.pdf

http://www.el34world.com/charts/Schematics/ampeg/AmpegV4_V22.Service%20Manual.pdf

1.  The first one reminds me that you should verify that the speaker impedance SW is set properly to match your cab.  A mismatch could account for low vol.

2.  The second one has a voltage chart towards the end. Joe Piazza draws nice clean schematics but he doesn't state plate voltages and other pins' voltages.  Doug has some official Ampeg schematics posted.  They're hard on the eyes but contain more info.  Or check the schematic inside your amp for specific voltages.  Match them to your actual readings.  The 6K11 tube seems to be a cathode driver for the (rather complex) mid-circuitry.  Weak mids will make the amp seem underpowered.  Maybe:  a) that tube itself is weak, or b) the plate(s) is loosing voltage from bad resistor(s) between the plate pins and the B+ rail.

3.  Your MV circuit should be treated as a suspect.

[tubesornothing]

Whats the problem with the amp?  I.e.  what are you hearing when it is cranked?

If its just that the PI is showing a strange waveform on the scope, then perhaps just disconnect the scope.  (sorry, I could not resist).  Perhaps post a picture of the strange waveform (preferably with a signal generator input, but a nicely struck guitar note will do if you dont have a generator).

[antieatingactivist]

1.  The first one reminds me that you should verify that the speaker impedance SW is set properly to match your cab.  A mismatch could account for low vol.

My volume isn't actually low. It sounds like about 50 watts. I didn't mean to imply that it was quiet. I have tried several cabs, check to make sure the secondary leads are in the right place. Everything seems to be ok in that area.

Also, i didn't realize that the pin voltages were not listed. I feel like an idiot. I will copy them off of the schematic on my amp.


3.  Your MV circuit should be treated as a suspect.

I added the master volume to try and correct the problem. I know that treating the symptom isn't the right thing to do,  but I have tried everything.

I will scope some things and take pictures of what I see.

I have been posting on 2 different boards several times about this same amp. I apologize if it has been annoying. I've done a few rebuilds on worn out amps and have had great results, except for this one. The main problem is that after playing at full volume, the inside power tube starts to turn orange around the edges. When I test the AC on the power tube grids, it is way high. Up to 100 VAC (with a 200mv sine wave). I used the MV to limit the signal going into the PI. It helps a lot, but I still get some red plate action. It also loses a lot of tone.

I have systematically replaced all of the caps in the amp. No tube has gone un-swapped. I changed the power tube sockets and the PI socket, and Re-flowed every solder joint. I also did the quick test on the OPT.

I will post all of the voltages from the schematic. The only odd one is pin 6 on the 6k11, which is supposed to be 194v, it is 149v. Thanks again for the suggestions.

[jjasilli]

My volume isn't actually low. It sounds like about 50 watts. Most people claim these amps are too loud!  (I'll add too heavy, but I love my V4)

I feel like an idiot.  Don't worry.  You are not alone!!!

I added the master volume to try and correct the problem. ??? An MV, or any pot or fixed resistor, can never make signal stronger, only weaker.  It takes active GAIN to get a boost.  Passive components can only cause a LOSS. 

the inside power tube starts to turn orange around the edges. Does this happen with any tube placed in THAT particular socket?  We may be onto something. 

[tubesornothing]

"the inside power tube starts to turn orange around the edges" - OK, so this is the problem with the amp?

To paraphrase you:  one of your tubes is redplating.  If so, then its not the signal going to the power tube, its the bias is screwed up.  However, if your tube is really 30ma through the plate/cathode, even at 550V this isn't enough to cause a problem unless one of the power tubes is bad.

So... how did you measure the current through the power tube?

[antieatingactivist]

An MV, or any pot or fixed resistor, can never make signal stronger, only weaker.  It takes active GAIN to get a boost.  Passive components can only cause a LOSS.


Yes, I installed a MV to limit the signal going into the power amp.

the inside power tube starts to turn orange around the edges. Does this happen with any tube placed in THAT particular socket?  We may be onto something. 

Yes, it is always the inside socket. I've used numerous sets of tubes. Right now I have a pair of those factory second winged C 6l6's that tube depot is selling.


To paraphrase you:  one of your tubes is redplating.  If so, then its not the signal going to the power tube, its the bias is screwed up.  However, if your tube is really 30ma through the plate/cathode, even at 550V this isn't enough to cause a problem unless one of the power tubes is bad.


This is interesting. I have been under the impression that driving the tube with too much amplitude will cause it to run too hot. If I have been wrong, then that is great. If I take the MV out and run the amp at full blast, it actually does sound quite nice. I just know that if I keep running it at this state, its bye bye power tubes. I assume it is the AC signal because the INSIDE tube always gets the highest amount of AC voltage.

So... how did you measure the current through the power tube?

Bias probe with the 1 ohm resistor in it. I've used it on several other amps, so I know the setup works. If I turn the current down to 20ma, there are still problems.

The redplating is relatively slow to happen. You have to hit an e-chord for about 20 seconds before you get any red plating. It just starts on the edge and then spreads. When you stop playing, it cools down. I would bet that the outside tube would red plate if I kept on going.

The big question that I have now is:
Too much AC signal (within reason of course) on the control grid of the power tube. Can this cause the tube to redplate? or do I just have a separate power amp related problem on my hands?

[tubesornothing]

The bias sets the idle current.   Then if the bias is OK, you can hit it with a big signal and the tube will be within its operating range and will not red plate.  If the bias is too high, a large signal will cause the tube to reach max dissipation quicker and it will start to cook and turn red.

If the bias is OK, and the signal going to the power tubes is too big - no problem - it will just clip  (saturate or cut-off).  It won't redplate.

So the goal is:  solve the red plating issue. Focus on the bias.

So, you say it only happens on one power tube.  A couple of questions and things to try:

(1)  was this amp modified?  How?

(2)  try swapping the power tubes - what happens?  This will try to eliminate a tube or a circuit problem.

(3)  When you measure bias, was it for each individual power tube?

[antieatingactivist]

(1)  was this amp modified?  How?

Master Volume, i took it off.
Bias pot, could be an issue, but the negative voltage stays fairly steady. This was there before I got the amp.
Cap farm replaced stock cap cans. been over this circuit many times and I cannot find any errors. Also done before I got the amp.
1K5W screen resistors.
Blue diamond tolex. (its a joke, sorry)


(2)  try swapping the power tubes - what happens?  This will try to eliminate a tube or a circuit problem.

I did this on several tube pairs. It is always the inside socket that red plates first.


(3)  When you measure bias, was it for each individual power tube?

yes, they are usually within a MA or two from each other.

[tubesornothing]

well, blue diamond tolex has been known to cause the runs, but I am not sure about red plating though...

OK, if this happens to any power tube in that one socket, then its one of the surrounding components. 

Wild ass guess:  the coupling cap coming from the PI is leaky, and its throwing off the bias.

Easy stuff first:

- retension the socket
- reflow all the surrounding solder joints

If that doesn't do it, start to replace components - one at a time.

- coupling cap
- screen resistor
- grid leak to bias resistor
- socket
- plate diodes

Do it one at a time then test each time, so you know, for sure what the problem was.  Boring, but very useful information.

[antieatingactivist]

- retension the socket

I replaced it a while ago, no better no worse

- reflow all the surrounding solder joints
I've done this, but I will check to see if I missed any


- coupling cap
I changed them all because they were all in such poor shape, is there a chance that I got a bad one from the factory?
- screen resistor
Check, tried 470 ohm and 1K
- grid leak to bias resistor
I did this, I haven't changed the grid stoppers because i didn't have the right values
- socket
check
- plate diodes
check

If something is throwing off the bias, would I always be able to see it by probing the control grid on DC with a signal? Is it supposed to stay constant or get more negative? more positive?

[RicharD]

>It is always the inside socket that red plates first.

Replace that socket.  Retensioning (is that a word??) may work but that's like putting a bandage on a cut that needs a couple of stitches.  If the problem is not following the tube, then obviously it isn't the tube.  Red plating is usually caused by bias being too close to zero, but it could be poor grounding at the cathode, a bad connection at the screen, or a related resistor such as a screen resistor or bias resistor or even an open grid leak.  I've seen a couple of amps that did this.  I replaced the socket and screen resistor and I haven't seen the amps since so either their fixed or their respective owners hate me.   

[antieatingactivist]

Replace that socket.  Retensioning (is that a word??) may work but that's like putting a bandage on a cut that needs a couple of stitches.  If the problem is not following the tube, then obviously it isn't the tube.  Red plating is usually caused by bias being too close to zero, but it could be poor grounding at the cathode, a bad connection at the screen, or a related resistor such as a screen resistor or bias resistor or even an open grid leak.  I've seen a couple of amps that did this.  I replaced the socket and screen resistor and I haven't seen the amps since so either their fixed or their respective owners hate me.   
angel

Both sockets are brand new. I replaced them to try to fix the problem, and it didn't get better or worse, so my hack solder job isn't to blame. My bias is at -71. I get a fraction of an ohm when probing the cathode to ground. I replaced the caps and diode in the bias circuit, but not the resistors, or the pot. grid leaks are new, screen resistors are new. I guess I will test the bias voltage again while playing through the amp and see where the bias goes when the tube starts to get hot. I feel like i've done this before, but its worth another try.

[tubesornothing]

OK, this is just a double (sanity) check.  Can you describe the red-plating.  Is it like this?

[antieatingactivist]

yep, although I haven't let it get that intense even though I'm sure it would.

[RicharD]

>The redplating is relatively slow to happen. You have to hit an e-chord for about 20 seconds before you get any red plating. It just starts on the edge and then spreads. When you stop playing, it cools down.

Do you gotta have the amp cranked to make it RP or will it do it at "normal" levels?  Them Ampegs are known for being kinda hard on tubes.  Uncle Ned can tell you about it
http://www.triodeel.com/7027.htm

I'm kinda at a loss.  Seems like a flaky connection at that socket.  You got 2 bias probes and meters?  Slap em in there and wail on it.  When it starts to RP, compare the 2 meters.  What's the difference?  It'd bee good to know what B+ and the screens are doing.  How much is B+ sagging?  How do the screen voltages compare when wailing on it?

[antieatingactivist]

Do you gotta have the amp cranked to make it RP or will it do it at "normal" levels?  Them Ampegs are known for being kinda hard on tubes.  Uncle Ned can tell you about it
http://www.triodeel.com/7027.htm

I'm kinda at a loss.  Seems like a flaky connection at that socket.  You got 2 bias probes and meters?  Slap em in there and wail on it.  When it starts to RP, compare the 2 meters.  What's the difference?  It'd bee good to know what B+ and the screens are doing.  How much is B+ sagging?  How do the screen voltages compare when wailing on it?



It has to be cranked. I have winged c's in it now. Is it possible that current production tubes just aren't cut out for this thing? I'd be willing to lower the B+, or try a different tube type. NOS is a little too rich for my blood. This amp is cool, i'm pretty sure my soldano will always sound better. And boy do those winged c's sound great in it.

[tubesornothing]

You have already established it is not the tubes.  So getting NOS wont help the problem.

[antieatingactivist]

I have some more data that might give some clues.

Plate Voltage at rest on both tubes = 567
Red plating tube with 200mv sine wave = 485
non red plating tube = 508

Bias voltage at rest = -71
Red plater w/signal = -84
non red plater w/signal =  -106

screen at rest = 566
red plate w/signal = 439
non red plate with signal = 429

way more plate sag and way less negative voltage on the grid. I'm still baffled by this one.

[craneclimber]

At this point it might be a good idea to verify all resistor values in this part of the circuit.
Don't forget to doublecheck the bias resistors also.

[JayB]

I'm betting it's a leaky coupling cap. They might not leak much when at idle but get the voltage swinging high enough, they will leak at the peaks and screw with the bias. It's all down hill after that. The weakest tube will red plate first.

[antieatingactivist]

I guess I will change out the bias circuit next. Now that I think about it, the .047 bias supply cap looked like it was changed out, so I didn't change it when I changed all the other caps. Some of the resistors are carbon comps, so I should change those too.

[tubesornothing]

I'm betting it's a leaky coupling cap. They might not leak much when at idle but get the voltage swinging high enough, they will leak at the peaks and screw with the bias. It's all down hill after that. The weakest tube will red plate first.

He did say that he checked them, but I am inclined to suggest that all components be scrapped and replaced one at a time until the problem goes away.  Cracked resistor or leaking cap sounds like it.

[tubesornothing]

I guess I will change out the bias circuit next. Now that I think about it, the .047 bias supply cap looked like it was changed out, so I didn't change it when I changed all the other caps. Some of the resistors are carbon comps, so I should change those too.

The bias supply is common to both tubes, so I don't think that's it.  However, good idea to check though.

[antieatingactivist]

Cracked resistor or leaking cap sounds like it.

Would a leaking cap only affect the stage after it? or could any cap in the preamp be causing DC on the grids? The 2 .33 caps coming off the PI have been changed, but could be faulty maybe? I used Solen Fast Caps for these. I will try switching their positions to see if the problem switches sides.

As far as the cracked resistor theory goes, I haven't changed the 47k grid stoppers because I ran out. I have heard this is a fairly sensitive component. They test fine, but it could be a possibility.

[jcm-jmp]

I'm betting it's a leaky coupling cap. They might not leak much when at idle but get the voltage swinging high enough, they will leak at the peaks and screw with the bias. It's all down hill after that. The weakest tube will red plate first.

+1

[JayB]

Cracked resistor or leaking cap sounds like it.

Would a leaking cap only affect the stage after it? or could any cap in the preamp be causing DC on the grids? The 2 .33 caps coming off the PI have been changed, but could be faulty maybe? I used Solen Fast Caps for these. I will try switching their positions to see if the problem switches sides.

As far as the cracked resistor theory goes, I haven't changed the 47k grid stoppers because I ran out. I have heard this is a fairly sensitive component. They test fine, but it could be a possibility.

The stage after it. If there is a leaky cap on the PI that feeds the power tubes, it can screw with the bias.

Those grid stoppers I didn't think about. If they gone bad and drifted out of spec, it could cause the problem too. I have seen grid and screen resistors measure out just fine on the meter but heat them up and pass a decent voltage or signal through them, they go wacky.

Cold solder joint on that grid can do it also. I went through that problem before. It would be just fine at low volume but crank it wide open, it would red plate every time. A cold solder joint can act like a bad resistor.

[antieatingactivist]

Those grid stoppers I didn't think about. If they gone bad and drifted out of spec, it could cause the problem too. I have seen grid and screen resistors measure out just fine on the meter but heat them up and pass a decent voltage or signal through them, they go wacky.

Cold solder joint on that grid can do it also. I went through that problem before. It would be just fine at low volume but crank it wide open, it would red plate every time. A cold solder joint can act like a bad resistor.



great, that is just what I wanted to hear. They both measure 47k, but maybe they go wacky at normal operation. I will change them with something close and check those joints.

[Structo]

I guess I will change out the bias circuit next. Now that I think about it, the .047 bias supply cap looked like it was changed out, so I didn't change it when I changed all the other caps. Some of the resistors are carbon comps, so I should change those too.

I would replace that 10K fixed resistor with a 10K bias pot.

[antieatingactivist]

update:

I changed the coupling caps from the PI, the grid stoppers, and any other resistors in the bias supply and the PI. No change at all.

Switching the wires to the power tube control grids makes the problem switch sides!

The offending power tube draws about twice the current (using bias probe) when a signal is applied. This would explain why the plate voltage sags so much.

[tubesornothing]

Hmmm... this is very interesting.  I wonder about ultra sonic parasitic oscillation.  Got a scope handy?

[antieatingactivist]

Hmmm... this is very interesting.  I wonder about ultra sonic parasitic oscillation.  Got a scope handy?

I have a 50mhz tektronix, I'm a novice though. I get a strange wave coming out of the suspicious side of the PI that kind of looks like a sine wave with a pulsating nipple on top.

[tubesornothing]

That sounds very suspect.  Can you take a picture and post it?

Make sure to attach your scope **after** the coupling cap on the PI.  Do not attach it directly to the plates or you will fry your scope.