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Transformer Diodes

carbonfold

carbonfold

Oct 13, 2006
1,009
323
Dallas, Texas
#1
Ok guys, got a tech question for ya.

Now, I understand that a tube amp transformer diode can fail if you have a output resistance issue (such as not properly providing the correct cab resistance) or voltage spikes in the tubes from not allowing them to warm up long enough before playing on them.

However, if all the steps are met properly, could there be another issue that could be causing the diodes to fail?

Maybe gain spikes causing tube voltage spikes? Bad power as well maybe? :hmm:

Just curious what everyone thinks or have had any issues with tube amp diode issues.
 
#2
With respect, I'm sorry but your post makes little sense to me. High voltage rectifiers don't fail because you've got the wrong cabinet attached and you cannot harm a tube amp by playing before it's fully warmed up. It'll simply make no noise and then distort until the tube heaters warm sufficiently for the tubes to start conducting. That'll be around 15 - 20 seconds. Voltage spikes from the AC line can cause rectifiers to fail as can a runaway output stage.

To the point in hand, it sounds like you have a tube amp giving you trouble. What amp and what symptoms???

Paul
 
#3
Well, in a nutshell, I've been battling F2 blowing out like crazy on my VB-2. So thinking the tubes were the issues and fuse, both were replaced (via by tech). However, after pushing through a 1.5 hour rehearsal, the F2 fuse blew out again.

So, the tech looked at the amp again and Peavey recommended that the transformer diodes were failing. I have been told that the diodes are there to provide protection for the tubes and output transformer if the amp is run without a load connected and that they can short out if a tube fails. I was also instructed that the diodes can prematurely fail from voltage spikes. (Voltage spike that occur within either a new tube not broken in or from not allowing the tubes to properly warm up.) Also was informed this could occur by over biasing (too hot). Again, that is just some friendly information provided to me from different techs :) . Whether or not it is true, just what I have been told.

While I waited 1 month for the second repair of the transformer diodes on the VB-2, I ran my same rig on another tube amp without any problems. So, I know that I'm running my rig right and I have double checked everything to a T.

Now, so far, I haven't blown F2 yet. I watch my biasing and I don't overbias the amp. I'm just curious to hear what ya'll may have to say and shed light on preventing future issues.
 
#4
The VB2 has a switch mode power supply which is a completely different animal to the so called linear (rectifier/capacitor) power supplies of most tube amps. Not having seen a schematic of the VB2, which I would love to see, I am unable to comment specifically but many tube amps have two or three reverse connected rectifier diodes placed in series from the output tube plates to ground. These are intended to shunt any spikes to ground and prevent them from possibly damaging the output transformer. If these are failing then you are essentially shorting the B+ power line thus blowing the high voltage fuse. If these are failing regularly I would suspect a possible bad batch of diodes. If so I would think that Peavey would be aware of this and be ready with the fix. This being so hopefully your troubles are past.

Paul
 
#5
No, the VB2 has a regular rectifier/capacitor power supply. Maybe you're thinking of another amp.

BassmanPaul said:
The VB2 has a switch mode power supply which is a completely different animal to the so called linear (rectifier/capacitor) power supplies of most tube amps. Not having seen a schematic of the VB2, which I would love to see, I am unable to comment specifically but many tube amps have two or three reverse connected rectifier diodes placed in series from the output tube plates to ground. These are intended to shunt any spikes to ground and prevent them from possibly damaging the output transformer. If these are failing then you are essentially shorting the B+ power line thus blowing the high voltage fuse. If these are failing regularly I would suspect a possible bad batch of diodes. If so I would think that Peavey would be aware of this and be ready with the fix. This being so hopefully your troubles are past.

Paul
Click to expand...
 
#7
Yes, he is talking about the "catch" diodes that are connected from ground to the tube plates...... if "one end" of the transformer is going too high in voltage, the "other end" is going below ground, and the 'catch" diodes do just that, they "catch" the winding and 'clamp" the voltage. transformer action holds the other 'end" of the winding to only 2 x the supply voltage, which is normal.

Those diodes should not fail from overcurrent, since they are generally capable of more than the maximum tube current. So it must be from voltage.

Either the series diodes are not rated high enough, and are not 'sharing" the voltage so that one gets excess voltage, or the voltage is too high for anotehr reason......

if one side has OPEN diodes, the other side may see crazy voltages........ if the tech looks only for SHORTS, he might fail to see the open, which might be from bad solder, etc.....

There is also a remote possibility of a transformer with too much "stray" or "uncoupled" inductance. That would be a bad design, or a winding defect. There is never perfect coupling, but usually the stray inductance is in the 1% or less range... If it for some reason is larger, then the "catch" diodes won't hold the other end of the winding to 2X the supply, and the diodes may be killed by very high spikes.

I doubt that is teh case, but if it is a problem with that particular model, it's a possibility.
 
#8
Some amps have the type of diode protection that Jerrold has described. They protect the transformer and output tubes from spikes.

Typically three 1N4001 are chained together and run from plate (banded end) to ground. There is one set on each side of the primary of the output transformer.

These diodes should be replaced if one of the output tubes has shorted. If you are getting spikes generated within the amp that are blowing these diodes, you need to track down the source. Sometimes you can hear a crackling sound. The cause of the spikes can be almost anything, an oscillation caused by bad lead dress of the wires, a bad component, even a bad cable. They can be difficult to locate.
 
#11
Hello All,
If the op is talking about the clamp diode string connected to the output tranny primary side on each side connected to the plate outputs. These are designed to clamp the tube cutoff derived back EMF from the collapsing magnetic field on the OPT tranny. The primary function of these clamp diodes is to prevent tube base inter pin flashover, ie high voltage arc. Also internal flashover of the tubes, dependent on tube type.

The energy contained in the back EMF is limited though once it initiates a flashover the DC supply will take over and constant arc is commenced, and possible s/c of primary OPT to supply, its the OPT that needs the greatest protection, tubes are easly replaced, even base sockets once "carboned" will need changing as well.

One effective prevention of OPT short to supply is a 10ohm resistor in the cathode to 0V or "earth" if you must. This suicide resistor will let go before OPT is damaged. To prevent fire hazard a fusable type should be used.

HTH
 
#12
Jerrold Tiers said:
oscillations... YES.... I should have mentioned that... it is a known killer of these diodes......
Click to expand...

Not sure if we are talking about audible oscillations or super/sub sonic? Not so much the unwanted oscillations or wanted ( if your playing well that night :p ). Even at maximum power output the diodes ought not to conduct, with sine oscillations, rather it is the speed at which the half of the output tube is cut off, ie a rapid cutoff will induce high back EMF and clamp diode conduction. When conducting the usual 3 diode string will begin to clamp at about 1.8V once conducting the current flow is small or total power loading on these diodes is small. they are not normally in great danger for most well designed amps. Others may have a different view or experience.

HTH
 
#13
If you get transient or continuous high frequency oscillations, it can heat up and kill the diodes, because they are not really "fast" enough to take that.

The regular current rating is well over what is needed, and they don't normally conduct, so something unusual has to be involved. If they fail regularly, either they were not replaced with suitable parts, (should be high voltage, 1000V), or something like oscillations etc is happening.
 
#14
Jerrold Tiers said:
If you get transient or continuous high frequency oscillations, it can heat up and kill the diodes, because they are not really "fast" enough to take that.

The regular current rating is well over what is needed, and they don't normally conduct, so something unusual has to be involved. If they fail regularly, either they were not replaced with suitable parts, (should be high voltage, 1000V), or something like oscillations etc is happening.
Click to expand...


Oh i see what your saying now. Possible i suppose, though these unwanted transients or as you mentioned HF oscillations would need to be inaudible to have a sustained effect on the flyback suppression diodes. UF1007 would sort that quickly.

Thank you for the reply, regards.
 

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