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Diodes On Rectifier Question

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Trout

Trout

Member
Joined 2004
#1
I was just looking over the service manual diagram for the Re-issue 1957 5E3 Deluxe and saw something I do not recall ever seeing before.

A pair of avalanche rectifier diodes inline with the 5Y3 rectifier tube?

BYD33V data sheet

What would be the purpose of these diodes in this location?

Trout
 

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#2
Back EMF protection? Bizarre though, as the 5Y3 itself is a diode, but perhaps a pre-emptive strike in case of arcing from a cap further down the line (unlikely or what!! But flashover at >300V can't be nice at any time) and not to fry the trafo?
 
#3
I found it odd, because the 1950's units of course did not have these, and many are still functioning perfectly after 50+ years.

I have built a few 5E3 amps from kits and from scratch and the only thing I see different is the diodes in the new units.

another noteworthy item would be the original units did not have a standby switch.
On all of the ones I built it is common to get a POP when flipping the standby switch.

On the Re-Issue unit they interrupt the HV CT, I have tried that setup as well as interrupting the B+ lead on pin 8 of the 5Y3, both techniques pop when entering standby unless you put a .05/630V cap across the switch.

Maybe they felt the diode was a cheaper way to protect the PT instead of a fuse?

Trout
 
#4
On the Re-Issue unit they interrupt the HV CT
Click to expand...

You have found the reason for the avalanche rated diodes. Some users had FRED diodes short out when flipping the standby switch on the SimpleSE amplifiers. One user had a 5AR4 self destruct. Some glitch capturing with a storage scope revealed some serious transients generated form the collapsing magnetic field in the power transformer. It is transformer dependent, and spikes of over 3KV have been seen on some Hammond transformers. The avalanche rated diodes allow most of the transient to be dissipated before becoming a lightning bolt inside the 5Y3.
 
#5
Sweeet, this might actually be useful information.

I have on several occasions seen arcs in the rectifer tubes on these old amps after adding standby switches. I'll have to order a few and try it out.

Thanks for the scoop.

Trout
 
#11
jacking for a second here:

Wait so does breaking the CT cause major problems with certain trannys? I have always wondered this because one of my amps uses this type of interupt so as not to stress a standard switch with HV DC on its contacts. I see that it can cause arcing in rectifiers, would this be a good idea with any tube rectifier with a standby switch before the DC part of the tube?

Also a thougt to ponder....do we really need standby switches? It seems they cause more problems than they do good. If you have a tube rectifier you already have a soft start. I can see guitarists wanting their tubes to "last longer" (ignoring the muting part-that can be done other ways.) but in real world does it really make much of a difference to tube life?
 
#12
ThSpeakerDude88 said:
jacking for a second here:

Wait so does breaking the CT cause major problems with certain trannys? I have always wondered this because one of my amps uses this type of interupt so as not to stress a standard switch with HV DC on its contacts. I see that it can cause arcing in rectifiers, would this be a good idea with any tube rectifier with a standby switch before the DC part of the tube?

Also a thougt to ponder....do we really need standby switches? It seems they cause more problems than they do good. If you have a tube rectifier you already have a soft start. I can see guitarists wanting their tubes to "last longer" (ignoring the muting part-that can be done other ways.) but in real world does it really make much of a difference to tube life?
Click to expand...

I went through several rectifiers before I figured out the arc was coming from opening and closing the CT style standby switch. If you hear an audible pop, Chances are it is or will happen at some point.

On the CT interrupt standby switch I cleared it up by putting a .05/630V film cap across the switch. Knock on wood, after about 9 months of heavy use, no problems.

When I saw the diode solution it seemed like a convenient idea, diodes generally costing a few pennies instead of a $1.00 cap.
Added bonus was you can put them on the 5Y3 socket easily as shown in the picture below.


I have another phenomenon that has yet to be explained when using CT interrupt style standby circuits.

Power up delay from standby. Once the amp is in standby mode for any significant time (2-10 minutes) when you go live, there is a 3-15 second delay before sound comes out of the amp.

On a B+ interrupt standby circuit, you get near instant response, But for some reason beyond my knowledge the CT style has a delay, and output sounds distorted and rises slowly to normal operation.

This concerns me, but I have not had anything fail in any obvious fashion yet.

IMO, it is a good idea to have a standby switch under some applications, I use mine a lot when in the studio or playing out. If you have a solid state rectifier it is more likely an asset than in a tube powered circuit.

I guess it save a bit of tube life but more likely saves on the filaments. New tubes are as reliable as light bulbs , they only have so many cycles.
 

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#13
I just thought I would mention I have been using UF4007 in this same topology for over 10yrs and it is very effective at preventing arc-over in the 5AR4 and other rectifiers with close cathode to plate spacing. I have not had a single rectifier tube failure due to arc over since I started doing this.

Special diodes don't seem to be required for this purpose. I have not had a problem with anything rated at least 1KPIV or above. (1N4007/UF4007, any of the types George mentioned.)

I generally don't use standby switches in my hifi amplifiers, but in the few instances where I have this seems to work fine. Also protects against hot power cycling (my main issue as a commercial amp builder) and other gremlins like sag/surge/dropout on the mains.
 
#14
kevinkr said:
I generally don't use standby switches in my hifi amplifiers, but in the few instances where I have this seems to work fine. Also protects against hot power cycling (my main issue as a commercial amp builder) and other gremlins like sag/surge/dropout on the mains.
Click to expand...


When you do use a standy switch, which type are you using?

Interrrupt the B+ or interrupt the CT?

I am still concerned with the delay and distorted sound on power up from standby on the CT type standby circuit. It sounds scarey, like I mentioned above, I have not had it fail and detect not symptoms of a failure, But the fact that it is a 4-10 second delay leads me to think that there might be a yet unseen issue.

Trout

Edit, this symptom occurs on both tube and solid state rectified amps.
 
#15
Hi Trout,
The issue you mention is probably the time constant of the supply capacitors and inductors to charge up after the standby switch is closed. I've never noticed because I usually don't play anything until I am sure the supplies are all the way up. Bear in mind that the rectifier if tube, and the secondaries have an appreciable amount of resistance and the charging current while large is finite.

I've used high voltage relays (omron I think) and alco switch toggles of various descriptions. I have/have not included snubbers across them as whim dictated.. 😀

What you describe is probably normal. (in my experience..) YMMV... 😀
 
#19
Hi guys

This is a very old thread to resurrect but could someone enlighten me whether installing these two diodes would be necessary if using a solid state diode rectifier?

I know there is a faster/higher voltage surge usually with a diode rectifier. Would adding these two external diodes across the rectifier pins provide added protection against a possible arc or is it not necessary ? I am thinking the arc that is caused usually is inside a tube rectifier glass but can there be an arc and dangerous zap across a solid state rectifier that these diodes could help prevent?

Thank you
 
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