Sunn Model T rectifier diodes

[Burnt Finger]

I wish to replace the 3 rectifier diodes in an old Sunn model T. One is a common IN4004, but the other two are marked MR 1-1600 on the diodes themselves and identified as 22-0416 on the schematic. I can find no reference to these part numbers elsewhere, so no specs.

Can anyone tell me what commonly available diode can be used here?

Thanks for any help!

[Chuck H]

Why do you want to replace those diodes. They either work, or they don't. If you just want to replace them for the sake of having new parts in the power supply that's fine. But just so you know, if the amp is working now, you shouldn't expect an improvement.

One of the diodes you referenced isn't a rectifier diode. It feeds the power tube grid bias voltage circuit. A regular 1N4007 (or a 1N4004) is fine there. Be sure to observe correcy polarity.

As for the two "MR1-1600" diodes, those ARE the rectifier. I know there are some burly diodes out there, but I don't know the part numbers and a search with the numbers you provided wasn't very effective. I would just use 6 1N4007 diodes. Three in series on either side of the PT HV secondary.

Chuck

[Burnt Finger]

Chuck,

Thanks for your response to my question. Please pardon my hasty and imprecise statement of the problem. I was aware that one of the diodes was for the bias supply, not the rectifier.

The amp was recently given a typical overhaul: replaced all electrolytics in the power supply, retubed, "death cap" removed, etc. It worked fine and sounded great for a couple weeks, including several days use during a marathon session in the recording studio. Then the owner turned it on one day and the circuit breaker tripped. Hooked up to the current limiter/test lamp, it shows a short even with all tubes removed. My understanding is this would indicate a problem in the power supply, transformer, rectifier, filter caps, possibly a bad standby switch (?). I figured a logical and easy place to start would be with the rectifier.

It looks like a typical full wave rectifier, but only has one of the "unidentifiable" diodes on each secondary from the transformer. I'm not sure there's room to neatly put in a string of 3 diodes, so I was hoping to determine more exactly what I need. Would one ECG 558 be the same as 3 IN4007 in series? This beast runs a quad of 6550s so I think the voltages can run fairly high.

The 'MR 1-1600' that are in there are small - look a bit smaller than the 4007; I'd have thought that greater power handling capacity would require a larger component. Perhaps size doesn't matter after all.

It's probably obvious I'm not an expert tech. I have a fair amount of practical experience, but limited in scope - troubleshooting is not my strong point. My reading suggests that even if the rectifier diodes test OK on a DMM they can malfunction at operating voltages.

Any comments or further suggestions very much appreciated.

[Chuck H]

I would think bigger is better also (with respect to diodes, my wifes opinions not withstanding) And the 1n4007 is all I ever use as it seems to be a good part for most applications. So I'm not familiar with diodes that can handle more PIV. But I do think that series diodes are a much better idea than a single bigger diode. From a design perspective, equipement designed with series resistors and/or diodes seem to hold up better than when a single higher rated part is used. Just an observation. And it's very true that diodes can test good but still under perform at operating voltage. So replacing them under your circumstances is valid. I suspect a filter cap. Even though they were replaced it is possible that a faulty cap got in there. I am biased (pun intended) because it happened to me. So take that for what it's worth.

The ECG 558 doesn't generate much info with a google search. I gather it's similar to a 1N4007 but with a PIV of 1500V. It should work fine in your application, but the series 1N4007 circuit is more common and performs well AND has stood the test of time. I would string a three diode per side arrangement in there even if I had to float them on the solder joints before I would rewire it with single diodes per side. JM2C.

Other than that, is there any extra hum in the amp lately? It could still be a filter.

Check the filament voltage to see if it's in spec. This would show any fault in the PT primary.

Chuck

[Enzo]

With the amp off, just measure those two diodes for resistance. They are either shorted or not. if they test OK, then they are most likely OK. I often find caps that test OK at low voltage and fail at high voltage, but I don;t remember seeing a diode that tested that way. I am sure it can happen, but over the last few decades, not to me.

Do you know what shotgun and easter egg repairs are? The shotgun approach is the just replace every part in a circuit. Assuming the trouble was a bad part, that will get it. An Easter egg hunt is where you go down the row replacing parts one at a time until you find the bad one. That is not very efficient.

Isolate the problem. Right now, you don;t even know if the high voltage circuit is where it is, do you? What if you were to replace the entire B+ circuit only to find a dead short across the 6v heater winding?

Your light bulb lights bright instead of taking out a breaker or fuse, so if it is bright, something is loading things down.

Does it still blow fuses - I know you are not losing fuses, but it is easier to write than light bulb lighting up brightly - when the power switch is off? Hopefully not. More important does it blow fuses when the power is on but the standby stays off?

If it blows even with standby in off position, then you could have a shorted rectifier, a shorted transformer, a shorted heater line, even a shorted pilot light assembly.

Not common, but I have seen this happen. You standby switch itself could fail internally leaving one of hte contact pieces touching the metal frame of the switch, thus grounding the high voltage. Easy enough to check.

To check the transformer, disconnect the red high voltage wires from those diodes, and the two green heater winding wires from wherever they are. Now apply power. If it no longer blows fuses, then the transformer should be OK. Then reconnect the heater wires and power up. If fuses blow, the heater wiring has a problem, (That hum balance pot could be shorted across its terminals) but if it holds, that is OK. Then you can connect the red wires back up and we are back where we started.


If however the standby must be on for fuse blowing, then it points to something in the amp circuits and yes, B+ related.But if it only blows fuses when the standby switch is ON, then all those things are probably OK. The rectifier diodes are before that switch, and they would blow fuses regardless of the standby switch position. SO with standby required on for trouble, we need again to further isolate the problem.

COUld be the supply itself or the circuits it feeds.

With powr off and standby off, measure resistance to ground across the filter caps. Any shorted? A short at no voltage won;t clear itself up when voltage is applied, but as you mentioned, a cap that seems OK to a meter might not be at 500v. But you could also have one of the chokes shorted to its frame. And that would show up as the + end of a filter cap being grounded. Likewise, the output transformer could be shorted to frame. Same deal.

No obvious shorts? Disconnect the red center tap wire from the output transformer. That leaves the chokes and the first two filter cap stages - the power suspply itself. I tend not to think the later caps are the problem. Why? Because after the first two B+ nodes, there is a 10k resistor to the next cap. If that next cap were shorted to ground, that would put the 10k resistor across the 500v supply. That would only draw 50 milliamps - not enough to blow fuses. SO I'd focus on C17, C16 and leave it at that for now.

Does it blow fuses with the OT center tap off? If not, then the output transformer or tube circuits are suspect. If still blows, then those caps and the chokes are about all that is left.

Just because a cap is new, doesn;t mean it can;t have failed. If we have narrowed this down to the two caps and choke, then disconnect the caps one at a time and see if either makes the difference. The amp would work poorly without both caps, but we are not using the amp now, we have the tubes out and we are ONLY looking for what blows fuses.

If the fuses hold, we are left with the disconected output transformer. We wamnt then to power off, measure resistance from that red wire to ground - should not show continuity to ground. Also check from the red wire the resistance to the blue wire and the blue/white wire - the wires to the tube plates. You could also measure resistance from pin 3 of each power tube to ground - should measure open.

That was a tutorial on troubleshooting this thing. You can do it.

[Burnt Finger]

Gentlemen, thank you so much for all the assistance. It turns out one of the rectifier diodes is indeed shorted. Hopefully that solves it. If not, I now have Enzo's excellent tutorial to guide my further efforts.

To clarify, in response to Enzo's questions, it does not 'blow fuses' with the power off. It does 'blow fuses' while on standby. And, if I switch the current limiter/test lamp out of the mains supply it immediately trips the circuit breaker (this is with all tubes removed).

I have another question regarding the series diodes, which will be "floating" on a terminal strip (this amp is true point to point - no circuit board). Can I slip a piece of insulation over the whole string of 3 diodes, or do they generate some heat, while operating, which must dissipate? While they will probably be fine without it, they will be close enough to other components that I'd feel more comfortable with some insulation.

If that seems to fix it, I'll check voltages per your suggestions just to make sure multiple problems don't exist.

Again, guys, thanks for your help. Enzo, I'll be studying your troubleshooting guide carefully for future reference.

Cheers,

Jim

[Enzo]

By definging the problem as power switch on and standby off, we isolated the problem to the immediate area between them.

Your dioides don;t get particularly hot, but they don;t really need insulation either. The wire terminals are exposed, the wire leads of resistors and caps at high voltage are exposed. Nothing special about those couple parts.

By the way, the Fender reissue of this amp uses 1N5062 diodes all over.

[Axtman]

As a side question. Why are the B+ diodes placed 3 in a series?

[Enzo]

The voltage breakdown adds up. 1N4007 has a 1000v reverse voltage rating - 1000PIV for Peak Inverse VOlts.

The filter cap in this amp will charge to +519v at low powr, probaby even higher with the tubes removed. That means there is always +519v on the cathode end of the rectifier diodes. The transformer is putting out roughly that much voltage peak on each cycle. That means each cycle goes from zero up to about +519v and then back down through zero to about -519v, and then back to zero. that is one complete cycle. 60 times a second the AC does that.

When the transformer voltage is at -519v, the diode blocks it, as it only lets current flow through one direction. So that means there is +519v on one end of the diode, and the other end has - 60 times a second - -519v. SO 60 times a second, each diode has 1038 volts across it backwards. The diode must try to block this. With its 1000v reverse rating, the 1N4007 is under rated for the job. SO we would either have to find higher voltage diodes - 1500v maybe - or we can use several 1N4007 in series. Three of them would require 3000v to breach them.

[Burnt Finger]

Replacing the diodes did the trick. The Model T is back home in time for a gig this weekend. The owner is happy.

By definging the problem as power switch on and standby off, we isolated the problem to the immediate area between them.

Point taken!

Enzo, thanks again for the troubleshooting lesson.

Cheers,

Jim