Need Help Finding These Replacement Caps!

[iccaros]

This is a better drawing
http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

They moved the connections from 4 and 6 to pin 5 and 7 as they are unused and connected back with the diodes

[Paul Marossy]

I think the first picture you drew is the way to do it. The picture above is for a 5AR4 type which looks like it has different pinouts than the 5U4G does...

[iccaros]

same pin out,  and the drawing are the same, showing same connections, this second one is just using unused pen so that connection goes to pin 5 (unused by the tube) and goes over the diode to pin 4, just like the schematic drawing.  the diffrence between the two is that the 5AR4 is not directly heated, but that means nothing for this type of hook up. But which ever is clearer for you.

[Paul Marossy]

Oh I see. I did a quick check and it looked like they had different pinouts, but I was in a hurry to get out the door which I got to do right now.

Thanks for the help!

EDIT: One thing, though. Will a 1N4007 be OK current-wise? If I interpret the data sheet correctly, it looks like it's good for 1 amp of current. Is that enough for a 100 watt amp that has a rectifier tube, four power tubes and up to eight preamp tubes? Seems kind of on the edge to me...

[iccaros]

Oh I see. I did a quick check and it looked like they had different pinouts, but I was in a hurry to get out the door which I got to do right now.

Thanks for the help!

EDIT: One thing, though. Will a 1N4007 be OK current-wise? If I interpret the data sheet correctly, it looks like it's good for 1 amp of current. Is that enough for a 100 watt amp that has a rectifier tube, four power tubes and up to eight preamp tubes? Seems kind of on the edge to me...

This is only for the 4 EL34 and not even their heaters so you are @ 300ma max, looks like D1, D2,D3 and D4, using 1N4007's for those, rectify the rest of the tubes already so that there is no load from the 5U4 to the rest of the amp... .. , so you will be fine, R.G. or Paul or amptramp would be sure to let us know if I am wrong on this, which I hold every right to be.


You can see when you chop away the schematic that the 5U4 only goes to point  A, which is the center tap of the OT, which is in all effect the load resistor for the plates of the EL34's.  and that it.. hope it works out, It would be cool if you could post audio clip of this amp

[Paul Marossy]

OK, I see what you are saying. But diodes D1 thru 4 don't look like a full wave rectifier to me (compare to D5 thru D9), they are just two diodes in series with a resistor bypassing them, which is also strange to me. I was looking at those diodes earlier this morning as was kind of puzzled by them. What they heck are they supposed to do? But I get the idea of protecting the PT from a shorted rectifier tube. That's a big deal because there are no replacement PTs available anymore.

I can try to make some audio of the amp, but I don't really quit have the right stuff to make real good recordings.

[SteveG]

Diodes 1-4 are a full wave rectifier. They work with the transformer centre-tap. Diodes 6-9 are a bridge-rectifier, which does not need the transformer to be centre-tapped (although, in this case, it is).

[gmoon]

OK, I see what you are saying. But diodes D1 thru 4 don't look like a full wave rectifier to me (compare to D5 thru D9), they are just two diodes in series with a resistor bypassing them, which is also strange to me. I was looking at those diodes earlier this morning as was kind of puzzled by them. What they heck are they supposed to do? But I get the idea of protecting the PT from a shorted rectifier tube. That's a big deal because there are no replacement PTs available anymore.

The tube rectifier only supplies power to the power tubes (A), so the SW3 must be the standby... The other PS taps (B, C, D, E) are rectified by the 1N4007s, off the same PT (!). They must be HT for the preamp. I guess using the tube rect for the power tubes only is a sag thing.

Resistor or cap between the SS rectifiers--either way it's intended to equalize the voltage over both rectifiers. Lot of the older Fender SS rectified PS schematics use three in series (and no resistors or caps between them), but with Fender you never know if they thought they might substitute 1N4004's if necessary...

I don't see why you couldn't use SS rectifiers to backup the tube like you plan. I'd use two 1N4007's in series like the schematic (a pair for each anode). I've personally tried using a single 1N4007 for a PS with about 400V on a 30 watt amp, and it failed.

That is one cool, strange amp!

[Paul Marossy]

The tube rectifier only supplies power to the power tubes (A), so the SW3 must be the standby... The other PS taps (B, C, D, E) are rectified by the 1N4007s, off the same PT (!). They must be HT for the preamp. I guess using the tube rect for the power tubes only is a sag thing.

Yeah, "A" is the voltage at the standby switch. Most schematics I look at I can follow, but this one I am still trying to understand.  

Resistor or cap between the SS rectifiers--either way it's intended to equalize the voltage over both rectifiers. Lot of the older Fender SS rectified PS schematics use three in series (and no resistors or caps between them), but with Fender you never know if they thought they might substitute 1N4004's if necessary...

OK, I get the idea. I don't remember seeing that on any other schematics that I can remember (ones that would apply to me anyway). Interesting.

I don't see why you couldn't use SS rectifiers to backup the tube like you plan. I'd use two 1N4007's in series like the schematic (a pair for each anode). I've personally tried using a single 1N4007 for a PS with about 400V on a 30 watt amp, and it failed.

That is one cool, strange amp!

It is a very cool amp indeed. They sound really great when all is working properly.

I was initially thinking of using some kind of "super rectifier" diode (1N5408 or similar?) that can handle much more continuous current than a 1N4007 can, which is what prompted me to ask the question. I suppose two 1N4007s in series would also work, though. It was RG's suggestion, and I don't want anything to happen to an irreplaceable PT.

[amptramp]

If your standby switch, S3, is open, and the rectifier shorts from plate to filament, the 1N4007 will have its cathode connected to the peak positive voltage and the anode will go to the peak negative voltage.  If the peak in each direction exceeds 500 volts, this will overstress the diodes, which are rated for 1000 volts.  Two diodes in series connected to each plate would be good enough.  Note that the peak is greater than the average value of rectified DC.

[gmoon]

Merlin writes that an SS rectifier on a fullwave, center-tapped PT setup needs to be rated at 2.8 times RMS (of one side of the PT). You gotta give some thought to that, but it makes sense.

Paul, check out the later Twins, the Showmans and even the late Bassmans. They've all got three (unspecified) SS rectifiers in series for each side of the PT.

RE: the amp's PS--The thought of two different types of rectifiers together in parallel--on the same PT, feeding different stages--is intriguing.

Were (are) there reliability issues with the design? All those module edge connectors. Man. Like an early home computer.

[Paul Marossy]

If your standby switch, S3, is open, and the rectifier shorts from plate to filament, the 1N4007 will have its cathode connected to the peak positive voltage and the anode will go to the peak negative voltage.  If the peak in each direction exceeds 500 volts, this will overstress the diodes, which are rated for 1000 volts.  Two diodes in series connected to each plate would be good enough.

Aha. OK, I get the logic behind that now.

Merlin writes that an SS rectifier on a fullwave, center-tapped PT setup needs to be rated at 2.8 times RMS (of one side of the PT). You gotta give some thought to that, but it makes sense.

Paul, check out the later Twins, the Showmans and even the late Bassmans. They've all got three (unspecified) SS rectifiers in series for each side of the PT.

RE: the amp's PS--The thought of two different types of rectifiers together in parallel--on the same PT, feeding different stages--is intriguing.

Were (are) there reliability issues with the design? All those module edge connectors. Man. Like an early home computer.

The early ones had some issues, but nothing major. It was minor stuff, mostly circuitwise, but it was stuff that annoyed the average joe musician. Mine appears to be one of the first ones made - the transformers, filter caps and cooling fan all have 1984 dates, which was the year it was introduced. The main thing I remember Kevin Beller with Seymour Duncan telling me once (he was one of the designers of the amp) is that he recommended using NOS rectifier tubes because the new ones were prone to failure and sometimes it took out the power transformer. Ten years ago I wasn't too concerned about it because you could still get a replacement PT, but now you can't. I don't know why they didn't think of putting these diodes on the rectifier as a protective measure. Maybe they felt that it affected the "sag" or something? I don't really know for sure.

[Earthscum]

Hey Paul, (or anyone, for that matter), in case you're interested, I keep seeing these at my local shop and thought of you today and snapped a couple pics for you:


http://img3.imageshack.us/img3/5626/image01072012112635.jpg



http://img214.imageshack.us/img214/5171/image01072012112705.jpg

They are at Mountain States Electronics

They have a couple other types, but these are the only 2 types that there were multiples of. There are probably about 10 of the paper ones, and about 5 or 6 of the silver can ones. Just an FYI, since I came across them. He has them marked at $4 each

[PRR]

The rectifiers must be rated *twice* the no-load DC.

Since they used 350V+350V worth of caps, we should assume there could be as much as 700V DC, and must use 1,400V diodes.

The cheapest "1,400V" diode is two 1N4007 in series. We see this on the preamp supply, so it makes sense.

If you must back-up the bottled rectifier with sand-state, this also must be 1,400V to hold things "if" the rectifier goes short or gassy.

Back in the old days we'd never do that. A 5U4 was clearly more reliable than silly-state diodes. However Silicon has improved some, while tube production moved to rundown factories in far corners of the world.

[gmoon]

There's a lot of discussion online about various ways to "bulletproof" tube amps--fuses on the B+ HT voltage, fuses on the heaters, etc., backup rectifiers, circuits to automatically sense/adjust bias voltage, etc. But you don't see much of this on production amps. Cost vs. benefit thing, I suppose. So many old, abused tube amps survive with minimal "safeties."

The early ones had some issues, but nothing major. It was minor stuff, mostly circuitwise, but it was stuff that annoyed the average joe musician. Mine appears to be one of the first ones made - the transformers, filter caps and cooling fan all have 1984 dates, which was the year it was introduced. The main thing I remember Kevin Beller with Seymour Duncan telling me once (he was one of the designers of the amp) is that he recommended using NOS rectifier tubes because the new ones were prone to failure and sometimes it took out the power transformer. Ten years ago I wasn't too concerned about it because you could still get a replacement PT, but now you can't. I don't know why they didn't think of putting these diodes on the rectifier as a protective measure. Maybe they felt that it affected the "sag" or something? I don't really know for sure.

I doubt adding diodes would have much effect on the efficiency of the 5u4, since they'd be wired before the tube. There are faster diodes, of course, but I'm sure the 1N4007 has a shorter recovery time than the 5U4. The SS diodes might introduce some noise spikes, etc.--maybe the tube's response would smooth those out...

[Paul Marossy]

Well it's an easy mod to do. If it has negative effects with noise spikes or anything like that I can just take them out! 

[MikeH]

They do it on the Stray Cat too:  http://ceriatone.com/images/layoutPic/matchlessLayout/StrayCat30Ceriatone.jpg

[Paul Marossy]

Well I turned on the amp yesterday to test it out some more and suddenly the sound went away after about 15 minutes, and there was a burning smell. The fuse did not pop. Tried a different rectifier tube, same behaviour. So I pulled out the chassis again, took out the main PCB again   and poked around. Checked B+ at the standby switch and it was 600V, but further on down the line in the filter section I was getting 0V.

From what I can gather, it appears that the original 150K 2W resistors I replaced at the can type caps are getting very hot. I had to replace those resistors a long time ago with a pair of NTE flame proof 150K 2 watt resistors because they broke (my own dumb mistake), and it seems that these new ones can't handle it. I guess I'll put 5W resistors in their place this time so I don't have this aggravation anymore.

I know it has something to do with the resistors overheating because when I put the amp back together again it was working again. But I bet if I leave the amp on for a while the same thing might happen again. So I think I've narrowed down the original problem to this pair of resistors. At least I fixed a whole bunch of other minor problems in the process.  

Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...

[R.G.]

The rectifiers must be rated *twice* the no-load DC.
Since they used 350V+350V worth of caps, we should assume there could be as much as 700V DC, and must use 1,400V diodes.
The cheapest "1,400V" diode is two 1N4007 in series. We see this on the preamp supply, so it makes sense.
If you must back-up the bottled rectifier with sand-state, this also must be 1,400V to hold things "if" the rectifier goes short or gassy. 

Yep. The measured B+ at 600V means that a 1200V diode is marginal. I'd like to see 2kV in an equivalent situation in my own amps if that happened.
Mouser has the 747-DSA1-18D  which is 1800V, 3.6A, but it costs $4.56 each. There are probably other choices in the industry. I didn't do a massive search. You're right, 2x1N4007 is cheapest, but this setup needs help.

It is important, but often ignored, to provide some equalizing of the reverse voltage on series devices for high voltages. That's what the resistors in parallel with series diodes do. It's a good first order fix, and forces the reverse voltage to be equal across, for instance, two series diodes, by providing a leakage path that completely swamps out the differences in reverse leakage of the two series diodes.

That works fine for DC, but what about transients? In that case, the reverse voltage divides by the inverse of the ratio of the diode capacitances (and inductances, which I'm choosing to ignore for the moment). In that case, you need to put a cap across the diodes that is greater than 10x the reverse capacitance of the diodes to equalize them for AC conditions. If you use a low-R + C snubber, often the snubber can do this for you. But it's something most people haven't run into.

There's a lot of discussion online about various ways to "bulletproof" tube amps--fuses on the B+ HT voltage, fuses on the heaters, etc., backup rectifiers, circuits to automatically sense/adjust bias voltage, etc.

Yeah. A lot of it from me.  I like the idea of solid state minions making the world safe for tubes.

I doubt adding diodes would have much effect on the efficiency of the 5u4, since they'd be wired before the tube.

There is substantially zero effect. The tube rectifier has a 30-60V forward drop, the solid state a 0.7 - 1.4V drop, and the same current, so the power loss is in the ratio of the voltages. Call it about 2% more loss in rectifiers only for a SS diode in series with a tube.

There are faster diodes, of course, but I'm sure the 1N4007 has a shorter recovery time than the 5U4. The SS diodes might introduce some noise spikes, etc.--maybe the tube's response would smooth those out...

The 1N4007 would follow the tube down in conduction, so the spikes and ringing from a standard silicon diode slamming off would not happen when it's in series with the tube. The turn-off transient only happens when normal diodes quit conducting abruptly and excite an RF ring in the associated wiring. Snubbers can largely eliminate this; this is why you often see caps and R-C networks around rectifiers. Fast+soft turn off diodes also prevent it without snubbers. So would a series tube. 

Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...

Simple enough. Open up the wires after the filter caps to the rest of the amp. Clip meter leads to the caps, get your hands out of it, then turn it on, and off. The B+ will go up to the no-load voltage, then start decaying. It will decay at the R-C rate of the caps and equalizing resistors.

[Paul Marossy]

Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...

Simple enough. Open up the wires after the filter caps to the rest of the amp. Clip meter leads to the caps, get your hands out of it, then turn it on, and off. The B+ will go up to the no-load voltage, then start decaying. It will decay at the R-C rate of the caps and equalizing resistors.

Yeah, I thought of trying to measure what kind of current might be happening at that point. But the way the amp is constructed, it makes it extremely difficult to try to test anything of this nature. All the connections are under the PCB, and to get the PCB out, you literally have to dismantle the entire amp. This is what you have to do to even get to those filter caps: