Advise needed on some rectifier diodes...

[Paultergeist]

This should be quick......

Rebuilding the power-supply of an old amp. The amp uses a "voltage doubler" type of rectifier -- this basically consists of (2) diodes and (2) electrolytic capacitors. I would like to replace the diodes (among other parts), but I am having trouble sourcing them. The old schematic describes the diodes as being "5W-05C" diodes and they are kind of unusual-looking (little round things). I was thinking that 1N4007s would probably work.....but I am looking for confirmation...

Can anyone advise me on diode selection for the rectifier (voltage doubler) of this circuit?

http://www.larryjohnmcnally.com/Teisco/ckmt20_sch.pdf

Thanks!

[martin manning]

I don't know what the voltage on the PT secondary is, but the schematic shows 250V caps in the doubler so 1N4007 at 1kV should do it.

[Paultergeist]

Thanks Martin. I appreciate te reply.

One question: assuming I want to make the power supply a bit mre robust and err on the side of caution, is there any benefit or problem with doubling up te diodes? In other words, use (2) of the 1N4007s each place where currently (1) diode resides?

Thanks very much, once again.

[martin manning]

No problem running two diodes in series. There will be a reduction in the rectified voltage (~1.4V), but that shouldn't be an issue.

[pdf64]

By doubling up, do you mean putting diodes in parallel or series?

On a side note, the standby appears to work by shorting out the OT secondary, which could be quite hard on the power tubes if, say, the guitar was unplugged, thereby putting a buzz signal through the amp.
Pete

[Stevem]

In series for greater voltage, but if your springing for new diodes anyway than if you can,buy UF5403, or UF5408 if you really want to go overboard,these are fast recovery type diodes and will take out the noise hash from the power supply with no need to put them in series, but if you can only find 1n4007 types than adding a .01uf 600 volt ceramic cap across each diode will be the next best thing to fast recovery types.

[Paultergeist]

By doubling up, do you mean putting diodes in parallel or series?

I was thinking of putting (2) diodes in parallel. Upon further research, it would appear that the idea -- while probably not catastrophic -- is not all that recommended. Unless the two paralleled diodes are exactly even matched, one will tend to conduct more than the other -- this leads to it becoming hotter and conducting even more, etc. I would be better off simply using one diode for each of the two legs of the rectifier/voltage-doubler circuit. I am just trying to make certain that the 1N4007s could handle it -- as the original schematic calls for a "5W" diode (and the things are rather large), so I was hoping to err on the side of caution. Frankly, i cannot find anything other than the 1N4007s -- anything bigger is no longer an axial diode, and I start finding heat-sinked zeniers, etc. I'll probably just end up using (1) 1N4007 on each of the two legs of the original circuit.

On a side note, the standby appears to work by shorting out the OT secondary, which could be quite hard on the power tubes if, say, the guitar was unplugged, thereby putting a buzz signal through the amp.
Pete

Well, I DO want to essentially re-build the power supply side of this thing. I confess that I am not seeing that which you describe. The "power" switch is this weird rotary thing that combines "Standby" with the main "Power" functions -- the amp's owener loves the look, but it is a pain. Can you possibly elaborate on this point a little -- such that I might be able to understand a better way to wire the amp.

I should point out that the lower-left corner of the schematic is hard for me to decipher. I plan on scrapping out the "death cap," as well as eliminating a "110V-220V" power selector switch which IS built into the amp but DOES NOT appear on the schematic.

Thanks to all for the assistance!

[Paultergeist]

......you can,buy UF5403, or UF5408 if you really want to go overboard,these are fast recovery type diodes and will take out the noise hash from the power supply with no need to put them in series....

This is exactly the info I was looking for -- thank you! Assuming I understand the datasheets correctly (forgive me, I am still learning) the 1N4007 and the UF5408 are both rated for a max voltage of 1000V, but the UF5408 has a max current rating of 3A, compared to the 1N4007 having a max current rating of 1A. It is probably over-kill, but if I can find them, I will sleep easier at night knowing that I over-built the power supply side of things.

Thanks again.

[ToneMerc]

I think in this circuit you fine with the 1N4007, I use two DO-41 1K 2A UF diodes in series in my MusicMan style ODS supplies.

TM

[darefugee]

So a 5R4 drops more voltage than a 5U4? This is good info - does anyone know of a chart comparing the common diode power tubes as far as typical voltage drop goes, or is there a way to predict this from their specs?

[lord preset]

So a 5R4 drops more voltage than a 5U4? This is good info - does anyone know of a chart comparing the common diode power tubes as far as typical voltage drop goes, or is there a way to predict this from their stats?

Here's one -

http://www.300guitars.com/articles/rect ... rop-chart/

[darefugee]

So a 5R4 drops more voltage than a 5U4? This is good info - does anyone know of a chart comparing the common diode power tubes as far as typical voltage drop goes, or is there a way to predict this from their specs?

Here's one -

http://www.300guitars.com/articles/rect ... rop-chart/

Thanks very much!

[Phil_S]

I found this somewhere (maybe here?) and find it is a very useful chart.

[Paultergeist]

.......adding a .01uf 600 volt ceramic cap across each diode will be the next best thing to fast recovery types.

With respect to the use of rectifier diodes, I have seen this placement of capacitors across the diodes (in parallel) before -- in fact -- this particular amp has this design feature (although not shown on the schematic).

What is the purpose/function of the adding of small-value capacitors in parallel with rectifier diodes?

[teemuk]

They decrease switching noise from diodes.

See, a diode doesn't immediately stop conducting when it's reverse biased, there's a finite time during which it still conducts and the leakage inductance of the power transformer stores down all the conducted energy. Then, when the diode finally stops conducting, the energy is suddenly released in a transient voltage burst. It can have a very high amplitude but it will only last maybe a few microseconds.

If you have a super fast recovery diode, like a modern Schottky, then it will have - like name implies - faster reverse recovery characteristics, the stored energy is decreased and the magnitude of the voltage burst is again decreased. Such diodes absolutely should not have capacitors connected in parallel because it detriments their operation.

If you slow down diode's operation with a parallel capacitor the ringing frequency of the resonant circuit decreases and the result is that the noise isn't as easily radiated around and picked up by surrounding circuitry. The magnitude of the transients may also decrease. This trick works well with diodes that have slow reverse recovery times.

If you have a toroidal power transformer it probably has lower leakage inductance than a generic EI or C -core, which again decreases the magnitude of voltage transients.

It is also possible to build an RC or RLC "snubber" circuit, which is tuned at the resonant frequency of ringing. This actually requires that you know the resonant frequency of the power supply, or own a scope, so you can tweak the other resonant filter to damp the ringing.

See, many ways to tackle this, and many factors and design choices affecting if the parallel capacitors are neccessary.

And before you go on a rave how you can't hear any excessive switching nose then do note that this is really not so much an issue of hearing it: The noise is easily picked up by close by radio receivers and such, and it can also find it's way down to mains line. It's just good design practice to build stuff that doesn't make excessive switching noise.