| whitelabrat |
I have a Lite LS7b (Marantz 7 clone) board that I bought from Hong Kong. I've read that starting this thing up the first time can fry it's rectifier tubes and the high voltage power supply caps should be precharged.
I suspect I'd have to build a circuit to precharge the caps before firing this up first? Anyone suggestions? |
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| FastEddy |
" ... I've read that starting this thing up the first time can fry it's rectifier tubes and the high voltage power supply caps should be precharged. ..."
?? I have never heard of "pre-charging" capacitors prior to first time operation on any modern tube or solid state device.
Please enlighten us on where you may have read this.
If a device is suseptable to sudden influx of power during startup, either the first time or at any time, I would say that the design and execution is pretty poor. Did you get a warranty?
:hot: |
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| whitelabrat |
I read it at this forum. See bottom of the page.
http://www.echoloft.com/cgi-bin/YaB...830397&start=15
I suspect that you are correct about poor implementation as it would have been wise to use a beefier rectifier or something solid state that can handle the rush. |
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| FastEddy |
I know you want to do the right thing as if there are no problems you may have gotten a cool amp.
Many years ago I would "smoke test" new devices by hooking them to a Variac (variable voltage transformer that varies from 0 VAC to 125 VAC) that would let you slowly ramp up the voltage to virgin or new designs ...
These are a little hard to find now ... but they would do the job = slowly load up the caps ...
:smash: |
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| d3imlay |
When the rectifier in tube amps was a tube the rectifier output voltage came up slow as the tube heated up. With solidstate rectifiers the output voltage from the rectifier comes up immediately. That puts high voltage on the audio tubes before they heat up and can damage the tubes. It's also possible that a solidstate replacement for the rectifier tube would allow a higher DC voltage on the filter caps and could exceed they're rating.
NOS capacitors can fail if they've been sitting on the shelf for more than a couple years although it is rare. The best thing to do is "reform" them with a current limited source. A quick Google on "capacitor reform" brings up a wealth of info.
"Precharge" circuits charge up a bank of caps each time a power supply is started to limit the inrush current. Given a sufficiently stiff source such as a large industrial battery, it's possible to draw thousands of amps of current when slamming on a 50,000mf cap bank. |
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| AndrewT |
Hi,
that link in post3 didn't exactly make it easy to find the quote you referred to!
D3's ref to "reforming" may be closer to the mark.
But, that warning about the order of firing up tubes when SS rectifiers are used is very important. |
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| Richard Ellis |
First off lets' refine a definition...perhaps "Pre-charging" is an incorrect term...kinda redundant, lets' try straightforward "Charging" the caps.
If you will recall, the larger the capacitance say 20,000 microfarads versus say 470 microfarads, the actual time that the supply "sees" a dead short trying to charge up these caps is longer with the greater capacitance. The time is measured in microseconds & with immense capacitance the time may be too long an interval that the diodes, transformer, et.al. cannot supply this current load.
This is usually why an L/C smoothing circuit is used rather than a straight capacitance. To achieve a suitable ripple voltage with just capacitance in a smoothing circuit the capacitance needs to be immense (20,000) versus a rather small capacitance (20) linked with a coil as in an L/C circuit.
If there is some notation as to the surge being a problem then yes something is very wrong with the circuit design or implementation.
__________________________________Rick......... |
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| cliff |
| quote: | Originally posted by FastEddy
Many years ago I would "smoke test" new devices by hooking them to a Variac (variable voltage transformer that varies from 0 VAC to 125 VAC) that would let you slowly ramp up the voltage to virgin or new designs ...
These are a little hard to find now ... but they would do the job = slowly load up the caps ...
:smash: |
Not hard to find! I would not be without mine - keeps the smoke in :) |
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| FastEddy |
" ... When the rectifier in tube amps was a tube the rectifier output voltage came up slow as the tube heated up. With solidstate rectifiers the output voltage from the rectifier comes up immediately. That puts high voltage on the audio tubes before they heat up and can damage the tubes. ..."
Ah, yes ... it all comes clear now. I just guess I had forgotten so much about older tube equipment (amps, etc,) that I just let this one get passed me. Live and learn ...then learn again .... Cliff, pass me another Alzhiemers Lite please ...
:smash: |
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| whitelabrat |
I appreciate everyone's post to this subject. I'm sure folks searching this forum will find it useful!
The same guy who sold me the Marantz 7 clone board, also sells a "Power Soft Start" circuit that sits after the AC power switch. Slowly ups the power over 5-8 seconds. Not much detail otherwise.
If it works well with the preamp I may use it with my Marantz 8B to bring it up. It hasn't been powered on in a couple of decades and I suspect that there could be some smoke emissions otherwise. |
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| jackinnj |
High value caps mean high inrush current as the rectifier is warming up. Some of the older rectifier tubes can't take the pain. when these amps and preamps were in vogue the filtration was usually some tens of microfarads. Switch mode power supplies have benefitted tubies because the input filtration caps are hundreds of microfarads at hundreds of volts.
If and when you go solid-state and use high cap values, the voltages post rectification and filtration are higher than specified.
In 1981 Walt Jung and Jim Voorhis did a pair of articles on modifying the Marantz 7C power supply and the PAS. It's a variant of the Maida regulator which can be found on National Semi's website. There was a gb of the LastPAS regulator boards which I organized about a year ago, I still have a couple left if you have any interest. |
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| whitelabrat |
| Anyone have suggestions on how to properly use a metered variac when bringing a tube amp up? I've also got a Marantz 8B that hasn't been powered on in decades and could possibly benefit from this. |
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| FastEddy |
| Yes, those variacs can be expensive. You might consider an isolation transformer 2:1 ... primary = your local household voltage / secondary 1/2 local voltage ... with an inline circuit breaker of just a few amps and, of course, good insulation practices. |
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| Stocker |
variacs are not hard to find or expensive, if you are willing to bid on ebay.com...
when using one to reform capacitors, slowly raise the voltage from zero to full mains level with the variac. The best method involves monitoring the current flow. Alternative method is turn it up a little every x minutes. |
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| cliff |
| quote: | Originally posted by whitelabrat
Anyone have suggestions on how to properly use a metered variac when bringing a tube amp up? I've also got a Marantz 8B that hasn't been powered on in decades and could possibly benefit from this. |
Bring the voltage up slowly!
Depending on what measurement equipment you have, watch the supply rails and current.
If no equipment, watch fror smoke ;) |
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| whitelabrat |
I can assure everyone that going through this forum has put the fear of smoke in me!
I did pick up a cheap variac on ebay. It is rated up to 5A which should be fine for my application. I've also gotten another multimeter, but I'm a bit nervous about working around the high voltage while in operation! Zap! |
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| FastEddy |
" ... I did pick up a cheap variac ..."
Good thinking ... :smash: |
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