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  #1  
Old 11-26-2010, 02:33 PM
polyphase polyphase is offline
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Moto VT71: a better ballast?

Here is a home-brew ballast that pin for pin is a direct substitute for the 17A485459 ballast in my VT71. It uses capacitors in the filament strings instead of resistors. The result is 80% less heat dissipated and elimination of the current surge at turn-on. The tubes instead enjoy a slow, gentle life extending warmup. A pic of this contraption, (along side its predecessor) and a schematic are shown below.

The math, in spread sheet format, is as follows:

reactance X of cap =10^6/(2*PI()*F*C)
for F=60Hz and C=10uF, X=265ohms

resistance R of HOT filament string =260ohms.

X in series with R results in impedance Z:
Z=SQRT(X^2+R^2)=370ohms, so current at 110Vac line voltage is 110/370=.3amp

The COLD resistance of the filament string is about 50ohms.
Substituting 50 for R in the above formula, the result is about .4amp, +30% of nominal. This is trivial compared to the .71amp (+150%) surge that occurs at turn-on with the original 105ohm resistor ballast.

note: The caps shown are Panasonic ECQ-E2106KF, avail. from Digikey
ballast17A.post.jpgbalschem.jpg
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Last edited by polyphase; 11-30-2010 at 12:45 PM. Reason: move attachments to post
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  #2  
Old 11-26-2010, 07:03 PM
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bandersen bandersen is offline
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Very clever! I've seen talk use of using a series diode + resistor to improve efficiency, but never a capacitor.

I wonder if adding a thermistor like a CL-90 would completely eliminate the surge ?
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  #3  
Old 11-26-2010, 08:03 PM
kvflyer kvflyer is offline
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Very nice. I am a sucker for the VT-71. I have five of them and one is restored so far. I am lucky to have ballasts for them but I like your idea.
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  #4  
Old 11-26-2010, 09:31 PM
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Phil Nelson Phil Nelson is offline
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Tidy construction. Did you reuse an original base & ventilated can?

Phil Nelson
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Old 11-26-2010, 10:33 PM
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Red Raster Red Raster is offline
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How about an ugly ballast contest

Necessity can be the the father of invention and in this case, bastardization



Pretty ugly


Can you beat this?


Like Frankenstien's monster it is ugly but it works!

Note! this not of my doing, just another fully restored set on ebay

Last edited by Red Raster; 11-26-2010 at 10:38 PM.
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  #6  
Old 11-27-2010, 08:48 AM
polyphase polyphase is offline
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Quote:
Originally Posted by Phil Nelson View Post
Tidy construction. Did you reuse an original base & ventilated can?

Phil Nelson
Yes I did. A very tight squeeze..
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Old 11-27-2010, 09:16 AM
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John Folsom John Folsom is offline
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Very elegant. Any issue with dissipation in the film capacitors?
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Old 11-27-2010, 09:44 AM
polyphase polyphase is offline
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Quote:
Originally Posted by John Folsom View Post
Very elegant. Any issue with dissipation in the film capacitors?
None whatsoever. The 2 resistors in the circuit together dissipate about 5 watts. If there is any dissipation in the capacitors, it would be measured in milliwatts. The overall power consumption of the set is reduced by 20 Watts although VA stays the same at about 113.

Last edited by polyphase; 11-27-2010 at 09:53 AM. Reason: clarification
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  #9  
Old 11-27-2010, 11:02 AM
polyphase polyphase is offline
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Quote:
Originally Posted by bandersen View Post
Very clever! I've seen talk use of using a series diode + resistor to improve efficiency, but never a capacitor.

I wonder if adding a thermistor like a CL-90 would completely eliminate the surge ?
I looked at the specs for the CL-90, and you may very well be right.
Have you tried it with a conventional ballast? Shall I?
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  #10  
Old 11-27-2010, 11:45 AM
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kx250rider kx250rider is offline
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This thread is why I feel inadequate for never having taken engineering courses...

I like the "soft start" idea on the filament string; especially since 7JP4s are now so scarce (with good emission), and I've seen plenty of them with open filaments, which this should prevent from happening.

And aesthetically, it is the best I've seen! I think it's a MUST for any VT-7x which will be more than just a display piece, and a MUST for anyone who is repairing sets for other collectors, and doesn't want to explain to an annoyed customer or friend, why a 7JP4 was "good" before, and now isn't...

Charles
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Old 11-27-2010, 05:32 PM
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Quote:
Originally Posted by polyphase View Post
I looked at the specs for the CL-90, and you may very well be right.
Have you tried it with a conventional ballast? Shall I?
I used one in my Hallicrafters series strung set. It definitely provided a soft start. It does get a little hot during operation though so mount appropriately.
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Old 11-27-2010, 05:43 PM
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Very nice work, thank you for posting it.

I'll have to try it out on my VT-71.
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  #13  
Old 04-13-2012, 09:07 AM
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I've been working with Big Dave since 2009 toward developing commercial replacements for the still-available Amperite 17A485459 and the Unobtainium 7A470303 into which we plan to incorporate a few advanced features. The goal we have set for ourselves is to achieve the ability to offer rebuilding of customer-supplied "dud" ballasts of either version by the end of this year. We have four proposed circuits (two for each VT-71 Ballast type) undergoing evaluation for use in the final version. We plan to include some pretty slick features in our final version: inrush limiting; overload protection; visual fault warning; and provisions for servicing instead of outright replacement in the event of a Ballast failure. The technologies we are evaluating for performance-to-cost ratio are Reactance versus Rectification, however, field trials of both technologies are still in progress.

Since most of the work we do is for set owners who are not experienced at post-Restoration troubleshooting, we hope to make subsequent Tech Support as simple as possible and eliminate as much Shipping as possible. Hopefully, developing the ability to determine by phone or email whether post-Restoration servicing (during or after warranty) will require the entire set or only the Ballast Tube to be returned for servicing. With a bit of creativity, we hope to make it easy for the customer to provide meaningful feedback based upon Ballast appearance to facilitate quick troubleshooting of a variety of Fault conditions for which some manner of visual indicator has been built right into the Ballast Tube.

Last edited by jshorva65; 04-13-2012 at 09:37 AM.
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  #14  
Old 04-13-2012, 03:23 PM
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Use of capacitors for filament string voltage dropping does result in a very soft start. I have seen about twenty seconds before voltage came from an initial approximately 60 up to just below line voltage on a string, so in such cases a CL90 wouldn't be needed. CL90's are good where you don't need to drop voltage (although they do drop a couple of volts) but want to just give a soft start. It's also a good idea to put a high value resistor across the dropper capacitors, around 100K, to quickly discharge them at shutdown. A quick turn off/turn on could send a surge through the string so a quick discharge helps.
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Old 04-13-2012, 10:11 PM
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Quote:
Originally Posted by Reece View Post
Use of capacitors for filament string voltage dropping does result in a very soft start. I have seen about twenty seconds before voltage came from an initial approximately 60 up to just below line voltage on a string, so in such cases a CL90 wouldn't be needed. CL90's are good where you don't need to drop voltage (although they do drop a couple of volts) but want to just give a soft start. It's also a good idea to put a high value resistor across the dropper capacitors, around 100K, to quickly discharge them at shutdown. A quick turn off/turn on could send a surge through the string so a quick discharge helps.
It seems like there are some really good ideas evolving in this thread. Our developmental circuits appear to be very similar to those being discussed here, but we're using thermistor types other than the CL-XX series for inrush limiting along with a combination of an internal master fuse plus fusible-link devices on each of the three main current branches. The master fuse inside the Ballast Tube has a slightly larger rating than and is in series with the under-chassis fuse we include in standard Restoration work, intended to be a tamper-resistant backup for the standard fuse. The fusible-link devices are designed to show failure visibly and are positioned to allow inspection under a strong light source through the ventilated metal shell of the Ballast Tube.

Our standard Restoration procedure includes pigtail fuses under the chassis for AC Line and B+ branch circuits or a Custom option substituting a fuse block mounted in an inconspicuous location under the chassis or inside the HV cage making the fuses user-replaceable like tubes upon special request. Therefore, we designed our Ballast Tube substitute in such a manner as to utilize its internal protective devices as tamper-resistant emergency backup devices.
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