200 Ohm resistors

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200 Ohm resistors

Postby Guybanez on Fri Apr 03, 2009 5:04 am

Hi Greg,

Is it common for the 200 Ohm resistors in the Wurlitzer 200 power amp to get hot? I believe the amp is Class A in its operation to a point? which I guess would explain heat dissipation. The amp all works / sounds fine but I'm more concerned with the long term effects on the solder joins etc. Is this a known / normal condition?
Any help or advice would be appreciated.
Thanks

Guy
Guybanez
 
Posts: 6
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Re: 200 Ohm resistors

Postby GregH on Tue Apr 07, 2009 2:28 pm

Yes. The bias current from these resistors dissipates almost 5 watts and is normal to be hot. Long term this can result in solder connections and the board being burned if the resistor is not high enough above the board by at least 1/4 inch. For this reason the resistors are suspended above the board. The operation of the power stage is CLASS AB which means TR9 and TR10 each take up half of a waveform but so that there is minimal crossover distortion a high bias current forces both transistors on simultaneously. The output stage also passes a small current through the speakers and this is used to supply negative feedback for distortion purposes but can damage speakers if a transistor goes bad. The output voltage at the junction of TR9/TR10 will be about 20 volts at no signal so the 200 ohm resistor going from this point to the emitter of TR7 will dissipate at least 1.7 watts and so will be hot. For increased reliabilty we use 220 ohms resistors which reduces the bias by 10% and of course they are also hot. You can order these resistors for your amplifier from VV if you wish.
GregH
 
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Re: 200 Ohm resistors

Postby Guybanez on Tue Apr 07, 2009 3:44 pm

Hi Greg, thanks for your help. I replaced the ceramic resistors that had drifted with Philips WW 5W comparisons of the same value. As they are new they were a bit smelly but they have settled down now. The solder joins on the underside of the board as you mentioned had gone cold so I desoldered and cleaned it all up, I even lay a component wire off cut along the track as reinforcement. I replaced all caps with Sprague Electrolytics etc, also the three pre transistors. The Wurlitzer is now dead quiet and all voltages are bang on. Thanks for again your help, very much appreciated.
Guybanez
 
Posts: 6
Joined: Mon Mar 30, 2009 8:52 pm
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