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help with schematic
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Author:  riven67 [ 08 Apr 2018, 19:18 ]
Post subject:  help with schematic

hello Im new here but have been trying to teach myself as much as possible. I'm still not great reading schematics, so i was hoping someone would look at this schematic for an amp i own and tell me what kind of biasing scheme it has. It's an obscure little amp from Shell Electronics model 2020P Manhasset. I would like to specifically know if its cathode biased.

Manhasset Schematic

thanks for any help!

Author:  Soundbrigade [ 09 Apr 2018, 05:44 ]
Post subject:  Re: help with schematic

Yes it is - both channels 200Ohms//100uF. You could split the cathode R and C and use 400Ohms and 47uF (or preferable 100uF) per channel.

Author:  riven67 [ 12 Apr 2018, 13:14 ]
Post subject:  Re: help with schematic

Soundbrigade wrote:
Yes it is - both channels 200Ohms//100uF. You could split the cathode R and C and use 400Ohms and 47uF (or preferable 100uF) per channel.


Hmm...yes but it looks like the same resister is used for the 12ax7 heater voltage not sure if it can be changed.

Author:  gofar99 [ 13 Apr 2018, 15:33 ]
Post subject:  Re: help with schematic

Hi, I have seen that it older amps. The cathode current is used to provide a DC source for the preamp heaters. Usually a rather large filter cap to keep the signal component from modulating the heater of the preamp tube. I have replaced a number of the resistors in that sort of circuit. When they fail nothing works right and usually the fuse does not blow. The amp will not work if the preamp tube is removed either. It can really mess your mind if you have not seen this type of circuit before. Common in Eico amps.

Good listening
Bruce

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