[R-390] OT: Transistor Amplifier Question

[Barry]

Hi Charles,

That sounds good to me.  I rebuilt a Marantz 1030 a few years ago and it, also, has similar caps.  The odd thing about the 1060 boards is that, instead of having separate holes for some of those caps, they inserted them in the same holes as the transistor's leads.  It's as if it was something they decided was needed after the boards had been manufactured which made them seem a bit odd to me but it wouldn't be a surprise if it was something found after production started and they changed the assembly process after-the-fact.

Thanks for the feedback!

I'm just hoping that the heater voltages for the transistors I'm using to replace the old ones with with are correct.  ;)

Barry - N4BUQ

----- Original Message -----
> From: "Charles Steinmetz" <csteinmetz at yandex.com>
> To: r-390 at mailman.qth.net
> Sent: Wednesday, May 20, 2020 11:28:38 AM
> Subject: Re: [R-390] OT: Transistor Amplifier Question
> 
> Barry wrote:
> 
> > On the preamp board, those 50pF caps are across base and collector, not
> > base and emitter.
> > On the phono board, some transistors have that across base and emitter
> > while others have that across base and collector.
> 
> Caps from base to collector reduce the stage bandwidth much more than
> caps from base to emitter, owing to the Miller effect.
> 
> As I said before, the B-E caps were almost certainly intended to
> mitigate RF interference (which Marantz alwys had a problem with, for
> some reason).
> 
> The B-C caps were almost certainly added to kill oscillations in the
> phono and line amplifiers (which Marantz solid-state gear also often had
> problems with).  It is possible that the KSC1845s you are using will
> upset this balance, and require some adjustment to the B-C capacitor
> values.  Be sure to check with an oscilloscope when you are done to make
> sure none of the amplifier modules is oscillating.
> 
> Best regards,
> 
> Charles
> 
> 
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