[AMRadio] 32V-2 speech amp question


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Sun Jan 22 20:04:21 EST 2006


Keep in mind, the input impedance is very high, 1 meg at the mike
input stock radio. No transformer on the mike input in the 32v series.
I thought the driver transformer was adequate, its very large for a driver
transformer, the
DX100 has a postage stamp, the 32v had a fist sized driver transformer.

I use a pair of KT88 tubes as modulators and don't use the driver
transformer 
(gave it away a long time ago). If you run the high impedance mike preamp in
the 
rig and want some hifi, you have to run the mike preamp tube off DC.
I just did 1/2 wave (1 diode) and an electrolytic cap. The size of the cap
sets 
the voltage the filament runs at. 
The DC does not need to be pure, even rough DC will eliminate hum.

Neg feedback, regulated voltage on the mod tube screens, large coupling caps
will get you a long way to a good sounding 32V.
Brett
N2DTS



> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John 
> Coleman ARS WA5BXO
> Sent: Sunday, January 22, 2006 11:03 AM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] 32V-2 speech amp question
> 
> 	I have not driven it externally before Craig but have some
> experience with the circuitry and the driver XFMR is a weak 
> link in the
> circuit the XFMR barely has enough iron and coupling 
> coefficiency in it
> to pass the low frequencies that the rig is designed for.  As a matter
> of fact if the cathode resistor of the driver stage drops in 
> resistance,
> as they or known to do with heat, the driver plate current 
> will quickly
> saturate the driver XFMR causing the bass frequencies to look like a
> trapezoid instead of sine wave.  If you were going to go with and
> outboard amplifier I would include a better driver XFMR as part of the
> external circuit and go to the grids directly.
> 
> John
> Coleman
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Craig Roberts
> Sent: Saturday, January 21, 2006 10:19 PM
> To: Discussion of AM Radio
> Subject: [AMRadio] 32V-2 speech amp question
> 
> Has anyone tried driving the mod input transformer of the 
> 32V-2 (or V-3)
> 
> directly with an outboard mic preamp?  If so, what were the results,
> please?
> 
> Many thanks and 73,
> 
> Craig
> W3CRR
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