|[AMRadio] Johnson Valiant Audio Driver Xfmr - Wake Up Call|
garyschafer at comcast.net
Mon Oct 22 22:01:25 EDT 2007
Someone mentioned using the driver transformer from a Viking 11. It may work
ok if there is enough audio from the driver. The difference is that in the
Viking 11 the 807 modulators run AB2 and draw some grid current on peaks.
This requires a low impedance drive source so the driver transformer in
there is a (if I remember right) 3:1 step down transformer to provide a
somewhat low impedance source at the modulator grids.
6146 modulators are usually run in class AB1 because they will produce
almost maximum power in AB1 compared to AB2. The 807 must be run into AB2 to
get much power out of it. So, the AB1 6146's are usually fed with a step up
transformer rather than a step down transformer to increase the drive
voltage. Less power is needed from the modulator driver tube as no current
needs to be supplied, only voltage to the 6146 grids.
Someone also mentioned using a 12AT7 phase splitter in place of a driver
transformer. This works well with the 6146,s as only voltage is needed to
drive the grids. It will not work in a Viking 11 where grid current is drawn
as the 12AT7 is a high impedance source. Bias is supplied to the 6146 grids
thru high value resistors.
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of w0ng at comcast.net
> Sent: Monday, October 22, 2007 7:04 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Johnson Valiant Audio Driver Xfmr - Wake Up Call
> Thanks for the advice. I'll check out the AES website. I'm thinking you're
> referring to the Hammond 124D or 124E interstage driver xfmrs they sell.
> My Valiant uses the 12AU7 driver tube just like your Ranger but mine feeds
> a pair of 6146's whereas your Ranger uses 1614's. I'm not enough of an
> expert at this to know if the impedance, etc. at the modulator grid is the
> same in both 6146 and 1614 modulators or not. Because of my lack of
> specific knowledge on this, choosing the correct replacement driver xfmr
> is a mystery to me for sure.
> The driver xfmr in my Valiant does have a center-tapped secondary, but
> from some info I found on the Peter Dahl website, they say the primary DC
> resistance is 230 ohms (12AU7 parallel driver plates) and the secondary DC
> resistance is 800 ohms C.T. (PP 6146 modulator grids). I haven't yet
> found anything online that matches that particular spec. Guess I'll have
> to keep looking. Any other suggestions appreciated.
> -------------- Original message --------------
> From: "Greg" <bluebirdtele at embarqmail.com>
> > About a year ago I fixed a Ranger with one of the transformers from
> > Electronics supply. I think what you're looking for is a common 3:1 or
> > interstage with a center tap. Right?
> > Go to the Tubes and More website and they should have them listed there.
> > I'm not mistaken it's the transformer that is a reproduction of a big
> > American manufacturer's offering from way back when, The price was
> > $20. If you can't find it, let me know and I'll take a look and see if I
> > ID it for you and let you know.
> > 73's
> > Greg
> > WA7LYO
> > Kinston NC
> > ----- Original Message -----
> > From:
> > To:
> > Sent: Monday, October 22, 2007 5:51 PM
> > Subject: [AMRadio] Johnson Valiant Audio Driver Xfmr - Wake Up Call
> > > In my quest for a replacement for the interstage audio driver
> > > (T4) for my 1957 Valiant I transmitter I got a "wake up call" as to
> > > expensive a new one from Peter Dahl can actually be.
> > >
> > > The quote was $189.66 plus $11.94 for freight and handling. Delivery
> > > 4-6 weeks!
> > > I had no idea that a transformer weighing less than one pound could be
> > > that expensive.
> > > Certainly way out of my budget for sure. I love my Valiant but not
> > > much, hi.
> > >
> > > I'm hoping that someone on this list has been in this situation before
> > > has discovered a cost-effective replacement that they would be willing
> > > share with me and the list membership.
> > > I'm getting kind of frustrated, I guess. Any help would be greatly
> > > appreciated.
> > >
> > > P.S.... I did contact Gary Brown, WZ1M, to see what he can do for me.
> > > my fingers crossed.
> > > Thanks, Bill, w0ng
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