[Johnson] Valiant mods........more questions

[Roy Morgan]

At 06:38 PM 2/4/02 -0500, you wrote:
>Hi All,
>I thought I was a member of this group but my posts dont appear so Ill try 
>posting through the general email.
>
>I plan on doing the audio mods recommended in ER number 137, "A Valiant 
>for the 21st Century".

Bill,

That article is not handy, so I can't comment on most of it but:

>...  will the KT88's have the power to modulate the rig. I like KT88's in 
>audio amps and hope he is correct.

Another question could be, why switch tubes?  What is wrong with the tubes 
used in the Valiant already (6146's, right)?

A pair of KT-88's run properly will deliver 75 watts of audio at 0.1 
percent distortion.. (e.g the McIntosh MC-275 amps) and probably 150 watts 
in class B modulator service.  I predict that using KT-88's in the Valiant 
will modulate it quite nicely.. I have no idea if the Valiant modulation 
transformer has the primary impedance to properly load the KT-88's as they 
are run.  The existing transformer is apparently rated at 7300 OHMS 
P-P.  The plate to plate load depends on final RF stage loading, of 
course.   Does the article call for a different, new modulation transformer?

I suggest you check the engineering of the modification before you stuff 
KT-88's in there.. and satisfy yourself that his choice of tubes is based 
on good engineering and not on audio mania for the legendary KT-88.  Also 
check tube prices.. and see what is in both your "junque boxe" and your 
pocket book.  New Sensor gets $34 each for the Sovtek KT-88, according to 
their web site.  Or at Groove Tubes, you can pay $150 for a pair.

One GOOD thing is that there are in fact many KT-88 and similar tubes being 
*manufactured now*, and in good supply.. Some are junk, some are very 
good.  If you buy the junk ones and put 750 volts on the plates, stand by 
to replace your modulation transformer soon.  Run a camcorder trained on 
your modulator tubes to catch the fireworks.  That's only a problem of 
money, of course: Peter Dahl will make you a new one for only $145.00 Plus S&H.

One thought comes to mind: make up adapters to let you run KT-88's in the 
transmitter before you do the permanent wiring..  TRY it first.  Of course 
then you are left with the problem of how to decide if they are working 
well or not.  Signal reports on 3885, maybe.  heheh

You realize that the accessory socket on the back of the transmitter lets 
you modulate the thing with a completely separate external modulator.... 
Now THERE is an idea ..... build the ultimate modulator as a separate unit!

Darn, you've got me thinking.. now where is that issue of ER, and where are 
those Tung Sol beauties I've got stashed here somewhere... hmmmm....  Let's 
see... Oil caps in the B+ supply, silver hookup wire, hand wound coupling 
caps, Vishay resistors in the speech amp.. ahhh.....

Roy
Whose Valiant is stock, but whose Ranger is in for some serious upgrades.




- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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