[AMRadio] 75TH vs. 75TL (was Class C stuff)

[John Coleman]

That is an interesting lash up Brett.  You may want to expiriment with a
pair of 12-20 volt filiment XFMRs as driver XFMR.  Hook the 110 volt
windings in series to drive the grids and you can parrallel or series the
low volt windings to connect to the 8 ohm driver source.  Some of the
filiment XFMRS you find may be open frame with separate E-I sections you may
want to knock the I section off and magnetically couple the two E sections
together with some straping material to hold them together.  You may be
pleasantly supprised.  I have done all of this but none is compared to the
cathode or emmiter followers type direct coupled circuits. No transfromers.

GL, 73, WA5BXO, John


-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of Brett Gazdzinski
Sent: Thursday, March 21, 2002 12:30 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] 75TH vs. 75TL (was Class C stuff)


John,
In some cases, I have the transformers driving the grids in AB1
(4x150/4cx250), so I am only looking for voltage swing.
In the 811 modulator, 8 ohms drives the transformer,
and the high impedance side does the grids, and seems to
work fine there.

In the ppp 100th deck, the transformer has a tapped high impedance
side, and the impedance is lower (3000 ohms?).

I suppose the transformer in the 100th deck is wrong for the job,
as four 100th tubes take a lot of driving power, and
would likely benefit from a lower impedance driver transformer
and more power to boot.

Unfortunately, I don't think I have anything better in a hi fi
8 ohm to grid transformer.

Perhaps I should plug in some 100tl,s and see if the high impedance
grids do better...270 volt swing on the tl's compared to the
150 volt swing on the th's....

I can hear additional distortion in the mod monitor using the
ppp 100th deck, but part of that is likely in the mod transformer
caused by the different (slightly) impedance ratio.
(7500 to 8700 ohms)

I almost always use the 4x150 mod deck, but cant bring myself
to take apart the ppp 100th deck as it came out so well....

Interesting to note, I use the same cvm5 mod transformer for both
decks, and both decks have the plates hooked up to the mod
transformer.
If I turn one deck on, it modulates the pair of 813's,
the other deck is off and static, and vice versa....

I always ask people if they hear any difference between decks,
and no one has ever said they did.

Its interesting how different they sound in the mod monitor
though...


Brett
N2DTS


>
> It's been a long time since I used a XFMR to drive modulator
> grids where
> grid current is to be drawn but as I recall a lot of class B
> grids were
> driven with a dual 500 ohm secondary windings or something
> with very low
> impedance, and never several thousand ohms.
>
> I must admit, I'm unsure of what your doing so I may be
> misinterpreting
> here.
>
> John, WA5BXO

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