[Johnson] VII downward modulation

[Gary Schafer]

The Viking II is designed to limit in the modulators before 100% modulation
is reached. The ratio of the modulation transformer is such that it does not
couple heavily enough (load) the modulators. They will go into clipping
before you get enough power out. Backing down on the final plate current and
running less power than they recommend helps a lot with the limited amount
of audio available.

807/1625s need to run into class AB2 in order to get much power out of them
too. Your 3:1 step down transformer may be too much to get enough drive to
the modulators.

Do you notice if the RF wave form shows clipping (flat topping) at maximum
modulation you can get?

Poor 6146's will show up as low modulation also. When you tune the plate
tuning off resonance momentarily they should draw around 400 ma or so if
they are good tubes. They may not quite make it there if the screen clamp is
set too high though. Poor emission on the finals will not allow the peak
current that is necessary for full modulation.

73
Gary  K4FMX

> -----Original Message-----
> From: johnson-bounces at mailman.qth.net [mailto:johnson-
> bounces at mailman.qth.net] On Behalf Of Robert Jefferis
> Sent: Thursday, June 21, 2007 9:12 PM
> To: jtml at vla.com; johnson at mailman.qth.net
> Subject: RE: [Johnson] VII downward modulation
> 
> John,
> 
> Thanks for the tips. Everything you say makes sense and, I have studied
> the
> DX-100 design because it is such a close parallel to the Viking II design.
> I
> did rotate/interchange the 807s - no effect on symptoms. I do have 2 pair
> of
> NOS 807s in the wings. The VII does modulate the 6146 screens, but I have
> not checked this for proper, or optimum operation. I suppose the clamper
> tube could come into play here too if it were not set up quite right, or
> not
> working properly. I did check it statically, and set the threshold
> according
> to manual instructions. It does shut the 6146s down when I remove PA grid
> excitation. But, I have not checked its behavior with modulation. I fear
> you
> are right, that there is a significant problem, not a subtle one or, a
> subtle one causing horrendous symptoms?.
> 
> I also suspect that I will wind up with a triode or triode connected
> pentode
> in V2. Before I do that though, I will have to change T3. The Hammond
> transformer installed does give me the option of putting the 2 primary
> sections in parallel to get a smaller step-down ratio and half the winding
> resistance. Without the change, there is simply not enough gain available
> I
> in the V2 stage in a triode configuration. This project goes in fits and
> starts, taking aback seat to higher priorities. Thanks again.
> 
> 73,
> Bob KF6BC
> 
> -----Original Message-----
> From: johnson-bounces at mailman.qth.net
> [mailto:johnson-bounces at mailman.qth.net] On Behalf Of John Lyles
> Sent: Thursday, June 21, 2007 12:02 PM
> To: johnson at mailman.qth.net
> Subject: [Johnson] VII downward modulation
> 
> Bob
> 
> I haven't done this (switched drivers) on a Johnson, but I have spent
> considerably time playing with a DX100, which had 1625's, equivalent of
> 807s
> for modulators, and ran class AB2 when pushed to peak. If I switched the
> positions of the two 1625s I could cause the modulation to flatten out on
> the opposite polarity. I think the lack of emission on some of the tubes
> was
> a culprit of low modulation peaks, but also the finals needed to be fully
> modulated via the screen dropping R.
> 
> I noted that the original Heathkit design 12BY7 was triode connected and
> run
> through a 2:1 to the grids of the 1625s. I replaced it with a 12B4, which
> is
> a low mu triode, that was close to compatible with a minor socket change.
> 
> Here is what I wrote to the AM Window forum, on March 8, 2005:
> 
> "Steve - last night I did just that. Strange you should mention it, I
> noted
> the 12B4 was the minuature version of the old 6B4G and 2A3 in my RCA tube
> databook. I made a special socket adaptor so I could A/B test the two
> tubes
> as drivers in the DX100. It is a 9 pin tube plug and socket mounted
> vertically, so that I could make the wiring changes in the adapter. There
> are only a few wires needing to be moved to fit a 12B4 into 12BY7 socket.
> Both tubes look incredibly similar, and have same filament ratings.
> 
> What i discovered is that the 12B4 does provide better drive to the 1625s
> when grid current flows in AB2. The 12BY7s crapped out about 50 V p-p on
> each grid. The 12B4 went out to 100 V p-p before noticable distortion
> cropped up. It does have lower gain however, and the 12AX7 is required to
> run at higher levels to make up for it. Oddly enough, the overall THD
> (without feedback) is best with the triode connected 12BY7, not the 12B4!
> It
> was consistently better. I think the 1625s and mod transformer crap out
> before the difference in drivers is noticable.
> 
> I added Neg feedback from the plate of the 12BY7 to the plate of 12AX7 #2,
> through 200 K and a 0.33 mylar cap. The DX100 is very sweet now...."
> 
> So, I would first take a look at your 807s and substitute a pair. Then
> make
> sure your RF output tubes are screen and plate modulated, combined. The
> effect of the 6AK6 substitution may help a minor amount, but your problem
> sounds like a more gross problem. But I would try all things, being an
> experamentalist at heart.
> 
> 73
> John Lyles
> K5PRO
> 
> 
> > After many component replacements, repair of bad chassis ground
> connections,
> > and general cleaning,  the VII is up and running at normal power levels.
> I
> > am at square 1. The previous owner made a half hearted attempt to follow
> the
> > "level 1" mods from the ER article by Mr. Bono. The T3 inter-stage
> > transformer is a Hammond 124E wired in a 3:1 step-down configuration.
> This
> > transformer has adequate inductance for good fidelity, but winding
> > resistances are very high (3.8k Ohm on the primary as wired). Grid drive
> to
> > the 807s is ample and undistorted up to the point of getting into A2
> drive
> > mode. Between 150 Hz and 2kHz I could initially get about 50% downward
> > modulation accompanied by only about 25% upward modulation before
> serious
> > distortion set in at 1kHz. I removed C55 on the primary of the mod
> > transformer (T4) and modulation symmetry improved dramatically, but
> still
> > not perfectly symmetrical. I know that V2 (6AU6) in this configuration,
> > i.e., wired in pentode mode (pseudo-triode mode does not have enough V2
> > stage gain for this T3 setup) presents a source impedance far too high
> to
> > expect A2 operation.
> >
> >
> >
> > I have carefully redone the meter shunts for PA and modulator current.
> When
> > I back the 40M PA loading off to 180 ma I can get about 75% downward
> > modulation undistorted out to about 3.5 kHz. Upward modulation almost
> tracks
> > but not quite. This is with ~300VDC on the 807 screens (I can get better
> > modulation at higher screen voltages, but.)
> >
> >
> >
> > Now to the questions: I have seen a couple of one-line references to
> > substitution of 6AK6s for V2. It has lower transconductance than the
> 6AU6
> by
> > about a factor of 2, but a far lower plate resistance (about 200k Ohm
> vice
> > about 1M Ohm). Is this worth trying? Does my description sound like
> sound
> > like something else is broken? Or, am I simply flogging a dead horse?
> >
> >
> >
> > All comments welcome. Thank you.
> >
> >
> >
> > Bob, KF6BC
> 
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