[Johnson] VII downward modulation

[John Lyles]

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