1625 Class B Modulator Schematic DX-100 Modification

[W7XXX]

Simplify your DX-100 class AB2 1625 modulator ... zero bias (eliminate current draw off 6146 grid bias supply) ... no screen grid current draw off PS ... clean class B triode audio.

[Steve - WB3HUZ]

What about the driver?

[WA1GFZ]

yup, you are going to need about 10 watts to motivate it.

[W7XXX]

What about the driver?

No change required. The 12BY7 has plenty of drive. According to 1625 tube data, in class B with 750 v on plates with current peaking at 240 ma, output is 120 watts with 5.3 watts drive. On the DX-100's I have modified with 120 watts out RF, I could modulate them 100% with gain up about half and about 80- 85 watts out of the modulator. Maybe 3.5 watts drive ... no problem for the 12BY7.

Compared to the .2 watts required in class AB2, yes it is quite an increase in driving power, but it presents no problem. I have never tried wiring the 12BY7 as a triode, but I know of one old ham that rewired the 12BY7 driver circuit to a 12AT7 driver for all triode audio. He had plenty of drive. I never had any problem with the gain control cranked up more.

I drive my 813's in class B with push pull 6L6's in class B at 6 watts out drive and 500 watts peak out of the 813T modulator. The 6L6T's are driven by a 12AU7. I originally used only a half of the 12AU7 and later wired both sections together and noticed little difference.

[Steve - WB3HUZ]

Power will be required. How much? And the 12BY7 has a rather high plate impedance to be used for a Class B driver (something like 90k Ohm). A 12B4 would be a good replacement (only 1k plate resistance).

yup, you are going to need about 10 watts to motivate it.

[W7XXX]

Remember the 12BY7 has 360 volts on the plate and is still operating as a pentode and it is no problem for it to pull a few more mills to put out the 3-4 watts needed for driving the class B 1625's to modulate the 6146's 100%. The DX-100 low voltage supply has plenty of power rated 360 volts at 150 ma. The 12BY7 at this plate voltage can put out up to 7 watts. The driver xfmr ratio is still close enough to be efficient.

Bill Orr claims the driving power required is lower than what the tube handbook suggest.

[Opcom]

may or may not include transformer/mismatch losses or extra margin.

[N3DRB The Derb]

I actually did the 12B4 mod in the Derb-100. 10X better....... if you dont mind cutting a octal socket you can stuff a 6as7 in there which is more or less 2 6B4's in he same glass. It's either sa7 or as7, I cant remember. look it up to make sure.

go push pull with a phase inverter as soon as you can if you want to dispense with the driver transformer.

the plate impedance has got to be as low as possible to drive power to the grid of a triode. Personal opinion :

class ab1 sux, ab2 is marginally better provided you regulate the piss out of the screens and mod them as well with a tickler winding and anything other than triodes suck for audio work.

[KM1H]

A 12BH7 is another readily available choice, either PP or parallel and giving plenty of breathing room. Save the 12BY7's for ricebox drivers, theyre getting scarce.

I used 12B4's as modulators back in a 60's 6M mobile rig. A HP tech clued me in on those and gave me a pair. They used a bunch of them in the 606 sig gen as regulators.

Carl
KM1H

[WU2D]

6BQ5 triode connected and the 6S4 are also good 9 pin choices.

Mike M. WU2D

[W3RSW]

W7XXX (cool call) said;

Compared to the .2 watts required in class AB2, yes it is quite an increase in driving power, but it presents no problem. I have never tried wiring the 12BY7 as a triode, but I know of one old ham that rewired the 12BY7 driver circuit to a 12AT7 driver for all triode audio. He had plenty of drive. I never had any problem with the gain control cranked up more.

Go ahead and lash it up.  If it works well with the 12BY7A (and through the existing driver transformer? ) then we'll quit mentioning every triode under the sun and beating your idea to death.

FWIW my Harmon Kardon Citation II has six 12BY7's and 4 KT88's.  ...the 12BY7's are pentode connected, of course.     Two 12BY7's are used in each channel as a novel phase inverter/driver stage.