Danelectro/Silvertone 1335 - Challenger 89 tremolo very weak

[Saransk]

Glutton for punishment -
Got the Danelectro 89 / Silvertone 1335 working 90%.
There is a real difference between a set of 6L6's at low power vs either 6V6's or El84's
However, even though the voltages and wiring are correct - the tremolo just doesn't work.  Very weak
One schematic for this circuit shows 220K "bias" resistors that connect the tremolo, the other has 390K resistors.  Not sure if that would make a great difference, maybe less dramatic slope.
I'm thinking it's the 6SJ7 tube.  While the Silvertone 1335 shows this tube, and my chassis had the socket and wiring for it, the Danelectro 89 shows a 6AU6.  Even with a NOS tube I'm wondering if, since everything else is, aside from one resistor, identical, the old octal tube really isn't up to the task.
Not sure what the main difference between the 6SJ7 and the 6AU6 is electrically, but it does look like the later versions of this circuit all went to the smaller tube.
Other than that, any ideas why the tremolo is so weak - as it works on the screen inputs (modified bias), it should be fairly strong.
Thanks
Mike

[sluckey]

6L6s are hard to modulate with that tremolo circuit. They require a very healthy oscillator signal. The value of the grid resistors is not likely a factor. C8, C9, and C10 can affect the strength of the tremolo signal. The tube should be the first suspect. Measure voltages on the 6SJ7 and compare to the schematic. Check plate resistor.

[Saransk]

I'm thinking that's why there was a switch to the 6AU6.
Also, depending on which "version" of the schematic looked at, Danelectro/Silvertone/Airline, the plate resistor doesn't change regardless of the tube.
1 - Silvertone 1335 - 6SJ7 - 68K plate & 1 Meg resistor to Strength control
2 - Airline GDR 8516 - 6AU6 - 68K plate & 1 Meg resistor
3 - Danelectro 89 - 6AU6 - 68K plate & 220K resistor to Strength Control, also 220K Grid resistors.
In fact, until the larger/new generation Silvertone amps came along with the optical coupled tremolo, Danelectro/Silvertone seemed to have used this one-tube tremolo in every amplifier with both 6V6 and 6L6 output tubes.
So I'm first going to check the voltages, then try changing the resistor to the Strength control to the 220K in the one schematic.  Finally, since the voltages and circuit is basically the same, I'll build an adapter and stick a 6AU6 in it.
We'll see where that goes

[PRR]

Yes, check voltages. Always a good first step.

6SJ7 was mildly popular. 6AU6 became incredibly popular. Also miniatures and their sockets cost less to make than Octals. When the first rush for 6AU6 declined, their price was probably well below 6SJ7. No-brainer to change the plan for lower amplifier cost.

The 6AU6 is capable of more gain than 6SJ7 in radio stages but in this plan the difference is probably moot.