Shared cathode Caps/Resistors & Hoffman AB763 1 channel Tremolo oscillator ckt

[RoadShow]

I have three questions, the first regarding a comment in an old thread by Sluckey:

"Parallel Cathode Caps and Shared cathode Caps/Resistors"
https://el34world.com/Forum/index.php?topic=21034.0
Reply #8

Edited to add, the typical Blackface circuit uses a dedicated cathode cap and resistor for V1 triode 1 but shares a cathode cap and resistor for V1 triode 2 with V2 triode 2. If you remove the second channel and build a 1 channel BF amp, is it best to have a dedicated cathode cap and resistor for V1 triode 2 or can it be shared with V1 triode 1?

Use separate cathode R/C. Do not share cathode R/C with two consecutive gain stages. You may just create an oscillator.

Using the Hoffman AB763 1 channel design as an example:
el34world.com/Hoffman/files/Hoffman_AB763_1.pdf

Would it be OK to share cathode R/C with V1-A and V3-B, dropping the resistor to 820?
Second question, if 750 is half of 1.5k and readily available, why are 820 commonly used for this?

And, the third and last question,
When I examine the Tremolo oscillator circuit in the above 1 channel design, it appears to be the circuit in the Fender Vibroverb 6G16, but there is no 0.02 cap going from cathode to plate on V4-A. Is it not necessary?

Thanks for your time...

[pdf64]

Would it be OK to share cathode R/C with V1-A and V3-B, dropping the resistor to 820?

I strongly suggest not to do that. As per sluckey’s statement, don’t share cathodes of cascaded (common cathode) stages. At high control settings instability would be likely.

if 750 is half of 1.5k and readily available, why are 820 commonly used for this?

I can confirm that 750 is half 1k5 
But E24 and higher resistor values have only become commonplace (and cheap!) in the past 2 or 3 decades https://en.m.wikipedia.org/wiki/E_series_of_preferred_numbers
So for E12 series resistors, 820 is within tolerance for 1k5 / 2.

there is no 0.02 cap going from cathode to plate on V4-A. Is it not necessary?

It’s probably more of a ‘nice to have’, helps the modulation waveform to be a bit closer to sinusoidal.

[sluckey]

Using the Hoffman AB763 1 channel design as an example:
el34world.com/Hoffman/files/Hoffman_AB763_1.pdf

Would it be OK to share cathode R/C with V1-A and V3-B, dropping the resistor to 820?

I would say NO. Why would you want to? To save 50 cents? Hoffman is a very smart guy. Build it as he designed and don't worry.

Second question, if 750 is half of 1.5k and readily available, why are 820 commonly used for this?

Because that's what Leo used. Although 750Ω is common today, it was not so common back in the '50s and '60s.

And, the third and last question,
When I examine the Tremolo oscillator circuit in the above 1 channel design, it appears to be the circuit in the Fender Vibroverb 6G16, but there is no 0.02 cap going from cathode to plate on V4-A. Is it not necessary?

Correct. The 6G11 also did not use that cap.

[RoadShow]

Thanks for the replies guys...

[PRR]

......it was not so common back in the '50s and '60s.

1/2W 10% were 10 cents, 1/2W 5% were 20 cents. Why pay more?