Hoffman AB763 Question

[schoolie]

Hello all,
  Hope you had a great holidays!    I'm starting a one-channel AB763 deluxe reverb, using a Hoffman board, but I'm making a few changes.  I want to use a tweed-style bias a'la 5f6a because I want to use two smaller-value film caps.   I'm also planning to use the 6G16 tremolo.  Can you tell me if the trem circuit has a bias or is it 0V RMS?  I guess I'm wondering what I should use for the voltage divider resistor values.    I'm thinking 32KOhm between the two 10uF caps and 25K pot and 33KOhm resistor, in series, as R2, to ground.  This should produce between -28V and -41V. Will this bias voltage range work?

I have a second question about the reverb dwell pot.  Does that only have one wire from the 500pF-V2a-grid turret to the wiper and one leg connected to ground?

Thanks!

Rob

[sluckey]

Can you tell me if the trem circuit has a bias or is it 0V RMS?

I don't understand, unless you mean "does the trem circuit use the negative bias like a real AB763?" If so, then no.

I'm thinking 32KOhm between the two 10uF caps and 25K pot and 33KOhm resistor, in series, as R2, to ground.  This should produce between -28V and -41V. Will this bias voltage range work?

That should be fine for 6V6s. If too low for 6L6s, just tweak the resistors a bit.

Does that only have one wire from the 500pF-V2a-grid turret to the wiper and one leg connected to ground?

Not quite. The 500pF connects to the top of the pot. The bottom is grounded. And the wiper goes to the parallel grids of the 12AT7 driver.

Here's a schematic for Doug's AB763 that may be useful to you...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

[schoolie]

Thank you, Steve.  Very helpful and thank you for the schematic!

Can you tell me if the trem circuit has a bias or is it 0V RMS?

I don't understand, unless you mean "does the trem circuit use the negative bias like a real AB763?" If so, then no.

I guess I just wondered if the tremolo affects the average  bias voltage to the grids, but the answer is no.

I'm thinking 32KOhm between the two 10uF caps and 25K pot and 33KOhm resistor, in series, as R2, to ground.  This should produce between -28V and -41V. Will this bias voltage range work?

That should be fine for 6V6s. If too low for 6L6s, just tweak the resistors a bit.

I'm using 6v6 tubes, so thanks for the confirmation.

Does that only have one wire from the 500pF-V2a-grid turret to the wiper and one leg connected to ground?

Not quite. The 500pF connects to the top of the pot. The bottom is grounded. And the wiper goes to the parallel grids of the 12AT7 driver.

That's what I read, but in the pictures of Doug's AB763 project on el34world I only saw one wire going to the dwell pot and a wire from the cap to the 12AT7 grids.  Also, I just noticed an extra filter cap!


Here's a schematic for Doug's AB763 that may be useful to you...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

Thanks! very useful

[sluckey]

I guess I just wondered if the tremolo affects the average  bias voltage to the grids, but the answer is no.

Correct, the average bias does not change. But the low frequency tremolo sine wave will vary the grid bias above and below the static (average) dc level established by the bias pot. (See attached pic)

That's what I read, but in the pictures of Doug's AB763 project on el34world I only saw one wire going to the dwell pot and a wire from the cap to the 12AT7 grids.

Ah so! My mistake. Doug simply replaced the stock 1Meg grid return resistor with a 1Meg pot wired as a variable resistor. That will work too. But it will have the side effect of acting like a simple treble bleed tone control too. With the dwell set to zero, you have a 500pF cap directly between the dry guitar signal path and ground. That will kill some of the highs that AB763s are known for. I prefer the voltage divider pot wiring as I described above. Doug's board layout remains the same regardless of how you wire the dwell pot.

[schoolie]

  Thank you for clearing up the dwell pot wiring, Steve!

--Rob

[EL34]

Here's a schematic for Doug's AB763 that may be useful to you...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

Nice schematic Steve
Can I use it on my web site?

[sluckey]

Sure.

[EL34]

Thanks