Is there a capacitor missing in Tremolo section of the Hoffman AB763 Circuit?

[dbishopbliss]

I have a '67 Bandmaster I plan on restoring using the Hoffman AB763 Circuit. Since a Bandmaster does not have reverb and it has a solid state rectifier I decided to re-draw the schematic "Fender Style".  While I was redrawing the schematic I was looking at the Fender 6G16 schematic an noticed that the Hoffman AB763 schematic does not have a 0.02uF capacitor that is included in the original Vibroverb circuit. 

Can someone tell me if this capacitor was omitted on purpose (e.g., I do not need it) or was it left out by mistake.

I have included my drawing as an attachment.  You will see the location of the 0.02 capacitor in red.

Oh... if you see any other issues with my schematic, please feel free to point them out.

[sluckey]

Can someone tell me if this capacitor was omitted on purpose (e.g., I do not need it) or was it left out by mistake.

IMO it was omitted on purpose. It's not needed. And Most Fender tremolo circuits don't have that cap. Look at the 6G9. But, you'll have to send Doug a pm and find out for sure what his intentions were.

I drew that schematic you linked to. I just used Doug's AB763 layout to reverse engineer the schematic. There is another missing cap on the original Hoffman layout that is very important to good trem operation. See note 2 on that schematic. I think it was omitted on the layout drawing by mistake.

[sluckey]

if you see any other issues with my schematic, please feel free to point them out.

Not really an issue, but if you will be using Hoffman's circuit you may want to include a bias pot.

[dbishopbliss]

You are correct... I accidentally extended the connection to the capacitor after the diode in the bias circuit instead of the center of the 10K-L resistor.  Thanks for catching that.

I just noticed I left out the 1R resistors from the cathodes of the power tubes as well.  Why are those needed?

While I'm asking questions... I essentially copied the Fender locations for AC Switch and Fuse, but I notice the Hoffman schematic has them in series.  I will change that as well.

[sluckey]

I just noticed I left out the 1R resistors from the cathodes of the power tubes as well.  Why are those needed?

They are not needed. But they provide a very safe/convenient method to measure tube current to facilitate bias adjustments.

While I'm asking questions... I essentially copied the Fender locations for AC Switch and Fuse, but I notice the Hoffman schematic has them in series.  I will change that as well.

Back in the 60s the Fender method was fine. And it still works well today. But, with so much focus on 'proper' AC wiring today, it's generally considered better to not put a fuse or switch in the neutral leg of the input AC. I'm not aware of any "Thou Shalt" laws governing how to do this inside an amp/appliance, but there are strict rules governing this for commercial/residential power wiring. NEC (National Electric Code) sets the rules and they say "Thou shall NOT put a switch or fuse in the neutral leg of a circuit." It's a good idea based on safety and it makes sense to do it inside an appliance also. NEC has no jurisdiction inside an amp/appliance.

[EL34]

you'll have to send Doug a pm and find out for sure what his intentions were.

I don't remember, that was many years ago
All I know is mine works just fine

The .022 may not be needed but you can add it very easily across pins 3 and 1 on the back of the tube socket
see the .022 in red in my diagram below

The other difference is I left off a 25/25 cathode cap because I did not need it on my build
see the red 4.7k resistor in my drawing

You can add both parts and report back if you have time
Do a before and after so you can give a good report

[dbishopbliss]

Thanks for the updates.  Here's my latest schematic with the following changes:

• Changed the tone stack of the Normal channel to use the values from a 5F6A Bassman so I can have some variety
  
  • .1 cap becomes .01
  • .1 cap becomes .02
  • 100K resistor becomes 220K
  • .047 cap becomes .01
• Changed the labels of the channels to Tweed and Blackface
• Added the 1R resistors to the cathodes of the power tubes
• Moved the location of the fuse to be inline with the AC switch
• Corrected the location of the connection to the bias pot.
• Added Tremolo Notes section

Please let me know if I've missed anything or should make corrections.

[EL34]

I don't know
Most of that is all personal mods of yours
To each his own on all that stuff

My notes are:
I already have 1 ohm resistors on the power tubes in all my fixed biased Hoffman boards
The Fuse is always the first thing the power cord goes to the Hoffman layouts
It's Vibrato on that circuit, not tremolo

[dbishopbliss]

Thanks for the feedback.  I realize the tone stack is my own thing... the rest is the Hoffman AB763 schematic redrawn for a Bandmaster.  In other words, solid state rectifier and no reverb.  I know it probably doesn't make sense, but I'm so used to looking at the old Fender schematics and layout diagrams that I get confused by newer looking drawings. I kept all the Fender labels and stuff just because I like the way it looks. Redrawing the schematic from scratch has really helped me understand the circuit better. 

While reading some other posts on this board, I came across the Bias circuits page.  I figured, while I am at it I will include the "better bias system" in my build.

Unless someone can find some issues with this version of the schematic, I will put together a BOM an start buying parts. 

[EL34]

I would be interested in your test results of changing the parts in notes 1-4
If they can be documented as true and useful, I would add them to my AB763 notes page

[dbishopbliss]

I would be interested in your test results of changing the parts in notes 1-4
If they can be documented as true and useful, I would add them to my AB763 notes page

I took those notes from the schematic on the AB763 page.  I plan on building stock to begin with but perhaps I will alligator clip those parts in instead of soldering so I can report back.

Sorry, one more question... (maybe this should be a new thread), Why are there three diodes a side (6 total) in series for the full wave rectifier?  I have built a number of audio amplifiers and have used 1N4007 diodes for full wave bridge rectifiers, but I have never doubled or tripled them.  Do I need all three or will a single diode be sufficient?

[sluckey]

Why are there three diodes a side (6 total) in series for the full wave rectifier?  I have built a number of audio amplifiers and have used 1N4007 diodes for full wave bridge rectifiers, but I have never doubled or tripled them.  Do I need all three or will a single diode be sufficient?

Back in the '60s when that amp was designed there were no consumer affordable 1000V diodes. The more common diodes had a voltage rating of 400V, maybe 600V. Voltage ratings are additive when putting diodes in series. It took 3 diodes to get a safe voltage rating.

But now we have a very cheap 1N4007 rated for 1000V. You really only need two  1N4007 diodes in series to be safe with that amp. A single diode in each leg is not safe.