Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gradyclouse on June 17, 2023, 04:55:55 pm
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I intend to build the Robinette "Princeton Delete" design into a 6g2 chassis (long story - I have an existing cabinet on-hand).
I would like to integrate the bias board components onto the "main board" - as shown in this image of a Hoffman Princeton: https://el34world.com/Hoffman/images/Img_8413.jpg (also attached).
I can't find a Hoffman layout .diy file that shows these components on the main board that I can copy when I modify the Robinette layout file. If someone can point me to one that would be great. Otherwise I will modify the Robinette layout file to add the bias components "from scratch". Thanks!
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Hoffman's PR layout was done with Visio. I don't believe a diy layout exists. Maybe you can look at the Visio drawing to add the components to your layout. You may need to buy the trimmer pot from Hoffman or adapt to whatever trimpot you use. Here's Hoffman's layout...
https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf
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Ok, here you go. The DIYLC file should be a help. But!
Buyer beware . . . A couple of caveats.
The schematic is close to Robrob's, but not identical. Specifically, the bias circuit is different, and the power section is adapted for 6L6GC's or 5881's.
While I am reasonably confident that the schematic is correct (for the PT I am using), I blew a 5881 and a 12AX7 15 min after powering up despite using an LBL, and despite nominal voltages. First time this has happened to me ever. So I am in the process of tracking down what might have gone wrong. Literally, this happened yesterday and I probably won't be able to work on it until some time this week.
The DIYLC board is an adaptation of Hoffman's PR board, while reusing a chassis and cab. It could be (far) more efficient, but this was my first go at this particular project. Given the "fail" after powering up, you will have to take a close look at the board design. I will be doing so but probably won't post on this until later this coming week.
As a guide, the "solid blue" lines are above board traces; green and dotted blue are below board insulated traces.
I normally use MANY extra turrets for the connections to off board components, because I can't stand unwrapping and desoldering wire from posts when I need to take the board out -- as will be done in this case. These extra turrets can be shxxcanned of course.
One design choice that escapes my understanding is the remnant of the reverb circuit in between C6 and the grid of V2-A. I've blindly copied Robrob's schematic for that portion, but my gut tells me that this can (should) be simplified. That will be an item to tinker with.
I recently switched from a stereo setup (PR + ODS 124) to a wet/dry rig. This amp is intended to be the dry amp, and I'm trying to design it for the max CLEAN output from 6L6GC's. This current iteration will likely not deliver that . . . suspect that the third amplification stage will induce distortion in either or both of the PI and the power tubes. But we'll see. :icon_biggrin:
Hope this helps.
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Oh, one other thing about the trimmer pot:
Piher trimpots mostly are 5mm across the base, with the wiper being 10mm from the base. You do have to pay attention to the dimensions though, when ordering from Digi-key or Mouser. Do NOT get these from ePay; most there are cheap knockoffs, and I won't tell you how I know this.
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I intend to build the Robinette "Princeton Delete" design into a 6g2 chassis (long story - I have an existing cabinet on-hand).
I would like to integrate the bias board components onto the "main board" - as shown in this image of a Hoffman Princeton: https://el34world.com/Hoffman/images/Img_8413.jpg (https://el34world.com/Hoffman/images/Img_8413.jpg) (also attached).
I can't find a Hoffman layout .diy file that shows these components on the main board that I can copy when I modify the Robinette layout file. If someone can point me to one that would be great. Otherwise I will modify the Robinette layout file to add the bias components "from scratch". Thanks!
.diy files are in the library
You don't need the parts showing, just the holes that get drilled by my CNC machine
The library .diy files are just the boards with holes
Just take the main board and the bias board and join them in DIYLC to make one board
One of my post in the DIYLC board shows how to join two .diy files
https://el34world.com/Forum/index.php?board=26.0 (https://el34world.com/Forum/index.php?board=26.0)
All the Hoffman turret boards are in the .diy library and many of the Rob files also
All the Fenders are there too